Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia and Parts of Maryland and North Carolina

Prepared in cooperation with the Virginia Department of Environmental Quality

Scientific Investigations Report 2013–5116

U.S. Department of the Interior U.S. Geological Survey Cover. Outcrop of Potomac aquifer sediments at Drewry’s Bluff on the James River, approximately 7 miles downstream from Richmond, Virginia. Typical sediment lithologies are locally diverse and complexly distributed. At top, coarse-grained quartzo-feldspathic sands are cross bedded and iron stained. Near center, a black 2-foot wide angular rip-up boulder of organic-rich clay is flanked by quartzite gravel. Underlying tan medium-grained quartz sand is weakly bedded. At bottom, beds of black organic-rich clay and white kaolinized feldspar sand are broken and truncated by quartzite gravel.

The outcrop is one of a series of sparse exposures of Potomac aquifer sediments, limited and localized along most of the Fall Zone in Virginia to incised major rivers. The bluff forms a 90-foot escarpment along the cut bank of the James River on the outside of a meander bend. The location during the Civil War was the site of Fort Darling, a major Confederate garrison and artillery installation that blocked Union advance along the James River toward the Richmond Capitol, and which is now part of the National Park Service Richmond Battlefields Park. Site access courtesy of Tim Mount, National Park Service. Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia and Parts of Maryland and North Carolina

By E. Randolph McFarland

Prepared in cooperation with the Virginia Department of Environmental Quality

Scientific Investigations Report 2013–5116

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation: McFarland, E.R., 2013, Sediment distribution and hydrologic conditions of the Potomac aquifer in Virginia and parts of Maryland and North Carolina: U.S. Geological Survey Scientific Investigations Report 2013–5116, 67 p., 3 attachments, 2 plates, http://pubs.usgs.gov/sir/2013/5116/. iii

Contents

Abstract...... 1 Introduction...... 2 Purpose and Scope...... 5 Description of the Study Area...... 6 Geologic Setting...... 6 Groundwater Conditions...... 7 Methods of Investigation...... 7 Description of the Potomac Aquifer...... 8 Hydrogeologic Designations...... 8 Configuration...... 9 Flow ...... 14 Sediment Distribution of the Potomac Aquifer...... 17 Lithologic and Depositional Characteristics...... 17 Borehole Sediment Intervals...... 20 Regional Lithologic Trends...... 22 Depositional Subareas...... 26 Norfolk Arch...... 26 Salisbury Embayment...... 26 Albemarle Embayment...... 34 Sediment Age Relations...... 34 Sediment Pollen-Age Zonation...... 34 Hydrostratigraphy of Cretaceous Age Sediments in Southeastern Virginia and Northeastern North Carolina...... 35 Conceptual Depositional Model...... 42 Spatial Controls...... 42 Virginia Fluvial Depositional Complex...... 44 Deposition Over Time...... 44 Hydrologic Conditions of the Potomac Aquifer...... 45 Sediment Hydraulic Properties...... 45 Transmissivity...... 47 Hydraulic Conductivity...... 47 Specific Storage and Storativity...... 50 Vertical Hydraulic Connectivity...... 50 Vertical Hydraulic Gradients...... 53 Regional Connectivity Trends...... 54 Water-Resource Management Applications...... 57 Aquifer Characterization...... 57 Regulatory Implications...... 57 Information Needs...... 58 Summary and Conclusions...... 60 Acknowledgments...... 62 References Cited...... 62 iv

Attachments (available at http://pubs.usgs.gov/sir/2013/5116/)

Attachment 1. Borehole sediment-interval data Attachment 2. Summary of reported aquifer-test results Attachment 3. Summary of calculated vertical hydraulic gradients

Plates (available at http://pubs.usgs.gov/sir/2013/5116/)

1. Map showing locations of boreholes and hydrogeologic section areas 2. Hydrogeologic sections showing distributions of coarse- and fine-grained borehole sediment intervals

Figures

1. Map showing locations of boreholes and hydrogeologic sections...... 3 2. Diagram of stratigraphic relations among hydrogeologic units...... 4 3. Generalized hydrogeologic section and directions of groundwater flow...... 5 4. Geohydrostratigraphic chart of sediments of Cretaceous through early Tertiary age....10 5–6. Maps showing the configuration and groundwater chloride concentration of— 5. The top of the Potomac aquifer...... 12 6. The bottom of the Potomac aquifer...... 13 7–8. Maps showing simulated— 7. Pre-development hydraulic head...... 15 8. Hydraulic head during 2003...... 16 9. Photographs showing examples of sediment lithologies and sections of drill core...... 18 10–12. Diagrams showing— 10. Simplified and generalized fluvial sedimentary depositional environment...... 20 11. Generalized composite electric and gamma borehole geophysical logs...... 21 12. Coarse- and fine-grained borehole sediment intervals...... 23 13–16. Maps showing— 13. Mean borehole-interval thicknesses composed of coarse-grained sediment...... 24 14. Mean borehole-interval thicknesses composed of fine-grained sediment...... 25 15. Percentages of borehole lengths composed of coarse-grained sediment...... 27 16. Borehole intervals of 40 feet or greater thickness exhibiting preserved massive or graded bedding...... 28 17. Hydrogeologic sections...... 29 18–24. Maps showing approximate altitudes and configurations of— 18. The top of the middle confining unit...... 30 19. The bottom of the middle confining unit...... 31 20. The top of the lower confining unit...... 32 21. The bottom of the lower confining unit...... 33 22. The pollen-zone I-II boundary surface...... 38 23. The pollen-zone II-III boundary surface...... 39 24. The top of the upper Cenomanian confining unit...... 41 25. Diagram of simplified structural features of the North American Continental Margin...... 42 v

26–31. Maps showing— 26. Fluvial depositional complexes...... 43 27. Aquifer-test estimates of transmissivity...... 46 28. Open-interval lengths of aquifer-tested wells...... 48 29. Hydraulic conductivity...... 49 30. Aquifer-test estimates of storativity...... 51 31. Specific storage...... 52 32. Conceptual sectional flow net of generalized hydraulic head distribution...... 53 33–35. Maps showing— 33. Historical mean vertical hydraulic gradients within the Potomac aquifer...... 55 34. Historical mean vertical hydraulic gradients between the Potomac aquifer and overlying aquifers...... 56 35. Percent of thickness penetrated by boreholes...... 59

Table

1. Published pollen-zone ages of sediment composing the Potomac aquifer...... 36 vi

Conversion Factors Inch/Pound to SI Multiply By To obtain Length inch (in.) 2.54 centimeter (cm) inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Flow rate inch per year (in/yr) 2.54 centimeter per year (cm/yr) foot per year (ft/yr) 0.3048 meter per year (m/yr) gallon per minute (gal/min) 0.06309 liter per second (L/s) million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s) Aquifer hydraulic properties per foot (/ft) 3.2808 per meter (/m) foot per day (ft/d) 0.3048 meter per day (m/d) foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Altitude, as used in this report, refers to distance above or below the vertical datum. Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). All photographs are by the author. Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia and Parts of Maryland and North Carolina

By E. Randolph McFarland

Abstract medium- to coarse-grained channel fills and point bars segregated from fine-grained overbank deposits. The Virginia Sediments of the heavily used Potomac aquifer broadly depositional complex merged northward across the Salisbury contrast across major structural features of the Atlantic embayment subarea with another complex in Maryland. Here, Coastal Plain Physiographic Province in eastern Virginia and additional sediments were received from schist source rocks adjacent parts of Maryland and North Carolina. Thicknesses that underwent three cycles of initial uplift and rapid erosion and relative dominance of the highly interbedded fluvial followed by crustal stability and erosional leveling. sediments vary regionally. Vertical intervals in boreholes of Because of the predominance of coarse-grained sedi- coarse-grained sediment commonly targeted for completion ments, transmissivity, hydraulic conductivity, and regional of water-supply wells are thickest and most widespread across velocities of lateral flow through the Potomac aquifer are the central and southern parts of the Virginia Coastal Plain. greatest across the Norfolk arch depositional subarea, but Designated as the Norfolk arch depositional subarea, the entire decrease progressively northward with increasingly fine- sediment thickness here functions hydraulically as a single grained sediments. Confining units hydraulically separate interconnected aquifer. By contrast, coarse-grained sediment the Potomac aquifer from overlying aquifers, as indicated intervals are thinner and less widespread across the northern by large vertical hydraulic gradients. By contrast, most of part of the Virginia Coastal Plain and into southern Maryland, the Potomac aquifer internally functions hydraulically as a designated as the Salisbury embayment depositional single interconnected aquifer, as indicated by uniformly small subarea. Fine-grained intervals that are generally avoided for vertical gradients. Most fine-grained sediments within the completion of water-supply wells are increasingly thick and aquifer do not hydraulically separate overlying and underlying widespread northward. Fine-grained intervals collectively coarse-grained sediments. Across the Salisbury embayment as thick as several hundred feet comprise two continuous depositional subarea, however, hydraulic separation among confining units that hydraulically separate three vertically the vertically spaced subaquifers is imposed by the intervening spaced subaquifers. The subaquifers are continuous northward confining units. but merge southward into the single undivided Potomac The Potomac aquifer is the largest and most heavily used aquifer. Lastly, far southeastern Virginia and northeastern source of groundwater in the Virginia Coastal Plain. Water- North Carolina are designated as the Albemarle embayment level declines as great as 200 feet create the potential for depositional subarea, where both coarse- and fine-grained saltwater intrusion. Conventional stratigraphic correlation has intervals are of only moderate thickness. The entire sediment been generally ineffective at accurately characterizing com- thickness functions hydraulically as a single interconnected plexly distributed fluvial sediments that compose the Potomac aquifer. A substantial hydrologic separation from overlying aquifer. Consequently, the aquifer’s internal hydraulic con- aquifers is imposed by the upper Cenomanian confining unit. nectivity and overall hydrologic function have not been well Potomac aquifer sediments were deposited by a fluvial understood. Water-supply planning and development efforts depositional complex spanning the Virginia Coastal Plain have been hampered, and interpretations of regulatory criteria approximately 100 to 145 million years ago. Westward, for allowable water-level declines have been ambiguous. persistently uplifted granite and gneiss source rocks sustained An investigation undertaken during 2010–11 by the a supply of coarse-grained sand and gravel. Immature, high- U.S. Geological Survey, in cooperation with the Virginia gradient braided streams deposited longitudinal bars and chan- Department of Environmental Quality, provides a comprehen- nel fills across the Norfolk arch subarea. By contrast, across sive regional description of the spatial distribution of Potomac the Salisbury and Albemarle embayment subareas, mature, aquifer sediments and their relation to hydrologic conditions. medium- to low-gradient meandering streams deposited Altitudes and thicknesses of 2,725 vertical sediment intervals 2 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina represent the spatial distribution of Potomac aquifer sediments Introduction in the Virginia Coastal Plain and adjacent parts of Maryland and North Carolina. Sediment intervals are designated as Groundwater in the Atlantic Coastal Plain Physiographic either dominantly coarse or fine grained and were determined Province in eastern Virginia (fig. 1) is a heavily used resource. by interpretation of geophysical logs and ancillary information The rate of groundwater withdrawal is estimated to have been from 456 boreholes. Sediment-interval and borehole summary close to zero during the late 1800s, but increased continuously statistical data indicate regional trends in sediment lithology during the 20th century. By 2003, withdrawal rates from and stratigraphic continuity, upon which three structurally Coastal Plain aquifers in Virginia totaled approximately based and hydrologically distinct sediment depositional 117 million gallons per day (Mgal/d) (Heywood and Pope, subareas are designated. Broad patterns of sediment deposition 2009). As a result, groundwater levels have declined by as over time are inferred from published sediment pollen-age much as 200 feet (ft) near large withdrawal centers. Flow gra- data. Discrepancies in previously drawn hydrostratigraphic dients have been reversed from a previously seaward direction to a landward direction, creating the potential for saltwater relations between southeastern Virginia and northeastern intrusion. Increasing withdrawals are likely, which could result North Carolina are partly resolved based on borehole in further water-level declines and intrusion potential. geophysical logs and a recently documented geologic map To manage the groundwater resource, the Virginia and corehole. A conceptual model theorizes the depositional Department of Environmental Quality (VA DEQ) regulates history of the sediments and geologically accounts for their groundwater withdrawals throughout the Virginia Coastal distribution. Documented pumping tests of the Potomac Plain. Withdrawals greater than 300,000 gallons per month aquifer at 197 locations produced 336 values of transmissivity must be approved under the VA DEQ Groundwater With- and 127 values of storativity. Based on effective aquifer thick- drawal Permit Program, which requires groundwater users nesses, 296 values of sediment hydraulic conductivity and to submit withdrawal-related information that is needed to 113 values of sediment specific storage are calculated. Vertical evaluate the potential effects of the withdrawals on the aquifer hydraulic gradients are calculated from 9,479 pairs of water system. levels measured between November 17, 1953, and October 4, The VA DEQ relies on a sound scientific understanding 2011, in 129 closely spaced pairs of wells. of Virginia Coastal Plain geology and hydrology to make Borehole sediment-interval and related data provide groundwater-management decisions. The U.S. Geological a means to achieve high yielding production wells in Survey (USGS) has been advancing knowledge of the geology the Potomac aquifer by site-specific targeting of drilling and hydrology of the Virginia Coastal Plain since the begin- operations toward water-bearing coarse-grained sand and ning of the 20th century. A widely recognized description of gravel. Advance knowledge of the potential of different parts the hydrogeology of the Virginia Coastal Plain resulted from of the aquifer also aids in planning optimal groundwater- the USGS Regional Aquifer-System Analysis (RASA) and development areas. Depositional subareas further provide a related investigations completed during the 1980s. A hydro- possible context for resource management. Current (2013) geologic framework was developed (Meng and Harsh, 1988) regulatory limits on water-level declines are relative to top sur- to provide a basis for construction of a digital computer model faces of subdivided upper, middle, and lower Potomac aquifers of groundwater flow (Harsh and Laczniak, 1990). Although across the entire Virginia Coastal Plain, but have the potential originally developed as a means for scientific analysis of the to exceed the same limit relative to a single undivided aquifer system, the RASA framework and model were adopted Potomac aquifer. By contrast, designation of the sediments as in an updated form by the VA DEQ as an aid in managing a single aquifer in the Norfolk arch and Albemarle embayment the groundwater resource (McFarland, 1998) and currently subareas—and as a series of vertically spaced subaquifers (2013) are used to evaluate the potential effects of existing and and intervening confining units in the Salisbury embayment proposed withdrawals. subarea—best reflects understanding of the Potomac aquifer Following the USGS RASA investigation and related efforts, additional findings provided a basis for major changes and can avoid the potential for excessive water-level declines. in the description of hydrogeologic conditions, with the most Simulation modeling to evaluate effects of groundwater significant being discovery of the largest known meteor- withdrawals could be designed similarly, including vertical impact crater in the United States buried beneath the lower discretization and (or) zonation of the Potomac aquifer based Chesapeake Bay (Powars and Bruce, 1999; Powars, 2000). on depositional subareas and a geostatistical distribution of Knowledge of the impact crater and other recently recognized aquifer properties derived from borehole sediment-interval geologic relations led to a complete redefinition of the data. Further resource-management information needs extend aquifer system, including a refined hydrogeologic framework beyond the developed part of the Potomac aquifer, particularly (McFarland and Bruce, 2006), a revised digital computer across the Northern Neck and Middle Peninsula where only model of the groundwater-flow system (Heywood and Pope, the shallowest part of the aquifer is known, and include 2009), and an updated description of groundwater chemical structural aspects such as faults, basement bedrock, and the quality (McFarland, 2010). Large amounts of new information Chesapeake Bay impact crater. were synthesized to provide a refined regional perspective Introduction 3

77° 76°

PRINCE WILLIAM A’ MD

MARYLAND VA

STAFFORD NC

Atlantic Coastal Plain Fredericksburg Location of the study area in B Virginia, Maryland, and North Carolina KING GEORGE

Potomac Oak SPOTSYLVANIA Grove WESTMORELAND MARYLAND River

NORTHERN NEC CAROLINE 38° NORTHUMBERLAND ACCOMACK ESSEX Pamunkey RICHMOND B’ Mattaponi K E E.G. Surprise Taylor Rappahannock Hill LANCASTER KING AND QUEEN CHESAPEAKE Chickahominy Piankatank HANOVER MIDDLE PENINSUL River BAY River Chesapeake MIDDLESEX KING WILLIAM River VIRGINIA EASTERN SHOR Bay West Impact Point A Richmond River NEW KENT Crater GLOUCESTER MATHEWS HENRICO River

JAMES CITY CHARLES CITY Mobjack YORK-JAMES PENINSUL York CHESTERFIELD A Bay Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James Appomattox NORTHAMPTON River Petersburg Newport News Rowanty Cr. Poquoson DINWIDDIE Blackwater SURRY

Nottoway River Hampton 37° Little

Elizabeth Creek River River

ISLE OF WIGHT SUSSEX Norfolk EXPLANATION River SOUTHEASTERN VIRGINIA Portsmouth A A’ Nansemond R. Virginia Beach Hydrogeologic section

North Landing Aquifer margin SOUTHAMPTON Emporia Borehole location Meherrin River Chesapeake Franklin Suffolk

River GREENSVILLE

ATLANTIC C’ OCEAN

NORTH CAROLINA

Chowan

Albemarle Hope River Sound C 36° Plantation A

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 1. LocationsFigure 1. Locationsof boreholes of boreholes and hydrogeologic and hydrogeologic sections sections of the Potomac of the Potomac aquifer aquifer in Virginia in Virginia and parts and ofparts Maryland of Maryland and North Carolina, and of the Chesapeakeand North Carolina, Bay impact and ofcrater the Chesapeake (from Powars Bay and impact Bruce, crater 1999). (from Further Powars details and Bruce,are shown 1999). on Further plate 1.details Hydrogeologic are shown sections are shown in figureon plate 17. 1. Hydrogeologic sections are shown in figure 17. 4 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina tailored toward meeting the future needs of surficial aquifer water-resource management. Yorktown confining zone The above works provide the most current and comprehensive understanding of the Yorktown-Eastover aquifer geologic relations, configuration, composition, Saint Marys confining unit and hydrologic aspects of a stratified series Saint Marys aquifer of 19 aquifers, confining units, and confining Calvert confining unit zones (collectively termed hydrogeologic Piney Point aquifer units) that compose the Virginia Coastal Plain Chickahominy confining unit aquifer system (fig. 2). Among these, the Potomac aquifer is the largest and most heavily Exmore Matrix confining unit used source of groundwater. The Potomac Exmore Clast confining unit aquifer extends across the entire Virginia Nanjemoy-Marlboro confining unit Coastal Plain except for the inner part of the Aquia aquifer Chesapeake Bay impact crater. The aquifer Peedee confining zone is several thousand feet thick and occupies Peedee aquifer the lowermost stratigraphic position within Present only in the hydrogeologic framework (figs. 2 and 3) Virginia Beach confining zone Southeastern Virginia Greater detail (McFarland and Bruce, 2006). Approximately Virginia Beach aquifer shown in figure 4 three-fourths of the groundwater used in the Upper Cenomanian confining unit Virginia Coastal Plain is produced from the Potomac confining zone Potomac aquifer at a rate of approximately Potomac aquifer 90 Mgal/d (Heywood and Pope, 2009) to supply major industries, many towns and cities, Basement bedrock and low density residential developments in Figure 2. SimplifiedFigure stratigraphic 2. Simplified stratigraphicrelations among relations hydrogeologic among units of the rural areas. Industrial withdrawals at the two Virginia Coastal Plain hydrogeologic (adapted fromunits of McFarland the Virginia Coastal and Bruce, Plain 2006). Associations largest pumping centers in the Virginia Coastal (adapted from McFarland and Bruce, 2006). among some hydrogeologic units that cross stratigraphic boundaries are not shown. Plain are from the Potomac aquifer, resulting Associations among some hydrogeologic units that cross stratigraphic boundaries are not shown. in two regional cones of depression that have water-level declines as great as 200 ft. The cones of depression dominate the hydraulic head distribution across most of the aquifer system (Heywood the USGS RASA investigation subdivided them into upper, and Pope, 2009). Additional large withdrawals from the middle, and lower Potomac aquifers separated by intervening Potomac aquifer are located in metropolitan areas along the confining units (Meng and Harsh, 1988). Subdivision of the middle and lower reaches of the York-James Peninsula and Potomac aquifer was necessary for development of a ground- southeastern Virginia, many for municipally operated public water-flow model (Harsh and Laczniak. 1990). The aquifer water systems. At some systems along the western margin of was vertically discretized into separate model layers that were the Chesapeake Bay impact crater, desalinization of brackish needed to simulate vertical flow within the aquifer system and groundwater is being developed to address growing demands. were combined into a single groundwater-flow model of the Additionally, numerous withdrawals from the Potomac aquifer entire North Atlantic Coastal Plain (Leahy and Martin, 1993). supply diverse commercial, agricultural, community, and Subsequently, data collected from a large number of additional individual domestic uses across most of the Virginia Coastal boreholes have indicated that hydraulically effective confining Plain (Pope and others, 2008). units are not widely present (McFarland and Bruce, 2006). The spatial distribution of Potomac aquifer sediments Thus, a single Potomac aquifer currently (2013) is considered determines the internal hydraulic connectivity and overall to function across a continuous expanse at the regional scale, hydrologic function of the aquifer within the Virginia Coastal but can locally exhibit discontinuities where flow is impeded Plain aquifer system. Conventional stratigraphic correlation, by fine-grained beds. however, has been generally ineffective at accurately char- The complex spatial distribution of contrasting Potomac acterizing the sediment distribution across much of Virginia. aquifer sediments has hindered a full understanding of its The Potomac aquifer consists of fluvial sediments of varied internal hydraulic connectivity and overall hydrologic function compositions that are complexly distributed. Sediments of within the Virginia Coastal Plain aquifer system. Moreover, differing textures contrast sharply and commonly are highly hydrogeologic characterization of the Potomac aquifer is interbedded. Individual beds typically extend only short necessary for water-resource management for two reasons. distances of several hundred feet or less. Numerous studies First, water-supply planning and development efforts in of groundwater in the Virginia Coastal Plain designated these Virginia are hampered by uncertainty associated with the sediments as a single Potomac aquifer or equivalent, until design and siting of production wells in the heterogeneous Introduction 5

WEST EAST PIEDMONT PHYSIOGRAPHIC PROVINCE COASTAL PLAIN PHYSIOGRAPHIC PROVINCE

500 Fall zone 500 Surﬁcial aquifer Outcrops SUFFOLK SCARP CHESAPEAKE BAY IMPACT CRATER

Yorktown confining zone Chesapeake Bay 0 Potomac confining zone 0 Aquia Calvert confining unit Yorktown-Eastover aquifer Nanjemoy-Marlboro confiningPiney unit Point aquifer Saprolite aquifer

–500 –500 Potomac Chickahominy confining unit Potomac aquifer confining zone Exmore Matrix confining unit

Bedrock Direction of –1,000 groundwater Exmore –1,000 ﬂow Altitude, in feet above or below NGVD 29 Clast Fractures conﬁning unit

zone Saltwater-transition –1,500 –1,500 VERTICAL SCALE GREATLY EXAGGERATED 0 10 20 MILES

0 10 20 KILOMETERS Figure 3. Generalized hydrogeologic section and directions of predevelopment groundwater ﬂow in the Virginia Coastal Plain (altitudeFigure 3. Generalizedrelative to Nationalhydrogeologic Geodetic section Vertical and directions Datum of of predevelopment 1929) (from McFarland groundwater and ﬂow Bruce, in the 2006). Virginia Coastal Plain [altitude relative to National Geodetic Vertical Datum of 1929] (from McFarland and Bruce, 2006).

sediments of the Potomac aquifer. Well drillers, environmental of the water level before development and the top of the aqui- consultants, and water-resource managers have short-term, fer being withdrawn. This criterion currently (2013) is applied practical needs for local scale information to facilitate relative to the subdivided upper, middle, and lower Potomac construction of water-supply wells in the Potomac aquifer. aquifers as delineated by the USGS RASA investigation. Effective design of high yielding wells requires site-specific Many withdrawals, however, would likely exceed the limit targeting of productive water-bearing beds composed of for the same criterion relative to a single undivided Potomac coarse-grained sand and gravel, and avoidance of low yielding aquifer because the top of the aquifer generally is higher than fine-grained beds. Production-well drilling projects, however, those of the earlier designated middle and lower aquifers. are currently undertaken without the potential benefit of an More fundamentally underlying the regulatory criterion, organized and easily used source of information on water- a general understanding of how the spatial distribution of bearing beds that are intercepted by existing boreholes. As a contrasting sediments controls internal hydraulic connectivity result, borehole advancement often entails a largely uncharted within the Potomac aquifer is needed to determine the overall exploration of conditions specific to each drilling site. Project function of the aquifer system. designs that incorporate advanced knowledge of likely target To address these needs, an investigation was undertaken depths for well completion would more effectively optimize by the USGS in cooperation with the VA DEQ during drilling operations and reduce costs. More broadly, water- 2010–11 to provide a comprehensive regional description of resource planners require a means to identify optimal areas the Potomac aquifer in the Virginia Coastal Plain. Existing for groundwater development. Understanding broad trends in groundwater data were compiled and analyzed to characterize the composition of Potomac aquifer sediments is contingent the spatial distribution of Potomac aquifer sediments and upon distinguishing areas dominated by relatively dense interpret their relation to hydrologic conditions. concentrations of coarse-grained beds from concentrations of fine-grained beds. Purpose and Scope A second need for water-resource management is designation of the sediments either as a single aquifer or a The Potomac aquifer is characterized here to address series of multiple aquifers, which has bearing on the regulation information needs for water-resource management in the of groundwater withdrawal in Virginia. The VA DEQ limits Virginia Coastal Plain. Both specific information to support water-level declines resulting from regulated withdrawals to water-supply planning and development and a broadened no more than 80 percent of the difference between an estimate perspective to understand the hydrologic function of the 6 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Potomac aquifer are provided. First, a generalized description Richmond. A large metropolitan area, collectively referred to is given to discuss various hydrogeologic designations of as Hampton Roads, occupies the southeast and consists of six the Potomac aquifer, its configuration, and distribution of counties (Gloucester, Isle of Wight, James City, Southampton, groundwater flow. Surry, and York) and 10 cities (Franklin, Hampton, Newport Next, a detailed geologic characterization of the Potomac News, Norfolk, Poquoson, Portsmouth, Williamsburg, Chesa- aquifer is presented. Reader familiarity with basic geologic peake, Suffolk, and Virginia Beach). The latter three cities are terminology and processes is assumed. Lithologies and associ- large—comparable to the counties in land area—and include ated depositional processes of fluvial sediments composing rural as well as urban land uses. The remainder of the Virginia the Potomac aquifer are described. The spatial distribution of Coastal Plain is mostly rural and fairly evenly divided between aquifer sediments is then presented in the form of altitudes and cropland and forest. Small towns are widely scattered, many thicknesses of vertical sediment intervals intercepted by exist- of which serve as county seats. Residential development is ing boreholes located in the Virginia Coastal Plain and adja- increasing by conversion of farmland in proximity to urban cent parts of Maryland and North Carolina. Sediment intervals centers and along waterfronts. are designated as either dominantly coarse or fine grained, as The Virginia Coastal Plain is characterized by rolling determined from interpretations of borehole geophysical logs terrain and deeply incised stream valleys in the northwestern and ancillary information. Borehole sediment-interval data are part and gently rolling-to-level terrain, broad stream valleys, presented in maps and hydrogeologic sections, and tabulated and extensive wetlands in the eastern and southern parts. in digital data-spreadsheet files. Trends among sediment Topography is dominated by valleys of major rivers including intervals and borehole summary statistical data are examined the Potomac, Rappahannock, York, James (fig. 1), and others. to infer broad contrasts between parts of the Potomac aquifer Lowlands consisting of terraces, floodplains, and wetlands dominated by either coarse- or fine-grained sediments. Three occupy valley floors and are flanked by broad uplands along hydrologically distinct depositional subareas are designated basin boundaries. Relict erosional scarps associated with the on the basis of relations among sediment texture, stratigraphic rivers bound the uplands and lowlands (Johnson and Ramsey, continuity, and regional structure. Sediment-age relations 1987), but are obscured in places by the present-day tributary are discussed, and a new conceptual model is introduced drainage pattern. Land-surface altitude ranges from over 300 ft to theorize the depositional history of the sediments and to across some western uplands to 0 ft along the Atlantic coast. geologically account for sediment distribution. Major rivers receive flow from dense and extensive Various hydrologic conditions are related to the spatial networks of tributaries that extend across their entire drainage distribution of Potomac aquifer sediments. Hydraulic proper- basins. These rivers collectively drain to the east and southeast ties of the sediments are analyzed from documented aquifer into Chesapeake Bay, a large estuary formed by submersion pumping tests and are related to contrasting lateral ground- of the Susquehanna River Valley as a result of rising sea level. water velocities inferred from published groundwater age- Major rivers draining from the west also become estuarine dating studies. In addition, vertical hydraulic connectivity upon entering the Coastal Plain. Distinct landmasses defined among sediments within the Potomac aquifer and with by the estuarine rivers include, from north to south, the overlying aquifers is characterized on the basis of vertical Northern Neck, Middle Peninsula, York-James Peninsula, hydraulic gradients calculated from water levels measured in and southeastern Virginia (fig. 1). Chesapeake Bay separates closely spaced pairs of wells. these parts of the Virginia Coastal Plain to the west from the Relevance of the above information to water-resource Virginia Eastern Shore to the east. management is discussed. Guidance is provided for using borehole sediment-interval data to target productive water- bearing beds for construction of water-supply wells in the Geologic Setting Potomac aquifer and to identify optimal parts of the aquifer for The Coastal Plain is underlain by a seaward-thickening groundwater development. In addition, various aspects of the wedge of regionally extensive, generally eastward-dipping sediment distribution are examined with regard to the regula- strata of unconsolidated to partly consolidated sediments of tion of groundwater withdrawal and evaluating its effects and Cretaceous, Tertiary, and Quaternary age that unconformably further information needs. overlie a basement of consolidated bedrock (fig. 3). The sediment wedge extends from Cape Cod, Massachusetts, southward to the Gulf of Mexico and offshore to the Conti- Description of the Study Area nental Shelf. Sediment thickness in Virginia ranges from 0 ft at its western margin to more than 6,000 ft along the Atlantic The Virginia Coastal Plain occupies an area of approxi- coast. The sediments were deposited by seaward prograda- mately 13,000 square miles (mi2) (fig. 1). The climate is tion of fluvial plains and deltas along the North American temperate and humid, with annual precipitation of more than Continental Margin, followed by a series of transgressions 40 inches (in.; National Weather Service, 1996). The region and regressions by the Atlantic Ocean in response to changes generally is heavily vegetated. Major urban centers along in sea level. A thick sequence of nonmarine strata primarily the western margin of the area include Fredericksburg and of Cretaceous age is overlain by a thinner sequence of marine Introduction 7 strata of Tertiary age, which is in turn overlain by a veneer of units are present along the margin of the Chesapeake Bay nearly flat-lying terrace and floodplain deposits primarily of impact crater. None of the hydrogeologic units extends across Quaternary age. the entire Virginia Coastal Plain. Coastal Plain sediments in Virginia were further affected Hydrogeologic conditions in the Coastal Plain are distinct during the Tertiary Period by the impact of an asteroid or from the Piedmont. In the Coastal Plain, groundwater is present comet near the mouth of the present-day Chesapeake Bay in pores between the sediment grains. By contrast, groundwater (Powars and Bruce, 1999). The buried Chesapeake Bay in the Piedmont is present mostly in fractures in the bedrock impact crater is greater than 50 miles (mi) in diameter and and in pores in weathered residuum overlying the bedrock. extends across a large part of the southeastern Virginia Coastal Groundwater in the Coastal Plain is recharged principally Plain (fig. 1). The crater was formed within the preexisting by precipitation that infiltrates the land surface and percolates sediments and contains a unique assemblage of impact-related to the water table. Most of the unconfined groundwater material as deep as basement bedrock. Subsequent sediment flows relatively short distances and discharges to nearby deposition has buried crater-fill sediments approximately streams, but a small amount leaks downward to recharge 1,000 ft below the present-day land surface. the deeper confined aquifers (fig. 3) primarily along the Fall The area to the west of the Coastal Plain is the Piedmont Zone and beneath surface-drainage divides between major Physiographic Province (Piedmont) (fig. 3), which is charac- river valleys. Because aquifers in the Fall Zone are shallow terized by rolling terrain. Residual soils range from nearly 0 to and subcrop along major rivers, flow interactions with the 100 ft thick and are underlain by igneous and metamorphic rivers result from direct hydraulic connections to the land bedrock of late Proterozoic and early Paleozoic age, along surface (McFarland, 1999). The basement bedrock imposes a with fault-bounded structural basins containing sedimentary relatively impermeable underlying boundary. Localized flow and igneous bedrock of Triassic and Jurassic age. Shallow interactions are theorized to occur between bedrock fractures alluvial deposits of Quaternary age are localized in stream val- and the overlying sediments, but the extent and magnitude of leys. The transitional part of the Coastal Plain adjacent to the these interactions are not well known. Piedmont is designated as the Fall Zone. Numerous falls and Because of the stratification of the Coastal Plain sedi- rapids are present in streams where gradients increase across ments, horizontal hydraulic conductivity generally is greater the transition from resistant bedrock onto more easily eroded than vertical hydraulic conductivity. Hence, flow through sediments. From the Fall Zone, the Piedmont bedrock slopes the confined aquifers is primarily lateral in the down-dip eastward beneath the sediment wedge to constitute the base- direction to the east (fig. 3) toward large withdrawal centers ment that underlies the Coastal Plain. The Fall Zone encom- and major discharge areas along large rivers and the Atlantic passes a belt several miles wide with an intricate configuration coast. Contrasting density between freshwater and saltwater of surface exposures of sediment and bedrock. Streams have to the east causes the confined groundwater to discharge by eroded through Coastal Plain sediments to expose Piedmont upward leakage across intervening confining units. In addition, bedrock in their valley floors, whereas interstream divides are hydraulic boundaries along the Chesapeake Bay impact crater capped by uneroded sediments overlying the bedrock (Mixon have been theorized to cause a lateral divergence of flow to and others, 1989). either side of the impact crater (McFarland, 2010).

Groundwater Conditions Methods of Investigation Sediments of the Virginia Coastal Plain are represented by a hydrogeologic framework of aquifers, confining units, Altitudes and thicknesses were determined for 2,725 and confining zones, collectively termed hydrogeologic units vertical sediment intervals within the Potomac aquifer that (fig. 2) (McFarland and Bruce, 2006). Permeable sediments are designated as either dominantly coarse or fine grained. that act as regionally extensive conduits for groundwater flow Geophysical logs and descriptions of sediment cuttings and are designated as aquifers, and less permeable sediments that cores were interpreted from 456 boreholes located in the partly restrict flow are designated as confining units. Less Virginia Coastal Plain and adjacent parts of Maryland and distinct transitional intervals between aquifers and confining North Carolina (fig. 1; pl. 1). Borehole geophysical logs and units are termed confining zones. Parts of some aquifers ancillary information were obtained from records at the USGS provide a widely used supply of water in Virginia. Virginia Water Science Center, a dataset provided by the A complex history of sediment deposition has produced Maryland Geological Survey (MGS), and a North Carolina numerous lateral variations in lithology. Consequently, the Department of Natural Resources (NCDNR) Web site at positions of hydrogeologic-unit margins do not coincide, http://www.ncwater.org/Data_and_Modeling/Ground_Water_ and their areal distribution has a complex, overlapping Databases/logaccess.php. Sediment intervals were determined “patchwork” configuration. In particular, some units pinch from all geophysical logs that span the Potomac aquifer in out westward toward the Fall Zone (fig. 3), where the vertical the revised hydrogeologic framework (McFarland and Bruce, sequence of sediments varies widely compared to other parts 2006), as well as additional logs obtained since the framework of the Coastal Plain. In addition, major discontinuities among was completed. 8 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Hydraulic properties of Potomac aquifer sediments 1973; Virginia State Water Control Board, 1973; Brown and were analyzed from results of 197 aquifer pumping tests Cosner, 1974; Lichtler and Wait, 1974; Cosner, 1975, 1976; documented in diverse sources. Values of vertical hydraulic Siudyla and others, 1977; Ellison and Masiello, 1979; Newton gradient were calculated from 9,479 pairs of water levels and Siudyla, 1979; Harsh, 1980; Hopkins and others, 1981; measured between November 17, 1953, and October 4, 2011, Larson, 1981; Siudyla and others, 1981; Wigglesworth and in 129 closely spaced pairs of wells. Historical water-level others, 1984). data were obtained from the USGS National Water Inventory Common among all of the above studies, Potomac System (NWIS) for Virginia, Maryland, and North Carolina, Formation sediments were designated as a single aquifer. By and from the NCDNR Web site at http://www.ncwater.org/ contrast, the USGS RASA investigation and several closely Data_and_Modeling/Ground_Water_Databases/wellaccess. collaborative studies divided these sediments in Virginia into php. Vertical gradients between wells open to the Potomac upper, middle, and lower Potomac aquifers separated by inter- aquifer were distinguished from gradients between wells in the vening confining units (fig. 4) (Hamilton and Larson, 1988; Potomac aquifer and wells in overlying aquifers. Laczniak and Meng, 1988; Meng and Harsh, 1988; Harsh and Laczniak, 1990). Subdivision of the Potomac aquifer was largely based on a single, then recently completed, corehole Description of the Potomac Aquifer at Oak Grove in Westmoreland County (Reinhardt and others, 1980) (fig. 1). This corehole was heavily relied on as the type stratigraphic section to define the sediment sequence for the Potomac aquifer sediments have been described using entire Virginia Coastal Plain across areas more than 100 mi various geologic and hydrogeologic designations. The to the south. Designation of the confining units that separate Potomac aquifer exhibits distinct structural and hydrologic the subdivided aquifers was based on stratigraphic correlation features. Flow through the Potomac aquifer is a function of among vertical intervals theorized to contain relatively dense relations among sediment hydraulic properties, recharge and concentrations of local-scale beds rather than regionally discharge areas, and hydrologic boundaries and stresses. extensive fine-grained beds. Correlated intervals were viewed as functioning hydraulically across a continuous expanse as Hydrogeologic Designations regional barriers to flow between intervening coarse-grained sand and gravel of the three aquifers. No explicit account Coastal Plain sediments exhibit widely varying was made, however, of the depositional history and related compositions and stratigraphic relations as a result of long processes that would have concentrated the fine-grained and complex depositional histories. The sediments have been sediments that constitute the confining units. designated by numerous studies as geologic formations or The same subdivision of the sediments was made by groups of formations to characterize their depositional history integrated RASA investigations by the USGS in Maryland and and as hydrogeologic units (such as aquifers and confining North Carolina (fig. 4). In Maryland, continuity was inferred units) to characterize their transmission and storage of water. between the Patapsco aquifer and the upper and middle Because the composition of the sediments is closely related Potomac aquifers in Virginia, and the Patuxent aquifer and to their depositional history and hydraulic properties, corre- lower Potomac aquifer (Vroblesky and Fleck, 1991). The same sponding names have traditionally been used to designate both relation has been more recently applied (Drummond, 2007). geologic formations and closely related hydrogeologic units, In North Carolina, continuity was inferred between the upper although their boundaries commonly do not exactly coincide. Cape Fear aquifer and the upper Potomac aquifer in Virginia, Stratigraphic relations that result from the depositional history the lower Cape Fear aquifer and middle Potomac aquifer, of the sediments must be considered conjunctively with their and the lower Cretaceous aquifer and lower Potomac aquifer hydraulic properties to provide context and impose constraints (Winner and Coble, 1996). Similar designations were made on interpretations of their hydraulic connectivity. in Delaware, New Jersey, and New York, and subsequently a McGee (1886) is widely cited as having made the first single hydrogeologic framework was developed for the entire designation of the Potomac Formation for sediments of North Atlantic Coastal Plain (Trapp, 1992). fluvial origin that crop out along the western margin of the Subdivision of the Potomac aquifer facilitated each of Coastal Plain in Virginia and Maryland. Because outcrops the integrated RASA investigations to vertically discretize of the Potomac Formation are relatively sparse in Virginia, the aquifer for development of a series of groundwater-flow descriptions of the Potomac Formation are not included models of the Coastal Plain in Virginia, Maryland, and North in many surficial geologic maps and similar studies of the Carolina (Harsh and Laczniak, 1990; Fleck and Vroblesky, Virginia Coastal Plain. Numerous subsurface studies, however, 1996; Giese and others, 1997). Modeling techniques of that have designated these sediments as the Potomac Formation, time required the designation of separate aquifers to enable Potomac Group, Potomac aquifer, or principal artesian aquifer representation as separate model layers in order to simulate (Clark and Miller, 1912; Sanford, 1913; Cederstrom, 1939, vertical flow within the aquifer system. Moreover, a uniform 1941, 1943, 1945a, 1945b, 1946a, 1946b, 1957, 1968; Sinnott, aquifer classification was used among the integrated studies 1969b; Commonwealth of Virginia, 1973, 1974; Teifke, for the layers to be combined into a single groundwater-flow Description of the Potomac Aquifer 9 model of the entire North Atlantic Coastal Plain (Leahy and Configuration Martin, 1993). Following the USGS RASA investigations, the The top surface of the Potomac aquifer in Virginia slopes hydrogeologic framework of the Virginia Coastal Plain was generally to the east (fig. 5). Offshore from the Atlantic coast, revised (McFarland and Bruce, 2006) to explicitly represent the sediments extend beneath the present-day Continental newly discovered features of the aquifer system, including the Shelf to terminate at the Continental Slope. Both onshore and Chesapeake Bay impact crater (Powars and Bruce, 1999), the offshore sediments are partly to wholly excavated within the James River structural zone, and other geologic relations in Chesapeake Bay impact crater. Fault-bounded megablocks southeastern Virginia (Powars, 2000). A revised groundwater- consisting of Potomac Formation sediments are currently flow model (Heywood and Pope, 2009) and hydrochemical designated (McFarland and Bruce, 2006) to compose the characterization (McFarland, 2010) that was based on the Potomac aquifer within the partially excavated outer part of revised framework indicated that the entire aquifer system the impact crater. Little information currently exists, however, is strongly affected by the impact crater, which has a major on the spatial distribution and hydrologic effects of megablock influence on groundwater flow and the saltwater-transition bounding faults, as well as possible internal disruption of zone. sediments within megablocks. Rotation and (or) overturning of The revised hydrogeologic framework (McFarland and megablocks is indicated by an inverted sequence of sediment Bruce, 2006) designates a single Potomac aquifer in Virginia pollen-age zones exhibited among samples of sediment core on the basis that laterally continuous and hydraulically from borehole 58F 50 located in the city of Newport News effective confining units within Potomac Formation sediments (see section “Sediment Pollen-Age Zonation”). Faulting are not widely present. A large number of additional boreholes and other structural features also likely extend an unknown revealed many locations where previously designated distance beyond the impact crater across a more widespread confining-unit intervals include predominantly coarse-grained disruption zone. A small number of faults have been mapped sediment. Moreover, both coarse- and fine-grained beds by other studies and (or) inferred by large vertical offsets commonly exhibit highly variable thicknesses and large among hydrologic-unit contacts between closely spaced vertical offsets across relatively short distances, which boreholes (fig. 5). Potentially many more faults remain do not align with any consistent regional structural trend. unknown. Within the inner impact crater, the Potomac aquifer Conventional stratigraphic correlation to delineate regionally is wholly excavated to create a structural window to underly- continuous, lithologically uniform volumes of sediment within ing basement bedrock. Crater-fill sediments that are designated the Potomac aquifer is generally ineffective in Virginia. Most as other hydrogeologic units directly overlie bedrock within concentrations of fine-grained beds are of local extent of the inner impact crater. several miles or less. Hydraulic evidence further indicates that The bottom surface of the Potomac aquifer in Virginia Potomac Formation sediments function as a single aquifer. coincides with the top surface of basement bedrock, which Potentiometric surfaces mapped for the separate upper, thereby forms the lower boundary of the Potomac aquifer and middle, and lower Potomac aquifers (Hammond and others, the entire Coastal Plain aquifer system (fig. 3). Thin intervals 1994a, b, c) are broadly similar and do not indicate widespread of saprolite formed from weathered bedrock are preserved in large vertical hydraulic gradients that would result if confin- places. The bedrock slope is generally to the east, but the slope ing units were regionally continuous (see section “Vertical is steeper than that of the top of the Potomac aquifer, which Hydraulic Connectivity”). The most recent groundwater-flow has resulted in an eastward-thickening wedge shape. Bedrock model (Heywood and Pope, 2009) represents the single exhibits an undulating configuration that results from adjacent Potomac aquifer in Virginia with multiple model layers, which major structural features (fig. 6). The broadly uplifted area simulate vertical gradients and flow within the aquifer without across the central and southern parts of the Virginia Coastal requiring subdivision into multiple aquifers. Plain is known as the Norfolk arch. Structurally lower adjacent The most recent hydrogeologic characterizations of areas are known as the Salisbury embayment to the north and the Virginia Coastal Plain (McFarland and Bruce, 2006; the Albemarle embayment to the south. Further details of the Heywood and Pope, 2009; McFarland, 2010) are followed composition and structural features of bedrock are among the here and consider the Potomac aquifer in Virginia to consist least well known aspects of the aquifer system. Bedrock is of all lower Cretaceous to lowermost upper Cretaceous fluvial commonly assumed to be impermeable relative to the overly- sediments of the Potomac Formation (fig. 4). Both undisrupted ing sediments, but some permeable zones may exist where Potomac Formation sediments of Cretaceous age outside of Mesozoic-age sedimentary basins are present. The spatial the Chesapeake Bay impact crater and megablock beds of the distribution and hydrologic effects of faults and fractures that Potomac Formation that formed within the crater during the likely pervade bedrock are also largely unknown. late Eocene Epoch are designated as composing the Potomac The Potomac aquifer is closest to land surface along aquifer. Also tentatively included here are lowermost lower the Fall Zone. Outcrops of Potomac Formation sediments in Cretaceous fluvial sediments of the Waste Gate Formation Virginia are areally extensive only north of Fredericksburg that directly overlie basement bedrock along the Atlantic coast (Mixon and others, 1989) along a relatively broad belt that (Hansen, 1982, 1984). extends northward into Maryland. Westernmost Potomac 10 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

AGE DESIGNATIONS GEOLOGIC INVESTIGATIONS HYDROGEOLOGIC INVESTIGATIONS ATLANTIC COASTAL PLAIN NORTH CAROLINA VIRGINIA MARYLAND NORTH CAROLINA VIRGINIA MARYLAND POLLEN ERA PERIOD EPOCH Powars and Bruce, 1999; Drummond, 2007; Winner and Coble, 1996; Lautier, Meng and Harsh and Hamilton and Larson, 1988; McFarland and Vroblesky and Fleck, 1991; ZONE Owens and Gohn, 1985 Weems and others, 2009 Powars, 2000 Hansen, 1982, 1984 1998; Gellici and Lautier, 2010 Harsh, 1988 Laczniak, 1990 Laczniak and Meng, 1988 Bruce, 2006 Drummond, 2007 late Aquia Formation Beaufort aquifer Aquia aquifer Aquia-Rancocas aquifer Paleocene not not Cenozoic Tertiary Brightseat upper Potomac upper Potomac upper Brightseat early used used Peedee Peedee Jericho Run Brightseat confining unit confining unit confining unit confining unit Paleocene Formation Formation confining confining 6 unit3 Brightseat aquifer zone Brightseat aquifer confining unit 5 Peedee confining unit Lower Brightseat confining unit6 sequence 6 clayey Peedee silty Monmouth Peedee aquifer3 aquifer 5 Peedee aquifer Peedee aquifer Severn aquifer6 2 sand3 VII Formation Group Black Creek Severn and sequence 5 confining upper confining organic-rich 6 younger Donoho unit4 confining Virginia Beach Virginia Beach unit clay3 Creek 3 Potomac Matawan aquifer6 Matawan Group Black Creek aquifer unit confining confining Late sequence 4 Caddin Formation Magothy confining unit6 glauconitic quartz confining ?1 Sheperd Grove Formation sand3 Magothy Formation 4 unit zone Magothy aquifer upper Cape Fear Pleasant Creek Formation Red unit confining V sequence 3 Beds not Patapsco Cape Fear unit Formation glauconitic sand aquifer 4 Virginia Beach aquifer Virginia Beach aquifer confining unit7 identified upper Cenomanian IV sequence 2 Clubhouse upper Cenomanian Mesozoic Cretaceous Formation Beds upper Potomac Brightseat-upper Upper Potomac confining unit upper Cape Fear aquifer III aquifer Potomac aquifer aquifer Potomac confining zone 5 Patapsco lower Cape Fear confining unit middle Potomac confining unit not Patapsco Formation lower Cape Fear II identified 7 confining middle Potomac aquifer aquifer Potomac Potomac unit Early Formation sequence 1 Arundel Formation aquifer lower Cretaceous confining unit4 lower Potomac confining unit Potomac confining unit I Patuxent Patuxent lower Formation Formation Cretaceous lower Potomac aquifer Patuxent aquifer pre-I Waste Gate Formation aquifer Jurassic Triassic basement basement Paleozoic undifferentiated Proterozoic 1Stratigraphic position of transition from zone V to zone VII in Virginia is uncertain. 2Presence inferred from Sohl and Owens (1991). 3Age relation of units in Virginia younger than pollen zone V to units in adjacent States is uncertain. 4Not recognized in Lautier (1998). 5Absent in Virginia in Lautier (1998). 6Not recognized in Drummond (2007). 7Subdivided into upper and lower units in Drummond (2007).

Figure 4. Expanded geohydrostratigraphic chart of sediments of Cretaceous through early Tertiary age in the Virginia Coastal Plain and adjacent parts of Maryland and North Carolina. Description of the Potomac Aquifer 11

77° 76°

–600 MD PRINCE WILLIAM A’

–150

0 VA

–100 –700 STAFFORD –50 NC –450 50 MARYLAND –550 –500 Fredericksburg Location of the study area in

–350 S A L I S B U R Y E M B A Y M E N T Virginia, Maryland, and B North Carolina KING GEORGE –650

–300 –400

–900 –950

–750 –1,050 Potomac

–1,150 Oak WESTMORELAND SPOTSYLVANIA Grove –1,250

0 –800 –250

–1,350 River –850 –1,750 –1,550 –1,650 CAROLINE –1,450 38° NORTHERN NEC

–200 NORTHUMBERLAND RICHMOND ACCOMACK –150 ESSEX B ’

–100 E –50 K Surprise E.G. Rappahannock Hill Taylor LANCASTER 250 KING AND QUEEN

HANOVER MIDDLE PENINSUL

Aquifer margin CHESAPEAKE River MIDDLESEX BAY KING WILLIAM 10,000 VIRGINIA EASTERN SHOR West Point A Richmond

–150 0 NEW KENT GLOUCESTER MATHEWS

50 HENRICO

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A 50 0 Aquifer Hopewell Williamsburg Colonial Heights YORK margin River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

100 River Hampton 50 37° N O R F O L K A R C H Chesapeake 0 Bay ISLE OF WIGHT SUSSEX Impact Norfolk Crater –50 SOUTHEASTERN VIRGINIA Portsmouth –100 –150 Virginia Beach10,000

SOUTHAMPTON –600 Emporia –650 Chesapeake Franklin Suffolk –1,250

GREENSVILLE –700 –750 –1,350 250 –800 –1,300

ATLANTIC –1,750 –250 –350 –950 –1,700 OCEAN

–1,000 –1,650 –1,600 –450 C’ –1,550 –400 –1,500 –300 –850 –1,450 –1,400 –200 NORTH CAROLINA –1,200 EXPLANATION

–1,150 Aquifer margin –1,100 50 Aquifer-top altitude contour. Contour interval

–1,050 is 50 feet. Datum is NGVD of 1929 –550

ALBEMARLEChowan EMBAYMENT 250 Chloride concentration contour, in milligrams per liter –500 Fault A A’ Hope River Hydrogeologic section C Plantation Albemarle

36° A –900 Sound

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 5. ApproximateFigure 5. altitudeApproximate and groundwater altitude and groundwater chloride concentration chloride concentration of the top of of the the Potomac top of the aquifer Potomac in aquiferVirginia in and parts of Maryland and NorthVirginia Carolina. and parts Adapted of Maryland from McFarland and North Carolina. and Bruce Adapted (2006) fromand McFarland and(2010). Bruce Location (2006) ofand Chesapeake McFarland (2010).Bay impact Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Hydrogeologic sections shown on figure 17. crater from Powars and Bruce (1999). Hydrogeologic sections shown in figure 17. Description of the Potomac Aquifer 13

77° 76°

PRINCE WILLIAM A’ MD

–1,750 –2,000 –2,250 –1,250 –2,500 VA –1,000 –1,500 –500 –3,250 Aquifer margin –750 –3,500 NC STAFFORD –250 –3,000 MARYLAND

Fredericksburg –2,750 Location of the study area in B Virginia, Maryland, and North Carolina KING GEORGE PotomacS A L I S B U R Y E M B A Y M E N T

–3,750 Oak SPOTSYLVANIA Grove WESTMORELAND

–5,750

–5,000

–5,500 –5,250 0

–4,250 –6,250

–4,000 –6,000

–4,750

CAROLINE River –4,500 38° NORTHERN NEC NORTHUMBERLAND ACCOMACK ESSEX RICHMOND B’

E K E.G. Surprise Taylor Rappahannock Hill LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond

NEW KENT GLOUCESTER MATHEWS HENRICO

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River Aquifer PRINCE GEORGE James 10,000 margin NORTHAMPTON Petersburg 250 Newport News Poquoson DINWIDDIE SURRY

0 N O R F O L K A R C H River Hampton 37° –250 Chesapeake Bay

–500 ISLE OF WIGHT SUSSEX Impact –750 Norfolk –5,500 Crater SOUTHEASTERN VIRGINIA Portsmouth –4,250 Virginia Beach –1,250 –3,750 –4,000

SOUTHAMPTON –1,500 –1,750 Emporia –1,000 –3,500 –2,000 –3,250 –2,750 –3,000 –6,000 –2,250 –2,500 –6,250 Chesapeake –5,750 –5,250 Franklin Suffolk –5,000 –4,750

–4,500 GREENSVILLE

ATLANTIC OCEAN C’

NORTH CAROLINA EXPLANATION Aquifer margin ALBEMARLE EMBAYMENT–250 Aquifer-bottom altitude contour. Contour interval is 250 feet. Datum is NGVD of 1929

Chowan 250 Chloride concentration contour, in milligrams per liter Aquifer margin A A’ Hope River Hydrogeologic section C A Albemarle 36° Plantation Sound

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 6. ApproximateFigure 6. Approximate altitude and altitude groundwater and groundwater chloride concentration chloride concentration of the bottom of the of bottom the Potomac of the Potomac aquifer inaquifer Virginia in and parts of Maryland andVirginia North Carolina. and parts Adapted of Maryland from and McFarland North Carolina. and Bruce Adapted (2006) from and McFarland McFarland and (2010). Bruce Location (2006) and of McFarlandChesapeake (2010). Bay impact Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Hydrogeologic sections shown on figure 17. crater from Powars and Bruce (1999). Hydrogeologic sections shown in figure 17. 14 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Formation sediments span upland areas to form caps overlying through beds of sand and gravel and is relatively stagnant in basement bedrock, within which the water table is positioned fine-grained beds. At the regional scale, the Potomac aquifer to constitute the unconfined surficial aquifer. Along most of is considered to function hydraulically across a continuous the Fall Zone in Virginia south of Fredericksburg, however, expanse, but locally can exhibit discontinuities where flow is the Potomac aquifer pinches out westward below land surface impeded by fine-grained beds. against basement bedrock (fig. 3). Consequently, outcrops Model simulation (Heywood and Pope, 2009) indicates are very sparse and limited to localized narrow cut banks of that prior to the onset of large withdrawals, groundwater incised major rivers (see report cover photograph). flowed through the Potomac aquifer generally eastward Compaction of sediments within the Potomac aquifer from recharge areas along its western margin to discharge and overlying hydrogeologic units resulted in subsidence areas along Chesapeake Bay and the Atlantic Ocean (fig. 7). of land surface across the Virginia Coastal Plain. Across the Hydraulic heads above sea level across much of the east Chesapeake Bay impact crater, ongoing subsidence has been resulted in upward leakage from the Potomac aquifer through induced by compaction of crater-fill sediments and settling of overlying sediments (fig. 3). Flow also was laterally diverted megablocks since the impact event took place (Powars and around a large stagnant zone across low-permeability sedi- Bruce, 1999). As a result, overlying post-impact sediments ments within the Chesapeake Bay impact crater (see section are downwarped across the impact crater, and land-surface “Configuration”). By contrast, as of 2003 and to the present topography has developed a series of seaward-facing scarps (2013), flow through the Potomac aquifer has been greatly that coincide closely with the crater rim. The formation and affected by numerous, large, and widespread groundwater alignment of Chesapeake Bay also may have resulted from withdrawals that have been imposed for much of the previous redirection of regional surface drainage toward the impact century. The steepest depressions in hydraulic head are near crater. More widely, sediment compaction and land subsidence the largest production wells, but broader regional depressions across most of the Coastal Plain in Virginia and beyond has span the entire aquifer system (fig. 8). Consequently, flow has been inferred to result from large groundwater withdrawals been redirected landward to create the potential for saltwater primarily from the Potomac aquifer (Pope and Burby, 2004). intrusion. In addition, head decline in the Potomac aquifer is Rates of subsidence, however, have only been directly greater than in overlying aquifers, thereby inducing downward measured at two locations during 1980–96. leakage throughout most of the aquifer system (see section Much of the Potomac aquifer contains fresh groundwater, “Vertical Hydraulic Connectivity”). Stagnant conditions dominated most widely by sodium cations and bicarbonate persist, however, within the impact crater. anions (McFarland, 2010). Toward Chesapeake Bay and the Both prior to and since the onset of large withdrawals, Atlantic coast, however, groundwater dominated by elevated steep hydraulic gradients indicate that recharge of the Potomac concentrations of sodium cations and chloride anions forms aquifer is focused along its western margin (figs. 7 and 8). the saltwater-transition zone, a major regional boundary The configuration of the Potomac aquifer, however, largely for the freshwater part of the aquifer system. The saltwater- precludes direct infiltration from land surface along most of transition zone in Virginia protrudes abnormally landward its western margin in Virginia. Other than north of Fredericks- across the Chesapeake Bay impact crater. Curved and closely burg, outcrops of the Potomac aquifer in Virginia exist only spaced contours of groundwater chloride concentration along a few narrow cut banks of incised major rivers (fig. 3), across the top of the Potomac aquifer (fig. 5) reflect how the which function as local discharge areas. Conversely, infiltra- saltwater-transition zone within the upper part of the Potomac tion is received primarily at the water table positioned in aquifer and overlying sediments is steeply sloping, warped, overlying sediments beneath adjacent uplands. Of an estimated and mounded across the impact crater. Conversely, widely 10 inches per year (in/yr) of recharge at the water table along spaced chloride-concentration contours across the bottom of the Fall Zone, approximately 9 in/yr travels only a relatively the Potomac aquifer (fig. 6) reflect that at greater depth the short distance before discharging to nearby streams and rivers saltwater-transition zone slopes more broadly landward. The (McFarland, 1997, 1999). Only the remaining 1 in/yr is not saltwater-transition zone is further affected by discrete, verti- intercepted by localized flow and discharge and, subsequently, cally inverted zones across which salty groundwater overlies is entrained in the regional flow system as recharge to deeper, fresh groundwater (fig. 3), resulting in a three-dimensionally downgradient parts of the Potomac aquifer. convoluted configuration. Prior to large groundwater withdrawals, dense salty groundwater along the coast induced eastward flowing fresh Flow groundwater to leak upward and discharge (fig. 3). Dispersive mixing between fresh and salty groundwater took place across Because of its varied sediment texture, the Potomac a density gradient along the saltwater-transition zone. Major aquifer is classified as a heterogeneous aquifer (McFarland withdrawals from the Potomac aquifer, however, include and Bruce, 2006). Hydraulic conductivity of coarse-grained large capacity production wells currently located within the sand and gravel is large, whereas that of fine-grained sand, silt, saltwater-transition zone that supply brackish groundwater to and clay is considerably smaller. Groundwater flows mostly desalination facilities. Consequently, monitoring of saltwater Description of the Potomac Aquifer 15

77° 76°

PRINCE WILLIAM MD

Aquifer margin NC 25 MARYLAND 75 STAFFORD S A L I S B U R Y E M B A Y M E N T Fredericksburg Location of the study area in

125 Virginia, Maryland, and North Carolina KING GEORGE

Potomac 0 Oak SPOTSYLVANIA100 Grove WESTMORELAND 75 River

CAROLINE NORTHERN NEC

38° 50 NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E E.G. Surprise Taylor Rappahannock Hill 125 LANCASTER KING AND QUEEN HANOVER

100 MIDDLE PENINSUL CHESAPEAKE 75 BAY KING WILLIAM MIDDLESEX River VIRGINIA EASTERN SHOR West Point A Richmond

NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON 125 Petersburg Newport News Poquoson DINWIDDIE SURRY

100 Chesapeake Bay N O R F O L K A R C H River Hampton Impact Crater 37° 125 75 ISLE OF WIGHT SUSSEX 50

100 Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk 25

GREENSVILLE

ATLANTIC OCEAN NORTH CAROLINA

75 Chowan

Aquifer margin

River ALBEMARLE EMBAYMENT

EXPLANATION Hope 25 Hydraulic head contour. Contour interval Albemarle is 25 feet. Datum is NGVD of 1929 36° Plantation Sound

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 7. SimulatedFigure 7.pre-development Simulated pre-development hydraulic head hydraulic in the Potomachead the Potomacaquifer in aquifer Virginia in and Virginia adjacents and adjacents parts of Marylandparts of and North CarolinaMaryland (from Heywood and North and Carolina Pope, 2009). (from Location Heywood of and Chesapeake Pope, 2009). Bay Location impact ofcrater Chesapeake from Powars Bay impact and Bruce crater (1999). from Powars and Bruce (1999). 16 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

STAFFORD NC

–50 –25 MARYLAND

Fredericksburg Location of the study area in Virginia, Maryland, and North Carolina KING GEORGE PotomacS A L I S B U R Y E M B A Y M E N T

Oak SPOTSYLVANIA Grove WESTMORELAND 25

75 50

100 River CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K –25 E E.G. 0 Rappahannock Surprise Taylor Hill LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE Aquifer margin BAY 50 KING WILLIAM MIDDLESEX River VIRGINIA EASTERN SHOR 75 West Point A –125 Richmond NEW KENT –100 –150 GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A 0 Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James 125 NORTHAMPTON Petersburg

Newport News

–50 –75

–25 Poquoson 100 DINWIDDIE SURRY Chesapeake Bay N O R F O L K A R C H River Hampton Impact Crater 37°

ISLE OF WIGHT SUSSEX

Norfolk 125 100 SOUTHEASTERN VIRGINIA Portsmouth –100 Virginia Beach

SOUTHAMPTON Emporia –150 Chesapeake Franklin Suffolk –125 GREENSVILLE

–50

ATLANTIC OCEAN 0 NORTH CAROLINA 75

50 –25 25 Aquifer margin ALBEMARLE EMBAYMENT

Chowan

EXPLANATION 25 Hydraulic head contour. Contour interval Hope River Albemarle is 25 feet. Datum is NGVD of 1929 Plantation 36° Sound

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 8. SimulatedFigure 8.hydraulic Simulated head hydraulic during head2003 induring the Potomac 2003 in the aquifer Potomac in Virginia aquifer and in Virginia parts of and Maryland parts of and Maryland North andCarolina (from Heywood and Pope,North Carolina2009). Location (from Heywood of Chesapeake and Pope, Bay 2009). impact Location crater fromof Chesapeake Powars and Bay Bruce impact (1999). crater from Powars and Bruce (1999). Sediment Distribution of the Potomac Aquifer 17 intrusion is critical to detect salinity increases above brackish 2012), ranging from approximately 3 ft/yr near upgradient concentrations. recharge areas to 0.1 ft/yr downgradient. Lateral velocities The configuration of the saltwater-transition zone resulted likely are slowed, and fresh groundwater flow is stalled from dynamic interactions between fresh and salty ground- progressively downgradient as upward leakage through water over geologically substantial periods of time (McFarland thick low-permeability sediments overlying the Potomac and Bruce, 2005; Sanford and others, 2009; McFarland, 2010). aquifer is induced by the saltwater-transition zone. Lateral Predominantly marine conditions have existed across most of flow velocities correspond to groundwater ages ranging from the Virginia Coastal Plain over the past approximately 65 mil- tens of thousands of years in Virginia to millions of years in lion years (m.y.) since the beginning of the Tertiary Period. Maryland. Thus, most of the Potomac aquifer contains fresh Seawater has been repeatedly emplaced within parts of the groundwater that was emplaced long before the most recent Potomac aquifer and other hydrogeologic units from recurrent glacial maximum. inundation by the Atlantic Ocean. The most recent regionwide series of inundations likely emplaced seawater within much of the Coastal Plain sediments during the Pliocene Epoch, approximately 2.6 to 5.3 million years ago (Ma). Sea level Sediment Distribution of the has since fluctuated at lower levels, but ultimately declined as Potomac Aquifer low as 390 to 470 ft below that of present day during the most recent glacial maximum of the Pleistocene Epoch from 18,000 The Potomac aquifer is composed of fluvial sediments to 21,000 years ago (Peltier, 1994; Bradley, 1999). With that exhibit distinct characteristics resulting from complex, emergence of the land surface, recharge resumed to emplace locally dynamic, and regionally evolving depositional environ- fresh groundwater and displace the saltwater-transition zone ments. Spatial distribution of the sediment is represented by eastward, probably to beyond its current position. Seawater borehole-interval data based on interpretations of geophysical was at least partly retained, however, within low-permeability logs and ancillary information. Regional trends among the sediments filling the Chesapeake Bay impact crater. Fresh sediment intervals are analyzed to describe and geologically groundwater flushed seawater from around the impact crater account for the distribution of sediments by theorizing their in a complex three-dimensional fashion, laterally bifurcating depositional history and related processes. to the north and south, and vertically focusing along discrete preferential pathways to produce inverted salinity zones. With renewed sea-level rise from approximately 18,000 years ago to Lithologic and Depositional Characteristics the present, modern seawater is reentering the sediments, and the saltwater-transition zone is migrating westward to merge “Lithology,” as used in this report, refers to (1) composi- with older saltwater in crater-fill sediments. Eastward flow of tion, including mineralogy of sediment grains, along with fresh groundwater has continued, but has been progressively other components such as fossil shells and organic material, stalled, truncated, and overridden by the westward migrating and (2) grain size distribution or texture. Potomac aquifer sedi- saltwater-transition zone. ments consist of coarse-grained quartz and feldspar sand and The configuration of the saltwater-transition zone remains gravel, interbedded with fine-grained sand, silt, and clay (see largely as it was prior to the onset of large groundwater with- photograph on report cover; fig. A–D9 ). Sediment lithology drawals because flow velocities are very slow, and ground- is locally variable and complexly distributed. Sediments of water has not yet been displaced over regionally appreciable different compositions and textures contrast sharply and are distances in response to the withdrawals. Instead, withdrawn highly interbedded, and individual beds extend only short water has primarily been released from storage. Velocities of distances typically several hundred feet or less. fresh groundwater that flows laterally through the Potomac Sediments of the Potomac aquifer in Virginia are of aquifer vary regionally as a result of differences in the texture fluvial origin. Sediment deposition in fluvial environments is and associated hydraulic conductivity of aquifer sediments locally dynamic. Coarse-grained sand and gravel are deposited (see section “Hydraulic Conductivity”). Estimates of lateral under high energy conditions along relatively narrow stream flow velocity have been based on ages of groundwater relative channels and adjacent bars (fig. 10). Fine-grained sand, silt, to the time of recharge, as determined among different and clay are deposited under low energy conditions across parts of the Potomac aquifer using an array of geochemical broad floodplains, swamps, and other overbank areas away tracers. In Virginia, a lateral flow velocity of approximately from channels. Rapid currents within the stream channels 10 feet per year (ft/yr) is apparent from ages of groundwater allow only the coarsest sediments to settle out, keeping finer samples collected from wells in the Potomac aquifer located sediments in suspension. At high flood stages, however, water in southeastern Virginia across the Norfolk arch (Nelms and spreads beyond the channels across overbank areas where it others, 2003). Ages of water samples collected from wells loses velocity, depositing fine-grained sand, silt, and clay. In located farther north in Virginia into the Salisbury embayment addition, channels and bars shift laterally over time as a result indicate a slower velocity of approximately 4 ft/yr. Yet slower of differential erosion of stream banks, as well as punctuated velocities are indicated in Maryland (Plummer and others, realignments during individual floods that rapidly cut new 18 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

A B

Figure 9. Examples of sediment lithologies. (A) Relation between coarse- and fine-grained sediments of the Potomac aquifer exposed at a sand and gravel operation near Petersburg, Virginia. Clay bed (center left) is approximately 10 feet thick and surrounded by coalescing, predominantly massive beds of coarse- grained sand and gravel. (B) Tough, dense, organic-rich clay is nearly impermeable and is in sharp contact with friable water-bearing fieldspathic Figuresand. Length 9. Example of pick sediment is 22 inches. lithologies. (A) relation between coarse- and fine-grained sediments of the Potomac aquifer exposed at Puddleduck Quarry, Petersburg, Virginia. Clay bed (center left) is approximately 10 feet thick and surrounded byC coalescing, predominatly massive bedsD of coarse-grained sand and gravel. (B) Etough, dense, organic-rich clay is nearly impermeable, and is in sharp contact with friable water-bearing fieldspathic sand. Length of pick is 22 inches.

Figure 9—Continued. Examples of sections of drill core. Core diameters range from approximately 2 to 4 inches, and major scale divisions are in tenths of feet where shown. Corresponding geophysical-log signatures are shown in figure 11, and borehole locations are Figure 9–Continued. Example sediment lithologies, sections of drill core. Core diameters range from shown on plate 1. (C ) Massive smectitic clay of the Potomac aquifer, typical of paleosol floodplain deposits, at a depth of approximately approximately 2 to 4 inches, and major scale divisions are in tenths of feet where shown. Corresponding 125 feet in borehole 54C 10 at Sebrell, Virginia. Lithology is varibly colored and commonly mottled as shown. (D ) Micaceous and variably geophysical-log signatures are shown on figure 11, and borehole locations on plate 1. (C) massive sandy organic-rich clay of the Potomac aquifer, typical of freshwater swamp deposits, at depth of approximately 795 feet in borehole smectitic clay of the Potomac aquifer, typical of paleosol floodplain deposits, at a depth of approximately 125 feet 60L 22 at Surprise Hill, Virginia. Note lenticular and wavy fine interbedding of sand and clay. (E ) Near-shore marine, glaucontitic and in borehole 54C 10 at Sebrell, Virginia. Lithology is varibly colored and commonly mottled as shown. micaceous, sandy and clayey silt of the upper Cenomanian confining unit, at depth of approximately 585 feet in borehole 58A 76 at Dismal (D) micaceous and variably sandy organic-rich clay of the Potomac aquifer, typical of freshwater swamp Swamp, Virginia. Note finely interbedded biofragmental sands and shells that include the diagnostic Exogyra woolmani. deposits, at depth of approximately 795 feet in borehole 60L 22 at Surprise Hill, Virginia. Note lenticular and wavy fine interbedding of sand and clay. (E) near-shore marine, glaucontitic and micaceous, sandy and clayey silt of the upper Cenomanian confining unit, at depth of approximately 585 feet in borehole 58A 76 at Dismal Swamp, Virginia. Note finely interbedded biofragmental sands and shells that included the diagnostic Exogyra woolmani. Sediment Distribution of the Potomac Aquifer 19

Figure 9—Continued. (F ) Relation between the Potomac aquifer and overlying Aquia aquifer exposed at a sand and gravel operation near Petersburg, Virginia. Weathered material has been scraped from surface of quarry wall across center of photograph to reveal sedimentFigure color 9–Continued and composition.. Example Light material sediment at bottom lithologies. is predominantly (F) relation quarzitic between coarse-grained the Potomac sand and aquifergravel that and constit overlayingute the PotomacAquia aquifer. aquifer Contact exposed with dark at glauconiticPuddleduck medium-grained Quarry, Petersburg, sand of the AquiaVirginia. aquifer Weathered is at position material “1.” Aquifer-on-aquifer has been scraped contact is lackingfrom intervening surface offine-grained quarry wall sediments across that center function of as photo a confining to reveal unit to sediment hydraulically color separate and thecomposition. aquifers. Overlyi Lightng intervalsmaterial with contactsat bottom at positionsis predominantly “2,” “3,” and quarzitic “4” exhibit coarse-grained three distinct phases sand of sediment and gravel networking comprising during deposition Potomac of aquifer. Aquia aquifer Contact with sedimentsdark thatglauconitic incorporate medium-grained underlying gravel and sand cobbles of Aquia from theaquifer Potomac is at aquifer. position Such “1”. intervals Aquifer-on-aquifer are likely widespread contact across ismuch lacking of theintervening Virginia Coastal fine-grained Plain and complicate sediments distinguishing that function the aquifers as a confiningfrom signatures unit on to borehole hydraulically geophysical separate logs. the aquifers. Overlying intervals with contacts at positions “2”, “3”, and “4” exhibit three distinct phases of sediment networking during deposition of Aquia aquifer sediments that incorporate underlying gravel and cobbles from channelPotomac segments aquifer. across Suchprevious intervals overbank are areas. likely As widespreaddeposi- gravels, across with much mineralogies of the Virginia that relatively Coastal directly Plain andreflect complicate those tion proceeds over periods of decades to millennia, locally of source rocks. With erosional leveling of sediment-source variabledistinguishing arrays of relict the channel aquifers and frombar deposits signatures develop, on boreholeareas, geophysical older depositional logs. systems are composed of mature, overlapping with relict overbank deposits. medium- to low-gradient meandering streams. Seasonal Distinct variations in the morphologies and sediment large magnitude flows and localized overbank flooding are lithologies of channel, bar, and overbank deposits develop as followed by relatively long intervening periods of stable fluvial environments evolve regionally over millennia. Newly base flow. Channel fills and point bars of medium- to formed depositional systems in proximity to actively eroding coarse-grained sands are segregated from large amounts of sediment-source areas are composed of localized networks fine-grained sediments that form various overbank deposits, of immature braided streams that undergo frequent, large including floodplains, swamps, and abandoned channels such magnitude, but short duration fluctuations in streamflow. as oxbow lakes. Sediment mineralogies are progressively Complexly overlapping and coalescing longitudinal bars dominated by clays produced by chemical weathering of and channel fills are dominated by coarse-grained sands and source rocks. 20 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Floodstage Normal stage

Stream

Floodplain channel Cut bank

Relict fine-grained overbank deposits

Actively depositing fine-grained overbank sediment

Actively depositing Relict coarse-grained bar and coarse-grained bar and channel deposits channel sediment

FigureFigure 10. 10. SimplifiedSimplified andand generalizedgeneralized fluvialfluvial sedimentary sedimentary depositional depositional environment. environment. Coarse-grained Coarse-grained sediments sediments are depositedare underdeposited high flow-energyunder high flow-energy conditions in conditions proximity to in stream proximity channels. to stream Fine-grained channels. sediments Fine-grained are deposited sediments under are depositedlow flow-energy conditionsunder low fartherflow-energy from stream conditions channels. farther Variations from stream in deposit channels. distributions Variations and ingeometries deposit distributions among streams and of geometries differing maturity areamong described streams in text.of differing Coalescing maturity concentrations are described of coarse-grained in text. Coalescing deposits concentrations (front right) comprise of coarse-grained major water-bearing deposits intervals within(front right)the Potomac comprise aquifer, major which water-bearing are sparse intervalsin parts of within the aquifer the Potomac dominated aquifer, by fine-grained which are deposits sparse in(front parts left of and the rear right). aquifer dominated by fine-grained deposits (front left and rear right).

Large fluvial depositional systems can consist of several localized and discontinuous in many places, and wholly absent or more individual drainage basins that change configuration elsewhere, which results in direct aquifer-on-aquifer contact. substantially over geologic time. Moreover, within a large Moreover, reworking of Potomac aquifer sediments into depositional system, immature braided streams in proximity to overlying intervals of the Aquia aquifer is likely widespread sediment-source areas can transition gradationally downgradi- and complicates distinguishing the two aquifers solely from ent to mature meandering streams farther from source areas. signatures on borehole geophysical logs. Interpretive uncer- Distal parts of the system adjacent to coastal areas can merge tainty associated with this stratigraphic relation is represented with subareal deltas, featuring broad networks of distributary by a transitional interval designated as the Potomac confining streams and extensive, perennially inundated interstream zone (McFarland and Bruce, 2006). By contrast, across the marshes and bays. southernmost part of the Virginia Coastal Plain, the Potomac Fluvial sediments of the Potomac aquifer were deposited aquifer is overlain by fine-grained, shelly marine sands of the over a period of approximately 45 m.y. and are overlain by upper Cenomanian confining unit (fig.E 9 ), which is readily younger sediments of mostly marine origin (for example, distinguishable from borehole geophysical-log signatures. fig. 9E and F). Marine sediment lithologies differ from The upper Cenomanian confining unit imposes a substantial fluvial sediments and commonly include components such hydraulic separation between the Potomac aquifer and overly- as glauconite, phosphate, and shells and other fossil mate- ing sediments (see section “Hydrostratigraphy of Cretaceous rial. In contrast to the locally dynamic conditions of fluvial Age Sediments in Southeastern Virginia and Northeastern environments, marine environments are broadly stable. Marine North Carolina”). sediments of relatively uniform lithology are widely deposited across large areas of the Continental Shelf. Hence, many of the sediments overlying the Potomac aquifer can be reliably Borehole Sediment Intervals correlated stratigraphically across distances of tens of miles or more. The spatial distribution of fluvial sediments that compose Across the central and northern parts of the Virginia the Potomac aquifer in the Virginia Coastal Plain and adjacent Coastal Plain, the Potomac aquifer is overlain by medium- parts of Maryland and North Carolina is represented by grained, glauconitic marine sand of the Aquia aquifer vertical intervals intercepted by a network of 456 boreholes (fig. 9F). Intervening fine-grained sediments that function as that penetrate the aquifer (fig. 1; pl. 1). Diagnostic signatures a confining unit to hydraulically separate the two aquifers are of borehole geophysical logs were used (fig. 11), along with Sediment Distribution of the Potomac Aquifer 21 descriptions of sediment cuttings and core, to determine Cross bedding is commonly exhibited in outcrop and sediment altitudes and thicknesses of 2,725 vertical intervals designated core. Blocky resistivity and spontaneous potential log signa- as either dominantly coarse or fine grained. tures indicate massive bedding, whereas tapered signatures Most boreholes are represented by electrical logs (fig. 11) indicate fining-upward graded bedding. Thick distinct bedding consisting of vertical profiles across the borehole of electrical sequences of either type are rarely preserved, however, resistivity and spontaneous potential that vary generally in because the sediments were frequently reworked by alternat- response to sediment texture. A small number of boreholes ing erosion and re-deposition under locally dynamic fluvial are represented by electromagnetic induction logs that are conditions. Hence, many coarse-grained intervals probably similarly interpreted. Many boreholes are further represented represent multiple stacked remnants of bedding sequences that by natural gamma logs, which vary in response to sediment have been partly beveled by erosion. mineralogies having different concentrations of radioactive Sediment intervals designated as fine grained are gener- elements. ally avoided for completion of water-supply wells and are Geophysical-log signatures of Potomac aquifer sediments commonly preserved either in bedded form or as rip-up clasts generally reflect several distinct lithologies. Sediment intervals (see photograph on report cover). Varied geophysical-log designated as coarse grained represent medium- to coarse- signatures reflect different lithologies. Massive clays exhibit grained channel and bar sands and gravels (see photograph distinctively flat and low resistivity and spontaneous potential on report cover; fig. A9 and B), which are commonly targeted signatures, commonly several tens of feet or more in thick- for completion of water-supply wells. The natural gamma ness, along with increased natural gamma (fig. 11) resulting signature (fig. 11) is generally low (to the left), reflecting the from clay minerals with relatively large concentrations of predominance of quartz, but can be slightly increased across radioactive elements. Such signatures on many logs represent some beds if potassium feldspar is sufficiently concentrated. widespread, dense, and commonly multicolored, smectitic

Hydrogeologic Borehole-interval Lithology designation interpretation

upper Marine sandy and clayey silt Cenomanian and shell confining unit

Fining-upward sand Coarse grained

Approximately Massive clay Fine grained 100 feet Potomac Massive sand aquifer Coarse grained and gravel

Fine sand, silt and clay Fine grained

Natural gamma

Increasing

Spontaneous potentia l Resistivity

Increasing Increasing

ro e Z

Figure 11. Generalized composite electric and gamma borehole geophysical logs. Diagnostic log signatures distinguish contrasting lithologies of the Potomac aquifer and overlying upper CenomanianFigure 11. Generalized confining unit. composite electric and gamma borehole geophysical logs. Diagnostic log signatures distinguish contrasting lithologies of the Potomac aquifer and overlying upper Cenomanian confining unit. 22 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina paleosols of floodplains (fig.C 9 ). Aggregated soil structure Borehole sediment-interval data are subject to limitations. and root traces are commonly exhibited in sediment core, Most boreholes terminate partway along their lowermost along with sharply contrasting mottled colors that result sediment interval, the bottom altitude of which is unknown from burrowing followed by post-depositional weathering. and designated as “not determined” (Attachment 1). Some Other massive clays were formed by in-situ kaolinization of interval top altitudes are likewise designated in boreholes that feldspathic sands, which can be distinguished in sediment are only partly spanned by geophysical logs. In both instances, core by the presence of preserved sand-grain nuclei and an individual interval thicknesses are “not determined.” Mean absence of soil-related features. Mineralogic analyses of interval thicknesses are also “not determined” for boreholes core samples indicate the transported floodplain clays to be that do not intercept at least one entire interval. Percentages of composed predominantly of smectitie and illite, whereas in the total length of each borehole composed of coarse-grained situ interstitial clays are predominantly kaolinite (Reinhardt sediments are “not determined” for boreholes that do not and others, 1980). intercept at least one entire coarse-grained and one entire Other sediment intervals designated as fine grained fine-grained interval. represent micaceous silts and fine sands interbedded with Coarse- and fine-grained borehole sediment-interval data black organic-rich clays (fig. A9 , B, and D). These intervals are graphically presented by six sectional views (pl. 2). The exhibit variable geophysical-log signatures among resistivity, sections are referenced to map areas (pl. 1) consisting of six spontaneous potential, and natural gamma (fig. 11), reflecting corresponding north-to-south oriented belts that are indexed to small scale variations among the proportions of the different USGS 7.5-minute series topographic quadrangle maps. Each textures. These sediments were deposited in freshwater section area spans two quadrangles from east to west and from swamps and abandoned stream channels such as oxbow lakes. 8 to 21 quadrangles from north to south. Sediment intervals Sediment core and cuttings commonly include readily visible are projected onto each section from the boreholes located plant fragments and lignite along with more finely dissemi- within the corresponding section area. In addition, traces of nated organic matter. the top surfaces of the Potomac aquifer and basement bedrock Coarse-grained sediment intervals were distinguished across the updip and downdip boundaries of each sectional from fine-grained intervals by identifying log signatures of area are projected onto the sections. prominent thickness and magnitude (fig. 11). Productive water-bearing beds composed of coarse sand and gravel are capable of yielding water at rates from several to multiple tens of gallons per minute (gal/min) or greater, depending on Regional Lithologic Trends well-construction characteristics. Designated coarse-grained intervals are thereby suitable for completing wells to supply Broad trends among borehole sediment intervals were water for uses ranging from domestic and other limited examined to describe the spatial distribution of Potomac demands to large commercial, municipal, and industrial opera- aquifer sediments. Visual examination of the sediment tions. Conversely, fine-grained intervals primarily represent intervals indicates a generally northward-fining/southward- fine sand, silt, and clay that are generally not suitable for coarsening trend that corresponds with major structural well completion, but within which small amounts of coarse- features of the Virginia Coastal Plain and adjacent parts of grained sediment can be interbedded. As designated, both Maryland and North Carolina (pl. 2; fig. 12). A predominance coarse- and fine-grained intervals can encompass multiple of coarse-grained intervals is apparent across the Norfolk arch, individual coarse- or fine-grained beds and, thus, are relatively and fine-grained intervals predominate across the Salisbury generalized. embayment. Coarse- and fine-grained borehole sediment intervals are Summary statistical data for the sediment intervals in tabulated in a digital data-spreadsheet file (Attachment 1). The each borehole corroborate the visually apparent trend. Values spreadsheet “borehole_summary” lists each borehole by num- of borehole mean coarse-grained interval thickness generally ber and includes ancillary information followed by summary are greater across the Norfolk arch than the adjacent Salisbury statistical data for the intervals in each borehole, including the and Albemarle embayments (fig. 13). Mean coarse-grained mean thicknesses of coarse- and fine-grained intervals and the intervals thicker than 80 ft are almost exclusively within the percentage of the total length of each borehole composed of area of the Norfolk arch. In addition, coarse-grained intervals coarse-grained sediments (see section “Regional Lithologic thicken eastward across the Norfolk arch. Almost all mean Trends”). These data are followed by altitudes of the top intervals thicker than 120 ft are along an eastward belt that and bottom surfaces of confining units where present (see spans from near West Point to the south-southeast through the sections “Depositional Subareas” and “Hydrostratigraphy northern part of the city of Suffolk. The eastward thickening of Cretaceous Age Sediments in Southeastern Virginia coarse-grained intervals coincide with eastward thickening of and Northeastern North Carolina”). Individual coarse- and the Potomac aquifer. fine-grained sediment-interval top and bottom altitudes and As a corollary to the above, values of borehole mean thicknesses in each borehole are listed in the spreadsheets fine-grained interval thickness generally are less across the “coarse-grained_intervals” and “fine-grained_intervals,” Norfolk arch than the adjacent Salisbury and Albemarle respectively. embayments (fig. 14). Mean fine-grained intervals of 20 ft Sediment Distribution of the Potomac Aquifer 23

Coarse-grained interval Fine-grained interval

ALBEMARLE EMBAYMENT ge n a i a r d

e c f a n d- su r a l f o n o i t and sediment transport c e r i D

COARSE-GRAINED

Emporia VIRGINIA NORTH CAROLINA NORTH

SEDIMENT-INTERVAL TREND FINE-GRAINED ARCH NORFOLK

North

Richmond SALISBURY EMBAYMENT

VIRGINIA

MARYLAND

sea level sea Present-day Coarse-grained (blue) and fine-grained (black) borehole sediment intervals in the Potomac aquifer, basement bedrock top surface (shaded and contoured), Coarse-grained (blue) and fine-grained (black) borehole sediment intervals in the Potomac aquifer, Fredericksburg

Location of the study area in Virginia Figure 12. and adjacent parts of Maryland North Carolina during inferred directions of land-surface drainage and sediment transport (tan arrows) in the Coastal Plain Virginia is to the northeast at an approximately 45-degree downward angle the early to earliest late Cretaceous period between approximately 100 and 145 million years ago. View exaggeration is 75 times. Basement bedrock dips to the east and exhibits an undulating configuration between uplifted Norfolk arch downwarped (see inset). Vertical Salisbury and Albemarle embayments. A visually apparent trend indicates predominantly coarse-grained sediment intervals southw ard overlying the Norfolk arch fine- grained sediment intervals northward overlying the Salisbury embayment. Coarse-grained (blue) and fine-grained (black) borehole sediment intervals in the Potomac aquifer, basement Figure 12. Coarse-grained (blue) and fine-grained (black) borehole sediment intervals in the Potomac aquifer, bedrock top surface (shaded and contoured), inferred directions of land drainage sediment transport and adjacent parts of Maryland North Carolina during the early to earliest (tan arrows) in the Coastal Plain of Virginia is to the northeast at an approximately late Cretaceous period between approximately 100 and 145 million years ago. View exaggeration is 75 times. Basement bedrock dips to the east, and exhibits 45-degree downward angle (see insert). Vertical an undulating configuration between the uplifted Norfolk arch and downwarped Salisbury Albemarle emabyments. A visually apparent trend indicates predominantly coarse-grained sediment intervals southward overlying the Norfolk arch, and fine-grained sediment intervals northward overlying the Salisbury embayment. 24 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ESSEX RICHMOND E.G. Taylor K E Surprise Rappahannock Hill LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond ACCOMACK NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

NORTH CAROLINA EXPLANATION Mean borehole-interval ALBEMARLEChowan EMBAYMENT thickness, in feet <80 >80–120 Hope Albemarle >120–200 Plantation River Sound >200 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 13. Mean borehole-interval thicknesses composed of coarse-grained sediment of the Potomac aquifer in Figure 13. MeanVirginia borehole-interval and parts of Maryland thicknesses and composedNorth Carolina. of coarse-grained Coarse-grained sediment intervals areof the generally Potomac thicker aquifer across in Virginia the and parts of Maryland and NorthNorfolk Carolina. arch than Coarse-grained elsewhere. Most intervals boreholes generally do not are penetrate thicker entire across aquifer the Norfolk thickness arch (see than ﬁgure elsewhere. 35). Location Most boreholes do not penetrateof entire Chesapeake aquifer Baythickness impact (seecrater ﬁgure from 35).Powars Location and Bruce of Chesapeake (1999). Bay impact crater from Powars and Bruce (1999). Sediment Distribution of the Potomac Aquifer 25

77° 76°

PRINCE WILLIAM MD

VA STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac

Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE BAY KING WILLIAM MIDDLESEX River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

River Hampton Chesapeake Bay 37° Impact Crater Aquifer margin N O R F O L K A R C H

ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

NORTH CAROLINA EXPLANATION Mean borehole-interval ALBEMARLEChowan EMBAYMENT thickness, in feet <20 >20–50 Hope Albemarle >50–80 Plantation River Sound >80 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 14. MeanFigure borehole-interval 14. Mean borehole-interval thicknesses composed thicknesses of composedfine-grained of fine-grainedsediment of sedimentthe Potomac of the aquifer Potomac in Virginia aquifer inand parts of Maryland and NorthVirginia Carolina. and parts Fine-grained of Maryland intervals and North generally Carolina. are Fine-grained thinner across intervals the Norfolk are generally arch than thinner elsewhere. across the Most boreholes do not penetrate entireNorfolk aquifer arch thickness than elsewhere. (see ﬁgure Location 35). Locationof Chesapeake of Chesapeake Bay impact Bay crater impact from crater Powars from and Powars Bruce (1999). and Bruce (1999). 26 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina or thinner are almost exclusively within the area of the ineffective at the regional scale, however, because of widely Norfolk arch. Fine-grained intervals also thicken eastward varying thicknesses and large vertical offsets among closely across the Norfolk arch, coinciding with eastward thickening spaced intervals and lack of their alignment to any consistent of the Potomac aquifer. By contrast, across the Salisbury regional structural trend. Thus, the entire sediment thickness embayment, mean fine-grained intervals thicker than 50 ft are functions hydraulically as a single interconnected aquifer (see common in boreholes located throughout the aquifer, includ- section “Vertical Hydraulic Connectivity”). ing parts of the aquifer to the far west. Most mean intervals thicker than 80 feet, however, are aligned in proximity to the Salisbury Embayment axis of the Salisbury embayment. Based on the above trends, the thicknesses of coarse- and The Salisbury embayment depositional subarea occupies fine-grained sediment intervals broadly contrast between the the northern part of the Virginia Coastal Plain, extending Norfolk arch and the Salisbury and Albemarle embayments. from the upper Middle Peninsula and lower Northern Neck Percentages of the total length of each borehole composed of northward into southern Maryland. Coarse-grained sediment coarse-grained sediments (fig. 15) represent the relative domi- intervals within the Potomac aquifer are thinner and less nance of coarse- and fine-grained sediments across the major widespread than in the Norfolk arch subarea and exhibit structural features independent of the eastward thickening of predominantly graded bedding where distinct sequences are the aquifer. On this basis, coarse-grained sediments are widely preserved. Conversely, fine-grained sediment intervals are more dominant across the Norfolk arch than the adjacent increasingly thick and widespread northward. embayments. Borehole lengths composed of greater than Unique to the Salisbury embayment subarea are 80 percent coarse-grained sediments are almost exclusively two continuous confining units that are delineated here within the area of the Norfolk arch. Moreover, boreholes with (figs. 17–21) on the basis of stratigraphic correlation. The greater than 85 percent coarse-grained sediments span most of confining units encompass fine-grained borehole sediment the area of the Norfolk arch, including areas of the arch to the intervals collectively as thick as several hundred feet and are far west. consistently aligned to a regional structural trend having a Bedding sequences of coarse-grained sediments, dis- generally north-south strike and eastward dip. Thin localized tinguished where preserved from geophysical-log signatures coarse-grained intervals intersected by some boreholes are as either massive or graded (fig. 11), also differ between the included where enclosed between thick fine-grained intervals. Norfolk arch and the Salisbury and Albemarle embayments The confining units approximately correspond to those (fig. 16). Thick (40 ft or greater) graded-bedding sequences mapped in southern Maryland (Drummond, 2007). From are generally widespread, whereas thick sequences of massive southern Maryland, the confining units correlate into Virginia bedding are present only across the Norfolk arch. to boreholes located along the northernmost Northern Neck and Virginia Eastern Shore. Positions of the southern margins of the confining units, however, are uncertain across Depositional Subareas the southernmost part of the Salisbury embayment subarea Thicknesses and relative dominance of coarse- and fine- because no boreholes there are deep enough to intercept the grained sediments composing the Potomac aquifer broadly confining units. Farther south, deep boreholes located along contrast across major structural features of the Virginia the northernmost part of the Norfolk arch subarea exhibit Coastal Plain and adjacent parts of Maryland and North widely varying thicknesses and large vertical offsets among Carolina. Accordingly, three hydrologically distinct sediment fine-grained intervals, which consequently cannot be corre- depositional subareas are designated here (figs. 12–17)—the lated with the confining units to the north. Moreover, margins Norfolk arch, Salisbury embayment, and Albemarle embay- of the confining units are likely indistinct because of region- ment—on the basis of regional trends in sediment lithology ally gradational changes in fluvial depositional conditions over and stratigraphic continuity. time. The confining units are inferred to extend across most of the Salisbury embayment subarea, based on the likelihood that mature meandering streams spanned the concave base- Norfolk Arch ment bedrock surface in the southern part of the subarea (see The Norfolk arch depositional subarea occupies the cen- section “Deposition Over Time”). Far southeastern parts of the tral and southern parts of the Virginia Coastal Plain, extending confining units possibly are also truncated by the Chesapeake from the upper York-James and lower Middle Peninsulas Bay impact crater. southward through southeastern Virginia to the North Carolina The confining units hydraulically separate coarse-grained line. Coarse-grained sediment intervals are among the thickest sediments within the Salisbury embayment subarea into three and most widespread within the Potomac aquifer and exhibit vertically spaced subaquifers—the upper, middle, and lower massive as well as graded bedding where distinct sequences Potomac aquifers (fig. 17, sectionsA –A' and B–B' ). Vertical are preserved. Both coarse- and fine-grained intervals thicken flow between the subaquifers is impeded by the confining eastward, but coarse-grained sediments dominate the entire units. The subaquifers are continuous northward with aquifers subarea. Stratigraphic correlation among sediment intervals is in southern Maryland, designated from shallowest to deepest Sediment Distribution of the Potomac Aquifer 27

77° 76°

PRINCE WILLIAM MD

STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

NORTH CAROLINA EXPLANATION Percentage of borehole

ALBEMARLEChowan EMBAYMENT length <80 >80–85 Hope Albemarle >85–90 Plantation River Sound >90 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 15. Percentages of borehole lengths composed of coarse-grained sediment of the Potomac aquifer in Figure 15. PercentagesVirginia and of borehole parts of Maryland lengths composed and North Carolina.of coarse-grained Coarse-grained sediment sediments of the Potomacare more dominantaquifer in across Virginia the and parts of Maryland and NorthNorfolk Carolina. arch than Coarse-grained elsewhere. Most sediments boreholes are domore not dominantpenetrate acrossentire aquifer the Norfolk thickness arch (see than figure elsewhere. 35). Most boreholes do not penetrate Locationentire aquifer of Chesapeake thickness Bay (see impact figure crater 35). Location from Powars of Chesapeake and Bruce (1999).Bay impact crater from Powars and Bruce (1999). 28 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

EXPLANATION NORTH CAROLINA Massive bedding Graded bedding

ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 16. Borehole intervals of 40 feet or greater thickness exhibiting preserved massive or graded bedding Figure 16. amongBorehole coarse-grained intervals of sediments 40 feet or of greater the Potomac thickness aquifer exhibiting in Virginia preserved and parts massive of Maryland or graded and North bedding Carolina. among coarse-grained sedimentsMassive of the Potomac bedding aquiferis exhibited in Virginia only across and parts the Norfolk of Maryland arch. Location and North of ChesapeakeCarolina. Massive Bay impact bedding crater is exhibitedfrom only across the Norfolk arch.Powars Location and Bruce of Chesapeake (1999). Bay impact crater from Powars and Bruce (1999). Sediment Distribution of the Potomac Aquifer 29 A ′ Nor th CH Be 57

′ C

CH Ce 57

East unit

unit e

D10A aquifer upper s aquifer

Potomac it eho l

un CH Ee 96 sediments D12V

conﬁning River

logged bo r

conﬁning Fault Site name

Potomac Potomac

MARYLAND lower Potomac aquifer

E13M interval boundary

middle VIRGINIA Sediment pollen-age ic l

55Q 6 l

I lower I

og I H i

e I B-B ′ I 8 Oak 54P 3 s e II I I I I I i

Potomac aquifer Grove I I og r SECTION

p dr r 60L 2

II I u

r hy S River

middle Chowan he EXP L A N T I ON

Ot ock

Salisbury embayment upper Cretaceous

55N 2

Cenomanian conﬁning unit be d r

be r n t A-A ′ G19B e SECTION nu m A - ’ left and C-C ’), truncated

upper Base m eho l s ’ left). i t u n C Bo r –

Sediment pollen-age H22I C sample location and zone VERTICAL EXAGGERATION 80X VERTICAL EXAGGERATION C (dashed where inferred) 55K 6 West

0 Hydrogeologic-unit contact

500 50 0 – –1,000 –1,500 –2,000 –2,500 –3,000 West Point 55J 13

′ B 56H 46 East I I I ic I I II I II I II I o g o l I 66M 1

e I

o g E.G. Taylor

d r 20 MILES VIRGINIA 30 KILOMETERS h y

56F 52 20

MARYLAND 10 Williamsburg 10 5 SO Dd 47 5

ock r VIRGINIA 0 0 River James h e

be d r

o t n t

Bay e Potomac aquifer Chesapeake

Base m

Hill 55D 5 60L 28

Surprise

i t

Norfolk arch VIRGINIA u n

55C 12 NORTH CAROLINA NORTH o g o l e

o g ’) and truncated against basement bedrock by a fault (section d r

upper Potomac aquifer C r h y 56A 9

middle conﬁning unit – Franklin

h e lower conﬁning unit C

middle Potomac aquifer O t lower Potomac aquifer

s pe r

n t ock u p e Cenomanian River conﬁning unit be d r

56N 7

Chowan se di m n t e

’ left and

A D18M – 55P 3 Base m A ous I I I I I I

c e

a A-A ′ t Oak

54P 3 I I Grove e I I SECTION

C r

B 9 C-C ′ G 1

53P 4 Albemarle embayment Albemarle SECTION pe r u p

’). The Potomac aquifer in North Carolina and southernmost Virginia is hydraulically separated from overlying upper Cretaceous sediments by the ’). The Potomac aquifer in North Carolina and southernmost Virginia H20S Hope B Plantation – 51Q 20 VERTICAL EXAGGERATION 80X VERTICAL EXAGGERATION 80X VERTICAL EXAGGERATION

B B A ou th West S 0 and northern most Virginia (sections ’ center). Fine-grained conﬁning units that subdivide the Potomac aquifer are limited to Salisbury embayment in Maryland and northern most Virginia 0 Hydrogeologic sections of the Virginia Coastal Plain and adjacent parts of Maryland North Carolina. Section locations are shown in ﬁgure 1. Additional sediment Hydrogeologic sections of the Virginia 500 50 0 A 50 0 50 0 – – –

–1,000 –1,500 –2,000 –2,500 –3,000 –3,500 –4,000 –4,500

–1,000 –1,500 –2,000 A

Altitude, in feet above or below NGVD 29 NGVD below or above feet in Altitude, Figure 17. Hydrogeologic sections of the Virginia Coastal Plain and adjacent parts Maryland North Carolina. Section locations show n in figure 1. Additional sediment pollen-age information on figs. 22–23 and table 1. In much of Virginia, the Potomac aquifer is composed m ostly coarse-grained sediments across the Norfolk arch (section A - ’ center). Fine-grained confining units that subdivide Potomac aquifer are limited to Salisbury embayment in Maryland and northern most Virginia (sections A - ’ right B-B ’). The Potomac aquifer North Carolina southern is hydraulically separated from overlying upper Cretaceous sediments by the Cenomanian confining unit (sections against basement bedrock by a fault (section C-C ’ left). below NGVD 29 NGVD below

’ right and Altitude, in feet above or above feet in Altitude, A – Figure 17. the Potomac aquifer is composed of mostly coarse-grained sediments across Norfolk arch pollen-age information is shown in ﬁgures 22–23 and table 1. In much of Virginia, (section A Cenomanian conﬁning unit (sections 30 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

–450 MD PRINCE WILLIAM –392 –832 A’ –716

– –607 VA

250

STAFFORD S A L I S B U–813 R Y E M B A Y M E N T NC MARYLAND –326 –985 –490 Fredericksburg –349 –322 Location of the study area B –786 in Virginia, Maryland, and KING GEORGE –303 – 1,700 – – North Carolina –367 – – 1,800 1,900 –342 600 800 –817

–447 – –433 2,000 –710 – 1,000 Oak –340 – – WESTMORELAND Potomac 900 2,100 SPOTSYLVANIA Grove –704 –

–534 1,100 – – – –

1,400 2,200 700 1,300

– 500 CAROLINE NORTHERN NEC –1,571 38° River NORTHUMBERLAND

– ACCOMACK ESSEX 300 – RICHMOND – 400 1,200 B’ –851 – – –2,309 K 1,600 E 1,500 Surprise E.G. Rappahannock Hill Taylor

LANCASTER –

2,300 KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE BAY KING WILLIAM MIDDLESEX River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN C’

NORTH CAROLINA EXPLANATION ALBEMARLE EMBAYMENT –250 Confining-unit top altitude contour. Dashed where inferred. Contour interval is 100 feet.

Chowan Datum is NGVD of 1929 Fault A A’ Hydrogeologic section –392 Hope Borehole location and confining-unit top altitude C A Plantation River 36° Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 18. ApproximateFigure 18. Approximate altitude and altitude configuration and configuration of the top of of the the top middle of the confining middle confining unit in the unit Potomac in the Potomac aquifer aquifer in Virginia and parts of Maryland andin North Virginia Carolina. and parts Location of Maryland of Chesapeake and North Carolina.Bay impact Location crater of from Chesapeake Powars andBay Bruceimpact (1999). crater Hydrogeologicfrom Powars sections shown and Bruce (1999). Hydrogeologic sections shown on figure 17. in figure 17. Sediment Distribution of the Potomac Aquifer 31

77° 76°

PRINCE WILLIAM –710 –968 MD –560 A’ –867 –789 VA

STAFFORD S A L I S B U R–1,041 Y E M B A Y M E N T NC MARYLAND –512 –1,405 –789 Fredericksburg –523 –540 Location of the study area B –1,055 –541 in Virginia, Maryland, and –655 – KING GEORGE – North Carolina 2,300 –1,103 2,400 –618

–577 – – 2,500 – 1,400 –478Oak 1,300 – 1,900 SPOTSYLVANIA Grove WESTMORELAND – 1,600 – Potomac – 1,000 1,500 – – 1,800 1,700

CAROLINE NORTHERN NEC –1,921 38° River NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

– –1,079 – B’ 400 2,000 –2,503 K E E.G. –1,079Surprise Rappahannock Hill Taylor LANCASTER KING AND QUEEN –

500 – HANOVER – MIDDLE PENINSUL 2,200 600 CHESAPEAKE

– – – 700 800 900 KING WILLIAM MIDDLESEX BAY – River 2,100 – VIRGINIA EASTERN SHOR 1,200 West – 1,100 Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY York YORK-JAMES PENINSUL CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN C’

EXPLANATION NORTH CAROLINA –400 Confining-unit bottom altitude contour. Dashed where inferred. Contour interval ALBEMARLE EMBAYMENT is 100 feet. Datum is NGVD of 1929

Chowan Fault A A’ Hydrogeologic section –968 Borehole location and confining-unit Hope bottom altitude C A Plantation River 36° Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 19. ApproximateFigure 19. Approximate altitude and altitude configuration and configuration of the bottom of the of thebottom middle of the confining middle confining unit in the unit Potomac in the Potomac aquifer in Virginia and aquifer in Virginia and parts of Maryland and North Carolina. Location of Chesapeake Bay impact crater from parts of Maryland and North Carolina. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Hydrogeologic Powars and Bruce (1999). Hydrogeologic sections shown on figure 17. sections shown in figure 17. 32 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76° MD PRINCE WILLIAM A’ –1,045 –782 –1,340 1,400 –

– –1,227 500 –1,070 1,500 – VA

1,600 –

STAFFORD – –1,367 NC

400 1,700 –445 – MARYLAND 1,800 –1,122 – Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area B –792 –1,446 in Virginia, Maryland, and KING GEORGE – – –1,690 2,600 2,700 North Carolina

–575 – – Oak 1,300 2,800

Grove –

2,900 –790 –

SPOTSYLVANIA 3,000 WESTMORELAND Potomac –

– 3,100 –

1,100

2,300 –

– 1,000

1,200 CAROLINE NORTHERN NEC –2,493 38° River NORTHUMBERLAND ACCOMACK ESSEX RICHMOND B’ –3,048 K – E –

2,400

2,500 – E.G. Surprise –

600 2,200

– Rappahannock

– Hill Taylor

700

– 800 900 LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR

– –

– West 1,900 2,000

2,100 Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN C’

NORTH CAROLINA EXPLANATION –400 Confining-unit top altitude contour. Dashed ALBEMARLE EMBAYMENT where inferred. Contour interval is 100 feet. Datum is NGVD of 1929 Chowan Fault A A’ Hydrogeologic section –790 Borehole location and confining-unit top altitude Hope C A River 36° Plantation Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 20. Approximate altitude and configuration of the top of the lower confining unit in the Potomac aquifer Figure 20.in VirginiaApproximate and parts altitude of Maryland and configuration and North Carolina. of the top Location of the lowerof Chesapeake confining Bay unit impact in the craterPotomac from aquifer Powars in Virginia and parts of Marylandand and Bruce North (1999). Carolina. Hydrogeologic Location of sections Chesapeake shown Bay on figureimpact 17. crater from Powars and Bruce (1999). Hydrogeologic sections shown in figure 17. Sediment Distribution of the Potomac Aquifer 33

77° 76°

MD PRINCE WILLIAM –1,372 A’ –1,162

– –907 600 –1,210 1,700 – VA

1,800 – STAFFORD –535 1,900 NC – MARYLAND –1,277 Fredericksburg Location of the study area – B 500 –898 S A L I S B U R Y E M B A Y M E N T – in Virginia, Maryland, and KING GEORGE 2,700 –1,896 – North Carolina –663 2,900 Oak – Grove 3,000 Potomac –

3,100 –938 –

– SPOTSYLVANIA WESTMORELAND 3,200 700 – –

2,600 3,300

–

River 2,500 –

1,000 NORTHERN NEC –2,718 CAROLINE –

1,400 2,000

38° –

1,500 NORTHUMBERLAND –

– ACCOMACK ESSEX1,300 RICHMOND B’

– – –3,216 K – E 1,600 2,800 2,200 2,100

– E.G. Surprise – 2,400

– Rappahannock Taylor 800

– Hill

900

– LANCASTER

–

1,100 KING1,200 AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River

VIRGINIA EASTERN SHOR –

West 2,300 Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN C’

EXPLANATION NORTH CAROLINA –400 Confining-unit bottom altitude contour. Dashed where inferred. Contour interval is 100 feet. ALBEMARLE EMBAYMENT Datum is NGVD of 1929

Chowan Fault A A’ Hydrogeologic section –663 Borehole location and confining-unit bottom Hope altitude C A Plantation River 36° Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 21. Approximate altitude and configuration of the bottom of the lower confining unit in the Potomac aquifer Figure 21. inApproximate Virginia and parts altitude of Maryland and configuration and North of Carolina. the bottom Location of the of lower Chesapeake confining Bay unit impact in the crater Potomac from Powarsaquifer in Virginia and parts of Marylandand and Bruce North (1999). Carolina. Hydrogeologic Location ofsections Chesapeake shown Bayon figure impact 17. crater from Powars and Bruce (1999). Hydrogeologic sections shown in figure 17. 34 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina as the upper Patapsco, lower Patapsco, and Patuxent (Drum- embayment was probably enhanced by a relatively large mond, 2007). The subaquifers merge southward into the Nor- proportion of clay minerals produced from schist source rocks folk arch subarea, where the entire thickness of the undivided by chemical weathering (see section “Deposition Over Time”). sediments functions hydraulically as a single interconnected aquifer (see section “Vertical Hydraulic Connectivity”). The distinction made here between divided and undivided Sediment Age Relations parts of the Potomac aquifer in Virginia provides a new The spatial distribution of sediments composing the understanding from previous hydrogeologic designations Potomac aquifer resulted from an approximately 45-m.y. of the aquifer sediments. Designation of upper, middle, and history of deposition within complex, locally dynamic, lower Potomac aquifers throughout the Virginia Coastal and regionally evolving fluvial environments (see section Plain by the USGS RASA investigation (Meng and Harsh, “Lithologic and Depositional Characteristics”). Accordingly, 1988) relied heavily on a single corehole at Oak Grove in further understanding of the distribution of the sediments can Westmoreland County (Reinhardt and others, 1980) as a type be inferred from differences among their ages. stratigraphic section to widely define the sediment sequence across areas more than 100 miles to the south (see section Sediment Pollen-Age Zonation “Hydrogeologic Designations”). The Oak Grove corehole is designated as borehole 54P 3 (pl. 1, Attachment 1) and is Numerous analyses of fossil pollen have been undertaken among the boreholes interpreted here to intercept confining during much of the past century as a means to estimate units (figs. 17–21). Thus, the stratigraphic section of the Oak the relative timing of sediment deposition. Since the rise Grove core is generally representative of the sequence of of terrestrial plants on Earth, pollen has been continually Potomac aquifer sediments across areas to the north and east, produced in large quantities and dispersed globally through the but probably less than 30 mi to the south. atmosphere. Subsequently, pollen incorporated within some sedimentary deposits has been preserved through geologic Albemarle Embayment time. Distinctions among pollen morphologies associated with the evolution of plant species have been chronologically The Albemarle embayment depositional subarea occupies systematized and applied to many sedimentary sequences only a small part of the Virginia Coastal Plain, extending worldwide. from far southeastern Virginia southward and southwestward Within the Atlantic Coastal Plain of North America, across northeastern North Carolina. Coarse-grained sedi- pollen-age zones of Cretaceous age designated pre-zone I ment intervals within the Potomac aquifer are thinner and through VII (from oldest to youngest) have been related to less widespread than in the Norfolk arch subarea. Unlike sediment ages, designated geologic formations and hydro- the Salisbury embayment subarea, however, delineation of geologic units, and interpreted depositional environments confining units by stratigraphic correlation among fine-grained (fig. 4). A pre-zone I pollen age was inferred for the Waste sediment intervals is ineffective. Fine-grained intervals are of Gate Formation (Hansen, 1982, 1984). Dominantly massive only moderate thicknesses and lack alignment to a consistent bedded, coarse-grained feldspathic sediments directly overly- regional structural trend. Thus, the entire sediment thickness ing basement bedrock along the Atlantic coast were interpreted functions hydraulically as a single interconnected aquifer to have been deposited by immature braided streams. Fluvial regionally. and deltaic sediments of the Patuxent and Arundel Formations The configuration of the Albemarle embayment subarea of Maryland were inferred to be of zone I pollen age and of contrasts with that of the Salisbury embayment subarea. the Patapsco Formation to be of zone II pollen age (Doyle The Albemarle embayment structurally represents a 100-mi and Robbins, 1977; Reinhardt and others, 1980). Zone III and southwestward elongation of the southern limb of the Norfolk younger sediments were further interpreted to be of estuarine arch. The top surface of basement bedrock exhibits a planar to marginal marine origin. The USGS RASA investigation and evenly southeastward sloping configuration (figs. 6 inferred pollen ages of lower Potomac aquifer sediments to be and 12), which continues with the northeastern limb of the pre-zone I and zone I, middle aquifer sediments to be zone II, Cape Fear arch farther south. By contrast, the shape of the and upper aquifer sediments to be zones III and IV (Meng and Salisbury embayment is distinctly concave. Despite the Harsh, 1988). relatively open structural configuration of the Coastal Plain A fundamental regional perspective was established by in northeastern North Carolina, the term embayment has been a comprehensive analysis of the entire Atlantic Coastal Plain used traditionally—but possibly inappropriately—to account (Owens and Gohn, 1985), in which six depositional sedimen- for a pronounced progradation of sediments far to the east tary sequences were described as spanning the Cretaceous across the Continental Shelf (Owens and Gohn, 1985). The Period. The Potomac Formation in Virginia represents the more closed configuration of the Salisbury embayment possi- first depositional sequence, consisting of early Cretaceous to bly was more conducive to accumulation of thick fine-grained earliest late Cretaceous age, primarily fluvial sediments of sediments (see section “Conceptual Depositional Model”). In pre-zone I through zone III pollen ages. These sediments span addition, the supply of fine-grained sediments to the Salisbury the entire Coastal Plain in Virginia and northward through Sediment Distribution of the Potomac Aquifer 35

Maryland, Delaware, and New Jersey, but extend southward relative ages of some pollen zone II sediments are denoted, only across northeastern North Carolina and are absent farther from oldest to youngest, by letter designations A, B, or C. south. Younger near-shore marine sediments of the second Published sediment pollen-age data were used to depositional sequence are not part of the Potomac Formation, delineate the approximate configuration of two surfaces but are present only farther north in Maryland and from within the Potomac aquifer that represent boundaries between far southeastern Virginia into northeastern North Carolina. sediments of zone I and zone II ages, and between zone II All remaining younger depositional sequences are entirely and zone III ages (figs. 22 and 23). Pollen-zone age-boundary absent from Virginia, southern Maryland, and northeastern surface altitudes at each borehole location were approximately North Carolina. bracketed vertically, on the basis of published sediment- Potomac Formation sediments have been delineated sample and borehole-interval pollen ages. Potomac aquifer to be of zone I pollen age across the entire Virginia Coastal sediments that probably are within megablocks disrupted by Plain, but in the northern and southern parts of the Virginia the Chesapeake Bay impact crater were not included because Coastal Plain, the Potomac Formation also includes two they do not reflect the sediment-age distribution resulting from separate overlying wedge-shaped masses of zone II pollen age original deposition. Boundary-surface altitude contours were (Powars, 2000). A thin overlying interval of glauconitic marine then broadly interpolated between the boreholes. Much of the sediments of zone III pollen age was further theorized to be area is generally lacking extensive time-synchronous fine- present in far southeastern Virginia. These sediments were grained sediments, however, that were relied on to constrain identified in only one borehole and, although designated as boundary-surface delineation in northern Delaware (Benson, part of the Potomac Formation, have not been included as part 2006). In addition, boundary-surface contours were projected of the Potomac aquifer. Overlying sediments of zones IV and across the excavated part of the impact crater where originally V pollen ages were also identified in far southeastern Virginia deposited Potomac aquifer sediments have been removed. but were designated as other formations. Following these Pollen-zone age-boundary surfaces broadly represent delineations, samples of sediment penetrated by borehole the sediment-age distribution within the Potomac aquifer. 58F 50 in proximity to the Chesapeake Bay impact crater Designated depositional subareas are not segregated by the (pl. 1) were analyzed to reveal an inverted sequence among boundary surfaces. Thus, the regional trend of coarse- and pollen zones I, II, and III, indicating overturning of a mega- fine-grained sediments among the depositional subareas block (see section “Configuration”). widely transgresses sediment-age boundaries. Within the A unique hydrologic application of sediment pollen-age Salisbury embayment subarea, however, the two confining analysis was undertaken in northern Delaware, where the units (figs. 17–21) are approximately aligned with the bound- Potomac Formation has not been subdivided because of the ary surfaces. lack of continuity among its highly interbedded sediments (Benson, 2006). Samples of sediment determined to have Hydrostratigraphy of Cretaceous Age Sediments in pollen ages ranging from zones I through III were stratigraphi- Southeastern Virginia and Northeastern North Carolina cally correlated to broadly distinguish early Cretaceous age from late Cretaceous age parts of the Potomac Formation. Across the central and northern parts of the Virginia Hydraulically continuous time-stratigraphic layers were then Coastal Plain, the stratigraphic relation between the Potomac extrapolated from the pollen-age boundaries throughout aquifer and the overlying Aquia aquifer is represented by a the Potomac Formation on the basis of correlations of transitional interval designated as the Potomac confining zone geophysical-log “marker” beds. This analysis was constrained, (McFarland and Bruce, 2006) (see section “Lithologic and however, by the required assumption that coarse-grained Depositional Characteristics”). Farther south, other sediments sediments are subordinate to and fully enclosed within aerially of late Cretaceous age separate the Potomac and Aquia extensive, time-synchronous fine-grained sediments. The aquifers (figs. 2 and 4). The late Cretaceous age sediments time-stratigraphic layers formed the basis for representing the have been alternatively interpreted by various studies. The sediments in a digital groundwater-flow model as three distinct lowermost among these sediments were most recently layers separated by the pollen-zone boundaries, but without designated geologically in Virginia as the upper Cenomanian intervening confining units. beds (Powars, 2000), and subsequently designated hydrogeo- Published pollen-age data representing zones pre-I logically as the upper Cenomanian confining unit (McFarland through III were compiled from some of the above and and Bruce, 2006). additional studies (table 1). The data represent pollen ages The upper Cenomanian confining unit represents a major described for sediments penetrated by several boreholes at hydrologic upper boundary on the Potomac aquifer in south- widely spaced locations in the Virginia Coastal Plain and eastern Virginia and imposes a substantial hydraulic separation southern Maryland. Similar data for northeastern North between the Potomac aquifer and overlying late Cretaceous Carolina are not readily available. Sediment pollen ages were age sediments of the Virginia Beach aquifer (McFarland either determined directly by microscopic examination of and Bruce, 2006). Sediment cores exhibit glauconitic and sediment samples or were inferred by the cited studies for micaceous fine shelly sands of near-shore marine origin designated intervals within boreholes. Subdivisions among (fig. 9E) of pollen-zone IV age (Powars, 2000) (fig. 4). On the 36 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Table 1. Published pollen-zone ages of sediment composing the Potomac aquifer of the Virginia Coastal Plain and adjacent parts of Maryland.—Continued

[Depths are in feet below land surface; borehole numbers and names refer to locations shown in figures 22–23; horizontal datum is referenced to the North American Datum of 1927; altitudes are in feet referenced to the National Geodetic Vertical Datum of 1929; sample – pollen age determined by microscopic examination of sediment; interval – pollen age inferred by cited study for designated borehole interval; NASA, National Aeronautics and Space Administration]

Pollen zone Designation type Depth Calis and Drummond, 2008 borehole CA Db 96 – Prince Frederick latitude 38.5456 longitude –76.5950 altitude 152 III sample 1,000 – 1,015 II or III sample 1,235 – 1,240 II-B sample 1,340 – 1,360 II-B sample 1,440 – 1,460 II-B sample 1,520 – 1,540 I sample 1,590 – 1,610 borehole SM Dd 72 – Paw Paw Hollow latitude 38.2739 longitude –76.6594 altitude 115 II-B or C sample 600 – 620 II-B or C sample 720 – 740 II-B sample 950 – 970 II-A sample 1,110 – 1,130 II-A sample 1,130 – 1,150 II-A sample 1,330 – 1,350 I sample 1,550 – 1,570 I sample 1,570 – 1,590 I sample 1,590 – 1,610 Doyle and Robbins, 1977 borehole 66M 1 – E.G. Taylor latitude 37.8842 longitude –75.5169 altitude 42 III interval 1,564 – 2,127* II interval 2,127 – 3,045* II interval 2,440 – 2,888 I interval 3,045 – 4,682* pre-I interval 4,682 – 6,272* pre-I and lowest I interval 5,128 – 6,086 III sample 1,616 III sample 1,860 II-B or C sample 2,214 lowest I sample 4,470 Edwards and others, 2010 borehole 59E 32 – Watkins Elementary School latitude 37.0755 longitude –76.4585 altitude 27 III interval 645 – 721.8 II-C interval 721.8 – 985.3 III sample 692.8 II-C sample 772.4 II-C sample 879.7 II-C sample 902.8 Sediment Distribution of the Potomac Aquifer 37

Table 1. Published pollen-zone ages of sediment composing the Potomac aquifer of the Virginia Coastal Plain and adjacent parts of Maryland.—Continued

Pollen zone Designation type Depth Fleck and Wilson, 1990 borehole CH Be 57 – Smallwood Drive latitude 38.6183 longitude –76.9656 altitude 212.26 II-B sample 635 borehole CH Bf 144 – Pinefield Production latitude 38.6536 longitude –76.8506 altitude 202.29 II-C or III sample 624 – 625 II-C or III sample 881 – 882 borehole CH Bf 149 – coreholes latitude 38.6508 longitude –76.8803 altitude 210 III sample 526.9 II-C sample 537.3 II-B sample 577.9 II-B sample 606 N.O. Fredericksen, U.S. Geological Survey, oral commun., 2001 borehole 56F 55 – James City Service Authority BGD-1B latitude 37.2483 longitude –76.7691 altitude 57 III sample 483.8 – 487 Fredericksen and others, 2005 borehole 59E 31 – NASA Langley latitude 37.0956 longitude –76.3858 altitude 8 I sample 1,464.7 Powars, 2000 borehole 58A 76 – Dismal Swamp latitude 36.6153 longitude –76.5556 altitude 33 III interval 593 – 728 II interval 728 – 1,383 I interval 1,383 – 1,827 borehole 61B 11 – Fentress latitude 36.7075 longitude –76.1297 altitude 15 III sample 1,055 III sample 1,303 Reinhardt and others, 1980 borehole 54P 3 – Oak Grove latitude 38.1694 longitude –77.0386 altitude 180 II-B sample 596.5 II-B sample 660 II-B sample 756 II-B sample 781 upper I or basal I-A sample 950 I sample 1,398 *Based on regional correlation. 38 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76° CH Bf 144 MD PRINCE WILLIAM CH Be 57 CH Bf 149 MARYLAND

CA Db 96 VA STAFFORD NC

–2,000

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area SM Dd 72 in Virginia, Maryland, and KING GEORGE North Carolina

–1,000 54P 3 Oak

Potomac –2,500 SPOTSYLVANIA Grove WESTMORELAND

–1,750

–2,750

–3,000

–1,500 CAROLINE 38° River –1,250 NORTHERNNORTHUMBERLAND NEC ACCOMACK ESSEX

RICHMOND –750 66M 1

E Surprise E.G.

Rappahannock K Hill –2,250 Taylor LANCASTER KING AND QUEEN CHESAPEAKE HANOVER MIDDLE PENINSUL BAY

MIDDLESEX KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT Aquifer GLOUCESTER MATHEWS HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK 56F 55 River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY 59E 31 59E 32 Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON 61B 11 Emporia Chesapeake Franklin Suffolk58A 76 GREENSVILLE

ATLANTIC OCEAN

EXPLANATION NORTH CAROLINA –750 Pollen-zone I-II boundary-surface altitude contour. Dashed where projected across altered or excavated sediments within the impact crater. Contour interval is 250 feet. Datum is NGVD of 1929 ALBEMARLEChowan EMBAYMENT58A 76 Borehole location and number having pollen-zone age samples or intervals Hope Plantation River 36° Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 22. Approximate altitude and configuration of the pollen-zone I-II boundary surface in the Potomac Figure 22. Approximateaquifer in Virginiaaltitude and partsconfiguration of Maryland of the and pollen-zone North Carolina. I-II boundary Location ofsurface Chesapeake in the PotomacBay impact aquifer crater in from Virginia and parts of Maryland andPowars North Carolina. and Bruce Location (1999). of Chesapeake Bay impact crater from Powars and Bruce (1999). Sediment Distribution of the Potomac Aquifer 39

77° 76°

CH Bf 149 CH Bf 144 MD PRINCE WILLIAM MARYLAND CH Be 57 CA Db 96 VA STAFFORD –250 NC

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area SM Dd 72 in Virginia, Maryland, and KING GEORGE North Carolina

Potomac Oak 54P 3 –2,000 SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND 66M 1

K E Surprise –1,500 E.G. Rappahannock Hill Taylor –1,250

MIDDLE PENINSUL LANCASTER –1,750 KING AND QUEEN HANOVER CHESAPEAKE

–750 BAY KING WILLIAM MIDDLESEX A River VIRGINIA EASTERN SHOR West Point Richmond NEW KENT –500 Aquifer GLOUCESTER MATHEWS HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A

–1,000 Hopewell Williamsburg Colonial Heights 56F 55 YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY 59E 31 59E 32 Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake 61B 11 Franklin Suffolk

GREENSVILLE 58A 76

ATLANTIC OCEAN

NORTH CAROLINA EXPLANATION –250 Pollen-zone II-III boundary-surface altitude contour. Dashed where projected across altered or excavated sediments within the impact crater. Contour interval is 250 feet. Datum is NGVD of 1929 ALBEMARLEChowan EMBAYMENT58A 76 Borehole location and number having pollen-zone age samples or intervals Hope Plantation River 36° Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 23. Approximate altitude and configuration of the pollen-zone II-III boundary surface in the Potomac Figure 23. Approximate altitude and configuration of the pollen-zone II-III boundary surface in the Potomac aquifer in Virginia and aquifer in Virginia and parts of Maryland and North Carolina. Location of Chesapeake Bay impact crater from parts of Maryland and North Carolina. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Powars and Bruce (1999). 40 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina basis of lithologic descriptions, these sediments were probably Previously published hydrostratigraphic interpretations included by the USGS RASA investigation as part of the upper among Cretaceous age sediments between southeastern Potomac aquifer (Meng and Harsh, 1988). The sediments are Virginia and northeastern North Carolina are inconsistent readily identifiable, however, based on a distinctive saw- with the relations established by Weems and others (2009). A toothed shaped geophysical-log signature (fig. 11) produced fundamental discrepancy exists in considering late Cretaceous by alternating thin beds of concentrated shells of the mollusk age sediments that compose some aquifers in North Carolina Exogyra woolmani as seen in sediment cores. as being continuous with early Cretaceous age sediments of The upper Cenomanian confining unit previously has the Potomac aquifer in Virginia. Direct hydraulic connectivity been delineated only in southeastern Virginia (McFarland between early and late Cretaceous age sediments is implau- and Bruce, 2006). In northeastern North Carolina, alternate sible with recognition of the substantial vertical hydrologic hydrogeologic interpretations have been made of sediments boundary imposed by the upper Cenomanian confining unit of both early and late Cretaceous age. The USGS RASA (see section “Vertical Hydraulic Connectivity”). investigation considered the upper, middle, and lower Potomac In order to partly resolve hydrostratigraphic relations aquifers in Virginia to be continuous in North Carolina with between early and late Cretaceous age sediments, the upper the upper Cape Fear, lower Cape Fear, and lower Cretaceous Cenomanian confining unit is delineated here as extending aquifers, respectively (Winner and Coble, 1996) (fig. 4). Sedi- from southeastern Virginia into northeastern North Carolina ments much younger than the Potomac aquifer, however, were (figs. 17 and 24). Geophysical logs of boreholes located in included in North Carolina, some as young as pollen-zone northeastern North Carolina generally exhibit the distinctive VII age. Subsequently, the lower Cape Fear aquifer in North signature of the upper Cenomanian confining unit, including Carolina was considered as continuous with both the middle sediments designated in the Hope Plantation core and subse- and lower Potomac aquifers in Virginia (Lautier, 1998). The quently mapped as the Clubhouse Formation (Weems and oth- upper Cape Fear, lower Cape Fear, and lower Cretaceous ers, 2007, 2009). The part of the upper Cenomanian confining aquifers were most recently considered as continuous with unit that extends from southeastern Virginia into northeastern a single Potomac aquifer in Virginia (Gellici and Lautier, North Carolina generally is consistently aligned to the regional 2010), but were delineated as extending individually across strike and dip, although the configuration is unknown toward southeastern Virginia and including sediments of pollen-zone the coast where no boreholes are deep enough to penetrate into IV age and younger. early Cretaceous age sediments. Apart from these studies, a fundamental geologic With recognition of the upper Cenomanian confining unit perspective was recently established for Cretaceous age in northeastern North Carolina, the following relations are sediments in northeastern North Carolina. Sediment lithologic apparent: and age relations were determined from a corehole at Hope 1. The Potomac aquifer deepens and thins southward Plantation (borehole H20S; pl. 1; fig. 1; fig. 17, section A–A' ) from southeastern Virginia into northeastern North (Weems and others, 2007). Subsequently, geologic mapping Carolina (fig. 17, sectionA –A' ) before being trun- of the Roanoke Rapids quadrangle (Weems and others, 2009) cated to the southwest against a fault (fig. 7; fig. 17, extrapolated these relations to some of the same boreholes section C–C' ). used by earlier hydrogeologic studies. Immature fluvial sediments of lower Cretaceous age that are continuous with 2. The substantial hydraulic separation imposed by the Potomac aquifer in Virginia are delineated in northeastern the upper Cenomanian confining unit between the North Carolina in the subsurface only and are truncated Potomac aquifer and overlying sediments in south- to the southwest against a fault. Overlying sediments are eastern Virginia continues southward across north- geologically designated separately as either the Clubhouse eastern North Carolina (see “Vertical Hydraulic Formation or Cape Fear Formation, both of late Cretaceous Connectivity”). age and including pollen zones IV and V, respectively (fig. 4). 3. Some late Cretaceous age sediments overlying the Clubhouse Formation sediments are of marine origin, which upper Cenomanian confining unit in northeastern separate and distinguish fluvial sediments of the overlying North Carolina possibly are continuous with the Vir- Cape Fear Formation from underlying fluvial sediments ginia Beach aquifer in southeastern Virginia. continuous with the Potomac aquifer. On the basis of lithologic composition, depositional origin, and sediment-age 4. Previous designations of the upper and lower Cape relations, the Clubhouse Formation in North Carolina is Fear aquifers in northeastern North Carolina include considered here as continuous with—and an extension of—the large parts of late Cretaceous age sediments that are upper Cenomanian confining unit in southeastern Virginia. not continuous with the Potomac aquifer in southeast- No previous hydrogeologic investigations in either State are ern Virginia, but possibly are partly continuous with known to have recognized this relation. the Virginia Beach aquifer. Sediment Distribution of the Potomac Aquifer 41

77° 76°

PRINCE WILLIAM A’ MD

STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area B in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND B’

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York A CHESTERFIELD Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX – Norfolk SOUTHEASTERN VIRGINIA 200 margin Portsmouth 1,000 Virginia Beach –

– SOUTHAMPTON Emporia

Chesapeake 1,050 Franklin Suffolk –

700 GREENSVILLE – 550 Confining-unit –

–226 850 –130 450 650 750 – –920 ATLANTIC – – – –100 150 400 800–823 OCEAN – –221 – 500 – C’ –455 – –548 –250 –1,104 –397 600 –

–300 –956 1,100 NORTH CAROLINA – –844 EXPLANATION – –50 Confining-unit top altitude contour. Contour –350 –369 950 – interval is 50 feet. Datum is NGVD of 1929

900 ALBEMARLEChowan EMBAYMENT –550 Fault –832 A A’ Hydrogeologic section –956 –430 Hope Borehole location in North Carolina and confining-unit top altitude A Plantation River 36° C –604 –674 Albemarle Sound Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 24. Approximate altitude and configuration of the top of the upper Cenomanian confining unit overlying Figure 24. Approximate altitude and configuration of the top of the upper Cenomanian confining unit overlying the Potomac aquifer in the Potomac aquifer in Virginia and parts of North Carolina. Configuration in Virginia adapted from McFarland Virginia and partsand of BruceNorth (2006).Carolina. Location Configuration of Chesapeake in Virginia Bay adaptedimpact crater from fromMcFarland Powars and and Bruce Bruce (2006). (1999). LocationHydrogeologic of Chesapeake Bay impact crater fromsections Powars shown and Bruceon figure (1999). 17. Hydrogeologic sections shown in figure 17. 42 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Conceptual Depositional Model Spatial Controls Ages of sediments composing the Potomac aquifer Sediment deposition coincided with geomorphic matura- provide an understanding of processes operating over geologic tion of the land surface as the form, function, and geographic time that resulted in the spatial distribution of coarse- and distribution of streams evolved (see section “Lithologic and fine-grained sediments. Following rifting of the supercontinent Depositional Characteristics”). During transport, coarse- and Pangea during the Triassic and Jurassic Periods, sediments fine-grained sediments were segregated eastward (fig. 25) were deposited along the eastern margin of the newly forming by fluctuations in streamflow energy through progressively North American continent, coinciding with initial opening mature stream morphologies. Coarse-grained sediments were of the present-day Atlantic Ocean during the early to earliest preferentially deposited in proximity to Piedmont source rocks late Cretaceous Period. Flexure associated with tension of under high energy flow conditions by immature, high gradient the Earth’s crust resulted in uplift and erosion of bedrock braided streams. Much of the fine-grained sediments remained across the Piedmont to the west and downwarp and sediment in suspension until transported farther from Piedmont source deposition across the Coastal Plain to the east (fig. 25). rocks and eventually were deposited upon entering low energy Sources of sediment were restricted to areas east of the Blue flow conditions of mature, medium to low gradient meander- Ridge Mountains that maintained an unbreached drainage ing streams. divide from areas farther west until the early Miocene Epoch, The distribution among coarse- and fine-grained based on fission-track dating of zircon sand grains (Naeser, sediments was further affected by structural features of the Naeser, Newell, and others, 2004, 2006). Erosional lowering Continental Margin. The top surface of basement bedrock of the Piedmont took place at approximately 0.0008 in/yr likely imposed a primary control on the locations of Piedmont as estimated from fission-track dating of apatite sand grains sediment-source rocks and on directions of stream drain- (Naeser, Naeser, Southworth, and others, 2004). Abundant age and sediment transport. The present-day undulating plant fossils and other organic matter in some sediments configuration of arches and embayments (figs. 6, 12, and 17) indicate a relatively heavily vegetated landscape and humid is theorized to have formed early in the development of the climate. Continental Margin (Owens and Gohn, 1985), possibly as

PIEDMONT COASTAL PLAIN West East

Sediment source area Early land surface B E D R O C K Erosion Intermediate land surface

Late land surface Sediment deposition area COARSE Deposition U P L I F T SEDIMENT FINE

D O W N W A R P

TENSION

Figure 25. Simplified structural features of the North American Continental Margin during the early to earliest late Cretaceous Period Figure 25. Simplified structural features of the North American continental margin during the early to earliest late between approximately 100 and 145 million years ago. Lateral tension associated with ongoing widening of the Atlantic Ocean induced Cretaceous Period between approximately 100 and 145 million years ago. Lateral tension associated with ongoing widening crustal flexure that uplifted the westward side of the margin and downwarped the eastward side. Sediment produced by erosion of of the Atlantic Ocean induced crustal flexure that uplifted the westward side of the margin and downwarped the eastward side. bedrock in uplifting source areas (corresponding to the present-day Piedmont) was transported eastward by rivers and streams into Sediment produced by erosion of bedrock in uplifting source areas (corresponding to the present-day Piedmont) was transported downwarped depositional areas (corresponding to the present-day Coastal Plain). Ongoing erosion and deposition progressively leveled eastward by rivers and streams into downwarped depositional areas (corresponding to the present-day Coastal Plain). the land surface. Ongoing erosion and deposition progressively leveled the land surface. Sediment Distribution of the Potomac Aquifer 43

a result of movement along landward extensions of ocean Maryland Coastal Plain (fig. 26). The Maryland depositional transform faults (Powars, 2000). Consequently, drainage and complex was comparable in size to major present-day deposi- sediment transport were not uniform eastward across the entire tional systems and possessed a bilaterally symmetric drainage Coastal Plain, but rather varied in local orientation. pattern that radiated southward to Virginia and eastward The spatial distribution of early Cretaceous age sediments to Delaware. Coarse-grained sediments were deposited by across the Maryland Coastal Plain has been theorized to immature braided streams across the center of the complex in result from segregation of coarse- and fine-grained sediments proximity to the present-day city of Baltimore. Increasingly during transport (Hansen, 1969, 1971). A broad regional trend fine-grained sediments were transported farther southward among sediments of the Patuxent and Patapsco Formations toward Virginia and eastward toward Delaware until being was produced by a fluvial depositional complex spanning the deposited by mature meandering streams.

78° 77° 76°

PENNSYLVANIA MARYLAND NEW JERSEY

WEST VIRGINIA

Baltimore Figure 26. Fluvial SEDIMENT depositional complexes of SOURCE I M M A T U R E B R A I D E D DELAWARE the Virginia Coastal Plain and AREA adjacent parts of Maryland 39° S T R E A M S and North Carolina during Maryland Fluvial the early to earliest late Washington Depositional D.C. Cretaceous Period between GENERALIZED Complex DIRECTION OF (modified from approximately 100 and SEDIMENT TRANSPORT Hansen, 1969 145 million years ago. and 1971) Depositional complexes likely M A T U R E M E A N D E R I N G consisted of several or more individual drainage basins that Fredericksburg changed configuration over S T R E A M S MD time. Predominantly coarse- 38° grained sediments were VA

generally deposited eastward L across the central and L southern Virginia Coastal Plain A F under high-energy conditions by immature, high-gradient braided streams in proximity Richmond to a persistently uplifted source area. Fine-grained SEDIMENT sediments were preferentially Virginia transported farther from 37° I M M A T U R E B R A I D E D Fluvial the source area to the SOURCE S T R E A M S Depositional Complex northeast and southeast, and were deposited under Emporia low-energy conditions VIRGINIA by mature, medium- to AREANORTH CAROLINA M A T U R E M E A N D E R I N G low-gradient meandering streams. The depositional S T R E A M S complex in Virginia merged northward with another similar depositional complex in Maryland described by Base adapted from U.S. Geological Survey, 1973 0 10 20 30 MILES State of Virginia, 1:500 000 Hansen (1969, 1971). 0 10 20 30 40 50 KILOMETERS

Figure 26. Fluvial depositional complexes of the Virginia Coastal Plain and adjacent parts of Maryland and North Carolina during the early to earliest late Cretaceous Period between approximately 100 and 145 million year ago. Depositional complexes likely consisted of several or more individual drainage basins that changed configuration over time. Predominantly coarse-grained sediments were generally deposited eastward across the central and southern Virginia Coastal Plain under high-energy conditions by immature, high-gradient braided streams in proximity to a persistently uplifted source area. Fine-grained sediments were preferentially transported farther from the source area to the northeast and southeast, and deposited under low-energy conditions by mature, medium-to low-gradient meandering streams. The depositional complex in Virginia merged northward with another similar depositional complex in Maryland described by Hansen (1969, 1971). 44 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Virginia Fluvial Depositional Complex medium- to low-gradient meandering streams onto the evenly sloping basement-bedrock surface that forms an extended A fluvial depositional complex spanning the Virginia southern limb of the Norfolk arch (see section “Albemarle Coastal Plain (fig. 26) is theorized here to have produced Embayment”). The Virginia depositional complex possibly the spatial distribution of sediments composing the Potomac merged with the northern margin of another depositional aquifer in Virginia during the early to earliest late Cretaceous complex positioned farther south in proximity to the Cape Period between approximately 100 and 145 Ma. The Fear arch. depositional complex represents a southern counterpart to the depositional complex theorized in the Maryland Coastal Plain (Hansen, 1969, 1971). Likewise, these are probably only Deposition Over Time two of a series of generally eastward oriented depositional Erosion of Piedmont bedrock and deposition of Coastal complexes that were arrayed from north to south along the Plain sediment throughout the Cretaceous Period led to entire Atlantic Coastal Plain. regionwide leveling across the North American Continental Within the Virginia depositional complex, distinct Margin for approximately 80 m.y. Areas of sediment deposi- variations in the morphologies and lithologies of sedimentary tion broadly alternated, however, in response to differential deposits are related to the three depositional subareas, vertical adjustments of the continental crust (Owens and which are based on borehole sediment intervals (see section Gohn, 1985). In Virginia, sediments that compose the Potomac “Depositional Subareas”). The Norfolk arch subarea was in aquifer were deposited only during the early to earliest late proximity to an uplifted, actively eroding sediment-source area Cretaceous Period between approximately 100 and 145 Ma. in the Piedmont to the west (fig. 26). Localized networks of Sediments deposited during the remainder of the Cretaceous immature, high-gradient braided streams deposited complexly Period in Virginia are preserved only to the far southeast and overlapping and coalescing longitudinal bars and channel fills are not directly hydraulically connected with nor considered dominated by coarse-grained quartz and feldspar sand and part of the Potomac aquifer (see section “Hydrostratigraphy gravel. Where not reworked and beveled by erosion, preserved of Cretaceous Age Sediments in Southeastern Virginia and sequences formed both massive and graded bedding. Some Northeastern North Carolina”). concentrations of feldspathic sands eventually weathered in The Virginia fluvial depositional complex evolved situ to kaolinitic clay. regionally over millennia, during which the configurations of Both the Salisbury and Albemarle embayment deposi- numerous individual drainage basins within the depositional tional subareas were farther from the sediment-source area complex probably changed substantially. Across the Norfolk in Virginia. Predominantly mature, medium- to low-gradient, arch and Albemarle embayment subareas, however, the broad broadly meandering streams deposited channel fills and point pattern of deposition remained relatively fixed over time. bars. Graded beds of quartzitic medium- to coarse-grained Sediment-source rocks primarily of granite and gneiss (Glaser, sand were segregated from fine-grained sediments in 1969) in proximity to the Norfolk arch were persistently overbank deposits. Immature braided streams in the Norfolk uplifted, sustaining the supply of coarse-grained feldspathic arch subarea transitioned progressively downgradient to sediments deposited throughout the period. mature meandering streams in the Salisbury and Albemarle By contrast, deposition across the Salisbury embayment embayment subareas. Likewise, distal parts of the Virginia subarea probably changed dynamically over time as a result depositional complex adjacent to coastal areas farthest to of punctuated adjustments of the Earth’s crust. Unique to the east merged with subareal deltas. Coeval sediments even the Salisbury embayment subarea, coarse-grained sediments farther east beneath the present-day Continental Shelf were composing three vertically spaced subaquifers are hydrauli- deposited under marine conditions. cally separated by thick fine-grained sediments composing Across the northernmost part of the Virginia Coastal two continuous confining units (see section “Salisbury Plain, the Virginia depositional complex likely merged with Embayment”). Piedmont rocks supplying sediment to the the southern margin of the Maryland depositional complex. Maryland depositional complex apparently underwent three Sediments were received from both depositional complexes distinct cycles, each consisting of initial uplift and rapid at different times and originated from two different source erosion followed by crustal stability and progressively slower areas located in Virginia and Maryland (fig. 26). Mineralogic erosional leveling (fig. 25). Predominantly coarse-grained analyses of sand grains from Potomac aquifer sediments sediments were generated and deposited during each uplift. reflect two distinct dominant Piedmont sediment-source rocks Increasingly thick and concentrated fine-grained sediments (Glaser, 1969). Erosion of granite and gneiss produced much were subsequently deposited as the source rocks were of the sediments deposited across the central and southern stabilized and leveled. Accumulation of fine-grained sediments parts of the Virginia Coastal Plain, whereas sediments depos- during quiescent periods was also probably enhanced by ited farther north and into southern Maryland were produced the predominantly schist source rocks (Glaser, 1969), which predominantly by erosion of schist. generated a relatively large proportion of clay minerals as The southward extent of the Virginia depositional products of chemical weathering. complex is uncertain. Sediment was deposited across the Across the southernmost part of the Salisbury embayment Albemarle embayment depositional subarea by mature, subarea, several factors potentially complicated the southern Hydrologic Conditions of the Potomac Aquifer 45 margins of the confining units over time. Alternating overlap transmitted through a unit volume of aquifer material resulting between the Virginia and Maryland depositional complexes from a unit change in hydraulic head and is expressed as probably supplied sediments from different source areas at length per unit time such as feet per day (ft/d). Coarse- different times. Moreover, as the configurations of individual grained sediments have relatively large and interconnected drainage basins within each depositional complex changed, pore spaces and thereby generally have greater hydraulic gradational transitions between immature braided streams and conductivities than fine-grained sediments. Transmissivity mature meandering streams also shifted. The configuration of (T) is the volumetric rate produced by the entire thickness of basement bedrock, however, possibly imposed a long-term the aquifer (b), equivalent to K × b, and is expressed as area structural control. Whereas immature braided streams broadly per unit time such as feet squared per day (ft2/d). Specific spanned the convex bedrock surface across the Norfolk arch storage (Ss ) describes the ability of sediment to store water. It subarea, development of mature meandering streams was is defined as the volume of water taken into or released by a confined to the structurally closed concave surface within the unit volume of aquifer material resulting from a unit change in Salisbury embayment subarea. Consequently, thick, fine- hydraulic head and is expressed as per length such as per foot grained sediments accumulated primarily within the Salisbury (/ft). Fine-grained sediments containing a large percentage of Embayment subarea most of the time. uncompacted clay are relatively compressible and thus can Following deposition of fluvial sediments that compose have higher specific storage than coarse-grained sediments. the Potomac aquifer, fine shelly sands of the upper Cenoma- Conversely, compacted clay can have low specific storage. nian confining unit were deposited under near-shore marine Storativity (S) is the volume taken or released by the entire conditions during the part of the late Cretaceous Period thickness of the aquifer (b), equivalent to Ss × b, and is unitless. corresponding to pollen zone IV (see section ”Hydrostratig- Hydraulic properties of the Potomac aquifer have been raphy of Cretaceous Age Sediments in Southeastern Virginia widely measured by aquifer tests. A well completed in the and Northeastern North Carolina”). Sediments of both fluvial aquifer is pumped at a known rate or rates, while water-level and marine origin were deposited during the remainder of drawdown is measured over time. Water levels can be the Cretaceous Period and have received various geologic measured either solely in the pumped production well or and hydrogeologic designations in Virginia, Maryland, and in the production well along with one or more unpumped North Carolina (fig. 4). The original geographic extents of observations wells. For a period following cessation of late Cretaceous age sediments, however, are not known. The pumping, water-level recovery can also be measured. A variety upper Cenomanian confining unit and overlying aquifers and of methods has been developed to analyze the water-level confining units are preserved only in southeastern Virginia drawdown and recovery data to determine values for aquifer and northeastern North Carolina. Other sediments of late transmissivity and storativity. Different analytical methods Cretaceous age compose a series of aquifers and confining address specific field conditions and test limitations, and are units in Maryland that pinches out approximately 20 mi constrained by various assumptions. Regardless of the analyti- north of Virginia, possibly as a result of downwarping and cal method used, values of storativity can only be determined (or) downfaulting of basement bedrock across the Salisbury from aquifer tests that include one or more observation wells. embayment (Hansen, 1978). Widespread deposition through- Values of transmissivity and storativity were compiled out the Virginia Coastal Plain would not be preserved until the from documented pumping tests of the Potomac aquifer. beginning of the Tertiary Period. Reported test results attributable to specific wells with known locations and construction characteristics were obtained from several sources. Results of many aquifer tests are cited among Hydrologic Conditions of the internal memoranda of the VA DEQ (S.W. Kudlas, Virginia Department of Environmental Quality, written commun., Potomac Aquifer 2010) that document technical evaluations of proposed groundwater withdrawals. Reports compiled by engineering The spatial distribution of sediments composing the and hydrogeologic consulting firms also document aquifer- Potomac aquifer affects groundwater levels, flow, and avail- test results among other information produced for various ability, including hydraulic properties and connectivity within groundwater-development projects. Additional aquifer-test the aquifer and with overlying aquifers. Information on the results from Virginia and Maryland were obtained from Potomac aquifer in this report can be applied to support efforts published scientific literature. Similar data for northeastern to manage the aquifer as a water resource. North Carolina were not readily available. Aquifer-test locations span the Virginia Coastal Plain west of Chesapeake Sediment Hydraulic Properties Bay and northward into Maryland, but are sparse across the upper Middle Peninsula and Northern Neck and in Sussex and Several hydraulic properties of aquifers are determined Surry Counties (fig. 27). by sediment composition (Freeze and Cherry, 1979). Aquifer-test results are tabulated in a digital data- Hydraulic conductivity (K) describes the ability of sediment spreadsheet file (Attachment 2). The file includes study-site or to transmit water. It is defined as the volumetric rate of water area names, documentation sources, and location information 46 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

STAFFORD NC S A L I S B U R Y E M B A Y M E N T MARYLAND

Fredericksburg Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E E.G. Surprise Rappahannock Taylor Hill LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake ATLANTIC Franklin Suffolk OCEAN GREENSVILLE

EXPLANATION Transmissivity, in feet squared per day <3,000 >3,000–6,000 NORTH CAROLINA >6,000–10,000 >10,000

ALBEMARLEChowan EMBAYMENT

Hope Plantation River Albemarle 36° Sound

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 27. Transmissivity values estimated from aquifer tests in the Potomac aquifer in Virginia and parts of Figure 27. Transmissivity values estimated from aquifer tests in the Potomac aquifer in Virginia and parts of Maryland and North Carolina. Maryland and North Carolina. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Hydrologic Conditions of the Potomac Aquifer 47 for 197 test sites. Production and observation wells are Hydraulic Conductivity identified by reported number and USGS local well number (where assigned). Rates and durations of pumped discharge Whereas transmissivity is partly a function of aquifer are listed where documented. Then listed are 336 values of thickness, hydraulic conductivity is an intrinsic property of aquifer transmissivity and 127 values of storativity. More than sediment texture and directly relatable to its distribution. one aquifer test is reported at some locations, and more than Hydraulic conductivity values can be calculated by dividing one analytical method and resulting set of hydraulic-property transmissivity values by aquifer thickness. Transmissivity values are reported for some tests. Storativity values are values represent the entire thickness of the aquifer, however, reported only for aquifer tests that included one or more only if tested wells fully penetrate the aquifer or—if tested observation wells. wells are partially penetrating—appropriate analytical Aquifer-test analytical methods determine values for methods have been applied. Most open intervals of tested transmissivity and storativity but not hydraulic conductivity or wells are small relative to aquifer thickness. Less than specific storage. Calculated values for these hydraulic proper- 20 percent of total aquifer thickness is penetrated by all but a ties are listed, along with supporting information on aquifer few wells, and many wells penetrate no more than 10 percent thickness, and production-well open intervals and aquifer of the aquifer (fig. 28). Moreover, analytical methods for many penetration. of the aquifer tests are not documented. Among those tests The aquifer tests likely represent only relatively small having documented analytical methods, only three indicate volumes of aquifer sediments. Pumping rates and durations that well partial penetration is accounted for but do not specify of aquifer tests were small, having a median discharge rate of values used for aquifer thickness. Analytical methods that are 325 gal/min and median duration of 48 hours where specified undocumented for many of the tests probably likewise do not among constant-rate tests (Attachment 2). Water levels account for well partial penetration. declined only in observation wells within a few hundred feet Because partial penetration is largely unaccounted for by of production wells, and at greater distances remained static. the aquifer tests, 296 values of hydraulic conductivity were Hence, drawdowns were generally propagated only short calculated (Attachment 2) by dividing reported aquifer-test distances through water-bearing beds directly connected to transmissivity values by the lengths of open intervals in pro- production-well open intervals. Hydraulic stresses probably duction wells. Open-interval length is assumed to approximate did not extend vertically through adjacent fine-grained the effective aquifer thickness that was hydraulically stressed sediments and into water-bearing beds above or below well during the tests and represented by the transmissivity values. open intervals. Hydraulic conductivity values were not calculated for 23 wells because open-interval data were not available. The distribution of hydraulic conductivity of sediments Transmissivity composing the Potomac aquifer is similar to the values of transmissivity, with large values across the Norfolk arch Aquifer transmissivity has been related to the sediment depositional subarea and small values toward the Salisbury distribution of the Maryland fluvial depositional complex embayment subarea (fig. 29). Unlike transmissivity values (Hansen, 1971) (see section “Conceptual Depositional that increase to the east, however, large hydraulic conductivity Model”). Large transmissivity values were correlated with values span all of the Norfolk arch subarea. Thus, hydraulic the predominance of coarse-grained sediments across the conductivity values are consistently aligned with relative center of the depositional complex, whereas small values were proportions of coarse-grained sediments throughout the correlated with increasingly fine-grained sediments toward the Potomac aquifer. margins of the complex. The texture and associated hydraulic conductivity of Similarly, transmissivity is generally aligned among sediments composing the Potomac aquifer control regional the depositional subareas of the Potomac aquifer designated velocities of lateral groundwater flow on the basis of trends here. Large transmissivity values broadly correlate with the in estimated ages of groundwater relative to the time of predominance of coarse-grained sediments across the Norfolk recharge (see section “Flow”). Assuming a relatively uniform arch subarea, and small values correlate with increasingly hydraulic gradient and sediment porosity, lateral flow velocity fine-grained sediments toward the Salisbury embayment is directly proportional to hydraulic conductivity. Groundwater subarea (fig. 27). Almost all transmissivity values greater than ages across the Norfolk arch depositional subarea (Nelms 6,000 ft2/d are within or close to the Norfolk arch subarea. and others, 2003) indicate a fast velocity of approximately Although the relative proportion of coarse-grained sediments 10 ft/yr, correlating with high hydraulic conductivity is great throughout the Norfolk arch subarea (fig. 15), trans- values and the predominance of coarse-grained sediments. missivity increases eastward coinciding with thickening of the Conversely, northward into the Salisbury embayment, Potomac aquifer. progressively slower velocities from approximately 4 ft/yr to 0.1 ft/yr are indicated by groundwater ages including parts of the Potomac aquifer in Maryland (Plummer and others, 2012), correlating with low hydraulic conductivity values and fine-grained sediments. 48 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

STAFFORD NC MARYLAND

Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E E.G. Surprise Rappahannock Taylor Hill LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

EXPLANATION <10 percent NORTH CAROLINA >10–20 percent >20–30 percent >30 percent ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 28. Open-intervalFigure 28. Screen lengths lengths of aquifer-tested of aquifer-tested wells wells as aas percentage percent of totalof total thickness thickness of the of thePotomac Potomac aquifer aquifer in Virginia in Virginia and parts of Maryland and partsNorth of Carolina. Maryland Location and North of ChesapeakeCarolina. Location Bay impact of Chesapeake crater from Bay Powars impact craterand Bruce from (1999).Powars and Bruce (1999). Hydrologic Conditions of the Potomac Aquifer 49

77° 76°

PRINCE WILLIAM MD

NC MARYLAND STAFFORD Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area KING GEORGE in Virginia, Maryland, and North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K Surprise E E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk

SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake ATLANTIC Franklin Suffolk OCEAN GREENSVILLE

EXPLANATION Hydraulic conductivity, in feet per day <50 >50–100 NORTH CAROLINA >100–300 >300

ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS Figure 29. Hydraulic conductivity of the Potomac aquifer in Virginia and parts of Maryland and North Carolina. Figure 29. HydraulicCalculated conductivity from values aquifer-test of the Potomacestimates aquiferof transmissivity in Virginia and and well parts screen of Maryland lengths. andLocation North of Carolina Chesapeake as calculated Bay from aquifer- test estimates of transmissivityimpact crater and from well Powars open-interval and Bruce lengths. (1999). Location of Chesapeake Bay impact crater from Powars and Bruce (1999). 50 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Specific Storage and Storativity Vertical Hydraulic Connectivity

Fewer storativity values are reported for the Potomac In confined stratified sedimentary aquifer systems such aquifer than transmissivity values because observation-well as in the Virginia Coastal Plain, groundwater primarily flows data were not available as required to determine storativity. laterally through aquifers with slower vertical flow as leakage Almost all storativity values are from aquifer tests located across confining units between aquifers (fig. 32). Additionally, within or close to the Norfolk arch depositional subarea, across much of the Virginia Coastal Plain, confining-unit leak- with the exception of one test located farther north within the age is generally downward because widespread water-level Salisbury embayment subarea in Virginia and a small number declines are greater in the Potomac aquifer than in overlying of additional tests in Maryland (fig. 30). The reported storativ- aquifers (see section “Flow”). In proximity to large production ity values do not exhibit any discernible regional trend. wells, substantial vertical flow within a single aquifer can be Whereas storativity is partly a function of aquifer locally induced toward the well open interval. thickness, specific storage is an intrinsic property of the The relations described above are reflected by vertical sediment resulting from its compressibility, and which is hydraulic gradients among different parts of the Virginia relatable to the sediment distribution. Specific storage can Coastal Plain aquifer system. Vertical gradients are large be calculated by dividing storativity values by thickness. across confining units that impose an effective hydraulic sepa- Accordingly, 113 values of specific storage were calculated ration between aquifers (fig. 32, far left side). Water levels in (Attachment 2) by dividing reported aquifer-test storativity wells completed in the upper aquifer are substantially higher values by the lengths of open intervals in production wells and than those in the lower aquifer. Conversely, vertical gradients were assumed to approximate the effective aquifer thickness away from and unaffected locally by large production wells that was hydraulically stressed during the tests (see section are generally small within each of the aquifers (fig. 32, left of “Hydraulic Conductivity”). Specific storage values were not center), and water levels in closely spaced wells completed calculated for 14 wells because open-interval data were not at different depths within a single aquifer differ only slightly. available. By contrast, at locations in proximity to large production Specific storage of sediments composing the Potomac wells, vertical gradients can be large within the pumped aquifer (fig. 32, right), and water levels in closely spaced wells aquifer across the Norfolk arch depositional subarea is gener- completed at different depths can differ substantially. ally greater than at the few locations with available data in the On the basis of the above information, variations among Salisbury embayment subarea (fig. 31). Lower specific storage values of vertical hydraulic gradients within the Virginia toward the Salisbury embayment subarea possibly could result Coastal Plain aquifer system indicate effects of contrasting from less compressible sediments there. Many fine-grained sediment texture on hydraulic connectivity. Large vertical sediments within the Potomac aquifer—notably widespread gradients between the Potomac aquifer and overlying aquifers floodplain paleosols and organic-rich swamp deposits (fig.C 9 result from an effective hydraulic separation imposed between and D )—are typically dense and highly compacted as a result the aquifers by fine-grained sediments that compose an of deep burial for more than 100 m.y. Pressurized extraction of intervening confining unit or units. Likewise, within parts of pore water from core samples of these sediments (McFarland the Potomac aquifer not affected locally by large production and Bruce, 2005) produced much smaller volumes (as little wells, large vertical gradients across intervals of fine-grained as an order of magnitude) than samples of younger, overlying sediments indicate that an effective hydraulic separation is and less compacted fine-grained sediments composing other imposed between overlying and underlying coarse-grained hydrogeologic units. By the same rationale, sediments having sediments. Conversely, fine-grained sediments that are too thin the largest specific storage values along the western margin of and (or) discontinuous to hydraulically separate overlying and the Potomac aquifer have been less deeply buried and possibly underlying coarse-grained sediments result in small vertical are less compacted. gradients within the Potomac aquifer. Hydrologic Conditions of the Potomac Aquifer 51

77° 76°

PRINCE WILLIAM MD

NC MARYLAND STAFFORD Fredericksburg S A L I S B U R Y E M B A Y M E N T Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk ATLANTIC GREENSVILLE OCEAN

EXPLANATION Storativity <0.0002 NORTH CAROLINA >0.0002–0.0005 >0.0005–0.001 >0.001

ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 30. StorativityFigure 30. values Storativity estimated values from estimated aquifer from tests aquifer in the tests Potomac in the aquifer Potomac in Virginiaaquifer in and Virginia parts andof Maryland parts of Maryland and North Carolina. Location of Chesapeakeand North Carolina. Bay impact Location crater of from Chesapeake Powars and Bay Bruce impact (1999). crater from Powars and Bruce (1999). 52 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

MD PRINCE WILLIAM

STAFFORD NC MARYLAND

Fredericksburg Location of the study area in S A L I S B U R Y E M B A Y M E N T Virginia, Maryland, and North Carolina KING GEORGE Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON ATLANTIC Emporia Chesapeake OCEAN Franklin Suffolk

GREENSVILLE

EXPLANATION Specific storage, in per foot <0.00001 >0.00001–0.00005 >0.00005–0.0001 >0.0001 NORTH CAROLINA

ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 31. SpecificFigure storage 31. Specific of the storagePotomac of aquiferthe Potomac in Virginia aquifer and in partsVirginia of Marylandand parts ofand Maryland North Carolina and North as Carolina.calculated from aquifer-test estimates of storativityCalculated and from well aquifer-testopen-interval estimates lengths. of Location storativity of andChesapeake well screen Bay lengths. impact Location crater from of Chesapeake Powars and Bay Bruce (1999). impact crater from Powars and Bruce (1999). Hydrologic Conditions of the Potomac Aquifer 53

Wells Wells Wells between within aquifer within aquifer aquifers without pumping with pumping

Lateral flow Small gradient

Interbed Equipotential

Large Vertical leakage gradient

Large gradient

Lateral flow Small gradient

Large gradient

Figure 32. Conceptual sectional flow net of generalized hydraulic head distribution across adjacent aquifers (light gray) Figureseparated 32. C byon ca epconfiningtual sec unittion (darkal flo wgray) net inof the gene Virginiaralized Coastal hydrau Plain.lic head Primarily distrib lateralution aflowcro sstakes adja placecent throughaquifer sthe (ligh aquiferst gray) sepaandr averticalted by leakagea confini throughng unit the(da rconfiningk gray) in unit. the AV irlargeginia downward Coastal P laihydraulicn. Prim agradientrily late rexistsal flow between takes p thelac eaquifers through (far the left). aqu ifers andWhere vertical not leina proximitykage thr oughto pumping, the con smallfining upward unit. A and large downward downward hydraulic hydrau gradientslic gradie existnt ex withinists be eachtween aquifer the aqu (leftife ofrs center). (far left ). WFine-grainedhere not in p interbedsroximity t withino pum ptheing aquifers, small up arew atoord discontinuousand downwar dto h imposeydraul ilargec grad verticalients e hydraulicxist within gradients. each aqu Inif eproximityr (left of ctoen ter). Fine-grainedpumping (right), interbeds large upward within t andhe aqu downwardifers are hydraulic too disc ongradientstinuous existto im withinpose l ar singlege ver aquifer.tical hy dEffectsraulic ofgr adboundaryients. I nconditions proximity to puaremp noting represented.(right), large upward and downward hydraulic gradients exist within a single aquifer. Effects of boundary conditions are not represented.

Vertical Hydraulic Gradients • measured in a single well lacking one or more closely spaced wells, Vertical hydraulic gradients were calculated on the basis of water levels measured in wells in the Virginia Coastal Plain • measured in pairs of wells having vertically overlapping and adjacent parts of Maryland and North Carolina. Historical open intervals, water-level data were obtained from the USGS National • measured more than 1 day apart between paired wells, Water Inventory System (NWIS) databases for Virginia, Maryland, and North Carolina. These data include water levels • dominated by large short-term fluctuations, and prob- measured by the VA DEQ and MGS. Additional water-level ably affected locally by large production wells (based on data from North Carolina were obtained from the NC DNR examination of water levels among wells having adequate Web site at http://www.ncwater.org/Data_and_Modeling/ lengths of record to support an evaluation), or Ground_Water_Databases/wellaccess.php. Vertical hydraulic gradients were calculated as the • from unconfined parts of the Potomac aquifer receiving difference in water-level altitude between closely spaced wells recharge by infiltration from the land surface. open across different intervals divided by the vertical distance A total of 129 pairs of wells were used to calculate 9,479 verti- between the open intervals. Resulting gradient values are cal gradient values based on water levels measured between dimensionless. Water levels were omitted that were November 17, 1953, and October 4, 2011. Periods of record 54 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina among most well pairs are brief, including 24 well pairs with than 0.1 downward. Thus, the Potomac aquifer is hydraulically only one set of measurements and 45 well pairs having mea- separated from overlying aquifers by intervening confining surements spanning less than 1 year. units, but most fine-grained sediments within the Potomac Historical mean vertical hydraulic gradients for each aquifer are too thin and discontinuous to hydraulically separate pair of wells were calculated from individual vertical gradient overlying and underlying coarse-grained sediments. Hydraulic values for each well pair and are tabulated in a digital data- continuity of the Potomac aquifer in Virginia has been further spreadsheet file (Attachment 3). Ancillary information on each indicated by potentiometric surfaces that were mapped for pair of wells is followed by summary statistics of individual the separate upper, middle, and lower Potomac aquifers of the gradient values for each pair, along with information on the USGS RASA investigation (Hammond and others, 1994a, b, water-level measurement record. c). The potentiometric surfaces are broadly similar and do not Wells open to the Potomac aquifer are distinguished indicate widespread large vertical gradients. from wells open to other aquifers on the basis of the most Data for vertical hydraulic gradients are relatively sparse recent hydrogeologic characterization of the Virginia Coastal across the Salisbury embayment depositional subarea and Plain (McFarland and Bruce, 2006). Historical mean verti- are only available for two locations in Virginia. Within the cal hydraulic gradients within the Potomac aquifer were Potomac aquifer (fig. 33), vertical hydraulic gradients are calculated for 52 pairs of wells having both wells open at small at Surprise Hill in Northumberland County, Virginia, different depths within the Potomac aquifer (fig. 33). At these which is more than 30 mi from large pumping centers and pos- well pairs, 4,954 individual gradient values were calculated sibly relatively isolated from widespread water-level declines. from water levels measured between June 25, 1969, and Small vertical gradients also exist far to the northwest near the December 29, 2010. Well pairs are sparse across the Northern aquifer margin among three well pairs with open intervals that Neck, upper Middle Peninsula, and in southern Maryland. are not separated by confining units. By contrast, several miles Only one well pair is located in northeastern North Carolina to the east in Maryland, large vertical gradients exist among because most observation wells in North Carolina are open to three wells pairs where intervals of fine-grained sediments upper Cretaceous sediments that are recognized here as being hydraulically separate overlying and underlying coarse- distinct from the Potomac aquifer. grained sediments. Pumping and drawdown in the middle part Historical mean vertical hydraulic gradients between the of the Potomac aquifer induces both upward and downward Potomac aquifer and overlying aquifers were calculated for gradients from underlying and overlying parts of the aquifer in 77 pairs of wells having one well open to the Potomac aquifer which water levels are higher. and the other open to an overlying aquifer (fig. 34). At these Consistent with the above, potentiometric surfaces well pairs, 4,525 individual gradient values were calculated mapped for the three vertically spaced subaquifers in from water levels measured between November 17, 1953, and Maryland (Curtin and others, 2010b, c, d) indicate that October 4, 2011. These well pairs are also sparse across the the subaquifers are hydraulically separated by intervening Northern Neck and upper Middle Peninsula, where water-level confining units (see section “Salisbury Embayment”). Cones measurements are generally lacking, and in southern Maryland of depression beneath pumping centers at different locations where few observation wells are closely spaced. Well pairs within each subaquifer are laterally offset by several miles, located in northeastern North Carolina include wells open to resulting in substantial vertical hydraulic gradients between upper Cretaceous sediments recognized here as distinct from the subaquifers. Hydraulic continuity of the subaquifers in sediments of the Potomac aquifer (see section “Hydrostratigra- Maryland—and their separation by confining units—also was phy of Cretaceous Age Sediments in Southeastern Virginia and corroborated by relating observed sediment sand percentages Northeastern North Carolina”). to numerical models of connectivity among individual sand bodies (Drummond, 2007). Hydraulic separation between the Regional Connectivity Trends subaquifers likely extends into Virginia to the northernmost Northern Neck and Virginia Eastern Shore where the confining Vertical hydraulic gradients indicate that sediments units have been identified in boreholes. Data are not adequate, composing most of the Potomac aquifer in Virginia function however, to determine how much farther south across the hydraulically as a single interconnected aquifer. Vertical gra- Salisbury embayment depositional subarea the confining units dients within the Potomac aquifer are generally small (fig. 33), extend and hydraulically separate the subaquifers. with an overall mean among historical well-pair mean Some vertical gradients between the Potomac aquifer and gradients of 0.006 downward. Moreover, gradients within overlying aquifers in the Salisbury embayment depositional most of the Potomac aquifer are neither distinctly upward nor subarea are large and upward (fig. 34). The Potomac aquifer downward, with all values in Virginia between 0.1 upward and in Maryland is not as heavily developed as in Virginia (Drum- 0.1 downward. By contrast, most vertical gradients between mond, 2007). The Aquia aquifer, however, is more heavily the overlying aquifers and the Potomac aquifer are large and developed in Maryland and exhibits large and widespread downward (fig. 34), with an overall mean among historical water-level drawdowns (Curtin and others, 2010a). Water well-pair mean gradients of 0.18 downward and values of levels in parts of the underlying Potomac aquifer are higher 29 well-pair mean gradients (38 percent of the total) greater than in the Aquia aquifer (Curtin and others, 2010b, c, d). Hydrologic Conditions of the Potomac Aquifer 55

77° 76°

PRINCE WILLIAM MD

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND Surprise K E Hill E.G. Rappahannock Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

Aquifer margin

ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

EXPLANATION Greater than 0.1 upward NORTH CAROLINA Up to 0.1 upward Up to 0.1 downward Greater than 0.1 to 1 downward Chowan ALBEMARLE EMBAYMENT Greater than 1 downward

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 33. HistoricalFigure mean 33. Historical vertical meanhydraulic vertical gradients hydraulic within gradients the Potomac within the aquifer Potomac in Virginia aquifer and in Virginia parts of and Maryland parts of and North Carolina. Location of ChesapeakeMaryland Bay and impact North Carolina.crater from Location Powars of andChesapeake Bruce (1999). Bay impact crater from Powars and Bruce (1999). 56 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

77° 76°

PRINCE WILLIAM MD

NC MARYLAND STAFFORD S A L I S B U R Y E M B A Y M E N T Fredericksburg Location of the study area in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E E.G. Rappahannock Surprise Taylor LANCASTER Hill KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater

Aquifer margin N O R F O L K A R C H

ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

EXPLANATION Greater than 0.1 upward NORTH CAROLINA Up to 0.1 upward Up to 0.1 downward ALBEMARLE EMBAYMENT Greater than 0.1 to 1 downward Chowan Greater than 1 downward

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 34. HistoricalFigure mean34. Historical vertical meanhydraulic vertical gradients hydraulic between gradients the between Potomac the aquifer Potomac and aquifer overlying and aquifers overlying in aquifers Virginia and parts of in Virginia and parts of Maryland and North Carolina. Location of Chesapeake Bay impact crater from Maryland and North Carolina. Location of Chesapeake Bay impact crater from Powars and Bruce (1999). Powars and Bruce (1999). Water-Resource Management Applications 57

Thus, withdrawals from the Aquia aquifer are inducing inconsistencies between previously used designations of upward gradients from the Potomac aquifer at some well pairs. aquifers and confining units, and explain the observed lack Across most of the Albemarle embayment depositional of stratigraphic continuity among the heterogeneous fluvial subarea, vertical hydraulic gradients between the Potomac sediments. Moreover, clear relations are demonstrated between aquifer and overlying aquifers are large and downward the spatial distribution of the sediments and their hydrologic (fig. 34). The upper Cenomanian confining unit hydrauli- function, including the hydraulic properties of the sediments cally separates the Potomac aquifer from overlying upper and vertical hydraulic connectivity within the Potomac aquifer Cretaceous sediments (see section “Hydrostratigraphy of and with other aquifers. Cretaceous Age Sediments in Southeastern Virginia and Northeastern North Carolina”). Regulatory Implications

The three structurally based depositional subareas of Water-Resource Management the Potomac aquifer designated here provide a possible context for resource management (see section “Depositional Applications Subareas”). Contrasting hydrologic characteristics among different parts of the Potomac aquifer are organized and Information on the Potomac aquifer presented here can geologically accounted for by the depositional subareas (see be used to support water-resource management efforts in the section “Conceptual Depositional Model”). Effects of current Virginia Coastal Plain. Applications of data and concepts and future groundwater withdrawals also likely differ among can aid characterization of the aquifer, provide a context for the subareas, and regulatory approaches could be applied regulation of groundwater withdrawal, and guide future data accordingly. Designation of the sediments as a single aquifer collection. in the Norfolk arch and Albemarle embayment subareas—and as a series of vertically spaced subaquifers and intervening Aquifer Characterization confining units in the Salisbury embayment subarea—best reflects understanding of the Potomac aquifer. By contrast, Water-supply planning and development can be aided the VA DEQ currently (2013) limits water-level declines to no by reducing uncertainty associated with the design and siting more than 80 percent from pre-development levels to the top of production wells in the heterogeneous fluvial sediments of surfaces of the subdivided upper, middle, and lower Potomac the Potomac aquifer. Design and construction of high yielding aquifers as delineated by the USGS RASA investigation wells can be facilitated by using the local scale information across the entire Virginia Coastal Plain. This criterion has presented here on water-bearing beds in existing boreholes. the potential to exceed the same limit relative to a single Specifically, borehole sediment-interval and related data can undivided Potomac aquifer. be applied toward site-specific targeting of productive beds The three Potomac subaquifers are known to extend composed of coarse-grained sand and gravel and avoidance of from Maryland into Virginia at least as far as the boreholes low yielding fine-grained beds. Illustrations and tabulated data on the northernmost Northern Neck and Virginia Eastern provide sediment textures and interval thicknesses (see section Shore in which the intervening confining units are identified “Borehole Sediment Intervals;” Attachment 1) and hydraulic (figs. 17–21). The confining units are theorized to extend properties (see section “Sediment Hydraulic Properties;” farther south across most of the Salisbury embayment subarea, Attachment 2). but data are lacking from boreholes deep enough to intercept More broadly, optimal locations for groundwater them. Consequently, subaquifer boundaries cannot currently development can be identified on the basis of the development be explicitly demarcated. Resource-management options for potential among planning areas. Data, illustrations, and designating areas to be regulated as an undivided Potomac descriptions presented here distinguish parts of the Potomac aquifer versus multiple subaquifers include recognition of aquifer dominated by relatively dense concentrations of the subaquifers (1) only as far south as the Northern Neck coarse-grained beds from concentrations of fine-grained beds and Virginia Eastern Shore boreholes, (2) as theorized across (see section “Regional Lithologic Trends”). Development- most of the Salisbury embayment subarea, or (3) beyond the project designs can thereby designate production-well Northern Neck and Virginia Eastern shore boreholes but across locations and estimate completion depths to optimize drilling only part of the Salisbury embayment subarea. Practical needs operations and associated costs. of the regulatory program could be met by approximating the In addition to specific information that supports water- extents of the subaquifers along county boundaries or other supply planning and development, a broadened perspective jurisdictional lines. on the Potomac aquifer provides accurate conceptualization On the basis of the above information, simulation of its hydrologic function that is fundamental to effective modeling performed by the VA DEQ to evaluate effects of management as a water resource. Characterizations of groundwater withdrawals could potentially base vertical Potomac aquifer sediments presented here largely resolve discretization of the Potomac aquifer on subaquifers present 58 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina across the Salisbury embayment subarea but undivided farther Consequently, conditions in the middle to upper part of the south. In addition, zonation of simulated hydraulic properties Potomac aquifer are generally best known because it has been of the Potomac aquifer could be based on the geographic the most heavily developed. Future broadening and deepening distribution of the subareas. Alternatively, a more complex of groundwater withdrawals are likely to increase information and detailed geostatistical distribution of sediment hydraulic needs for further development. Controls on groundwater properties could be derived from borehole sediment-interval levels and flow within the developed part of the Potomac data. Vertical hydraulic gradients presented here also provide a aquifer, however, are likely already being imposed by deeper possible basis for model calibration. parts of the aquifer for which data are largely unavailable. Characterization of the Potomac aquifer requires inclusion of Information Needs its deepest parts, even if groundwater is heavily developed only in shallower parts. Results presented here provide guidance for future Several aspects of the structural configuration of the data-collection efforts needed to support management of the Potomac aquifer remain relatively unknown. Many more Potomac aquifer as a sustainable water supply. Few existing faults probably exist throughout the Virginia Coastal Plain boreholes penetrate the total thickness of the Potomac aquifer that have not been identified, particularly in and around the as it deepens and thickens eastward (fig. 35). Consequently, Chesapeake Bay impact crater. A more complete determina- knowledge of the spatial distribution of sediments compos- tion is needed of the distribution of faults as well as their ing the Potomac aquifer lessens eastward. Across much hydrologic effects. In stratified sediments, faults can function of southeastern Virginia and northeastern North Carolina, as either conduits or barriers to groundwater flow depending conditions within the deepest part of Potomac aquifer are on specific structural details (Caine and others, 1996). unknown. Moreover, only the shallowest part of the Potomac Basement bedrock underlies the entire Potomac aquifer, yet aquifer is known across much of the Northern Neck and its composition, structure, and hydrologic function remain Middle Peninsula where borehole and observation-well data largely unknown. In addition to pervasive faults and fracture are generally sparse. As a result, the extent of subaquifers and zones in crystalline bedrock, permeable sediments associated intervening confining units in the southernmost part of the with buried Mesozoic-age basins could affect groundwater Salisbury embayment subarea cannot currently be accurately flow and quality substantially. characterized. Lastly, better understanding is needed of the Chesapeake Historically, information to support the needs of water- Bay impact crater. Specifically regarding the Potomac aquifer, resource management efforts in the Virginia Coastal Plain a more fully developed conceptualization of the megablocks has been derived from groundwater characterizations made could help assess their current designation as part of the Poto- in tandem with and reliant upon groundwater development. mac aquifer or alternatively as a separate hydrogeologic unit. Water-Resource Management Applications 59

77° 76°

PRINCE WILLIAM MD

NC MARYLAND STAFFORD

Fredericksburg Location of the study area S A L I S B U R Y E M B A Y M E N T in Virginia, Maryland, and KING GEORGE North Carolina Potomac Oak SPOTSYLVANIA Grove WESTMORELAND River

CAROLINE NORTHERN NEC 38° NORTHUMBERLAND ACCOMACK ESSEX RICHMOND

K E Surprise E.G. Rappahannock Hill Taylor LANCASTER KING AND QUEEN HANOVER MIDDLE PENINSUL CHESAPEAKE

MIDDLESEX BAY KING WILLIAM River VIRGINIA EASTERN SHOR West Point A Richmond NEW KENT GLOUCESTER MATHEWS Aquifer HENRICO margin

JAMES CITY CHARLES CITY YORK-JAMES PENINSUL York CHESTERFIELD A Hopewell Williamsburg Colonial Heights YORK River PRINCE GEORGE James NORTHAMPTON Petersburg Newport News Poquoson DINWIDDIE SURRY

Chesapeake Bay River Hampton 37° Impact Crater Aquifer margin N O R F O L K A R C H ISLE OF WIGHT SUSSEX Norfolk SOUTHEASTERN VIRGINIA Portsmouth Virginia Beach

SOUTHAMPTON Emporia Chesapeake Franklin Suffolk

GREENSVILLE

ATLANTIC OCEAN

EXPLANATION <25 percent NORTH CAROLINA >25–75 percent >75–99 percent 100 percent ALBEMARLEChowan EMBAYMENT

Hope Albemarle Plantation River Sound 36°

Base from U.S. Geological Survey, 1973 0 10 20 MILES State of Virginia, 1:500,000 0 10 20 KILOMETERS

Figure 35. PercentFigure of35. thickness Percent of penetrated thickness bypenetrated boreholes by inboreholes the Potomac in the aquifer Potomac in Virginiaaquifer in and Virginia parts andof Maryland parts of Maryland and North Carolina. Location of Chesapeakeand North Carolina.Bay impact Location crater of from Chesapeake Powars and Bay Bruce impact (1999). crater from Powars and Bruce (1999). 60 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Summary and Conclusions intervals exhibit widely varying thicknesses and large vertical offsets, and lack alignment to any consistent The widespread and several-thousand-foot thick Potomac regional structural trend. Thus, the entire sediment aquifer is the largest and most heavily used source of ground- thickness functions hydraulically as a single intercon- water in the Atlantic Coastal Plain Physiographic Province in nected aquifer. eastern Virginia. It accounts for approximately 90 Mgal/d or 2. The Salisbury embayment depositional subarea three-fourths of the groundwater withdrawn to supply major spans the northern part of the Virginia Coastal Plain industries, many towns and cities, and low density residential and extends into southern Maryland. The Salisbury developments in rural areas. Resulting water-level declines embayment subarea encompasses thinner and less as great as 200 ft near large withdrawal centers have reversed widespread coarse-grained sediment intervals than flow gradients to create the potential for saltwater intrusion. the Norfolk arch subarea. It also includes fine-grained Conventional stratigraphic correlation does not accurately sediment intervals that are increasingly thick and characterize the complex spatial distribution of highly widespread northward. Two continuous confining interbedded and sharply contrasting fluvial sediments that units are delineated that include fine-grained inter- compose the Potomac aquifer. Without a full understanding vals collectively as thick as several hundred feet. of the aquifer’s internal hydraulic connectivity and overall The confining units are consistently aligned to a hydrologic function, water-supply planning and development regional structural trend and stratigraphically cor- efforts are hampered, and interpretations of regulatory criteria relate from southern Maryland into Virginia to the for allowable water-level declines are ambiguous. northernmost Northern Neck and Virginia Eastern An investigation by the USGS in cooperation with the Shore. Although their southern margins are uncertain VA DEQ during 2010–11 provides a comprehensive regional because of inadequate boreholes depths, the confin- description of the Potomac aquifer in the Virginia Coastal ing units are inferred to extend across most of the Plain. Hydrogeologic data characterize the spatial distribution Salisbury embayment subarea. The confining units of Potomac aquifer sediments and their relation to hydrologic hydraulically separate three vertically spaced sub- conditions. The Potomac aquifer is considered to consist of aquifers between which vertical flow is impeded. The all lower Cretaceous to lowermost upper Cretaceous fluvial subaquifers are continuous northward with aquifers in sediments of the Potomac Formation, including undisrupted southern Maryland. Southward into the Norfolk arch sediments outside of the Chesapeake Bay impact crater and subarea, the subaquifers merge into a single undivided megablock beds of the Potomac Formation that formed within Potomac aquifer. The distinction between divided and the crater during the late Eocene Epoch. undivided parts of the Potomac aquifer in Virginia The spatial distribution of Potomac aquifer sediments in provides a new hydrogeologic understanding of the the Virginia Coastal Plain and adjacent parts of Maryland and sediments in the area. North Carolina is represented by altitudes and thicknesses of 2,725 vertical sediment intervals, designated as dominantly 3. The Albemarle embayment depositional subarea coarse or fine grained. Sediment intervals were determined spans southeastern Virginia into northeastern North by interpretation of geophysical logs and descriptions of Carolina and encompasses thinner and less wide- sediment cuttings and core from a network of 456 boreholes. spread coarse-grained sediment intervals than the Coarse-grained sediment intervals represent channel and bar Norfolk arch subarea. Fine-grained intervals are of sands and gravels, which commonly are targeted for comple- only moderate thicknesses. Closely spaced intervals tion of water-supply wells. Fine-grained sediment intervals exhibit widely varying thicknesses and large vertical represent floodplain paleosols, weathered interstitial clays, and offsets, and lack alignment to any consistent regional freshwater swamp and abandoned-channel fill deposits, which structural trend. Thus, the entire sediment thickness generally are avoided for completion of water-supply wells. functions hydraulically as a single interconnected A visually apparent trend among sediment intervals aquifer. The relatively open structural configuration along with borehole summary statistical data indicate that of the Albemarle embayment differs from the more thicknesses and relative dominance of coarse- and fine-grained closed configuration of the Salisbury embayment, sediments broadly contrast across major structural features of and possibly was not as conducive to accumulation of the Virginia Coastal Plain and adjacent parts of Maryland and thick fine-grained sediments. North Carolina. Three corresponding sediment depositional Published sediment pollen-age data indicate regionally subareas are designated on the basis of regional trends in uniform deposition of Potomac aquifer sediments across most sediment lithology and stratigraphic continuity. of the area over time. Delineated pollen-zone age-boundary 1. The Norfolk arch depositional subarea spans the cen- surfaces broadly represent the sediment-age distribution tral and southern parts of the Virginia Coastal Plain internally within the Potomac aquifer. The spatial distribu- and encompasses the thickest and most widespread tion of Potomac aquifer sediments transgresses the ages of coarse-grained sediment intervals. Closely spaced the sediments, although the two confining units within the Summary and Conclusions 61

Salisbury embayment subarea are approximately aligned with Across the Norfolk arch depositional subarea, transmissivity is the age-boundary surfaces. generally large and broadly correlates with the predominance Fine-grained marine sediments of the upper Cenomanian of coarse-grained sediments. Toward the Salisbury embayment confining unit are a major hydrologic upper boundary on subarea, transmissivity is generally small and correlates with the Potomac aquifer. The upper Cenomanian confining unit increasingly fine-grained sediments. Similarly, 296 values of was previously recognized only in southeastern Virginia and sediment hydraulic conductivity, calculated from transmissiv- is newly delineated as extending into northeastern North ity values and tested well open-interval length, consistently Carolina, based on distinctive borehole geophysical-log align with relative proportions of coarse-grained sediments signatures and a recently documented geologic map and among the depositional subareas. Hydraulic conductivity corehole. A substantial hydraulic separation is imposed by in turn correlates with and controls regional velocities of the upper Cenomanian confining unit between the Potomac lateral flow through the Potomac aquifer, based on published aquifer and overlying sediments across southeastern Virginia estimates of groundwater ages relative to the time of recharge. and northeastern North Carolina. Discrepancies in previously A velocity of approximately 10 ft/yr across the Norfolk arch drawn hydrostratigraphic relations among Cretaceous age subarea correlates with large hydraulic conductivity values sediments between southeastern Virginia and northeastern and predominance of coarse-grained sediments. Velocities North Carolina are partly resolved with recognition that the northward into the Salisbury embayment slow progressively Potomac aquifer deepens and thins southward before being from approximately 4 ft/yr to 0.1 ft/yr and correlate with small truncated to the southwest against a fault. Large parts of hydraulic conductivity values and increasingly fine-grained overlying late Cretaceous age sediments previously designated sediments. Storativity of the Potomac aquifer does not exhibit in northeastern North Carolina as the upper and lower Cape a regional trend. Specific storage calculated from storativity Fear aquifers are not continuous with the Potomac aquifer in values and effective aquifer thicknesses, however, broadly southeastern Virginia, but possibly are partly continuous with reflects the degree of sediment compaction resulting from the Virginia Beach aquifer. depth of burial. A fluvial depositional complex spanning the Virginia Effects of contrasting sediment texture are inferred on Coastal Plain produced the spatial distribution of Potomac vertical hydraulic connectivity within the Potomac aquifer aquifer sediments between approximately 100 and 145 Ma. and with overlying aquifers. Values of vertical hydraulic The depositional complex is a counterpart to an earlier gradient were calculated from 9,479 pairs of water levels theorized complex in Maryland and probably was among a measured between November 17, 1953, and October 4, 2011, series of depositional complexes arrayed along the Atlantic in 129 closely spaced pairs of wells. Mostly large downward Coastal Plain. Within the Virginia depositional complex, gradients between the Potomac aquifer and overlying aquifers coarse-grained sand and gravel from an uplifted source area indicate that hydraulic separation is imposed by intervening to the west was deposited across the Norfolk arch subarea confining units, including in northeastern North Carolina by by immature, high-gradient braided streams as longitudinal the upper Cenomanian confining unit. Conversely, generally bars and channel fills. From the Norfolk arch subarea, braided small vertical gradients within the Potomac aquifer indicate streams transitioned across the Salisbury embayment and that most of the sediments in Virginia function hydraulically as Albemarle embayment subareas to mature, medium- to a single interconnected aquifer. Most fine-grained sediments low-gradient meandering streams. Medium- to coarse-grained are too thin and discontinuous to hydraulically separate sand was deposited as channel fills and point bars, and fine- overlying and underlying coarse-grained sediments. Large grained sediments as overbank deposits. The broad pattern of vertical gradients in Maryland along with results of earlier deposition across the Norfolk arch and Albemarle embayment studies indicate that the three vertically spaced subaquifers are subareas remained relatively fixed over time by persistently hydraulically separated by intervening confining units at least uplifted granite and gneiss source rocks that sustained the as far as the northernmost Northern Neck and Virginia Eastern supply of coarse-grained sediments. Across the Salisbury Shore. embayment subarea, the Virginia depositional complex likely Information on the Potomac aquifer can be used to merged northward with the Maryland depositional complex. support water-resource management in the Virginia Coastal Sediments originating from two different source areas were Plain. Uncertainty associated with effective siting, design, received at different times. Deposition changed dynamically and construction of high yielding production wells can be as schist source rocks underwent three cycles. Initial uplift and reduced by using borehole sediment-interval and related rapid erosion generated coarse-grained sediments. With crustal data to target productive coarse-grained beds and avoid low stability and erosional leveling, thick fine-grained sediments yielding fine-grained beds. Optimal locations for groundwater were deposited by mature meandering streams spanning the development can also be identified on the basis of advance structurally closed concave basement bedrock surface. knowledge of the development potential of different parts of The spatial distribution of Potomac aquifer sediments the Potomac aquifer among planning areas. Characterizations controls variations among its hydraulic properties and of the Potomac aquifer presented here also largely resolve groundwater flow. Documented pumping tests of the Potomac inconsistencies between previously used designations of aquifer at 197 locations produced 336 values of transmissivity. aquifers and confining units, and demonstrate clear relations 62 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina between the spatial distribution of aquifer sediments and their The quality of this report was aided greatly from reviews hydrologic function. by David D. Drummond of the Maryland Geological Survey, The single undivided Potomac aquifer recognized in John R. Eggleston of the USGS Virginia Water Science Center, the Norfolk arch and Albemarle embayment depositional and Rodney A. Sheets of the USGS Water Science Field Team. subareas—and a series of vertically spaced subaquifers and Editorial and graphical quality of this report was greatly aided intervening confining units in the Salisbury embayment by Kay P. Naugle and Jeffrey L. Corbett of the USGS Science subarea—provide a possible context for resource management. Publishing Network. As currently (2013) delimited by the VA DEQ, water-level declines relative to previously subdivided aquifers have the potential to be exceeded relative to the single undivided Potomac aquifer. Given that subaquifer boundaries cannot References Cited currently be explicitly demarcated, resource-management options for designating areas to be regulated as an undivided Andreasen, D.C., 1999, The geohydrology and water-supply Potomac aquifer versus multiple subaquifers include recogni- potential of the lower Patapsco aquifer and Patuxent aqui- tion of the subaquifers (1) only as far as identified in boreholes fers in the Indian Head-Bryans Road area, Charles County, on the northernmost Northern Neck and Virginia Eastern Maryland: Maryland Geological Survey Report of Investi- Shore, (2) as theorized across most of the Salisbury embay- gations No. 69, 119 p. ment subarea, or (3) beyond the Northern Neck and Virginia Eastern Shore boreholes but across only part of the Salisbury Andreasen, D.C., 2004, Optimization of ground-water with- embayment subarea. Simulation modeling performed by the drawals from the lower Patapsco and Patuxent aquifers in VA DEQ to evaluate effects of groundwater withdrawals could the Bryans Road service area, Charles County, Maryland: be similarly based, including vertical discretization and (or) Maryland Geological Survey Administrative Report, 21 p. zonation of Potomac aquifer, or a geostatistical distribution Benson, R.N., 2006, Internal stratigraphic correlation of of aquifer properties derived from borehole sediment-interval the subsurface Potomac Formation, New Castle County, data. Delaware, and adjacent areas in Maryland and New Jersey: Hydrogeologic conditions of the Potomac aquifer are Delaware Geological Survey Report of Investigations less known eastward where less of the aquifer is penetrated by No. 71, 15 p. existing boreholes. Particularly, only the shallowest part of the Potomac aquifer is known across much of the Northern Neck Bradley, R.S., 1999, Paleoclimatology: Burlington, Mass., and Middle Peninsula. Consequently, southward margins of Harcourt/Academic Press, 613 p. confining units that hydraulically separate vertically spaced subaquifers are not accurately determined. Information needs Brown, G.A., and Cosner, O.J., 1974, Ground-water condi- for water-resource management extend beyond the developed tions in the Franklin area, southeastern Virginia: U.S. Geo- part of the Potomac aquifer to include its deepest parts, along logical Survey Hydrologic Investigations Atlas HA–538, with hydrologic effects of structural aspects such as faults, 3 sheets, scale 1:125,000. basement bedrock, and the Chesapeake Bay impact crater. Caine, J.S., Evans, J.P., and Forster, C.B., 1996, Fault zone architecture and permeability structure: Geology, v. 24, no. 11, p. 1025–1028. Acknowledgments Calis, Nadine, and Drummond, D.D., 2008, Hydrogeologic data from six test wells in the upper Patapsco and lower This study was supported by the Virginia Department of Patapsco aquifers in southern Maryland: Maryland Geologi- Environmental Quality (VA DEQ). Special thanks for program cal Survey Basic Data Report No. 22, 73 p. planning go to Scott Kudlas, Scott Bruce, and Mary Ann Massie of VA DEQ. Cederstrom, D.J., 1939, Geology and artesian-water resources Aquifer-test data compilation also was greatly assisted of a part of the southern Virginia Coastal Plain: Virginia by retrieval, organization, and transfer of test documentation Geological Survey Bulletin 51–E, p. 123–136. provided by Previn Smith of VA DEQ and groundwater- program staff of the VA DEQ Tidewater and Piedmont Cederstrom, D.J., 1941, Ground-water resources of the south- Regional Offices. eastern Virginia Coastal Plain: Virginia Geological Survey Thanks also are extended to the many drillers and owners Circular 1, 11 p. of water-supply wells who have provided well data from Cederstrom, D.J., 1943, Chloride in ground water in the across the Virginia Coastal Plain, particularly Sydnor Hydro, Coastal Plain of Virginia: Virginia Geological Survey Bul- Inc., for the majority of borehole geophysical logs on which letin 58, 36 p. interpretations are based. References Cited 63

Cederstrom, D.J., 1945a, Geology and ground-water resources Curtin, S.E., Andreasen, D.C., and Staley, A.W., 2010a, Poten- of the Coastal Plain in southeastern Virginia: Virginia Geo- tiometric surface of the Aquia aquifer in southern Maryland, logical Survey Bulletin 63, 384 p. September 2009: U.S. Geological Survey Open-File Report 2010–1201, 1 p. Cederstrom, D.J., 1945b, Selected well logs in the Virginia Coastal Plain north of James River: Virginia Geological Curtin, S.E., Andreasen, D.C., and Staley, A.W., 2010b, Poten- Survey Circular 3, 82 p. tiometric surface of the upper Patapsco aquifer in southern Maryland, September 2009: U.S. Geological Survey Open- Cederstrom, D.J., 1946a, Chemical character of ground water File Report 2010–1205, 1 p. in the Coastal Plain of Virginia: Virginia Geological Survey Bulletin 68, 62 p. Curtin, S.E., Andreasen, D.C., and Staley, A.W., 2010c, Poten- tiometric surface of the lower Patapsco aquifer in southern Cederstrom, D.J., 1946b, Genesis of ground waters in the Maryland, September 2009: U.S. Geological Survey Open- Coastal Plain of Virginia: Economic Geology, v. 41, no. 1, File Report 2010–1207, 1 p. p. 218–245. Curtin, S.E., Andreasen, D.C., and Staley, A.W., 2010d, Cederstrom, D.J., 1957, Geology and ground-water resources Potentiometric surface of the Patuxent aquifer in southern of the York-James Peninsula, Virginia: U.S. Geological Maryland, September 2009: U.S. Geological Survey Open- Survey Water-Supply Paper 1361, 237 p. File Report 2010–1209, 1 p. Cederstrom, D.J., 1968, Geology and ground-water resources Doyle, J.A., and Robbins, E.A., 1977, Angiosperm pol- of the Middle Peninsula, Virginia: U.S. Geological Survey len zonation of the continental Cretaceous of the Atlantic Open-File Report (unnumbered), 231 p. Coastal Plain and its application to deep wells in the Salis- CH2M Hill, 1990a, Injection and recovery testing cycles for bury embayment: Palynology, v. I, American Association the aquifer storage and recovery (ASR) well no. 4-1 pilot of Stratigraphic Palynologists, Proceedings of the Eighth project, Appendix B, Analysis of aquifer test data, 4 p. Annual Meeting, Houston, Texas, October 1975, p. 43–78. CH2M Hill, 1990b, Aquifer simulation modeling of potential Drummond, D.D., 2007, Water-supply potential of the Coastal expanded aquifer storage and recovery (ASR) systems at the Plain aquifers in Calvert, Charles, and St. Mary’s Counties, Western Branch well field, 38 p. Maryland, with emphasis on the upper Patapsco and lower Patapsco aquifers: Maryland Geological Survey Report of CH2M Hill, 1997, Chesapeake deep well project report, Investigations No. 76, 48 p. Appendix B, Aquifer testing data, 6 p. Edwards, L.E., Powars, D.S., Horton, J.W., Gohn, G.S., Self- Clark, W.B., and Miller, R.L., 1912, The physiography and Trail, J.M., and Litwin, R.J., 2010, Inside the crater, outside geology of the Coastal Plain Province of Virginia: Virginia the crater: Stratigraphic details of the margin of the Chesa- Geological Survey Bulletin 4, p. 13–322. peake Bay impact structure, Virginia, U.S.A., in Gibson, R.L., and Reimold, W.U., eds., Large Meteorite Impacts Commonwealth of Virginia, 1973, Ground water of the York- and Planetary Evolution IV, Vredefort Dome, South Africa, James Peninsula, Virginia: Virginia State Water Control August 17–21, 2008: Geological Society of America Special Board Basic Data Bulletin 39, 74 p. Paper 465, p. 319–394. Commonwealth of Virginia, 1974, Groundwater of southeast- Ellison, R.P., III, and Masiello, R.A., 1979, Groundwater ern Virginia: Virginia State Water Control Board Planning resources of Hanover County, Virginia: Virginia State Water Bulletin 261–A, 33 p. Control Board Planning Bulletin 314, 130 p. Consolvo, C.A., 2008, Aquifer test report, courthouse water Fleck, W.B., and Vroblesky, D.A., 1996, Simulation of system, New Kent County, New Kent, Virginia: GeoRes- ground-water flow of the Coastal Plain aquifers in parts ources, Inc., 10 p. of Maryland, Delaware, and the District of Columbia: Cosner, O.J., 1975, A predictive computer model of the Lower U.S. Geological Survey Professional Paper 1404–J, 40 p., Cretaceous aquifer, Franklin area, southeastern Virginia: 9 pls. U.S. Geological Survey Water-Resources Investigations Fleck, W.B., and Wilson, J.M., 1990, Geology and hydrologic Report 51–74, 72 p. assessment of Coastal Plain aquifers in the Waldorf area, Cosner, O.J., 1976, Measured and simulated ground-water Charles County, Maryland—Water-supply potential of the levels in the Franklin area, southeastern Virginia 1973–74: aquifers: Maryland Geological Survey Report of Investiga- U.S. Geological Survey Water-Resources Investigations tions No. 53, p. 39–96. Report 76–83, 5 sheets, scale 1:250,000. 64 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Fredericksen, N.O., Edwards, L.E., Self-Trail, J.M., Bybell, Hansen, H.J., 1978, Upper Cretaceous (Senonian) and Paleo- L.M., and Cronin, T.M., 2005, Paleontology of the impact- cene (Danian) pinchouts on the south flank of the Salisbury modified and impact-generated sediments in the USGS- embayment, Maryland, and their relationship to antecedent NASA Langley core, Hampton, Virginia, in Horton, J.W., basement structures: Maryland Geological Survey Report of Jr., Powars, D.S., and Gohn, G.S., eds., Studies of the Investigations No. 29, 34 p. Chesapeake Bay impact structure—The USGS-NASA Langely corehole, Hampton, Virginia, and related coreholes Hansen, H.J., 1982, Waste Gate Formation Part One—Hydro- and geophysical surveys: U.S. Geological Survey Profes- geologic framework and potential utilization of the brine sional Paper 1688, chap. D, 37 p. aquifers of the Waste Gate Formation, a new unit of the Potomac Group underlying the Delmarva Peninsula: Mary- Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Engle- land Geological Survey Open File Report, p. 1–50. wood Cliffs, N.J., Prentice Hall, 604 p. Hansen, H.J., 1984, Hydrogeologic characteristics of the Gellici, J.A., and Lautier, J.C., 2010, Hydrogeologic frame- Waste Gate Formation, a new subsurface unit of the work of the Atlantic Coastal Plain, North and South Potomac Group underlying the eastern Delmarva Peninsula: Carolina, in Campbell, B.G., and Coes, A.L., eds., Ground- Maryland Geological Survey Information Circular 39, 22 p. water availability in the Atlantic Plain of North and South Carolina: U.S. Geological Survey Professional Paper 1773, Hansen, H.J., and Wilson, J.M., 1990, Hydrogeology and stra- chap. B, p. 49–162. tigraphy of a 1,515-foot test well drilled near Princess Anne, Somerset County, Maryland: Maryland Geological Survey Giese, G.L., Eimers, J.L., and Coble, R.W., 1997, Simulation Open File Report No. 91–02–5, 59 p. of ground-water flow in the Coastal Plain aquifer system of North Carolina: U.S. Geological Survey Professional Paper Harsh, J.F., 1980, Ground-water hydrology of James City 1404–M, 142 p. County, Virginia: U.S. Geological Survey Open-File Report 80–961, 73 p. Glaser, J.D., 1969, Petrology and origin of Potomac and Magothy (Cretaceous) sediments, middle Atlantic Coastal Harsh, J.F., and Laczniak, R.J., 1990, Conceptualization and Plain: Maryland Geological Survey Report of Investigations analysis of ground-water flow system in the Coastal Plain of No. 11, 101 p. Virginia and adjacent parts of Maryland and North Carolina: U.S. Geological Survey Professional Paper 1404-–F, 100 p. Hamilton, P.A., and Larson, J.D., 1988, Hydrogeology and analysis of the ground-water flow system in the Coastal Heywood, C.E., and Pope, J.P., 2009, Simulation of ground- Plain of southeastern Virginia: U.S. Geological Survey water flow in the Coastal Plain aquifer system of Virginia: Water-Resources Investigations Report 87–4240, 175 p. U.S. Geological Survey Scientific Investigations Report 2009–5039, 115 p. Hammond, E.C., McFarland, E.R., and Focazio, M.J., 1994a, Potentiometric surface of the Brightseat-upper Potomac Hopkins, H.T., Bower, R.F., Abe, J.M., and Harsh, J.F., 1981, aquifer in Virginia, 1993: U.S. Geological Survey Open-File Potentiometric surface map for the Cretaceous aquifer, Report 94–370, 1 p. Virginia Coastal Plain: U.S. Geological Survey Open-File Report 80–965, 1 sheet, scale 1:500,000. Hammond, E.C., McFarland, E.R., and Focazio, M.J., 1994b, Potentiometric surface of the middle Potomac aquifer in Johnson, G.H., and Ramsey, K.W., 1987, Geology and geo- Virginia, 1993: U.S. Geological Survey Open-File Report morphology of the York-James Peninsula, Virginia, in Pro- 94–372, 1 p. ceedings of the Atlantic Coastal Plain Geological Associa- tion: Williamsburg, Va., College of William and Mary, 45 p. Hammond, E.C., McFarland, E.R., and Focazio, M.J., 1994c, Potentiometric surface of the lower Potomac aquifer in Klohe, C.A., and Kay, R.T., 2007, Hydrogeology of the Piney Virginia, 1993: U.S. Geological Survey Open-File Report Point-Nanjemoy, Aquia, and upper Patapsco aquifers, Naval 94–373, 1 p. Air Station Patuxent River and Webster Outlying Field, St. Marys County, Maryland, 2000–06: U.S. Geological Hansen, H.J., 1969, Depositional environments of subsurface Survey Scientific Investigations Report 2006–5266, 26 p. Potomac Group in southern Maryland: American Associa- tion of Petroleum Geologists Bulletin, v. 53, no. 9, Laczniak, R.J., and Meng, A.A., III, 1988, Ground-water p. 1923–1937. resources of the York-James Peninsula of Virginia: U.S. Geological Survey Water-Resources Investigations Report Hansen, H.J., 1971, Transmissivity tracts in the Coastal Plain 88–4059, 178 p. aquifers of Maryland: Southeastern Geology, v. 13, no. 3, p. 127–149. Larson, J.D., 1981, Distribution of saltwater in the Coastal Plain aquifers of Virginia: U.S. Geological Survey Open- File Report 81–1013, 25 p. References Cited 65

Lautier, J.C., 1998, Hydrogeologic framework and ground Meng, A.A., III, and Harsh, J.F., 1988, Hydrogeologic frame- water resources of the north Albemarle region, North Caro- work of the Virginia Coastal Plain: U.S. Geological Survey lina: North Carolina Department of Environment, Health Professional Paper 1404–C, 82 p. and Natural Resources, Division of Water Resources, 42 p. Mixon, R.B., Berquist, C.R., Jr., Newell, W.L., Johnson, Leahy, P.P., and Martin, Mary, 1993, Geohydrology and simu- G.H., Powars, D.S., Schindler, J.S., and Rader, E.K., lation of ground-water flow in the northern Atlantic Coastal 1989, Geologic map and generalized cross sections of the Plain aquifer system: U.S. Geological Survey Professional Coastal Plain and adjacent parts of the Piedmont, Virginia: Paper 1404–K, 81 p., 22 pls. U.S. Geological Survey Miscellaneous Investigations Series Map I–2033, scale 1:250,000. Lichtler, W.F., 1974, Report on the development of a groundwater supply at George Washington Birthplace National Naeser, C.W., Naeser, N.D., Newell, W.L., Southworth, C.S., Monument: National Park Service Administrative Report, Weems, R.E., and Edwards, L.E., 2004, Provenance studies 12 p. in the Atlantic Coastal Plain: What fission-track ages of detrital zircons can tell us about the source of sediments: Lichtler, W.F., and Wait, R.L., 1974, Summary of the ground- Geological Society of America Northeastern and Southeast- water resources of the James River basin, Virginia: U.S. ern Section Joint Meeting, Tysons Corner, Va., March 2004, Geological Survey Open-File Report 74–139, 54 p. 1 p. Mack, F.K., 1966, Ground water in Prince George’s County: Naeser, C.W., Naeser, N.D., Newell, W.L., Southworth, C.S., Maryland Geological Survey Bulletin 29, 101 p. Weems, R.E., and Edwards, L.E., 2006, Provenance studies McFarland, E.R., 1997, Hydrogeologic framework, analysis in the Atlantic Coastal Plain: What fission-track ages of of ground-water flow, and relations to regional flow in the detrital zircons can tell us about the erosion history of the Fall Zone near Richmond, Virginia: U.S. Geological Survey Appalachians: Geological Society of America Annual Meet- Water-Resources Investigations Report 97–4021, 56 p. ing, Philadelphia, Pa., October 2006, 1 p. McFarland, E.R., 1998, Design, revisions, and considerations Naeser, N.D., Naeser, C.W., Southworth, C.S., Morgan, B.A., for continued use of a ground-water-flow model of the and Schultz, A.P., 2004, Paleozoic to Recent tectonic and Coastal Plain aquifer system in Virginia: U.S. Geological denudation history of rocks in the Blue Ridge Province, Survey Water-Resources Investigations Report 98–4085, central and southern Appalachians—Evidence from fission- 49 p. track thermochronology: Geological Society of America Northeastern and Southeastern Section Joint Meeting, McFarland, E.R., 1999, Hydrogeologic framework and Tysons Corner, Va., March 2004, 1 p. ground-water flow in the Fall Zone of Virginia: U.S. Geological Survey Water-Resources Investigations Report National Weather Service, 1996, Climatological data, Virginia, 99–4093, 83 p. December 1996: National Oceanic and Atmospheric Admin- istration, v. 106, no. 12, 26 p. McFarland, E.R., 2010, Groundwater-quality data and regional trends in the Virginia Coastal Plain, 1906–2007: U.S. Geo- Nelms, D.L., Harlow, G.E., Plummer, L.N., and Busenberg, logical Survey Professional Paper 1772, 86 p. Eurybiades, 2003, Aquifer susceptibility in Virginia, 1998– 2000: U.S. Geological Survey Water-Resources Investiga- McFarland, E.R., and Bruce, T.S., 2005, Distribution, origin, tions Report 03–4278, 58 p. and resource-management implications of ground-water salinity along the western margin of the Chesapeake Bay Newton, V.P., and Siudyla, E.A., 1979, Groundwater of the impact structure in eastern Virginia, in Horton, J.W., Jr., Northern Neck Peninsula, Virginia: Virginia State Water Powars, D.S., and Gohn, G.S., eds., Studies of the Chesa- Control Board Planning Bulletin 307, 48 p. peake Bay impact structure—The USGS-NASA Langely Owens, J.P., and Gohn, G.S., 1985, Depositional history of corehole, Hampton, Virginia, and related coreholes and the Cretaceous series in the U.S. Atlantic Coastal Plain: geophysical surveys: U.S. Geological Survey Professional Stratigraphy, paleoenvironments, and tectonic controls of Paper 1688, chap. K, 32 p. sedimentation, in Poag, C.W., ed., Geologic evolution of McFarland, E.R., and Bruce, T.S., 2006, The Virginia Coastal the United States Atlantic margin: New York, N.Y., Van Plain hydrogeologic framework: U.S. Geological Survey Nostrand Reinhold, p. 25–86. Professional Paper 1731, 118 p. Peltier, W.R., 1994, Ice age paleotopography: Science, v. 265, McGee, W.J., 1886, Geologic formations of Washington, D.C., no. 5169, p. 195–201. and vicinity: American Journal of Science, 3d series, v. 31, p. 473–474. 66 Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia, Maryland, and North Carolina

Plummer, L.N., Eggleston, J.R., Andreasen, D.C., Raffens- Russnow-Kane and Associates, 2002, Report on the hydrogeo- perger, J.P., Hunt, A.G., and Casile, G.C., 2012, Old logic framework and well construction activities, Gloucester groundwater in parts of the upper Patapsco aquifer, Atlantic County brackish groundwater supply project production Coastal Plain, Maryland, U.S.A.: Evidence from radiocar- well no. 2, 6 p. bon, chlorine-36, and helium-4: Hydrogeology Journal, 26 p. Russnow-Kane and Associates, 2004, Report on the hydrogeologic framework and well construction activities at the Pope, J.P., and Burby, T.J., 2004, Multiple-aquifer character- groundwater treatment facility—Five Forks site, middle and ization from single borehole extensometer records: Ground- lower Potomac aquifers, 32 p. water, v. 42, no. 1, p. 45–58. Sanford, Samuel, 1913, The underground water resources of Pope, J.P., McFarland, E.R., and Banks, R.B., 2008, Private the Coastal Plain Province of Virginia: Virginia Geological domestic-well characteristics and the distribution of domes- Survey Bulletin 5, 361 p. tic withdrawals among aquifers in the Virginia Coastal Plain: U.S. Geological Survey Scientific Investigations Sanford, W.E., Voytek, M.A., Powars, D.S., Jones, B.F., Coz- Report 2007–5250, 47 p. zarelli, I.M., Cockell, C.S., and Eganhouse, R.P., 2009, Pore-water chemistry from the ICDP-USGS corehole in the Powars, D.S., 2000, The effects of the Chesapeake Bay impact Chesapeake Bay impact structure—Implications for paleo- crater on the geological framework and correlation of hydrology, microbial habitat, and water resources: Geologi- hydrogeologic units of southeastern Virginia, south of the cal Society of America Special Paper 458, p. 867–890. James River: U.S. Geological Survey Professional Paper 1622, 53 p., 1 pl. Sinnott, Allen, 1969a, Results of aquifers tests in sands of the Potomac Group in the Franklin area, southeastern Virginia Powars, D.S., and Bruce, T.S., 1999, The effects of the Chesa- (1949–50): U.S. Geological Survey Open-File Report peake Bay impact crater on the geological framework and 69–260, 80 p. correlation of hydrogeologic units of the lower York-James Peninsula, Virginia: U.S. Geological Survey Professional Sinnott, Allen, 1969b, Ground-water resources of the Northern Paper 1612, 82 p., 9 pls. Neck Peninsula, Virginia: U.S. Geological Survey Open- File Report 69–259, 269 p. Reinhardt, Juergen, Newell, W.L., and Mixon, R.B., 1980, Geology of the Oak Grove core, Part 1, Tertiary lithostratig- Siudyla, E.A., Berglund, T.D., and Newton, V.P., 1977, raphy of the core: Charlottesville, Virginia Division of Ground water of the Middle Peninsula, Virginia: Virginia Mineral Resources Publication 20, p. 1–13. State Water Control Board Planning Bulletin 305, 90 p. Russnow-Kane and Associates, 1995, Report on the hydro- Siudyla, E.A., May, A.E., and Hawthorn, D.W., 1981, Ground geologic framework and well construction activities, Lee water resources of the four cities area, Virginia: Virginia Hall water treatment plant test site, section 4, aquifer testing State Water Control Board Planning Bulletin 331, 90 p. program, 30 p. Sohl, N.F., and Owens, J.P., 1991, Cretaceous stratigraphy of Russnow-Kane and Associates, 1996a, Report on the hydro- the Carolina coastal plalin, in Horton, J.W., Jr., and Zullo, geologic framework and well construction activities at V.A., eds., The geology of the Carolinas: Carolina Geologi- the Kristiansand production well site, upper and middle cal Society, 50th anniversary volume, p. 191–220. Potomac aquifers well lot W-38, 24 p. Teifke, R.H., 1973, Geologic studies, Coastal Plain of Vir- Russnow-Kane and Associates, 1996b, Report on the hydro- ginia: Virginia Division of Mineral Resources Bulletin 83, geologic framework and well construction activities at pts. 1 and 2, 101 p. the Toano/Owens-Illinois production well site, upper and Trapp, Henry, Jr., 1992, Hydrogeologic framework of the middle Potomac aquifers well lot W-1, 20 p. northern Atlantic Coastal Plain in parts of North Carolina, Russnow-Kane and Associates, 1997, Report on the hydrogeo- Virginia, Maryland, Delaware, New Jersey, and New York: logic framework and well construction activities, Harwood U.S. Geological Survey Professional Paper 1404–G, 59 p., Drive (LH-1) and Lee Hall (LH-2) well sites, section 4, 13 pls. aquifer testing program, 39 p. Virginia State Water Control Board, 1973, Ground water of the Russnow-Kane and Associates, 2000, Report on the hydrogeo- York-James Peninsula, Virginia: Virginia State Water Con- logic framework and well construction activities, Gloucester trol Board, Bureau of Water Control Management, Basic County water treatment plant test/monitor well, 5 p. Data Bulletin 39, 74 p. References Cited 67

Vroblesky, D.A., and Fleck, W.B., 1991, Hydrogeologic Wigglesworth, H.A., Perry, T.W., and Ellison, R.P., III, 1984, framework of the Coastal Plain of Maryland, Delaware, and Groundwater resources of Henrico County, Virginia: the District of Columbia: U.S. Geological Survey Profes- Virginia State Water Control Board Planning Bulletin 328, sional Paper 1404–E, 45 p., 2 pls. 118 p. Weems, R.E., Lewis, W.C., and Aleman-Gonzalez, W.B., Wilson, J.M., 1986, Stratigraphy, hydrogeology, and water 2009, Surficial geologic map of the Roanoke Rapids 30' x chemistry of the Cretaceous aquifers of the Waldorf/ 60' quadrangle, North Carolina: U.S. Geological Survey La Plata area, Charles County, Maryland: Maryland Open-File Report 2009–1149, 1 sheet, 1:100,000 scale map. Geological Survey Open File Report No. 86–02–2, 66 p. Weems, R.E., Seefelt, E.L., Wrege, B.M., Self-Trail, J.M., Wilson, J.M., and Fleck, W.B., 1990, Geology and hydro- Prowell, D.C., Durand, Colleen, Cobbs, E.F., III, and logic assessment of Coastal Plain aquifers in the Waldorf McKinney, K.C., 2007, Preliminary physical stratigraphy area, Charles County, Maryland—Geology, hydrogeologic and geophysical data of the USGS Hope Plantation core framework, and water quality: Maryland Geological Survey (BE-110), Bertie County, North Carolina: U.S. Geological Report of Investigations No. 53, p. 1–38. Survey Open-File Report 2007–1251, 16 p. Winner, M.D., Jr., and Coble, R.W., 1996, Hydrogeologic Werkheiser, W.H., 1990, Hydrogeology and ground-water framework of the North Carolina Coastal Plain: U.S. Geo- resources of Somerset County, Maryland: Maryland Geo- logical Survey Professional Paper 1404–I, 106 p. logical Survey Bulletin 35, 156 p. Approved for publication on May 22, 2013.

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Or visit the Virginia Water Science Center Web site at: http://va.water.usgs.gov McFarland—Sediment Distribution and Hydrologic Conditions of the Potomac Aquifer in Virginia and Parts of Maryland and North Carolina—Scientific Investigations Report 2013–5116