Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia
United States Geological Survey Water-Supply Paper 2401
Prepared in cooperation witl Accomack County, Northampton County, and the Virginia Water Control Board Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia
By DONNA L. RICHARDSON
Prepared in cooperation with Accomack County, Northampton County, and the Virginia Water Control Board
U.S. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 2401 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Printed in the Eastern Region, Reston, Va.
UNITED STATES GOVERNMENT PRINTING OFFICE: 1994
For sale by U.S. Geological Survey, Map Distribution Box 25286, MS 306, Federal Center Denver, CO 80225
Library of Congress Cataloging in Publication Data Richardson, Donna L. Hydrogeology and analysis of the ground-water-flow system of the Eastern Shore, Virginia / by Donna L. Richardson. p. cm. (U.S. Geological Survey water-supply paper; 2401) Includes bibliographical references. Supt. of Docs, no.: I 19.13: 1. Water, Underground Eastern Shore (Md. and Va.) 2. Groundwater flow Eastern Shore (Md. and Va.) I. Title. II. Series. GB1025.V8R53 1994 551.49'09755'1 -dc20 92-33725 CIP CONTENTS
Abstract ...... 1 Introduction...... 2 Purpose and Scope...... 2 Location of Study and Model Area...... 3 Previous Studies...... 3 Methods of Investigation ...... 4 Acknowledgments ...... 4 Hydrogeology...... 4 General Geology...... 4 Cretaceous Sediments...... 5 Tertiary Sediments ...... 5 Quaternary Sediments ...... 5 Aquifers and Confining Units ...... 7 Columbia Aquifer ...... 7 Pleistocene Paleochannel Aquifers ...... 18 Yorktown-Eastover Aquifer System...... 18 Upper Yorktown-Eastover Aquifer and Confining Unit...... 20 Middle Yorktown-Eastover Aquifer and Confining Unit ...... 20 Lower Yorktown-Eastover Aquifer and Confining Unit...... 20 St. Marys Confining Unit...... 22 Ground-Water Hydrology ...... 22 Local Ground-Water-Flow System ...... 22 Regional Ground-Water-Flow System ...... 23 Ground-Water Use...... 26 Chloride Distribution...... 33 Analysis of the Ground-Water-Flow System ...... 33 Development of the Flow Model...... 33 Model Grid and Boundaries...... 46 Model Calibration...... 48 Transmissivity...... 50 Storage Coefficient...... 50 Vertical Leakance ...... 50 Steady-State-Model Simulation of Prepumping Conditions ...... 50 Transient-Model Simulation of Pumping Conditions ...... 61 Time Discretization and Ground-Water Withdrawals ...... 61 Results of Simulation...... 61 Application of Ground-Water-Flow Model...... 71 Southern Northampton County Scenario ...... 71 Simulation 1 ...... 71 Simulation 2 ...... 81 Northeastern Accomack County Scenario...... 85 Simulation 1: No-Flow Boundary...... 85 Simulation 2: Constant-Head Boundary...... 87 Permitted-Withdrawal Scenario ...... 94 Discussion of Model Results...... 95 Sensitivity Analysis...... 100
Contents III Model Limitations ...... 100 Summary...... 106 References Cited...... 107
PLATE [In pocket] I. Hydrogeologic sections for the Eastern Shore, Virginia
FIGURES 1. Map showing location of study and model area...... 3 2. Map showing location of control wells used in hydrogeologic framework analysis ...... 6 3-9. Hydrogeologic maps showing altitude of top of: 3. Upper Yorktown-Eastover confining unit...... 10 4. Upper Yorktown-Eastover aquifer ...... 11 5. Middle Yorktown-Eastover confining unit...... 12 6. Middle Yorktown-Eastover aquifer ...... 13 7. Lower Yorktown-Eastover confining unit ...... 14 8. Lower Yorktown-Eastover aquifer ...... 15 9. St. Marys confining unit...... 16 10. Schematic diagram of aquifers and confining units and generalized flow lines...... 17 II,12. Maps showing: 11. Bathymetry in the vicinity of the Eastern Shore...... 21 12. Locations of wells along transect A-A' in the Columbia aquifer...... 24 13,14. Graphs showing: 13. Water levels in selected wells along a transect A-A' in the Columbia aquifer ...... 25 14. Water levels in an irrigation pond and in a nearby well completed in the Columbia aquifer ...... 26 15. Map showing location of selected Virginia Water Control Board research-station well clusters ...... 27 16. Graphs of water levels in research-station well clusters (A) in a recharge area and (B) in a discharge area .. 30 17. Map showing location of observation wells and chloride concentrations in the upper Potomac aquifer for the Coastal Plain of Virginia...... 31 18. Graph showing annual ground-water withdrawal from model area ...... 32 19-22. Maps showing chloride concentrations in: 19. Columbia aquifer...... 34 20. Upper Yorktown-Eastover aquifer ...... 35 21. Middle Yorktown-Eastover aquifer ...... 36 22. Lower Yorktown-Eastover aquifer...... 37 23. Schematic diagram of model representation of the saltwater-freshwater interface tip and toe ...... 46 24. Map showing finite-difference grid and boundaries used in model analysis...... 47 25. Schematic diagram showing physical and model conceptualizations of ground-water-flow system...... 48 26. Map showing average water levels for the Columbia aquifer...... 49 27-29. Maps showing hydraulic conductivity based on model calibration: 27. Upper Yorktown-Eastover aquifer ...... 51 28. Middle Yorktown-Eastover aquifer ...... 52 29. Lower Yorktown-Eastover aquifer ...... 53 30-32. Maps showing simulated prepumping water levels in: 30. Upper Yorktown-Eastover aquifer ...... 55 31. Middle Yorktown-Eastover aquifer ...... 56 32. Lower Yorktown-Eastover aquifer...... 57 33-35. Maps showing simulated prepumping position of the saltwater-freshwater interface for: 33. Upper Yorktown-Eastover aquifer ...... 58 34. Middle Yorktown-Eastover aquifer ...... 59 35. Lower Yorktown-Eastover aquifer...... 60 36. Graph showing estimated annual withdrawal and average withdrawal for simulated pumping periods...... 62
IV Contents 37-39. Graphs showing simulated and measured water levels at selected observation wells in: 37. Upper Yorktown-Eastover aquifer ...... 64 38. Middle Yorktown-Eastover aquifer ...... 65 39. Lower Yorktown-Eastover aquifer...... 66 40-42. Maps showing simulated and measured water levels for 1988 in: 40. Upper Yorktown-Eastover aquifer ...... 67 41. Middle Yorktown-Eastover aquifer ...... 68 42. Lower Yorktown-Eastover aquifer...... 69 43-45. Maps showing simulated position of the saltwater-freshwater interface toe for a 1,000-year transient run using 1988 withdrawals in: 43. Upper Yorktown-Eastover aquifer ...... 72 44. Middle Yorktown-Eastover aquifer ...... 73 45. Lower Yorktown-Eastover aquifer...... 74 46. Map showing location of hypothetical withdrawals for the southern Northampton County scenario, simulation 1...... 75 47. Map showing water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 1 ...... 78 48,49. Maps showing water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe for the southern Northampton County scenario, simulation 1, in: 48. Middle Yorktown-Eastover aquifer ...... 79 49. Lower Yorktown-Eastover aquifer...... 80 50-52. Maps showing water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe for the southern Northampton County scenario, simulation 2, in: 50. Upper Yorktown-Eastover aquifer ...... 82 51. Middle Yorktown-Eastover aquifer ...... 83 52. Lower Yorktown-Eastover aquifer ...... 84 53. Map showing location of hypothetical withdrawals in the northeastern Accomack County scenarios ...... 86 54. Map showing water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 1...... 88 55,56. Maps showing water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe for the northeastern Accomack County scenario, simulation 1, in: 55. Middle Yorktown-Eastover aquifer ...... 89 56. Lower Yorktown-Eastover aquifer...... 90 57. Map showing water-level decline from simulated 1988 water levels, simulated position of the saltwater- freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 2...... 91 58,59. Maps showing water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe for the northeastern Accomack County scenario, simulation 2, in: 58. Middle Yorktown-Eastover aquifer ...... 92 59. Lower Yorktown-Eastover aquifer...... 93 60. Map showing location of permitted withdrawals ...... 96 61. Map showing water-level decline from simulated 1988 water levels, simulated position of the saltwater- freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, permitted-withdrawal scenario ...... 97 62,63. Maps showing water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe for the permitted-withdrawal scenario in: 62. Middle Yorktown-Eastover aquifer ...... 98 63. Lower Yorktown-Eastover aquifer...... 99 64-67. Maps showing the difference in simulated water levels for the southern Northampton County scenario, simulation 1, upper Yorktown-Eastover aquifer, resulting from: 64. A 50-percent increase in horizontal hydraulic conductivity ...... 101 65. A 50-percent decrease in horizontal hydraulic conductivity...... 102
Contents V 66. A 50-percent increase in confining unit leakance ...... 103 67. A 50-percent decrease in confining unit leakance...... 104
TABLES 1. Locations and depths of wells used to define the hydrogeologic framework and altitude of structural tops of hydrologic units for the Eastern Shore...... 8 2. Statistical summary of transmissivity and storage coefficients derived from aquifer-test results ...... 18 3. Statistical summary of well yield, specific capacity, transmissivity, and horizontal hydraulic conductivity derived from specific-capacity tests ...... 19 4. Vertical hydraulic conductivities derived from laboratory analyses of sediment cores from the Jenkins Bridge Research Station ...... 19 5. Well-construction data for wells completed in the Columbia aquifer in a transect A-A' near Townsend, Va. ... 23 6. Selected Virginia Water Control Board research-station well clusters on the Eastern Shore...... 28 7. Vertical distribution of chloride concentrations in ground water at Jenkins Bridge and Kiptopeke Research Station well clusters ...... 29 8. Chloride concentrations in the Columbia aquifer...... 38 9. Chloride concentrations in the upper Yorktown-Eastover aquifer...... 40 10. Chloride concentrations in the middle Yorktown-Eastover aquifer ...... 42 11. Chloride concentrations in the lower Yorktown-Eastover aquifer...... 44 12. Range of transmissivities estimated from specific-capacity data and from model calibration...... 54 13. Minimum and maximum values of model storage coefficient...... 54 14. Minimum and maximum values of model vertical leakance ...... 54 15. Withdrawals for each pumping period by aquifer...... 63 16. Simulated ground-water budgets for the confined freshwater-flow system ...... 70 17. Withdrawals for southern Northampton County scenario, simulation 1...... 76 18. Withdrawal by aquifer for model scenarios ...... 77 19. Maximum water-level decline from 1988 flow conditions for model scenarios ...... 77 20. Location of southern Northampton scenario withdrawals, simulation 2...... 81 21. Hypothetical withdrawals for the northeastern Accomack County scenario ...... 87 22. Permitted withdrawals as of January 1, 1990...... 94 23. Maximum water-level changes resulting from sensitivity runs ...... 105
VI Contents CONVERSION FACTORS AND VERTICAL DATUM
Multiply By To obtain Length inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square mile (mi2) 2.590 square kilometer (km2) Volume gallon (gal) 3.785 liter (L) Flow million gallons per day (Mgal/d) .04381 cubic meter per second (m3/s) Hydraulic conductivity foot per day (ft/d) .3048 meter per day (m/d) Transmissivity foot squared per day (ft2/d) .09290 meter squared per day (m2/d)
Water-quality units: Water-quality units are expressed in this report as milligrams per liter (mg/L). Hydraulic conductivity and transmissivity: In this report, hydraulic conductivity is reported in feet per day (ft/d), a mathematical reduction of the unit cubic foot per day per square foot [(ft3/d)/ft2]. Transmissivity is reported in feet squared per day (ft2/d), a mathematical reduction of the unit cubic feet per day per square foot times feet of aquifer thickness ([(ft3/d)/ft2]ft). Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
Contents VII
Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia
By Donna L. Richardson
Abstract estimated from specific-capacity data range This report presents the results of a study from 61 to 4,530 feet squared per day (ft2/d). of the hydrogeology and ground-water-flow sys Transmissivities generally are greater in the tem of the Eastern Shore in Virginia by the upper Yorktown-Eastover aquifer and decrease U.S. Geological Survey in cooperation with with depth in the middle and lower Yorktown- Accomack County, Northampton County, and Eastover aquifers. Annual ground-water with the Virginia Water Control Board. The Eastern drawals were compiled by aquifer for commer Shore of Virginia is a peninsula that includes cial, industrial, and municipal uses. Major Accomack and Northampton Counties and is the pumping centers are located near the towns of easternmost part of Virginia's Coastal Plain Accomac, Cape Charles, Cheriton, Chinco- physiographic province. Ground water provides teague, Exmore, Hallwood, and Oyster, Va. the sole freshwater supply to the Eastern Shore. Total ground-water use was estimated to be Water demands from increased industrial, com about 5 million gallons per day in 1988. The upper, middle, and lower Yorktown-Eastover mercial, municipal, and agricultural growth have caused water-level declines and concern aquifers supplied 36, 42, and 22 percent of the about the future of the ground-water resource. total withdrawal in 1988, respectively. Data on Detailed hydrogeologic information was chloride concentrations were compiled by aqui collected and incorporated into the ground- fer to provide information on the distribution of water-flow model. The data were used to chlorides in the study area. Chloride concentra develop an understanding of the way ground tions generally increase with depth; chloride water enters, moves through, and leaves the concentrations are greater in the lower multiaquifer system. A hydrogeologic frame Yorktown-Eastover aquifer than are found in the work of the aquifers and confining units con overlying middle and upper Yorktown-Eastover taining potable ground water was developed aquifers. from geophysical and lithologic information. A digital flow model was developed to The hydrogeologic framework consists of an aid in the analysis of the ground-water-flow unconfined aquifer (Columbia aquifer) and three system. The model incorporates the hydrogeo confined aquifers (upper, middle, and lower logic characteristics of the aquifers and confin Yorktown-Eastover aquifers) separated by inter ing units, simulates freshwater and saltwater vening confining units (upper, middle, and flow, and simulates the movement of the lower Yorktown-Eastover confining units). The saltwater-freshwater interface. The effects of ability of the aquifer and confining-unit sedi historical ground-water development were ments to transmit, store, and release water was examined by comparing simulations of pre- defined by estimating values for transmissivity, pumping with past pumping conditions. Model vertical leakance, and storage. Transmissivities results indicate that most of the ground water
Abstract withdrawn from the system comes from an fers consisting of sand, gravel, and shell material increase in the amount of water recharging the separated by confining units of silt and clay. The confined-aquifer system from the unconfined fresh ground water is limited to approximately the aquifer and a decrease in the amount of dis first 300 ft below land surface; the water at depths charge from the confined-aquifer system to the greater than 300 ft is salty (greater than 250 milli unconfined aquifer. The simulation of prepump- grams per liter (mg/L) chloride concentration). Beginning about 1965, increases in withdraw ing conditions indicates that about 11 million als for agricultural, commercial, and industrial uses gallons per day enter and exit the confined- have caused water-level declines and created cone- aquifer system. Given 1988 withdrawal condi like depressions in the water-level surface around tions, simulated flow into the confined-aquifer major pumping centers. In November 1976 the East system is increased to about 13 million gallons ern Shore was declared a Ground-Water Manage per day, and simulated flow out of the ment Area by the Virginia Water Control Board1 confined-aquifer system is reduced to 8.64 mil (VWCB). Under the management-area designation, lion gallons per day. The position of the simu a permit is required for ground-water users that lated saltwater-freshwater interface does not withdraw more than 300,000 gallons per month change in response to historical pumpage. (gal/month). Three model scenarios of hypothetical Increased water needs due to intensifying agri increases in withdrawals provide information on cultural, industrial, commercial, and urban develop the regional response of the ground-water sys ment could adversely affect the supply of fresh ground-water on the Eastern Shore. Potential prob tem to additional pumping. Results indicate that lems are (1) declining water levels, (2) decreased (1) water levels continue to decline as with freshwater discharge to nearshore estuaries, drawals increase and could result in well inter (3) intrusion of salty water into freshwater parts of ference among major ground-water users, aquifers, and (4) contamination of potable water by (2) increases in withdrawals result in a decrease the migration of pesticides and nitrates. A thorough in the amount of offshore freshwater discharge, knowledge of the ground-water-flow system is (3) water-level declines associated with needed to enable planners to minimize the detrimen increased withdrawals cause slight movement of tal effects that would result from increased use of the saltwater-freshwater interface over a 50-year the resource. In 1986 the U.S. Geological Survey simulation period, (4) increased withdrawals (USGS), in cooperation with the VWCB and the near the shoreline cause offshore water-level counties of Accomack and Northampton, began a declines and a reversal in the direction of comprehensive study of the ground-water resources ground-water flow that could induce vertical of the Eastern Shore of Virginia. leakage of saltwater into the freshwater parts of Purpose and Scope the uppermost confined aquifer, and (5) with drawals near the center of the peninsula cause The purpose of this report is to describe the less landward movement of the saltwater- hydrogeology and ground-water-flow system of the Eastern Shore of Virginia. The report includes dis freshwater interface than withdrawals near the cussions of (1) the hydrogeologic framework of shoreline. aquifers and confining units, (2) the flow of water through the multiaquifer system, (3) the hydraulic INTRODUCTION characteristics of aquifers and confining units, The Eastern Shore of Virginia includes Acco- (4) the distribution of chloride concentrations in the mack and Northampton Counties and is the eastern aquifers, (5) the digital model used to simulate most part of Virginia's Coastal Plain physiographic ground-water flow, and (6) the simulated effects of province. The Eastern Shore is a peninsula sur increased ground-water withdrawals. rounded on three sides by salty water and has no major fresh surface-water sources; therefore, ground water provides the sole freshwater supply. Fresh 1Predecessor of the Virginia Department of Environmental ground water is present in a layered system of aqui Quality Water Division.
2 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 84° 82° 80 78°
0 50 100 150 KILOMETERS
WEST VIRGINIA
37°-
Figure 1. Location of study and model area.
This study is primarily an evaluation of the The model area extends into Maryland and includes fresh ground-water-flow system of the Eastern offshore areas in the Atlantic Ocean and Chesapeake Shore; therefore, the hydrogeologic data compiled Bay, so that the effects of offshore saltwater flow for the study focus on the uppermost 300 feet (ft) could be incorporated into the model of the ground- of the system. Hydrogeologic data for aquifers and water-flow system. confining units of the Eastern Shore were collected, compiled, and analyzed. Hydraulic characteristics of Previous Studies the aquifers and confining units were estimated from hydrologic data. Water samples were collected and Previous studies provide information about the analyzed to determine the distribution of chloride ground-water resources of the Eastern Shore of Vir concentrations in each aquifer. These data were used ginia. Sanford (1913) was the first to document the to develop a digital model of three-dimensional flow geology and ground water throughout the Virginia that simulates ground-water movement and tracks Coastal Plain. Sinnot and Tibbitts (1954, 1957, the lateral movement of the saltwater-freshwater 1968) describe the ground-water resources of interface. Northampton and Accomack Counties. Gushing and others (1973) provide a comprehensive study of the ground water of the Delmarva Peninsula. Siudyla Location of Study and Model Area (1975) and Siudyla and others (1977, 1981) present The study area includes Accomack and ground-water information for the Eastern Shore from Northampton Counties in the easternmost part of a planner's perspective. Fennema and Newton Virginia's Coastal Plain physiographic province (1982) present a summary of ground-water informa (fig. 1). The two counties are collectively referred tion for the Eastern Shore, and Bal (1977) devel to as the Eastern Shore of Virginia. The Eastern oped the first digital ground-water-flow model for Shore is a peninsula that is about 70 mi long and the area. Mixon (1985) describes the stratigraphy covers approximately 695 square miles (mi2) of land and geomorphic framework of the uppermost Ceno- area. It is bounded on the east by the Atlantic zoic deposits in the southern Delmarva Peninsula. Ocean, on the west and south by the Chesapeake Knobel (1985) provides ground-water-quality data Bay, and on the north by the State of Maryland. for the northern Atlantic Coastal Plain including the
Introduction Eastern Shore. Harsh and Laczniak (1986) and rately reported; therefore, withdrawals for irrigation Meng and Harsh (1988) contribute to the under are not included in the pumpage estimates. standing of the ground-water resource by describing Data on chloride concentrations and distribu the hydrogeologic framework and conceptualization tions throughout the study area were compiled from of ground-water flow for the Virginia Coastal Plain. previous investigations. Additional water samples Kull and Laczniak (1987) compiled ground-water- were collected and analyzed for chlorides during this withdrawal data for the Virginia Coastal Plain. study. Several reports examine the distribution of SHARP, a quasi-three-dimensional, digital, saltwater in areas that include the Eastern Shore of ground-water-flow model, was used to simulate past Virginia. Cederstrom (1945) and Larson (1981) and present ground-water-flow conditions. The describe the distribution of chloride concentrations SHARP model simulates freshwater and saltwater in the ground water of the Virginia Coastal Plain. flow and tracks the lateral movement of the Back (1966) describes the patterns of ground-water saltwater-freshwater interface (Essaid, 1990a). Sim flow and the interface between freshwater and salt ulations of hypothetical withdrawal scenarios were water in the northern Atlantic Coastal Plain. Meisler used to assess potential changes in water levels, and others (1985) document the distribution of salty ground-water flow, and saltwater-interface position. ground water beneath the Atlantic Ocean in the These scenarios are intended to identify the general northern Atlantic Coastal Plain aquifer system. nature of the response of the hydrologic system to various stresses. The scenarios are not intended to predict specific future problems. Methods of Investigation
The report by Meng and Harsh (1988) pro Acknowledgments vided data that were used to develop the hydrogeo logic framework described in this study. Additional The author would like to thank Keith Bull, hydrogeologic data were obtained from local well former Northampton County administrator, for his drillers and the VWCB to refine the framework for support of this study. Terry Wagner, Virginia New the fresh ground-water system of the Eastern Shore. ton, Scott Bruce, and Eugene Powell of the Virginia Two clusters of observation wells were drilled by Water Control Board provided data and support. the VWCB to provide additional hydrologic infor Special thanks also are extended to local drillers for mation and further define the ground-water-flow providing well-construction data and other pertinent system. hydrogeologic information. Water levels were measured to provide infor mation on ground-water flow through the multiaqui- HYDROGEOLOGY fer system. An established water-level network was expanded to a total of 58 wells, and water levels The Eastern Shore of Virginia is the eastern were measured every 6 weeks by the VWCB. His most part of Virginia's Coastal Plain physiographic toric water-level data were compiled for use in province. The Coastal Plain consists of layered, model development. A transect of wells in the unconsolidated, sedimentary deposits that thicken unconfined aquifer was constructed across the penin and slope seaward. These deposits consist of inter- sula in southern Northampton County to improve the bedded clay, silt, sand, and gravel and variable understanding of ground-water flow in the uncon amounts of shell material that form a system of lay fined aquifer. Aquifer-test and specific-capacity data ered aquifers and confining units. were reviewed to define the hydraulic characteristics of the aquifers. General Geology Data obtained from the USGS water-use data base and the VWCB were reviewed for errors and The sedimentary deposits composing the East compiled by aquifer through 1988. Water-use data ern Shore generally thicken and dip northeastward for the Eastern Shore consist of pumpage for major and range in thickness from about 3,000 ft west of industrial, municipal, commercial, and public-supply the peninsula to about 7,500 ft east of the peninsula systems. Pumpage for agricultural use is not accu (Meng and Harsh, 1988). These Coastal Plain
Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia deposits overlie a hard-rock surface, commonly able from well 66M1, data are also provided by the referred to as "basement," that also dips northeast VWCB research stations at Jenkins Bridge (well ward. The geologic age of these unconsolidated sed 66M23, fig. 2), Accomack County, Va. These Late iments ranges from Early Cretaceous to Holocene. Cretaceous deposits vary in composition from The sediments have a varied depositional history. clayey, shelly, glauconitic sands to chalky marl and The lower 70 percent of the sediments are of Early range in thickness from 50 to 60 ft in the northeast to Late Cretaceous age and were deposited in fluvial ern part of Accomack County. environments (Robbins and others, 1975). The remaining 30 percent of the sediments are mostly of Tertiary Sediments Tertiary age and were deposited in marine environ The Late Cretaceous sediments are overlain by ments (Gushing and others, 1973). The Tertiary sed a sequence of marine sediments of Tertiary age. The iments are overlain by a thin veneer of sediments of Tertiary sediments underlying the Eastern Shore are Quaternary age that were deposited in various envi divided into a series of formations by depositional ronments (Mixon, 1985). Figure 2 shows the loca environment, texture, grain size, and lithology. As tion of control wells used in the development of the is true for the underlying Cretaceous sediments, hydrogeologic framework of aquifers and confining information is limited concerning the composition, units for the Eastern Shore. lithology, and nature of most Tertiary deposits beneath the Eastern Shore. If the Tertiary sediments Cretaceous Sediments are similar to those beneath the Virginia mainland, they are really extensive and homogeneous in char Most of the Cretaceous sediment underlying acter, forming layered sequences of clay, silt, and the Eastern Shore is commonly referred to as the sand and varying amounts of shell material. The Potomac Formation (Meng and Harsh, 1988) or the probable Tertiary formations, from oldest to young Potomac Group (Robbins and others, 1975). Infor est, are the Brightseat, Aquia, Nanjemoy, Piney mation is limited concerning the composition and Point, Chickahominy, Old Church, Calvert, Chop- lithology of these Cretaceous sediments beneath the tank, St. Marys, Eastover, and Yorktown Forma Eastern Shore. The most complete source of geo tions. Geologic data for these Tertiary units on the logic data available is a deep oil-test hole in Tem Eastern Shore are from the deep wells 66M1 and perance ville, Va. (66M1, fig. 2). The Potomac For 66M23. An additional source of information for the mation beneath Virginia's Eastern Shore is probably deep Tertiary sediments is a stratigraphic core hole similar in composition and lithology to that of sur rounding areas (Meng and Harsh, 1988; Glaser, (well 64J14, fig. 2) that was drilled by the USGS at the Virginia Truck Experimental Station north of 1969; Hansen, 1969; Robbins and others, 1975). Exmore, Va. (R.B. Mixon, U.S. Geological Sur These deposits in the Virginia Coastal Plain range in age from Early to early Late Cretaceous (Robbins vey, oral commun., 1986). Preliminary analyses of these cores indicate an extremely thick Eocene sec and others, 1975) and are characteristically hetero tion, overlain by a sequence of Oligocene, Miocene, geneous in composition, consisting of interlayered and intermixed clay, silt, sand, and gravel deposits and Quaternary deposits. In the Miocene sediments, that mainly are a result of fluvial deposition. Cur the Calvert Formation contains a sand facies over lain by a clay-silt facies. The thickness of the Terti rent interpretations suggest that the sediments in the ary sediments ranges from 1,000 to 1,500 ft. eastern part of the Virginia Coastal Plain (including the Eastern Shore) probably were deposited in a marginal-marine environment. The thickness of the Quaternary Sediments Cretaceous sediments beneath the Eastern Shore As sea levels fluctuated with the advance and ranges from about 2,000 to 5,600 ft. retreat of continental ice sheets during the Pleisto The Early and early Late Cretaceous sedi cene Epoch, the drainage patterns of the major river ments are overlain by late Late Cretaceous sedi systems in the Chesapeake Bay area were altered, ments deposited in marginal-marine to marine envi eroding channels into previously deposited sedi ronments. Information is limited concerning the ments. As sea levels declined with the advance of composition and lithology of these uppermost Creta the glaciers, streams flowed eastward across the ceous deposits; however, in addition to data avail Eastern Shore, deeply dissecting (more than 200 ft
Hydrogeology 38° -
45' -
CONTROL WELL AND IDENTIFIER
37° 15" -
Figure 2. Location of control wells used in hydrogeologic framework analysis.
6 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia below present sea level) or removing the Yorktown part of the ground-water-flow system are shown in Formation. As sea levels rose with the retreat of the figures 3-9. glaciers, the incised stream channels were infilled Aquifers beneath the Eastern Shore consist of with estuarine and marginal-marine deposits gener an unconfined aquifer underlain by a series of con ally of a composition different from the eroded sedi fined aquifers and intervening confining units ments. Mixon (1985) and Colman and Mixon (1988) (fig. 10). The Columbia aquifer is the uppermost describe such paleochannels that cut eastward across aquifer and is unconfined. The confined aquifers the peninsula at Cape Charles, Eastville, and shallower than approximately 300 ft contain fresh Exmore, Va. water and are named the upper Yorktown-Eastover, The remaining Quaternary sediments were middle Yorktown-Eastover, and lower Yorktown- deposited in marginal-marine and estuarine environ Eastover aquifers. These freshwater aquifers are the ments. The central uplands of the Eastern Shore are focus of this report. The previously defined flanked by broad, flat terraces and bordered by lin Yorktown-Eastover aquifer (Meng and Harsh, 1988) ear scarps. Mixon (1985) provides the stratigraphic has been refined for this report and divided into the nomenclature and describes the depositional history upper, middle, and lower Yorktown-Eastover aqui of Quaternary sediments on the Eastern Shore. Since fers. The Yorktown-Eastover aquifers are underlain the Pleistocene Epoch, sea levels have continued to rise along the margins of the Eastern Shore, and by aquifers and confining units that contain salty Holocene-age deposits make up the salt-marsh, water (water with chloride concentrations greater back-bay, and barrier-island sediments around the than 250 mg/L). peninsula. The thickness of the Quaternary sedi ments ranges from 40 to 150 ft. Columbia Aquifer The Columbia aquifer is unconfined through Aquifers and Confining Units out the Eastern Shore. It is defined as the saturated, chiefly sandy, surficial sediments that overlie the Sediments beneath the Eastern Shore have uppermost continuous clay-silt unit (Meng and been divided on the basis of hydrologic properties Harsh, 1988). The Columbia aquifer primarily con into a layered sequence of aquifers and intervening sists of Pleistocene sediments of the Columbia confining units. Aquifers consist of sand, gravel, Group. Holocene sediments, which overlie the Pleis and shell material of sufficient saturated thickness to tocene deposits around the margin of the Eastern yield significant quantities of water. Confining units Shore, are not used as a ground-water source and, consist of clay and silt that are continuous and of therefore, are not discussed further in this report. low permeability; confining units yield little water Lithologically, the Columbia aquifer has a large and retard the movement of water. Aquifers com range in composition, depending on the depositional monly contain interbedded clay and silt, whereas environment of its lithic units. The composition of confining units commonly contain interbedded sand, the Columbia aquifer ranges from very fine silty gravel, and shell material. An aquifer or confining sands to very coarse and gravelly clean sands, com unit can comprise part of a geologic formation, all monly consisting of thin, discontinuous, interbedded of a formation, or a combination of all or parts of clay and silt. Sinnott and Tibbitts (1968) character adjacent formations. ize the deposits that compose the Columbia aquifer The hydrogeologic framework of aquifers and as chiefly yellow sand and sandy clay, with minor confining units on the Eastern Shore has been delin lenses and beds of gravel. The thickness of the eated by correlating lithologic and geophysical logs Columbia aquifer and the depth to the water table and by analyzing water-quality and water-level data. generally vary with topography. Usually, land- The locations and depths of the wells used in this surface elevation is proportional to the thickness of analysis and the altitudes of the tops of aquifers and the Columbia aquifer and the depth to the water confining units are given in table 1. The relative table. Surface expressions of the water table in this positions of the hydrogeologic units throughout the aquifer are the ponds and streams throughout the peninsula are illustrated in the hydrogeologic sec Eastern Shore. tions shown in plate 1. The altitudes of the tops of The Columbia aquifer generally supplies suffi the aquifers and confining units in the freshwater cient quantities of ground water for domestic
Hydrogeology > 2 S W c CO LU in oo ^H «-i | y u ha I*! 1 S _VO - § JO C"^ -* O\ O\ ^« fNen^1 vooooo^ O< o\ooo\ csenoo^cn =2g §u «o CN en in o S cswi 1 O^enOO enowiOV) enoocSfNO 1 i i-^ ^-4 *-« I-N ol "-^ CN -* cs CN ^ CN en en en en en en en en en en TT ^ B en en en en en en en en en en en en en en en en en en en en en en en en en CO .3I 2* o* Q. >en> O osp~«oooenO O O O O 5 111 .2 60 60 0) CD a-s-l * -H en ^ 2 o O LL.M? ^7LL. >gC5 EC J EC H-, CO -C _ , _, cs_, v*j en en I-W a ii VO vo vo vo vo vo vS vS sS vS nb nb nb nb nb ^b ^b 8 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 1. Locations and depths of wells used to define the hydrogeologic framework and altitude of structural tops of hydrologic units for the Eastern Shore Continued Land Altitude of structural top of hydrogeologic unit Well Station surface Well (feet) number number Latitude Longitude altitude depth (feet) (feet) UYCU UYAQ MYCU MYAQ LYCU LYAQ STCU STAQ 65K10 374309075385801 374309 75 38 58 37 -293 -23 -81 -143 -165 -195 -218 -279 65K17 373735075400001 37 42 33 754429 17 -263 -13 -85 -135 -145 -191 -225 - 65K23 374442075432501 374428 75 43 28 13 -277 -33 -65 -115 -125 -163 -199 -275 - 65K29 374425075400003 374427 754000 35 -355 -7 -75 -145 -170 -195 -215 -295 ~ 65L6 374530075401001 37 45 30 754010 35 -250 -31 -105 -120 -135 -180 -202 - ~ 66K2 374320075380501 37 43 19 753654 10 -378 -24 -94 -152 -168 -210 -234 -318 66M1 375303075310101 375303 75 31 01 42 -6,220 -36 -106 -172 -210 -248 -286 -366 -588 66M7 375538075330201 37 55 38 753302 27 -424 -37 -91 -141 -171 -198 -227 -295 - 66M9 375256075332301 375256 753323 44 -251 -36 -90 -130 -162 -204 -230 - 66M12 375321075334401 37 53 21 753344 42 -278 -38 -88 -128 -162 -193 -230 - ~ 66M18 375723075344403 375723 753445 11 -338 -25 -99 -127 -141 -165 -209 -259 66M23 375610075361801 375610 753618 6 -1,202 -42 -72 -106 -130 -174 -202 -342 -494 67L2 375220075265401 37 52 20 752654 10 -172 -42 -120 ~ - « ~ - ~ 67M12 375635075271503 375635 752715 13 -267 -21 -95 -161 -189 -- - - - 67N1 380010075253401 380010 752534 25 -387 -47 -109 -195 -230 -303 -347 -382 ~ 68M2 375324075202501 37 53 24 752025 10 -790 -35 -144 -226 -258 -304 -378 -430 -748 68M4 375153075221001 37 51 53 752210 5 -284 -39 -135 -245 -277 ~ - ~ - MDCE42 380930075415601 380930 75 41 56 105 -206 -10 - ~ -48 -60 -116 -166 -218 MDDE28 380209075401801 380209 754018 5 -1,045 -35 -57 -73 -145 -185 -265 -395 MDFC46 380359075251501 380359 752515 39 -521 -54 -86 -152 -228 -280 -312 -370 ~ MDDE27 380455075433201 380455 75 43 32 5 -1,008 .. .. -171 -345 MDDG20 381427075081101 381427 750811 6 -593 -10 -54 -156 -266 -390 -414 -524 ~ I 76 C 45' 30' 75" 15' 38° 45' 30' EXPLANATION AREA WHERE UPPER YORKTOWN-EASTOVER UNIT IS MJSSING-Discussed in the section "upper Yorktown-Eastover aquifer and confining unit" STRUCTURE CONTOUR-Shows altitude of top of upper Yorktown-Eastover confining unit. Dashed where approximately 37" 15' located. Interval 20 feet. Datum is sea level CONTROL WELL 10 20 MILES I I 20 KILOMETERS Figure 3. Altitude of top of upper Yorktown-Eastover confining unit. 10 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 45' 30' 75° 15' 38" 45' 30' EXPLANATION STRUCTURE CONTOUR-Shows altitude of top of upper Yorktown-Eastover aquifer. Dashed where approximately 37° 15' located. Interval 20 feet. Datum is sea level CONTROL WELL 10 20 MILES j_I 20 KILOMETERS Figure 4. Altitude of top of upper Yorktown-Eastover aquifer. Hydrogeology 11 76 C 45' 30' 75° 15' 38 C 45' 30' EXPLANATION STRUCTURE CONTOUR--Shows altitude of top of middle Yorktown-Eastover confining unit. Dashed where approximately located. Interval 25 feet Datum is 37° 15' sea level CONTROL WELL 10 20 MILES 20 KILOMETERS 1 Figure 5. Altitude of top of middle Yorktown-Eastover confining unit. 12 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 38° - 45' - 30' EXPLANATION .75. STRUCTURE CONTOUR-Shows altitude of top of middle Yorktown-Eastover aquifer. Dashed where approximately located. Interval 25 feet Datum is sea level 37° 15' - CONTROL WELL 0 10 20 MILES .W.M.M- ' ' ' 20 KILOMETERS Figure 6. Altitude of top of middle Yorktown-Eastover aquifer. Hydrogeology 13 76° 45 45 EXPLANATION STRUCTURE CONTOUR--Shows altitude of top of lower Yorktown-Eastover confining unit. Dashed where approximately located. Interval 25 feet. Datum 37° 15 is sea level 20 KILOMETERS I I Figure 7. Altitude of top of lower Yorktown-Eastover confining unit. 14 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 38° - 45 1 - -75 STRUCTURE CONTOUR-Shows altitude of top of lower Yorktown-Eastover aquifer. Dashed where approximately located. Interval 25 feet. Datum is sea level 37° 15' - Figure 8. Altitude of top of lower Yorktown-Eastover aquifer. Hydrogeology 15 76° 45 30' - EXPLANATION STRUCTURE CONTOUR-Shows altitude of top of St. Mary confining unit. Dashed where approximately located. Interval 50 feet. Datum is sea level 37" 15' - APPROXIMATE LIMIT ST. MARYS CONFINING UNIT / Figure 9. Altitude of top of St. Marys confining unit. 16 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia WEST EAST EASTERN SHORE VIRGINIA MAINLAND ATLANTIC OCEAN Y \Upper mktown-Eastover aquifer e Yorktown-Eastover amufe Lower Yorktown-Eastover aquife $t. Marys-Choptank aquifer Brightseat aquifer Middle Pptorhac aquifer Lower Potbmac aquifer, ^^^ NOT TO SCALE EXPLANATION GENERALIZED FLOW LINE FRESHWATER SALTWATER CONFINING UNIT >] BASEMENT ROCKS Figure 10. Schematic diagram of aquifers and confining units and generalized flow lines. Hydrogeology 17 Table 2. Statistical summary of transmissivity and storage coefficients derived from aquifer-test results [ft2/d, foot squared per day; , no values reported] Analytical method Nonleaky analysis Nonleaky analysis of ofTheis(1935) Cooper and Jacob (1946) Yorktown- Eastover aquifer Transmissivity Storage Transmissivity Storage (ft 2/d) coefficient (ft 2/d) coefficient (dimensionless) (dimensionless) 13X10 3 93X10" Upper Maximum 3,960 670 2.0X10" 4 470 620 4.6X10" 9.7X10" 4 Median 1,670 Mean 1,940 8.6X10" 4 Number of tests 85X10" 4 Middle Maximum 2,650 Minimum 230 95X10" 5 Median 1,130 5.2X10" 4 Mean 1,290 4.9X10" 4 350 3.8X10" 4 Number of tests 4 4 1 1 Lower Maximum 1,360 9.4X10" 4 Minimum 120 2.6X10" * Median _ _ _ _ Mean _ _ _ _ Number of tests 2 2 purposes. Irrigation ponds in the Columbia aquifer southern Northampton County. The channel is cov provide much of the water needed for agricultural ered with a basal-gravelly sand unit that contains purposes. In upland areas, the quality of water in pebbles and small cobbles overlain by muddy sand this aquifer is generally within drinking-water stand and clay-silt, marginal-marine deposits. The sands ards if wells are not located downgradient of poten and gravels of the channel deposits are extremely tial sources of contamination. In low-lying and transmissive; however, their extent has not yet been poorly drained areas, the water quality is worse than defined, and the gravelly sands are overlain by a in upland areas, reflecting the nearness of saltwater poorly sorted mixture of mud, silt, and clay of vary bodies and contamination from land uses. ing thicknesses. Detailed study of the paleochannels is necessary to define the extents of the different Pleistocene Paleochannel Aquifers types of sediments and determine the hydraulic Evidence indicates the presence of subsurface properties associated with those sediments. For the erosional channels where all or part of the Yorktown purposes of this report, the channel sediments are Formation sediments have been removed and hydraulically connected to the surrounding York- replaced by marginal-marine deposits of Pleistocene town sediments and have been included as part of age. The sediments in these paleochannel areas are, the Yorktown-Eastover aquifer system. therefore, quite different from the Yorktown sedi ments that are typical of the rest of the Eastern Yorktown-Eastover Aquifer System Shore. The two major paleochannels that have been The Yorktown-Eastover aquifer system is a identified in the study area cut eastward across the multiaquifer unit consisting of late Miocene and Pli peninsula near Exmore and Eastville, Va. Mixon ocene deposits and is composed of the sandy facies (1985) describes the lithology of a type cross section of the Yorktown and Eastover Formations (Meng in the vicinity of the Eastville paleochannel in and Harsh, 1988). The Yorktown-Eastover aquifer 18 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 3. Statistical summary of well yield, specific capacity, transmissivity, and horizontal hydraulic con ductivity derived from specific-capacity tests [gal/min, gallon per minute; (gal/min)/ft, gallon per minute per foot; ft2/d, foot squared per day; ft/d, foot per day] Transmissiviry Horizontal hydraulic conductivity Yorktown- Statistic WeU Specific (ft'/d) (ft/d) Eastover yield capacity aquifer (gal/rain) [(gal/min)/ft] Unadjusted Adjusted 1 Unadjusted Adjusted1 Upper Maximum 315 17.5 1.000 4^30 17.2 60.4 nAnntnntn 5 2. 49 61 .9 33 Median 120 1.7 361 739 103 10.6 Mean 125 2.8 446 1,259 8.9 213 Number of tests 14 14 10 10 10 10 Middle Maximum 645 9.9 912 3,240 15.6 443 Rfltn iniiini 20 .7 186 206 3.8 42 Median 95 1.5 427 834 62 17.2 Mean 136 23 487 1,375 83 22.7 Number of tests 12 12 7 7 7 7 Lower Maximum 201 5.7 1,697 2,094 19.6 24.2 Ininimum 1 .1 24 95 .4 1.6 Median 34 1.0 209 353 53 8.8 Mean 53 1.8 35 724 7.6 10.9 Number of tests 10 10 4 4 4 4 Adjusted for effects of partial penetration. Table 4. Vertical hydraulic conductivities derived from coefficient, and vertical hydraulic conductivity are laboratory analyses of sediment cores from the Jenkins the principal hydraulic characteristics necessary for Bridge Research Station an analysis of ground-water flow. Transmissivities [ft/d, foot per day] and storage coefficients derived from aquifer-test Depth of sample Vertical hydraulic data for the freshwater-confined aquifers are summa below land surface Confining unit conductivity rized in table 2. Few aquifer tests are available that (feet) (ft/d) reflect the characteristics of an individual aquifer 1.39x10" * because most of the wells used for aquifer tests have 63.7- 64.7 Upper Yorktown-Eastover 348.7-349.7 St Marys 1.63X10" 3 screens that are open to more than one aquifer. The 368.4-369.4 St Marys 1.27X10" s aquifer-test data are supplemented by transmissivi- ties estimated from specific-capacity data (table 3). Table 3 provides a statistical summary of well yield, specific capacity, transmissivity, and horizontal system consists of a series of alternating sand and hydraulic conductivity estimated from specific- clay-silt units that form three distinct aquifers that capacity tests. A detailed description of the method generally are present throughout the Eastern Shore. and equations used to estimate transmissivities from These aquifers are identified as the upper, middle, specific-capacity data is presented by Laczniak and and lower Yorktown-Eastover aquifers. Correspond Meng (1988). A few point estimates for vertical ingly, each aquifer is overlain by the upper, middle, hydraulic conductivities are available from labora and lower Yorktown-Eastover confining units. The tory analysis of sediment cores from the Jenkins entire aquifer system is wedge shaped and thickens Bridge Research Station (well 66M23) (table 4). and dips eastward. The units extend eastward These data need to be interpreted and used with cau beneath the Atlantic Ocean to the continental shelf tion because (1) the core samples could be dis and westward underneath the Chesapeake Bay. turbed, (2) the core samples represent 1-ft intervals The hydraulic characteristics of the aquifers of thicker confining units, and (3) the values are and confining units determine their ability to store, local point values and cannot be interpreted as transmit, and release water. Transmissivity, storage regional estimates. Hydrogeology 19 Upper Yorktown-Eastover Aquifer and Confining Unit peake Bay (Hack, 1957; Colman and others, 1990). The Columbia aquifer is underlain by the In the model area where control wells exist, the upper Yorktown-Eastover confining unit. The con upper Yorktown-Eastover aquifer ranges in thickness from 15 ft at well 65L6 in central Accomack fining unit consists of gray, greenish-gray, or County to 110 ft at well 68M4 on Chincoteague brownish-gray clayey silt or silty clay. The confin Island. ing unit is continuous underneath the peninsula; however, incisement by present-day channels in the Middle Yorktown-Eastover Aquifer and Confining Unit Chesapeake Bay has likely removed part or all of the upper Yorktown-Eastover confming-unit sedi The upper Yorktown-Eastover aquifer is ments (figs. 3 and 11) west of the peninsula. In the underlain by the middle Yorktown-Eastover confin model area where control wells exist, the confining ing unit. The confining unit consists of gray, unit ranges in thickness from 26 ft at well 63F16 in greenish-gray, or brownish-gray clayey silt or silty southern Northampton County to 109 ft at well clay and ranges in thickness from 8 ft at well 63G24 68M2 on Chincoteague Island. A laboratory analysis in southern Northampton County to 76 ft at well of a sediment core from well 66M23 indicates a ver MDFC46 in Worcester County, Md. The confining tical hydraulic conductivity of 1.39 x 10~5 ft/d for unit is present throughout the study area. the upper Yorktown-Eastover confining unit. Analy The middle Yorktown-Eastover confining unit is underlain by the middle Yorktown-Eastover aqui ses of cores from the St. Marys confining unit, at fer. Estimated hydraulic properties are summarized the same site, indicated similar values (table 4). in tables 2 and 3. The middle Yorktown-Eastover Elsewhere on the mainland part of the Virginia aquifer consists of sediments from the Yorktown Coastal Plain, laboratory analyses of confining-unit Formation; therefore, the hydraulic properties of the sediments have ranged from 3.93 x 10~3 to 9.2 x middle Yorktown-Eastover aquifer are similar to 10" 1 ft/d (Harsh and Laczniak, 1986). those of the upper Yorktown-Eastover aquifer. The The upper Yorktown-Eastover confining unit middle Yorktown-Eastover aquifer is present is underlain by the upper Yorktown-Eastover aquifer throughout the study area. The characteristics and (figs. 3,4). Geologic data from the Exmore core extents of these units in offshore areas are unknown. (well 64J14) and the VWCB Jenkins Bridge It is likely that the western limit of the middle Research Station (well 66M23) indicate that the Yorktown-Eastover aquifer (fig. 6) extends beyond upper Yorktown-Eastover aquifer predominantly the western limit of the upper Yorktown-Eastover consists of Yorktown Formation (Pliocene) sedi aquifer (fig. 4) as a result of erosion by the ancient ments. Lithologically, the sediments of the York- Susquehanna River channel. In the model area, town are diverse, consisting of varying mixtures of where control wells exist, the middle Yorktown- fine-grained to very coarse-grained, white to Eastover aquifer ranges in thickness from 12 ft at greenish-gray, shelly, glauconitic, and pebbly quartz well MDCE42 in Somerset County, Md., to 124 ft sands (Meng and Harsh, 1988). Hydraulic properties at well 67N1 in northeastern Accomack County. of the upper Yorktown-Eastover aquifer are summa rized in tables 2 and 3. The range of fine-grained to Lower Yorktown-Eastover Aquifer and Confining Unit very coarse-grained sediments in the Yorktown For mation and the variable aquifer thickness result in The middle Yorktown-Eastover aquifer is an order of magnitude range in transmissivity val underlain by the lower Yorktown-Eastover confining ues. The upper Yorktown-Eastover aquifer extends unit. The lithology of the confining unit is similar to eastward to the continental shelf and westward that of the middle and upper confining units and underneath the Chesapeake Bay. The characteristics consists of gray, greenish-gray, or brownish-gray and extent of the upper Yorktown-Eastover aquifer clayey silt or silty clay. The lower Yorktown- are not known in offshore areas beneath the Atlantic Eastover confining unit ranges in thickness from 10 Ocean and the Chesapeake Bay. The upper ft at well 62F1 in southern Northampton County to Yorktown-Eastover aquifer is most likely truncated 74 ft at well 68M2 on Chincoteague Island (fig. 2). beneath the Chesapeake Bay by erosion from the The lower Yorktown-Eastover aquifer under ancient Susquehanna River channel and incised by lies the lower Yorktown-Eastover confining unit and the nearshore channels of the present-day Chesa primarily consists of sediments from the Miocene 20 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 37° 15' - 60 BATHYMETRIC CONTOUR-Interval, in feet, is variable Datum is sea level Figure 11. Bathymetry in the vicinity of the Eastern Shore. Hydrogeology 21 Eastover Formation. Mixon (1985) describes the and the Atlantic Ocean. The regional system of the Eastover sediments as chiefly fine-grained to very Eastern Shore consists of the confined aquifers fine-grained, greenish-gray, clayey, silty, and shelly beneath the lower Yorktown-Eastover aquifer. Infor quartz sands. Estimated hydraulic properties of the mation for these deep confined aquifers beneath the lower Yorktown-Eastover aquifer are summarized in Eastern Shore is limited; however, it is likely that tables 2 and 3. The Eastover Formation typically the lower Yorktown-Eastover aquifer is underlain by contains finer-grained sediments than the Yorktown the St. Marys-Choptank, Brightseat, upper Poto- Formation; therefore, the lower Yorktown-Eastover mac, middle Potomac, and lower Potomac aquifers aquifer generally is less transmissive than the upper (Meng and Harsh, 1988). These aquifers are hydrau- and middle Yorktown-Eastover aquifers. The lower lically separated from the overlying freshwater aqui Yorktown-Eastover aquifer is present throughout the fers by the thick St. Marys confining unit. The study area. Because the lower Yorktown-Eastover regional aquifers are continuous underneath the aquifer is at a greater depth, its limit probably Chesapeake Bay, and deep ground-water-flow extends farther west underneath the Chesapeake Bay beneath the Eastern Shore is affected by the regional than the middle and upper Yorktown-Eastover aqui Coastal Plain ground-water-flow system. fers (figs. 4, 6, and 8). The lower Yorktown- Eastover aquifer ranges in thickness from 22 ft at Local Ground-Water-Flow System well 63L1 on Tangier Island (fig. 2) to 140 ft at A schematic of ground-water flow in the local well 66M23 in Accomack County. ground-water system is presented in figure 10. St. Marys Confining Unit Freshwater recharges the local ground-water system primarily through precipitation that falls on the pen The St. Marys confining unit consists of the insula and infiltrates into the sediments, because predominantly clayey facies of the St. Marys For there are no major surface-water bodies on the pen mation and the lower clayey facies of the Eastover insula. Gushing and others (1973) estimated that 8.5 Formation. These sediments are middle to late Mio to 15 in. of the 43 in. of annual precipitation cene in age. The St. Marys confining unit is con recharges the unconfined aquifer; the remainder is formably overlain throughout the study area by the either surface runoff or evaporation. Using an aver lower Yorktown-Eastover aquifer. The sediments age recharge of 12 inches per year (in/yr) over a consist of interbedded silty and sandy clay and vary 450 square mile (mi2) recharge area (total land area ing amounts of shells, typically bluish-gray to gray minus wetlands) for the Virginia part of the Eastern in color (Meng and Harsh, 1988). Laboratory analy Shore, the estimated natural recharge to the uncon ses of sediment cores from the St. Marys confining fined aquifer is 257 Mgal/d. Precipitation infiltrates unit at well 66M23 indicate vertical hydraulic con into the ground and percolates to the water table of ductivities of 1.63 x 10~5 and 1.27 x 10~5 ft/d the Columbia aquifer. Water in the unconfined aqui (table 4). The St. Marys confining unit ranges in fer flows vertically into the lower parts of the thickness from 150 to 350 ft. This massive clay unit unconfined aquifer and laterally through the uncon is effectively a lower boundary for the fresh ground- fined aquifer toward discharge sites such as springs, water-flow system on the Eastern Shore. streams, marshes, estuaries, the Chesapeake Bay, and the Atlantic Ocean. The lateral direction of Ground-Water Hydrology ground-water flow generally is from the ground- water divide at the center of the peninsula to the The ground-water-flow system can be divided Chesapeake Bay and Atlantic Ocean. Eventually, into a local and a regional ground-water-flow system water that is moving vertically encounters the upper (fig. 10). The local ground-water-flow system con Yorktown-Eastover confining unit, and much of the sists of the unconfined aquifer (Columbia) and the flow is forced to move laterally through the uncon confined-freshwater aquifers (upper, middle, and fined aquifer. Under natural (prepumping) condi lower Yorktown-Eastover). The aquifers in the local tions, a comparatively small amount of water is able system contain freshwater that is recharged locally to flow through the less permeable confining unit by rainfall on the Eastern Shore and discharges into the confined-aquifer system. The predominant locally to estuaries, marshes, the Chesapeake Bay, movement of ground water is in a lateral direction 22 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia through aquifers and in a vertical direction through Table 5. Well-construction data for wells completed in the confining units. Where fresh ground water encoun Columbia aquifer in a transect A-A' near Townsend, Va. [Datum is sea level; well depth is in feet below land surface datum; ters salty ground water, the less dense freshwater is USGS, U.S. Geological Survey] forced upward. The upward-moving fresh ground water is again inhibited by confining units but even USGS Land-surface WeU tually discharges into marshes, estuaries, the bay, weU Station elevation depth and ocean. number number (feet) (feet) Water levels in wells in the Columbia aquifer 63F25 371145075565901 1238 6.6 indicate the direction of ground-water flow and the 63F26 371143075565801 1537 8.9 63F27 371133075570401 22.92 12.7 response of the system to recharge and discharge. 63F29 371121075565001 13.40 9.5 Well-construction information for wells along a 63F30 371128075572101 29.03 15.0 transect from the topographic high (ground-water 63F31 371136075580201 31.79 12.0 63F32 371136075574801 2855 12.0 divide) near U.S. Route 13 near Townsend, Va., to 63F38 371144075580201 22.00 pond the marsh adjacent to Magothy Bay (ground-water- 63F49 371125075570205 2735 16.8 discharge area) (fig. 12) is presented in table 5. Water levels fluctuate throughout the year in response to the amount of recharge to and discharge in figure 16. The water levels for the research sta from the system (fig. 13). Water-level declines in tion in a recharge area (fig. 16A) reflect downward this agricultural area during the spring and summer vertical flow from the unconfined aquifer (well indicate the effects of increased evapotranspiration. 64K10), to the upper Yorktown-Eastover aquifer Water levels are highest at well 63F31 near the cen (well 64K11), to the lower Yorktown-Eastover aqui ter of the peninsula and decline toward the coast fer (well 64K12). In contrast, the water levels pre (fig. 13). The water-level gradients indicate that sented in figure 16B indicate upward flow in a ground water flows from the topographic high in the coastal discharge area. Well 64J11 is screened in the center of the peninsula to the lowlands adjacent to lower Yorktown-Eastover aquifer, whereas wells Magothy Bay. Water levels from an irrigation pond 64J10 and 64J9 are screened in the middle and (63F38) and a nearby well (63F31) show the upper Yorktown-Eastover aquifers, respectively. response of the unconfined aquifer to pumping (fig. Except for the early period of record when water 14). The water level in well 63F31 shows little levels appear to be affected by pumping, heads in response to the greater than 4-ft decline in water the wells for this research station reflect vertical levels in the pond caused by pumpage during the 1989 growing season. Pumpage from the irrigation flow in an upward direction as ground water flows toward discharge sites in the coastal marshes, estu pond only has a local effect on ground-water levels because of the high permeability of the coarse aries, and ocean. grained sediments in the unconfined aquifer. Temporal water-level trends and vertical gradi Regional Ground-Water-Flow System ents in water levels provide additional information The regional ground-water-flow system con about the response of the ground-water-flow system sists of the confined aquifers beneath the lower to recharge, discharge, and pumpage stress. The Yorktown-Eastover aquifer (fig. 10). These aquifers VWCB has constructed a series of research stations are continuous underneath the Chesapeake Bay and on the Eastern Shore to monitor such responses (fig. are part of the westward-thinning wedge of uncon- 15). Each research station consists of a cluster of solidated sediments that make up the Coastal Plain wells with individual wells screened in different of Virginia. Ground-water flow in these deep con aquifers. Well identifiers, well location, and well- fined aquifers beneath the Eastern Shore is affected construction information for wells in selected by the Chesapeake Bay and regional ground-water research stations are summarized in table 6. Water flow from the Virginia mainland. Freshwater is levels from research-station wells provide informa recharged to the deep confined aquifers from precip tion about the vertical direction of flow between itation that falls on the Virginia mainland and infil aquifers. Water levels for two research stations on trates into the confined system. At the northern end the Eastern Shore that illustrate the vertical direc of the peninsula, fresh ground water flows farther tions of flow in this multiaquifer system are shown beneath the Chesapeake Bay and the Eastern Shore Hydrogeology 23 CONTOUR INTERVAL 5 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 EXPLANATION ^63F31 WELL AND Figure 12. Locations of wells along transect A-A' in the Columbia aquifer. 27 26 25 24 UJ 23 > UJ 22 UJ 21 CO HI 20 o CO 19 18 UJ UJ u. 17 16 UJ > 15 UJ _J 14 DC UJ 13 12 11 10 9 8 SONDJFMAMJJ ASONDJFMAMJJASONDJFMAMJ JASOND 1987 1988 1989 1990 Figure 13. Water levels in selected wells along transect A-A' in the Columbia aquifer. than at the southern end. The northern end of the in water from wells at approximately the same lon peninsula is closer to higher landmasses; therefore, gitude in the southeastern part of the Virginia main it has a nearby freshwater source. The freshwater land range from 1,360 to 1,900 mg/L in the upper heads at the southern part of the Virginia Coastal Potomac aquifer. Research-station well clusters on Plain are not high enough to force the saltwater out the Eastern Shore also indicate a stronger regional of the deep sediments beneath the Chesapeake Bay freshwater influence in the northern part of the East and Eastern Shore. ern Shore than in the southern part. The vertical Chloride concentrations support this conceptu chloride distribution is shown in table 7 for the two alization of ground-water flow. Chloride concentra research-station well clusters on the Eastern Shore tions in ground water from the upper Potomac aqui that have wells located in the upper Potomac aqui fer for the Virginia Coastal Plain are lower along fer. At the Jenkins Bridge Research Station, chlo the coast in the northeastern part of the Virginia ride concentrations are lower in the upper Potomac mainland than in the southeastern part (fig. 17). The aquifer (1,500 mg/L) than in the overlying St. chloride concentration is 150 mg/L in water from Marys-Choptank (3,800 mg/L) and lower well 63L4 on Tangier Island, which is screened in Yorktown-Eastover (2,100 mg/L) aquifers. The ver the upper Potomac aquifer. Chloride concentrations tical profile of chloride concentrations from the Hydrogeology 25 30 IRRIGATION POND 63F38 LU CO LU > o CO UJ 25 LU 20 APRIL MAY JUNE JULY AUGUST SEPTEMBER OCTOBER 1989 Figure 14. Water levels in an irrigation pond and in a nearby well completed in the Columbia aquifer. Jenkins Bridge Research Station indicates that the 24,000 mg/L indicates highly saline water at well origin of the freshwater in the upper Potomac aqui 63F52 and no fresh ground-water flow. fer is freshwater flowing beneath the Chesapeake Bay from the mainland of Virginia and Maryland Ground-Water Use (fig. 17, table 7). The vertical profile of chloride concentrations for the Kiptopeke Research Station at Prior to 1965, there were few large users of the southern tip of the Eastern Shore shows increas ground water on the Eastern Shore. By 1970, ing chloride concentrations with depth at this loca increased population combined with commercial and tion (table 7). Well 63F52 at the Kiptopeke industrial growth created a greatly increased demand Research Station is located farther west than well for the ground-water resource. Major pumping cen 66M23 at the Jenkins Bridge Research Station (fig. ters on the Eastern Shore are located near the towns 17); however, the freshwater flow beneath the Ches of Chincoteague, Hall wood, Accomac, Exmore, apeake Bay does not extend as far to the east at the Oyster, Cheriton, and Cape Charles, Va. southern tip of the peninsula as it does at the north Annual ground-water withdrawal data for the ern part of the peninsula. The chloride concentration model area were compiled by confined aquifer for in ground water in the upper Potomac aquifer of commercial, industrial, and municipal withdrawals 26 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 38° - 67M10 i 67M11 Chiricoteague 67M12 -* ^ ^ 67M13 67M14 65K27 Tangier Island 65K2e 0 65K29 65K30 45' - 30' - Cape EXPLANATION 63F15 Chartoi 63F16 37° 15' - 63F17 RESEARCH STATION AND WELL IDENTIFICATION 20 MILES i 20 KILOMETERS Figure 15. Location of selected Virginia Water Control Board research-station well clusters. Hydrogeology 27 Table 6. Selected Virginia Water Control Board research-station well clusters on the Eastern Shore [Latitude and longitude are reported in degrees, arc minutes, and arc seconds; USGS, U.S. Geological Survey; VWCB, Virginia Water Control Board] USGS VWCB Wen well wen Latitude Longitude depth Aquifer penetrated number number (feet) 66K4 SOW 101A 374320 0753656 152 Upper Yorktown-Eastover 66K3 SOW 101B 374320 0753805 220 Middle Yorktown-Eastover 66K2 SOW 101C 374319 0753654 292 Lower Yorktown-Eastover 64H6 SOW102A 372925 0754704 154 Upper Yorictown-Eastover 64H7 SOW102B 372921 0754705 220 Middle Yorktown-Eastover 64H5 SOW 102C 372921 0754705 306 Lower Yorktown-Eastover 63H6 SOW 103A 372705 0755559 37 Columbia 63H5 SOW 103B 372705 0755559 132 Upper Yorktown-Eastover 63H4 SOW 103C 372706 0755559 235 Lower Yorktown-Eastover 63G21 SOW104S 371709 0755608 36 Columbia 63G17 SOW 104A 371709 0755608 140 Upper Yorktown-Eastover 63G16 SOW 104B 371709 0755608 240 Middle Yorktown-Eastover 63G1S SOW104C 371709 0755607 310 Lower Yorktown-Eastover 63F15 SOW105A 371307 0755835 130 Upper Yorktown-Eastover 63F17 SOW 105B 371307 0755835 196 Middle Yorktown-Eastover 63F16 SOW 105C 371307 0755835 285 Lower Yorktown-Eastover 64K9 SOW106A 373845 0755225 37 Columbia 64K8 SOW106B 373845 0755225 95 Upper Yorktown-Eastover 64K7 SOW106C 373845 0755225 176 Lower Yorktown-Eastover 66L2 SOW 107A 375225 0753217 140 Upper Yorktown-Eastover 66L3 SOW107B 375225 0753217 206 Middle Yorktown-Eastover 66L1 SOW 107C 375225 0753217 305 Lower Yorktown-Eastover 64K10 SOW 108A 373932 0754527 50 Columbia 64K11 SOW 108B 373932 0754527 180 Upper Yorktown-Eastover 64K12 SOW 108C 373932 0754527 284 Lower Yorktown-Eastover 65K26 SOW109S 374442 0754325 25 Columbia 65K24 SOW 109A 374442 0754325 130 Upper Yorktown-Eastover 65K25 SOW 109B 374442 0754325 228 Lower Yorktown-Eastover 65K23 SOW109C 374428 0754328 290 Lower Yorktown-Eastover 66M19 SOW110S 375723 0753444 36 Columbia 66M16 SOW110A 375723 0753444 130 Upper Yorktown-Eastover 66M17 sow HOB 375723 0753444 178 Middle Yorktown-Eastover 66M18 sow iioc 375723 0753445 240 Lower Yorktown-Eastover 63G25 SOW 11 IS 371653 0755848 70 Columbia 63G22 SOW111A 371653 0755848 150 Upper Yorktown-Eastover 63G23 SOW 11 IB 371653 0755848 280 Lower Yorktown-Eastover 63G24 SOW 11 1C 371653 0755848 330 Lower Yorktown-Eastover 64J12 SOW112S 373059 0754845 47 Columbia 64J9 SOW112A 373059 0754845 135 Upper Yorktown-Eastover 64J10 SOW112B 373059 0754845 210 Middle Yorktown-Eastover 64J11 SOW112C 373059 0754845 313 Lower Yorktown-Eastover 6311 SOW113A 373216 0755407 120 Upper Yorktown-Eastover 63J2 SOW113B 373216 0755407 225 Middle Yorktown-Eastover 63J3 SOW113C 373216 0755407 290 Lower Yorktown-Eastover 65K30 SOW114S 374425 0754000 40 Columbia 28 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 6. Selected Virginia Water Control Board research-station well clusters on the Eastern Shore- Continued uses VWCB Wen weU well Latitude Longitude depth Aquifer penetrated number number (feet) 65K27 SOW114A 374425 0754000 160 Upper Yoiktown-Eastover 6SK28 SOW114B 374425 0754000 230 Middle Yoiktown-Eastover 65K29 SOW114C 374427 0754000 315 Lower Yoiktown-Eastover 67M10 SOW 1 ISA 375635 0752715 52 Columbia 67M11 SOW115B 375635 0752715 138 Upper Yoiktown-Eastover 67M12 SOW115C 375635 0752715 222 Middle Yoiktown-Eastover 67M13 SOW115D 375635 0752715 249 Middle Yoiktown-Eastover 67M14 SOW115E 375617 0752737 280 Middle Yoiktown-Eastover 66M23 SOW 181A 375610 0753618 1,300 Upper Potomac 66M24 SOW 18 IB 375610 075 36 18 508 St. Marys 66M25 SOW 181C 375610 0753618 340 Lower Yoiktown-Eastover 66M26 SOW 18 ID 375610 0753618 230 Lower Yoiktown-Eastover 66M27 SOW 18 IE 375610 0753618 30 Columbia 63F51 SOW 182A 370807 0755708 1,730 Middle Potomac 63F52 SOW182B 370807 0755708 1,332 Upper Potomac 63F53 SOW182C 370807 0755708 220 Lower Yoiktown-Eastover 63F54 SOW 182D 370807 0755708 60 Upper Yoiktown-Eastover 63F55 SOW 182E 370807 0755708 20 Columbia Table 7. Vertical distribution of chloride concentrations in ground water at Jenkins Bridge and Kip- topeke Research Station well clusters [USGS, U.S. Geological Survey; mg/L, milligrams per liter] USGS WeU Chloride weU depth Aquifer Date concentration number (feet) (mg/L) Jenkins Bridge Research Station 66M27 40 Columbia 10-29-87 31 08-29-88 23 66M26 230 Lower Yorktown-Eastover 10-30-87 1,000 08-29-88 810 66M25 340 Lower Yorktown-Eastover 10-30-87 2,100 08-29-88 2.100 66M24 508 St. Marys - Choptank 10-28-87 3,900 08-29-88 3.800 66M25 1^20 Upper Potomac 10-29-87 1,500 08-29-88 1,500 Kiptopeke Research Station 63F55 20 Columbia 11-16-89 24 63F54 60 Upper Yorktown-Eastover 11-16-89 32 63F53 220 Lower Yorktown-Eastover 11-16-89 59 63F52 1,332 Upper Potomac 11-29-89 24,000 Hydrogeology 29 55 I I I I I I LU > LU _l 5 45 CO 10 A UJ o /\ CO 35 LU U-J25 \ 64K12 QC LU I 15 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 B LU 10 LU LU< 8a CO g e mLU LU Sm LU o LU -2 LU QC LU S-6 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Figure 16. Water levels in research-station well clusters (A) in a recharge area and (B) in a discharge area. 30 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 77' 76° EXPLANATION OBSERVATION WELL-Upper number is well identifier Lower number is chloride concentration in milligrams per liter 20 MILES 38" 37" Figure 17. Location of observation wells and chloride concentrations in the upper Potomac aquifer for the Coastal Plain of Virginia. Hydrogeology 31 EXPLANATION UPPER YORKTOWN-EASTOVER AQUIFER DC HI MIDDLE YORKTOWN-EASTOVER AQUIFER 0. eo LOWER YORKTOWN-EASTOVER AQUIFER z o TOTAL z o DC O I I- 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 Figure 18. Annual ground-water withdrawal from model area. (fig. 18). Estimates do not include domestic or agri aquifer from which water was pumped. For wells cultural withdrawals. Domestic use is not included screened in multiple aquifers, water-withdrawal rates because currently there is no practical method of from each aquifer were estimated from the ratio of collecting such data by aquifer, and it is assumed to the length of screen in each aquifer to the total represent only a small percentage of nonreturned length of well screen. water. Agricultural use is significant on the Eastern The middle and upper Yorktown-Eastover Shore; however, agricultural users are not required aquifers have historically provided most of the to report withdrawals. As a result, the specific loca freshwater to users on the Eastern Shore. Prior to tions and aquifers tapped for agricultural withdraw 1968, the largest withdrawals were from the shal als are unknown. Most of the ground water used for lowest confined aquifer, the upper Yorktown- agricultural purposes is withdrawn from irrigation Eastover. By 1970, the middle Yorktown-Eastover ponds in the unconfined Columbia aquifer. All other aquifer was contributing more water than the upper ground-water users in Northampton and Accomack or lower Yorktown-Eastover aquifers, and pumpage Counties that withdraw over 300,000 gal/month are from the lower Yorktown-Eastover aquifer was required to report usage data to the VWCB. The increasing. Estimated ground-water use peaked in depth of the well screen was used to determine the 1974 at 6.96 Mgal/d. The decline in water use for 32 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia the period 1975-83 represents the loss of several nel (described in Pleistocene Paleochannel Aquifers major industrial users. Since 1985, water use has section). The erosion of the original aquifer and generally been steady. Total ground-water use was confining-unit materials combined with the different estimated to be about 5.04 Mgal/d in 1988. The hydraulic characteristics of the backfilled-channel upper Yorktown-Eastover aquifer supplied 36 per sediments could result in a better hydraulic connec cent of the withdrawal in 1988, and the middle and tion between the freshwater-flow system and the lower Yorktown-Eastover aquifers supplied 42 and saltwater-flow system in this area. A detailed study 22 percent, respectively. of the extent of the erosion and the hydraulic prop erties of the channel-fill sediments is needed for a complete understanding of saltwater-freshwater rela Chloride Distribution tions in this part of the ground-water-flow system. Elevated chloride concentrations in water in the Chloride concentrations were compiled by middle and lower Yorktown-Eastover aquifers also aquifer to provide information on the distribution of are present near Cape Charles, Va. This area could chlorides in the aquifers that currently are being be affected by the southernmost paleochannel or by used as a freshwater supply for the Eastern Shore incision from a nearshore channel in the present-day (figs. 19-22). The chloride concentrations presented Chesapeake Bay. on the maps are the most recent chloride analyses for each well. Individual chloride analyses are pre ANALYSIS OF THE GROUND-WATER- sented by aquifer in tables 8-11. Chloride concen FLOW SYSTEM trations typically are greater along the coast than in the center of the peninsula. Chloride concentrations The conceptualization of the physical charac in water collected from wells in the Columbia aqui teristics of the three-dimensional, multiaquifer, flow fer and the upper Yorktown-Eastover aquifer were system can be incorporated into a digital ground- less than the U.S. Environmental Protection Agency water-flow model. The model is a mathematical rep (USEPA) secondary drinking-water regulation of resentation of the natural system and includes many 250 mg/L (U.S. Environmental Protection Agency, simplifying assumptions. Model input parameters 1989). The line delineating the approximate limit of are based on the measured and estimated character the 250 mg/L chloride concentration in the Colum istics of the aquifers and confining units. The model bia aquifer was estimated to be the landward limit is calibrated by comparing simulated water levels to of the saltwater marshes and estuaries (fig. 19). All water levels measured at observation wells. Once chlorides from the upper Yorktown-Eastover aquifer calibrated, the digital model can be used within its were well below the 250 mg/L limit; therefore, the limitations to simulate changes in ground-water-flow limit line was estimated based on the understanding conditions that result from changes in hydrologic of the ground-water-flow system (fig. 20). Chloride stresses. A digital model can assist in analyzing a concentrations probably are less than 250 mg/L in ground-water system; however, it is important to the upper Yorktown-Eastover aquifer beneath all realize that the model is only an approximate repre major land surfaces on the peninsula. Chloride con sentation of the actual physical system. centrations generally increase with depth; greater chloride concentrations are found in the lower Yorktown-Eastover aquifer than in the overlying Development of the Flow Model middle Yorktown-Eastover, upper Yorktown- Eastover, and Columbia aquifers. Chloride concen A ground-water-flow model was developed for trations in water in the lower Yorktown-Eastover the Eastern Shore using SHARP (Essaid, 1990a), a aquifer are stratified; concentrations are less near the numerical finite-difference model that uses a quasi- top of the aquifer than near the bottom of the aqui three-dimensional approach to simulate freshwater fer. Data indicate an area of elevated chloride con and saltwater flow separated by a sharp interface. centrations in water in the middle and lower The sharp-interface approach assumes that saltwater- Yorktown-Eastover aquifer near Exmore, Va. between saltwater and freshwater is small relative to (figs. 21, 22). The elevated concentrations appear to the thickness of the aquifer. The approach does not be in the area of an ancient Pleistocene river chan provide information on the physical or chemical Analysis of the Ground-Water-Flow System 33 76 C 45' 30' 75° 15' 38° 45' 30' EXPLANATION CHLORIDE CONCENTRATIONS LESS THAN 250 MILLIGRAMS PER LITER __ __ APPROXIMATE LIMIT OF 250 MILLIGRAMS PER LITER CHLORIDE CONCENTRATION 37° 15' 23 WELL Number is chloride concentration in milligrams per liter 20 MILES 20 KILOMETERS Figure 19. Chloride concentrations in the Columbia aquifer. 34 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' o a: 38 C w 45' 30' EXPLANATION CHLORIDE CONCENTRATIONS LESS THAN 250 MILLIGRAMS PER LITER APPROXIMATE LIMIT OF 250 MILLIGRAMS PER LITER CHLORIDE CONCENTRATION 37° 15' WELL-Number it chloride concentration in milligrams per liter 10 20 MILES 10 20 KILOMETERS Figure 20. Chloride concentrations in the upper Yorktown-Eastover aquifer. Analysis of the Ground-Water-Flow System 35 75° 15' EXPLANATION CHLORIDE CONCENTRATIONS LESS THAN 250 MILLIGRAMS PER LITER ______APPROXIMATE UMTT OF 250 MILLIGRAMS PER LITER CHLORIDE CONCENTRATION 37° 15' - J°18 WELL-Number it chloride concentration in milUgnnu per liter Figure 21. Chloride concentrations in the middle Yorktown-Eastover aquifer. 36 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76' 45' 30' 75° 15' 38' 45' 3D 1 EXPLANATION WELL Number is chloride concentration in milligrams 37° 15' per liter 20 MILES 20 KILOMETERS Figure 22. Chloride concentrations in the lower Yorktown-Eastover aquifer. Analysis of the Ground-Water-Flow System 37 Table 8. Chloride concentrations in the Columbia aquifer [USGS, U.S. Geological Survey; VWCB, Virginia Water Control Board; latitude and longitude are reported in degrees, arc minutes, arc seconds; mg/L, milligrams per liter] USGS WeU Land- Chloride Date Sampling well Latitude Longitude depth surface concentration sampled agency number (feet) altitude (mg/L) (feet) 62F 4 371456 0760030 40 10 73 12-01-77 VWCB 63F 6 370806 0755718 74 10 54 09-27-55 USGS 63F19 370806 0755709 60 10 23 01-02-75 VWCB 63F20 370806 0755708 62 10 26 01-02-75 VWCB 63F21 370806 0755707 65 10 31 01-02-75 VWCB 32 02-12-75 VWCB 32 12-12-75 VWCB 63F22 370939 0755704 46 10 27 01-02-75 VWCB 36 12-12-75 VWCB 63G21 371709 0755608 36 30 66 10-03-77 VWCB 43 08-19-80 VWCB 32 08-04-86 VWCB 63G2S 371653 0755848 70 15 13 06-29-79 VWCB 15 08-19-80 VWCB 63H.6 372705 0755559 37 17 43 09-28-77 VWCB 30 05-11-79 VWCB 23 06-26-84 VWCB 631 4 373220 0755415 40 25 22 05-11-84 VWCB 64J12 373059 0754845 47 30 23 07-03-79 VWCB 18 08-21-80 VWCB 64J26 373200 0754917 58 35 41 03-01-67 VWCB 105 04-01-75 VWCB 92 09-19-78 VWCB 64K9 373845 0755225 37 2 78 09-21-77 VWCB 92 08-20-80 VWCB 363 06-26-84 VWCB 64K10 373932 0754527 50 45 6 08-20-80 VWCB 18 06-29-84 VWCB 65K21 374257 0754041 45 42 9 09-28-71 VWCB 65K26 374442 0754325 25 10 19 08-20-80 VWCB 65K30 374425 0754000 40 45 12 02-13-80 VWCB 21 08-26-80 VWCB 65K32 374232 0754342 52 30 19 08-07-81 VWCB 65K33 374249 0754207 55 40 15 08-07-81 VWCB 66M19 375723 0753444 36 10 15 08-26-80 VWCB 16 07-11-84 VWCB 66M21 375403 0753025 69 35 38 08-04-81 VWCB 66M27 375610 0753618 30 5 31 10-29-87 USGS 23 08-29-88 USGS 67M6 375624 0752836 45 30 10 08-17-48 VWCB 67M10 375635 0752715 52 15 14 08-13-81 VWCB 12 05-27-82 VWCB 16 08-13-84 VWCB nature of the transition zone; however, it does repre rated by confining units that are represented by a sent the overall ground-water flow in the system and vertical leakance term. Vertical gradients within will reproduce the general response of the interface aquifers are not represented; therefore, the modeling to applied stresses (Essaid, 1986). The model repre approach is not fully three-dimensional. The quasi- sents regional-scale ground-water-flow systems as a three-dimensional solution of the ground-water-flow layered sequence of two-dimensional aquifers sepa equation requires several assumptions: (1) flow in 38 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia the aquifers is in the lateral direction, (2) vertical ages (L *); flow is controlled by confining units, and (3) water cf>y, fys = freshwater and saltwater heads (L); released from confming-unit storage is negligible. n= effective porosity; These assumptions are considered valid when the f=time (7); and lateral and vertical hydraulic conductivities of the a=a parameter equal to 1 for unconfined aquifers are much greater than those of the confin aquifers and zero for confined aquifers. ing units, and simulation times are long enough to The flow equations for the freshwater and salt minimize the effects of water released from water zones are coupled by the interface boundary confining-unit storage. condition. Continuity of flux and pressure must be The Eastern Shore is a peninsula surrounded maintained at the interface; the fluid pressure of the by saltwater; therefore, the model's inclusion of freshwater must equal the fluid pressure of the salt saltwater-flow dynamics is of particular importance water (Bear, 1979). to an analysis of the ground- water system. Any change in offshore freshwater discharge induces (3) movement of the interface between freshwater and saltwater. The rate of interface movement is a func where tion of the flow conditions and aquifer properties of Pp Ps =*he freshwater and saltwater fluid pres the freshwater and saltwater flow domains. sures (ML-1r~2); The model uses two vertically integrated, par fy, ^=the freshwater and saltwater specific abolic, partial-differential flow equations, represent weights (MZT2r~2); and ing freshwater and saltwater flow, which must be £/=the interface elevation (L). simultaneously solved for freshwater and saltwater Solving for the interface elevation, head, as follows (Essaid, 1986): (4) dd>< T J where 8=-y//('yJ y/). at at at Once the freshwater and saltwater heads have been obtained from equations 1 and 2, the interface elevation can be calculated from equation 4. The SHARP model calculates and tracks the and positions of the interface tip and toe in the finite- difference grid for each aquifer. The interface tip is the intersection of the interface with the top of the aquifer, and the interface toe is the intersection of the interface with the bottom of the aquifer (fig. 23). The interface position will not always coincide with the grid block boundaries. The tip and toe are located by linearly projecting the interface, where based on the interface elevations at grid points. On Kfa, Ksx =the freshwater and saltwater hydraulic the freshwater side of the interface toe, the entire conductivities in the x direction (L7"" 1); thickness of the aquifer contains freshwater. Simi Kfy, Ksy =ihe freshwater and saltwater hydraulic larly, on the saltwater side of the interface tip, the conductivities in the v direction (LT l)\ entire thickness of the aquifer contains saltwater. In Qf-> Gj =the freshwater and saltwater source/sink the area between the interface tip and toe, the aqui terms (LT~ l); fer contains freshwater and saltwater. Qy, <2/.y=the freshwater and saltwater leakage The sharp-interface approach assumes that terms (LT 1), calculated using Darcy's saltwater and freshwater do not mix. Vertical leak law; age between saltwater and freshwater is restricted. Bp Bs =the thicknesses of the freshwater and salt Saltwater is not allowed to leak into the freshwater water zones (L); zone, and freshwater is not allowed to leak down Sf, Ss =the freshwater and saltwater specific stor ward into the saltwater zone. A node can contain Analysis of the Ground-Water-Flow System 39 Table 9. Chloride concentrations in the upper Yorktown-Eastover aquifer [USGS, U.S. Geological Survey; VWCB, Virginia Water Control Board; latitude and longitude are reported in degrees, arc minutes, arc seconds; mg/L, milligrams per liter] USGS Well Land- Chloride well Latitude Longitude depth surface concentration Date SflTTlTlllflg number (feet) gUtitnde (mg/L) sampled agency (feet) 62F3 371456 0760027 130 12 8 02-01-77 VWCB 63F15 371307 0755835 130 31 20 06-07-78 VWCB 25 08-11-80 VWCB 25 07-12-84 VWCB 63F18 37 13 12 0755915 112 15 18 04-05-80 VWCB 63F24 371302 0755807 140 37 67 01-08-81 USGS 63G9 372134 0755908 134 2 20 05-26-54 VWCB 63G17 371709 0755608 140 28 16 10-03-77 VWCB 24 08-18-80 VWCB 24 08-06-84 VWCB 19 02-28-89 USGS 63G22 371653 0755848 150 15 8 06-29-79 VWCB 8 08-19-80 VWCB 6 07-12-84 VWCB 63G37 372106 0755620 165 38 17 11-01-74 VWCB 14 11-28-78 VWCB 13 02-26-79 VWCB 15 05-02-79 VWCB 13 08-20-79 VWCB 12 12-06-79 VWCB 15 01-28-80 VWCB 15 08-28-80 VWCB 14 02-19-81 VWCB 17 08-24-81 VWCB 36 07-21-82 VWCB 16 06-16-83 VWCB 14 07-23-84 VWCB 63H5 372705 0755559 132 17 24 09-28-77 VWCB 24 05-11-79 VWCB 24 06-26-84 VWCB 28 01-25-88 USGS 35 03-02-89 USGS 63H10 372412 075 54 15 152 38 15 12-18-80 VWCB 63H11 372608 0755307 180 30 15 12-18-80 VWCB 63 Jl 373230 075 54 10 120 22 27 08-25-80 VWCB 24 06-28-84 VWCB 64H6 372905 0754740 154 6 52 06-01-77 VWCB 49 07-11-84 VWCB 51 03-01-89 USGS 64J 2 372235 0755335 190 34 14 10-27-69 USGS 15 01-29-70 USGS 14 02-18-75 VWCB 64J 9 373059 0754845 135 30 11 07-03-79 VWCB 12 08-22-80 VWCB 64J18 373503 0754920 167 31 13 02-01-75 VWCB 10 08-01-78 VWCB 10 02-19-81 VWCB 64J24 373045 0754829 130 7 48 08-02-79 VWCB 40 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 9. Chloride concentrations in the upper Yorktown-Eastover aquifer Continued USGS Wefl Land- Chloride wen Latitude Longitude depth surface concentration Date SfJtnpliTig number (feet) altitude (mg/L) sampled agency (feet) 64K8 373845 0755225 95 3 33 09-21-77 VWCB 41 08-20-80 VWCB 47 06-26-84 VWCB 34 03-02-89 USGS 64K11 373932 075 45 27 180 47 10 08-20-80 VWCB 9 06-29-84 VWCB 8 01-26-88 USGS 6SK6 373830 0754000 190 43 10 10-21-71 USGS 10 03-04-72 USGS 6SK9 374233 0754432 159 17 10 02-26-75 VWCB 9 09-10-75 USGS 6SK22 37 41 53 0754309 180 43 8 09-28-71 USGS 8 03-06-80 VWCB 8 02-19-81 VWCB 5 08-24-81 VWCB 10 02-11-82 VWCB 4 09-28-82 VWCB 9 03-29-84 VWCB 7 11-19-84 VWCB 65K24 374442 0754325 130 12 7 08-13-80 VWCB 65K27 374425 0754000 160 45 10 02-13-80 VWCB 9 07-09-84 VWCB 65L3 373730 0754000 160 40 7 04-22-60 VWCB 24 01-05-72 USGS 24 03-07-72 USGS 20 06-01-72 VWCB 22 06-27-77 VWCB 22 11-14-77 VWCB 23 02-23-78 VWCB 66K4 374320 0753656 152 10 8 OSO3-77 VWCB 8 07-10-84 VWCB 66L2 375225 0753217 140 5 6 09-21-77 VWCB 8 07-10-84 VWCB 66L4 374625 0753646 160 40 9 08-04-81 VWCB 66M16 375723 0753444 130 11 13 08-26-80 VWCB 14 07-11-84 VWCB 66M22 375920 0753205 132 21 15 03-30-82 VWCB 67M11 375635 0752715 138 14 28 03-29-81 VWCB 23 05-12-81 VWCB 19 05-27-82 VWCB 19 08-07-84 VWCB freshwater, saltwater, or both freshwater and salt a node that contains all saltwater, then all the fresh water. Upward freshwater leakage is distributed water leakage will be incorporated into the saltwater between the saltwater and freshwater zones based on zone (Essaid, 1990a). Vertical leakage of saltwater the volume of each type of water in the node receiv into freshwater is not directly simulated; evidence of ing the leakage. If freshwater is leaking upward into vertical saltwater intrusion is provided by examina a node that contains 80-percent freshwater and tion of the hydraulic gradients and areas of reversed 20-percent saltwater, then 80 percent of the leakage ground-water flow. will be incorporated into the freshwater zone and 20 The sharp-interface modeling approach percent of the leakage will be incorporated into the neglects hydrodynamic dispersion; therefore, the saltwater zone. If freshwater is leaking upward into interface position does not represent a particular Analysis of the Ground-Water-Flow System 41 Table 10. Chloride concentrations in the middle Yorktown-Eastover aquifer [USGS, U.S. Geological Survey; VWCB, Virginia Water Control Board; latitude and longitude are reported in degrees, arc minutes, arc seconds; mg/L, milligrams per liter] USGS Well Land- Chloride weH Latitude Longitude depth surface concentration Date Sampling number (feet) altitude (mg/L) sampled agency (feet) 62F 2 371456 0760030 210 12 12 12-01-77 USGS 62G 4 372250 0755335 210 12 24 01-04-72 USGS 62G 8 371540 0760121 200 12 170 10-17-75 USGS 62G 9 37 15 39 0760114 170 12 250 10-17-75 VWCB 62G15 371543 0760034 190 12 175 08-06-84 VWCB 62G16 371544 0760118 221 12 114 05-09-77 VWCB 130 08-01-77 VWCB 121 11-28-78 VWCB 124 01-28-80 VWCB 144 02-19-81 VWCB 129 07-22-82 VWCB 161 02-28-83 VWCB 147 01-23-84 VWCB 171 05-15-85 VWCB 63F10 371057 0755814 220 27 14 09-11-75 USGS 15 02-01-75 VWCB 63F17 371307 0755835 196 31 13 08-01-80 VWCB 11 07-12-84 VWCB 63G16 371709 0755608 240 28 72 10-03-77 VWCB 20 08-18-79 VWCB 31 09-26-79 VWCB 15 08-06-84 VWCB 14 01-25-88 USGS 14 02-28-89 USGS 63G27 37 17 10 0755522 185 5 18 08-25-80 VWCB 63G34 37 17 15 0755521 186 3 19 09-23-81 VWCB 15 07-22-82 VWCB 17 03-14-83 VWCB 63G35 37 17 15 0755521 186 5 25 04-07-75 VWCB 17 10-16-81 VWCB 63G36 37 17 11 0755524 185 6 19 02-12-75 VWCB 16 04-01-75 VWCB 15 03-26-80 VWCB 63G43 371620 0755815 215 15 11 03-03-89 VWCB 63J2 373230 0755410 225 22 1.400 08-06-86 VWCB 64H7 372905 0754740 220 6 400 06-01-77 VWCB 340 07-11-84 VWCB 400 08-05-86 VWCB 430 11-12-87 USGS 350 03-01-89 USGS 64H 9 372830 0755132 245 37 10 05-11-84 VWCB 64J 7 372245 0755335 228 34 17 06-02-65 VWCB 15 09-10-70 VWCB 12 12-19-72 VWCB 15 11-12-74 VWCB 64J10 373059 0754845 210 30 36 07-03-79 VWCB 35 08-21-80 VWCB 35 02-22-88 USGS 64J17 373507 0755155 180 30 10 08-07-81 VWCB 64J21 373159 0754915 229 35 208 03-01-67 VWCB 41 09-19-78 VWCB 53 08-01-79 VWCB 64J23 373146 0755047 190 28 10 08-31-81 VWCB 64K3 373756 0754906 210 25 12 12-31-06 USGS 42 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 10. Chloride concentrations in the middle Yorktown-Eastover aquifer Continued USGS Well Land- Chloride well Latitude Longitude depth surface concentration Date Sflmpling number (feet) altitude (mg/L) sampled agency (feet) 64K21 374029 0754925 185 6 17 02-02-89 USGS 64L 1 374559 075 45 15 135 5 69 02-02-89 USGS 65K28 374425 0754000 230 45 15 02-13-80 VWCB 12 07-09-84 VWCB 6SK34 373904 0754034 218 9 11 07-08-75 VWCB 6SK42 373750 0754415 225 41 9 02-02-89 USGS 65L9 374931 0753908 155 3 8 02-02-89 USGS 65L12 374809 0753818 220 36 9 02-02-89 USGS 65M 1 375537 0754318 115 3 14 08-18-48 USGS 65M2 375512 0754348 115 5 66 02-28-75 VWCB 65 03-25-75 VWCB 67 09-08-75 VWCB 66K3 374320 0753656 220 8 16 06-03-77 VWCB 8 07-10-84 VWCB 66L3 375225 0753217 206 5 5 09-30-77 VWCB 9 07-10-84 VWCB 8 02-02-88 VWCB 66L6 374900 0753524 246 53 10 08-09-78 VWCB 10 10-08-81 VWCB 66M17 375723 0753444 178 11 66 08-26-80 VWCB 68 07-11-84 VWCB 82 11-05-86 VWCB 66M20 37 53 32 0753300 240 42 8 08-04-81 VWCB 66M39 375623 075 30 19 180 25 7 02-02-89 USGS 67M 9 375626 0752723 256 19 125 02-27-75 VWCB 124 04-06-76 VWCB 107 07-12-76 VWCB 123 10-12-76 VWCB 129 04-08-77 VWCB 141 11-14-77 VWCB 164 04-21-81 VWCB 173 12-14-81 VWCB 167 05-27-82 VWCB 171 07-22-82 VWCB 183 01-25-83 VWCB 67M12 375635 0752715 222 13 79 03-29-81 VWCB 77 05-05-81 VWCB 73 05-27-82 VWCB 78 08-07-84 VWCB 81 11-06-86 VWCB 67M13 375635 0752715 249 16 135 03-29-81 VWCB 129 05-27-82 VWCB 135 08-07-84 VWCB 137 11-06-86 VWCB 67M14 375617 0752737 280 26 134 05-27-82 VWCB 144 08-07-84 VWCB 142 11-06-86 VWCB 140 01-26-88 USGS 67M24 375639 0752859 245 24 62 02-01-65 VWCB 68 10-27-69 USGS 64 07-01-70 VWCB 64 06-01-71 USGS 65 06-01-72 VWCB 56 02-27-75 VWCB 74 04-05-81 VWCB Analysis of the Ground-Water-Flow System 43 Table 11 . Chloride concentrations in the lower Yorktown-Eastover aquifer [USGS, U.S. Geological Survey; VWCB, Virginia Water Control Board; latitude and longitude are reported in degrees, arc minutes, arc seconds; mg/L, milligrams per liter] USGS Well Land- Chloride wdl Latitude Longitude depth surface concentration Date Sampling number (feet) altitude (mg/L) sampled agency (feet) 62F 1 371457 0760028 260 12 358 01-01-78 VWCB 63F 16 37 13 07 0755835 285 31 75 08-11-80 VWCB 73 07-12-84 VWCB 77 084)4-86 VWCB 92 09-27-87 VWCB 63G 15 37 17 09 0755608 310 28 148 08-06-84 VWCB 130 084)4-86 VWCB 130 02-28-89 USGS 63G23 371653 0755848 280 15 28 06-29-79 VWCB 9 08-19-80 VWCB 8 07-12-84 VWCB 9 01-25-88 USGS 9 03-01-89 USGS 63G24 371653 0755848 330 15 387 06-29-79 VWCB 640 08-11-80 VWCB 630 07-12-84 VWCB 750 084)4-86 VWCB 730 11-10-87 USGS 630 03-01-89 USGS 63H 4 37 27 05 0755559 235 17 297 09-28-77 VWCB 244 05-10-79 VWCB 246 06-26-84 VWCB 262 08-04-86 VWCB 290 03-02-89 USGS 63H 8 372620 0755255 295 33 13 05-02-79 VWCB 10 11-28-79 VWCB 12 02-19-81 VWCB 12 07-21-82 VWCB 14 09-12-83 VWCB 12 08-23-84 VWCB 12 02-19-85 VWCB 63J 3 373230 0755410 290 22 4.850 06-28-84 VWCB 6,200 11-18-87 USGS 64H 5 372922 0764701 306 6 2,217 06-01-77 VWCB 2,150 07-11-84 VWCB 2^50 08-05-86 VWCB 2,100 11-12-87 USGS 64J11 373059 0754845 313 30 1.598 07-03-79 VWCB 1,510 08-21-80 VWCB 1,900 11-10-87 USGS 64J15 373642 0754608 264 39 11 08-07-81 VWCB 64J16 373705 0754550 262 40 8 08-07-81 VWCB 64K 5 373828 0754509 290 45 7 02-28-75 VWCB 9 09-11-75 USGS 64K7 373845 0755225 176 8 320 09-30-77 VWCB 318 08-20-80 VWCB 306 06-26-84 VWCB 300 08-05-86 VWCB 310 03-02-89 USGS 44 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 11. Chloride concentrations in the lower Yorktown-Eastover aquifer Continued USGS Well Land- Chloride well Latitude Longitude depth surface concentration Date Sampling number (feet) altitude (mg/L) sampled____i_j agency (feet) 64K 12 37 39 32 0754527 284 47 12 08-21-80 VWCB 12 06-29-84 VWCB 11 01-26-88 USGS 65K 7 373805 0754000 295 36 43 09-21-71 USGS 12 02-20-75 VWCB 65K18 373800 0754000 283 40 37 09-20-71 USGS 33 02-27-75 VWCB 65K20 373810 0754000 295 43 55 09-21-71 USGS 65K23 374442 0754325 290 13 515 08-13-80 VWCB 800 02-01-88 USGS 65K25 374442 0754325 228 12 9 08-13-80 VWCB 15 01-27-88 USGS 65K29 374425 0754000 315 45 10 08-26-80 VWCB 226 07-09-84 VWCB 320 11-11-87 USGS 65M 3 37 55 12 075 43 48 195 5 430 02-01-75 VWCB 330 01-28-80 VWCB 343 01-20-82 VWCB 398 04-17-85 VWCB 66K 2 37 43 20 0753805 292 10 130 06-03-84 VWCB 45 08-06-86 VWCB 9 01-27-88 USGS 66L 1 375225 075 32 17 305 5 16 09-15-77 VWCB 12 07-10-84 VWCB 14 02-01-88 USGS 66M 5 375239 0753529 246 17 17 04-06-55 USGS 16 02-26-76 VWCB 25 03-23-76 VWCB 15 06-03-76 VWCB 19 08-31-76 VWCB 24 12-02-76 VWCB 18 03-31-77 VWCB 23 05-31-77 VWCB 18 09-15-77 VWCB 19 12-01-77 VWCB 17 03-06-78 VWCB 66M18 375723 0753444 240 11 790 08-26-80 VWCB 755 07-11-84 VWCB 1,675 11-05-86 VWCB 1,400 10-30-87 USGS 66M25 375610 0753618 340 6 2,100 10-30-87 USGS 2,100 08-29-88 USGS 66M26 375610 0753618 230 6 1,000 10-30-87 USGS 810 08-29-88 USGS chloride concentration. This approach is not shown that the sharp-interface toe tends to be farther intended to provide specific information concerning inland than the actual transition zone because the the physical and chemical nature of the transition effects of dispersion are neglected (Cooper, 1959; zone between freshwater and saltwater. A more Kohout, 1964; Volker and Rushton, 1982; Hill, detailed examination of the transition zone would 1988). The modeled saltwater-freshwater sharp require knowledge of the dispersive characteristics interface is a first attempt at understanding the of the aquifers. Comparisons between sharp- saltwater-freshwater-flow dynamics; the sharp inter interface and disperse-interface solutions have face provides information concerning the general Analysis of the Ground-Water-Flow System 45 GRID CELL COORDINATES ( I, j ) M i+1 /HP FRESHWA TER TOE Not to scale Figure 23. Model representation of the saltwater-freshwater interface tip and toe. response of the interface to applied stresses. A deri or both freshwater and saltwater. When the vation of the equations and a complete description saltwater-freshwater interface passes through a grid of the solution algorithm are provided in Essaid block, the grid block contains both saltwater and (1990a). freshwater. The model boundaries are designed to approx Model Grid and Boundaries imate the actual physical system. The western, east ern, and southern boundaries for the Eastern Shore The physical conceptualization of the ground- peninsula are the Chesapeake Bay and the Atlantic water-flow system is incorporated into a finite- Ocean and are represented as no-flow boundaries in difference model by dividing the system into a net the digital flow model. The boundaries are located work of rectangular grid blocks (fig. 24). Each grid far enough offshore to include the nearshore block is assigned values that represent the average saltwater-flow regime. The model simulates the aquifer characteristics and hydrologic stresses for position of the saltwater-freshwater interface bound that area. The spatial discretization for the Eastern ary condition. The location of this boundary changes Shore ground-water-flow model consists of a vari in response to changes in the saltwater-flow and able three-dimensional grid of 106 rows and 59 col freshwater-flow regimes. The Ghyben-Herzberg umns. The grid-block dimensions range from a min approximation was applied to current water-table imum of 0.49 mi to a maximum of 3.29 mi. head values for an initial estimate of the interface The model simulates flow only in the confined position (Heath, 1983). The lower boundary of the aquifers. Each of the three confined aquifers con model is simulated as a no-flow boundary and taining freshwater on the Eastern Shore was repre approximates the contact between the lower sented by a separate model layer. The unconfined Yorktown-Eastover aquifer and the 150- to 300-ft- aquifer was represented as a constant-head boundary thick St. Marys confining unit. This contact also is overlying the confined-aquifer system. Confining the lower limit of the freshwater-flow system. The units are not represented by layers but by vertical upper boundary of the model is simulated as a leakance terms assigned between layers. The physi constant-head boundary that represents the long-term cal and model conceptualizations of the ground- water-flow system are shown in figure 25. Model Figure 24. Finite-difference grid and boundaries used > grid blocks can contain all freshwater, all saltwater, in model analysis. 46 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 20 MILES KILOMETERS Analysis of the Ground-Water-Flow System 47 WEST EAST EASTERN SHORE PENINSULA NOT TO SCALE EXPLANATION F FRESHWATER VARIABLE-HEAD NODE NO-FLOW BOUNDARY B VARIABLE-HEAD NODE CONTAINING BOTH CONSTANT-HEAD BOUNDARY-- REPRESENTS SALTWATER AND FRESHWATER LONG-TERM HEADS IN COLUMBIA AQUIFER S SALTWATER VARIABLE-HEAD NODE DIRECTION OF FLOW WITHIN AQUIFER (HORIZONTAL) SALTWATER-FRESHWATER INTERFACE CONFINING-UNIT LEAKANCE (VERTICAL) Figure 25. Physical and model conceptualizations of ground-water-flow system. (quasi-steady-state) water table. This boundary is the basis of a physical feature. Therefore, this separated from the uppermost confined aquifer by a boundary is extended beyond the study area, and an confining unit and represents the vertical connection estimated flow line is represented by a no-flow between the unconfmed-aquifer and the confined- boundary. aquifer system. The relative consistency of water levels in the unconfined aquifer over the time and Model Calibration scale of simulation supports the use of this boundary condition. Constant heads representing the average The hydraulic properties of the aquifers and of the upper boundary in the onshore area were esti confining units are not uniform throughout the mated from pond elevations, stream elevations, and model area; therefore, the hydraulic characteristics water-level measurements in wells in the unconfined are allowed to vary by assigning average values to aquifer (fig. 26). Average elevations of surface each grid block. The hydraulic characteristics that water were estimated from USGS 7.5-min topo vary spatially in this analysis are transrnissivity, graphic maps. Heads in the offshore part of the storage coefficient, and vertical leakance. Data upper boundary were calculated as the freshwater quantifying these characteristics were not available equivalent of the saltwater head as indicated from for each grid block; values were estimated from the bathymetry on USGS 1:250,000 scale topo available measurements of physical and hydrologic graphic maps (fig. 11). The northern boundary is properties and laboratory analyses. A constant effec the only boundary that could not be delineated on tive porosity of 0.25 was assigned to each model 48 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75° 15' 38 C 45' 30' EXPLANATION WATER-LEVEL-CONTOUR-Shows altitude of water level Interval 10 feet. Datum is sea level 37° 15' 20 MILES _I -Fishermans Island 20 KILOMETERS Figure 26. Average water levels for the Columbia aquifer. Analysis of the Ground-Water-Flow System 49 layer. Model-sensitivity analyses indicated that compared with the range of transmissivities used in results were not sensitive to changes in effective the final model analysis is given in table 12. porosity. An iterative process was used to calibrate the Storage Coefficient ground-water-flow model. Initial estimates of The storage coefficient for each grid block hydraulic properties were used to run a steady-state was calculated by multiplying the estimated specific simulation for prepumping conditions. The initial storage of the aquifer by the average saturated thick steady-state results were used as a starting point for ness of the aquifer. A constant specific storage of a transient simulation of pumping conditions for the 1 x 10~6/ft is used in the model analysis; the value period 1940-88. The initial hydraulic properties for specific storage was not calibrated during model were adjusted by comparing the simulated water development. This value is commonly used in the levels to measured water levels. The process was literature to represent the specific storage of a con repeated until simulated and measured water levels fined aquifer and is considered reasonable if all were in close agreement at all observation wells. water released from aquifer storage results from The calibrated values used in the model analysis are the compressibility of water (Lohman, 1979). The stored on computer tapes at the Virginia District range of storage coefficients is listed by aquifer in Office of the USGS in Richmond, Va. table 13. Transmissivity Vertical Leakance The transmissivity for each grid block is cal The vertical leakance for each grid block was culated by multiplying the average thickness of the calculated by dividing the vertical hydraulic conduc aquifer by the average horizontal hydraulic conduc tivity of the confining unit by the average thickness tivity of the aquifer. The average thickness of the of the confining unit (figs. 3-9). A constant vertical aquifer was calculated for each grid block using hydraulic conductivity of 1.39xlO~5ft/d from labo maps of the tops of aquifers and confining units ratory analysis of core samples (table 4) was used to (figs. 3-9). Initial average horizontal hydraulic con calculate the initial vertical leakance used in the ductivities were estimated from specific-capacity and model calibration. Few core samples are available aquifer-test data. These initial values were adjusted for the Eastern Shore; therefore, initial estimates slightly during the transient-model calibration. were adjusted during transient-model development to Actual horizontal hydraulic conductivity data are estimate areal variations in vertical hydraulic con sparse; therefore, the values were held constant for ductivity. The range of final calibrated values for each layer except in areas where major regional geo vertical leakance is listed by confining unit in logic changes could be discerned. The final horizon table 14. tal hydraulic conductivities used in the model analy sis are 51.6, 43.2, and 8.6 ft/d for the upper, Steady-State-Model Simulation of middle, and lower Yorktown-Eastover aquifers, Prepumping Conditions respectively (figs. 27-29). The horizontal hydraulic conductivity in the northwestern corner of the model Prior to 1940, ground-water withdrawals on area was reduced to 1.3 ft/d to reflect fine-grained the Eastern Shore were minor. Ground-water use sediments and reduced water-bearing capabilities in consisted of a relatively small number of users with the western part of Somerset County, Md. drawing small amounts of water. The ground-water- (Werkheiser, 1990). The horizontal hydraulic con flow system at this time existed in an approximate ductivity was also reduced 1 order of magnitude state of hydraulic equilibrium (steady state). A near Exmore and near Cape Charles in the middle steady-state-flow condition is reached when recharge Yorktown-Eastover aquifer (4.3 ft/d) to reflect the to the system equals discharge from the system. presence of Pleistocene paleochannels in which the This condition implies that the water levels are original aquifer materials in these areas have been constant over time and that the change in storage in eroded and replaced by sediments with different the ground-water system is negligible. A steady- hydraulic properties. A summary of the range of state simulation was conducted using prepumping transmissivities estimated from specific-capacity data conditions for the Eastern Shore. The steady-state 50 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' 38 r 45' 30' EXPLANATION 37° 15' AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 1.3 FEET PER DAY AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 51.6 FEET PER DAY 10 20 MILES I_I 20 KILOMETERS Figure 27. Hydraulic conductivity of the upper Yorktown-Eastover aquifer based on model calibration. Analysis of the Ground-Water-Flow System 51 76 C 45' 30' 75° 15' 38° 45' 30' EXPLANATION AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 1.3 FEET PER DAY AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 37" 15' 4.3 FEET PER DAY AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 43.2 FEET PER DAY 1U 20 MILES 20 KILOMETERS Figure 28. Hydraulic conductivity of the middle Yorktown-Eastover aquifer based on model calibration. 52 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75° 15' 38" 45' 30' AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 37" 15' 1.3 FEET PER DAY AREA WHERE HYDRAULIC CONDUCTIVITY EQUALS 8.6 FEET PER DAY 20 MILES 20 KILOMETERS Figure 29. Hydraulic conductivity of the lower Yorktown-Eastover aquifer based on model calibration. Analysis of the Ground-Water-Flow System 53 Table 12. Range of transmissivities estimated from Table 13. Minimum and maximum values of model specific-capacity data and from model calibration storage coefficient [ft2/d, foot squared per day] [values, dimensionless, are not intended to imply accuracy to precision shown] Estimated transmissivity (ft*/d) Storage coefficient Aquifer Specific-capacity data Modd calibration Aquifer Minimum Maximum L52X10" 4 Upper Yorktown-Eastover 61-4.530 Upper Yoiktown-Eastover 1.01 X 10" 5 93-4.611 8J1X10" 1 Middle Yorktown-Easlover 206-3,240 26-3.588 Middle Yorktown-Eastover 1.06 X 10" 5 Lower Yorktown-Eastover 95-2,094 86-1,210 Lower Yoiktown-Eastover 1.07X10' 5 1.83X10"* simulation is an approximation of the natural Table 14. Minimum and maximum values of model ground-water system prior to any major stresses, vertical leakance and it provides a starting point for transient simula tions that examine the effects of increased ground- Vertical leakance (days"1) water withdrawals. Aquifer Maximum Simulated prepumping water levels for the 4.32 X10~ 7 Upper Yorktown-Eastover 4.52X10 ' Yorktown-Eastover aquifers are shown in figures Middle Yoiktown-Eastover 8.55 X 10" 7 5.18 X 10" * L24XlO~' 30-32. There are no reliable measurements of water Lower Yorktown-Eastover 3.95 X 10" * levels for the confined aquifers on the Eastern Shore prior to 1940; however, the conceptualization of prepumping ground-water flow for the Eastern Shore The simulated position of the tip and toe of assumes that water levels were a subdued replica of the saltwater-freshwater interface for each of the the land surface and that flow gradients were from Yorktown-Eastover aquifers for prepumping condi topographic highs in the center of the peninsula to tions is shown in figures 33-35. The position of the the Chesapeake Bay on the west and Atlantic Ocean interface is a function of the freshwater-flow and on the east. Simulated water levels were compared saltwater-flow regimes. The interface generally is with the prepumping water levels from a previous farthest offshore in the upper Yorktown-Eastover simulation of ground-water flow on the Eastern aquifer and is progressively farther inland in the Shore (Bal, 1977). The simulated water levels and middle and lower Yorktown-Eastover aquifers, flow directions are consistent with Bal's study and where the depths to the aquifers increase and the are in agreement with the conceptualization of freshwater heads decrease. The simulated position of ground-water flow during prepumping conditions. the saltwater-freshwater interface in the steady-state The calibration of the steady-state model was simulation is an equilibrium position; the actual evaluated by analyzing estimates of rate of recharge. position of the prepumping saltwater-freshwater Simulated recharge to the confined system on the interface is not known. Several studies indicate that Eastern Shore was compared with results from a in some coastal areas the saltwater-freshwater inter previous ground-water-modeling study in southeast face is still responding to long-term Pleistocene sea- ern Virginia (Hamilton and Larson, 1988). The level fluctuations and has not achieved equilibrium southeastern Virginia model of prepumping condi with the present-day sea level (Essaid, 1990b; tions estimated an average recharge rate to the Meisler and others, 1985). It is assumed for the pur confined-aquifer system of approximately 0.4 in/yr. poses of this study that the prepumping saltwater- The Eastern Shore ground-water-flow model for pre- freshwater interface is in equilibrium with the stressed conditions indicates a similar but slightly present-day sea level. This approach provides a con higher average recharge rate of approximately 0.6 servative estimate of saltwater-freshwater interface in/yr. The recharge rate estimated in the steady-state movement; a transitional interface would be moving calibration for the Eastern Shore model is consistent landward because sea levels have been rising since with a previous analysis of a similar system in the the late Wisconsin glacial maximum (Meisler and Coastal Plain of Virginia. others, 1985). The simulated prepumping position of 54 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' 38 C 45' 30' EXPLANATION SIMULATED WATER-LEVEL CONTOUR--Shows 37° 15' altitude of simulated water level. Interval 5 feet Datum is sea level 10 20 MILES MM- ' ' 20 KILOMETERS Figure 30. Simulated water levels in the upper Yorktown-Eastover aquifer for prepumping conditions. Analysis of the Ground-Water-Flow System 55 76° 45' 30' 75° 15' 38° 45' 30' EXPLANATION SIMULATED WATER-LEVEL CONTOUR-Shows 37° 15' altitude of simulated water level. Interval, in feet, is variable. Datum is sea level 20 MILES :0 KILOMETERS Figure 31. Simulated water levels in the middle Yorktown-Eastover aquifer for prepumping conditions. 56 Hydrogeology and Analysis of the Ground-Water-Flew System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' 38° 45' 30' EXPLANATION SIMULATED WATER-LEVEL CONTOUR--Shows 37° 15' altitude of simulated water level. Interval, in feet, is variable. Datum is sea level 10 20 MILES 20 KILOMETERS Figure 32. Simulated water levels in the lower Yorktown-Eastover aquifer for prepumping conditions. Analysis of the Ground-Water-Flow System 57 76° 45' 30' 75° 15' 38 C 45' 30' SIMULATED STEADY-STATE PREPUMPING INTERFACE TOE 37° 15' SIMULATED STEADY-STATE PREPUMPING INTERFACE TIP 20 MILES 20 KILOMETERS Figure 33. Simulated position of the saltwater-freshwater interface for the upper Yorktown-Eastover aquifer for prepump- ing conditions. 58 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76< 45' 30' 75° 15' 38 C 45' 30' EXPLANATION SIMULATED STEADY-STATE PREPUMPING INTERFACE TOE 37° 15' SIMULATED STEADY-STATE PREPUMPING INTERFACE TIP 20 MILES I 20 KILOMETERS Figure 34. Simulated position of the saltwater-freshwater interface for the middle Yorktown-Eastover aquifer for prepump- ing conditions. Analysis of the Ground-Water-Flow System 59 76' 45' 30' 75° 15' 45' 30' EXPLANATION SIMULATED STEADY-STATE PREPUMPING INTERFACE TOE 37" 15' SIMULATED STEADY-STATE PREPUMPING INTERFACE TIP 10 20 MILES i i i I 20 KILOMETERS Figure 35. Simulated position of the saltwater-freshwater interface for the lower Yorktown-Eastover aquifer for prepump- ing conditions. 60 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia the saltwater-freshwater interface is a useful refer tive of the total group of observation wells. Simu ence for examining the relative effects of withdraw lated water levels show reasonable agreement with als on the ground-water-flow system. measured water levels in all of the observation wells for the period of record. Some simulated water lev els are slightly higher than measured water levels Transient-Model Simulation of Pumping and some are slightly lower. Conditions Simulated water levels for 1988 are shown in A transient-model simulation was conducted to figures 40-42. Measured water levels are included simulate the effects of ground-water withdrawals on on these maps to allow comparison between simu the Eastern Shore from 1940 to 1988. The water lated and measured values. A comparison of simu levels and interface position generated in the pre- lated 1988 water levels with prepumping water lev pumping steady-state simulation were used as initial els (figs. 30-32) indicates a decline in water levels conditions in the transient-model analysis. Aquifer around the major pumping centers. The maximum and confining-unit hydraulic characteristics were simulated water-level declines in all three aquifers equivalent to those used when simulating prepump- occur near the town of Accomac. Maximum water- ing conditions. The transient simulation shows the level declines are 18, 30, and 53 ft in the upper, effects of historic withdrawals on the ground-water- middle, and lower Yorktown-Eastover aquifers, flow system. respectively. Drawdown cones associated with the major pumping centers indicate a change in ground- Time Discretization and Ground-Water water flow from prepumping conditions. Prior to Withdrawals ground-water withdrawals, flow was from the topo graphic highs in the center of the peninsula toward Pumpage has varied during the history of the Chesapeake Bay and Atlantic Ocean (figs. ground-water withdrawal on the Eastern Shore 30-32). By 1988, simulated water-level gradients (fig. 36). The transient changes in withdrawals are show that water is diverted toward the major pump accounted for in the model by dividing historical ing centers (figs. 40-42). Top-of-aquifer maps can pumpage into 12 pumping periods. Model-simulated be compared with the simulated water levels to pumping periods are the years 1940^4, 1945-46, identify areas in which the water levels are 1947-55, 1956-64, 1965-67, 1968-72, 1973-77, approaching the top of the aquifer. Water levels that 1978-79, 1980-81, 1982-84, 1985-86, and decline below the top of a confined aquifer cause 1987-88. Each pumping period starts on January 1 unconfined conditions in the aquifer and can result of its beginning year and ends on December 31 of in dewatering and associated irreversible changes in its final year. Simulated withdrawals were calculated the aquifer. Dewatering can contribute to compac by aquifer for each pumping period from annual tion of aquifer sediment and eventual decreases in withdrawal data (fig. 18) using an average for the aquifer yields. Simulated 1988 water levels are time period (fig. 36, table 15). Aquifer-top maps above the tops of the aquifers throughout the model (figs. 3-9) and well-screen depth information were area. used to assign the withdrawals to the appropriate The amount of ground-water flow through the aquifer. Withdrawals for multiaquifer wells were system also is changed as a result of withdrawals. determined by the percentage of the total screen The majority of the water for the increase in with present in each aquifer. drawals comes from an increase in the amount of water recharging the confined-aquifer system from Results of Simulation the unconfined aquifer and a decrease in the amount The transient simulation was evaluated by of discharge to the unconfined aquifer. In areas comparing simulated water levels to measured water where pumpage causes water levels to decline in the levels. This comparison was made for a network of confined aquifers, the head difference between the 48 observation wells distributed throughout the unconfined-aquifer and the confined-aquifer system model area. Water levels for 12 of the observation increases. The increased head difference causes an wells are presented in figures 37-39. The observa increase in vertical leakage through the confining tion wells selected are distributed throughout the unit, and some freshwater that was previously model area, and water-level changes are representa discharging from the unconfined aquifer to Analysis of the Ground-Water-Flow System 61 AVERAGE WITHDRAWAL SIMULATED FOR PUMPING PERIOD DC LU Q. CO Z o _l _l o z o ESTIMATED ANNUAL WITHDRAWAL DC n:Q 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 Figure 36. Estimated annual withdrawal and average withdrawal for simulated pumping periods. surface-water bodies is diverted and flows into the saltwater-freshwater interface, although significant confined-aquifer system. Any increase in withdraw changes in the flow system occur. The simulated als from the freshwater aquifers on the Eastern position of the saltwater-freshwater interface for Shore results in a reduction in offshore freshwater 1988 conditions is identical to the simulated inter discharge. A reduction in freshwater discharge face position for prepumping conditions (figs. affects the long-term position of the saltwater- 33-35). This result indicates that interface response freshwater interface in the aquifers and could cause is slow and takes place over long periods of time. increased salinity in sensitive estuarine environ Similar findings have been recorded in other ments. The steady-state prepumping simulation indi saltwater-intrusion studies (Bond and Bredehoeft, cates that 11.07 Mgal/d recharges and discharges the 1987; Essaid, 1990b; Meisler and others, 1985). confined ground-water-flow system (table 16). The investigation by Bond and Bredehoeft (1987) When 1988 withdrawals are simulated, the recharge using a two-dimensional solute-transport model to the confined aquifers increases to 13.11 Mgal/d, showed the main pathway for saltwater intrusion and natural discharge decreases to 8.64 Mgal/d. over short timeframes was downward vertical leak The transient simulation of conditions for the age of saltwater from surface-water bodies into the period 1940-88 shows no movement of the shallow aquifers. Simulated water-level gradients for 62 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia a i § S i i s 00 S J2 en ZH OO oo c< §p o\ " M " «o o ci 1 i S 1 oo o ON oo i i 1 00 oo s 00 S\ 1 § oo Ji en '"-en ^ en 1rt 5s f? o\ 00 VO oo 00 oo o\ "1 VO ^P " ~ 3~ j8 M 5? 1 -S 1 | ,? ^^ 1 1 1 °3'S s, ^2 oo s i s 2 2 o ^ i_ a y> 2 W">tn j o o ^H ^* !Q of ^ ^ 00 g 8 ^* ^^oK o ^^ -°>* "ON6 | 2 * i O) «J "i 1 ? o C B '5. « E S Q. | s 0 >^ s § -1 6 i i 6 o 8, W B «5 ^| 2 -a > { s o o « i |1 1 1 1 in .B UpperYoiktow MiddleYoikto YotktovLower _« S § 1 1 I Analysis of the Ground-Water-Flow System 63 WELL 67M14 10 i M I I I I l-14-L-LLi.LLLLLL1 ' ' ' ' ' ' ' ' 0 -10 I 20 i 40 uj WELL 64H7 > f o> < 6 HI C/3 3 -I 9 HI 0 20 KILOMETERS UJ 6 (0 _l o DC 3 S HI I! I WELL 66K3 3 EXPLANATION 5' 12 I I I I I I H-LJ.J _L I I I MEASURED WATER LEVEL \ SIMULATED WATER LEVEL 67M14 WELL AND IDENTIFIER 1940 1950 1960 1970 1980 1990 Figure 37. Simulated and measured water levels at selected observation wells in the upper Yorktown-Eastover aquifer. LU LU > > LU LU QC I I LU __ DC DC Z LU LU LU i i O Q Q LU LU tt t 3 < (0 d _J < i LU LU 5 2 53 X LU I 2 cr> CO Lit c I I i i _ - '' -L± - ~ - ' -^^ ~ _ -'^ - - - i s^ ~ :VV ~ CO - - F2:^ \ -Qi : (i : /> O \ i - -r/ , _ : i ^ - 1 i -i ii _i:i - i _- _i ~" - i -i CD - i - -i i - I - i -r-H r / - -f - _ i 1 - i I 1 ~~ - i - T ~ - ii - I _ | 4 - co - i f ~ - ii O) T CO L CM o i _ O _J co i CO co co co : co - co E co LU I LU : LU LU - i i i i T T T O) 13A3T V3S MO138 dO 3AC9V 133d Nl '13A31 U31VM Analysis of the Ground-Water-Flow System 65 WELL 66M18 6 3 0 ^v -3 -6 LJJ > LJJ WELL 63H4 8 £. I I I I M I M_[_M I I I I I I I I I M I M I I I | I M I I I I I I I I I I I I I I i r < ss.CO* LJJ C/3 7 o 6 (D O o 5 3 LJJ CO a CC 4 O 3 i LJJ O CO WELL 65K23 12 LJJ 8 LJJ 4 0 20 KILOMETERS LJJ -4 > LJJ V) -J -8 I I I I o CC LU s WELL 63G24 1111111 Hinj.LLLL1 ' I'' '' ''''' I'' '''''' ' I '''' '''' ' EXPLANATION 4 MEASURED WATER LEVEL SIMULATED WATER LEVEL 0 66M18 -4 WELL AND IDENTIFIER -8 -12 1940 1950 1960 1970 1980 1990 Figure 39. Simulated and measured water levels at selected observation wells in the lower Yorktown-Eastover aquifer. 76° 45' 30' 75° 15' 38° 45' 30' EXPLANATION SIMULATED WATER-LEVEL CONTOUR-Shows altitude of simulated water level. Hachures indicate depression. 37° 15' Interval, in feet, is variable. Datum is sea level WELL--Number is measured altitude of water level, in feet above sea level :o MILE: ?0 KILOMETERS Figure 40. Simulated and measured water levels in the upper Yorktown-Eastover aquifer, 1988. Analysis of the Ground-Water-Flow System 67 76 C 45' 30' 75° 15' 38 C 45' 30' EXPLANATION SIMULATED WATER-LEVEL CONTOUR-Shows altitude of simulated water level. Interval, in feet, is 37° 15' variable. Datum is sea level WELL Number is measured altitude of water level, in feet above or below sea level 20 MILES 20 KILOMETERS Figure 41. Simulated and measured water levels in the middle Yorktown-Eastover aquifer, 1988. 68 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45 45' - EXPLANATION SIMULATED WATER-LEVEL CONTOUR--Show altitude of simulated water level. Interval, in feet, is 37° 15' - variable. Datum is sea level WELL Number is measured altitude of water level, in feet above or below sea level 20 KILOMETERS Figure 42. Simulated and measured water levels in the lower Yorktown-Eastover aquifer, 1988. Analysis of the Ground-Water-Flow System 69 Table 16. Simulated ground-water budgets for the confined freshwater-flow system [Modeled values, in million gallons per day, are not intended to imply accuracy to precision shown; the small error between sources and discharges is due to numerical truncation in digital simulation; , budget component not applicable to the simulation] Simulation >logyAnalysisofandtl Budget 1988 Southern Southern Northeastern Northeastern Permitted component Prepumping conditions Northampton Northampton Accomack Accomack withdrawal simulation 1 simulation 2 simulation 1 simulation 2 Source heGround-Wa Recharge to 11.07 13.11 14.93 14.63 14.18 12.84 17.44 confined system through uppermost confining unit 5 Water released from .00 .64 .87 .96 1.09 1.09 1.92 Tj aquifer storage Lateral boundary 1.47 f constant-head nodes a Total 11.07 13.75 15.80 15.59 15.27 15.40 19.36 (D s Discharge Natural discharge 1 1 .08 8.64 7.52 7.29 8.11 8.27 5.65 from confined system O through uppermost 9 confining unit c3 Ground-water .00 5.05 8.24 8.26 7.10 7.10 13.70 5*3 withdrawals from wells Water taken into .00 .00 .00 .00 .00 .00 .00 aquifer storage "Lateral boundary .00 constant-head nodes Total 11.08 13.69 15.76 15.55 15.21 15.37 19.35 1988 conditions on the Eastern Shore show that Eastern Shore. Three scenarios of hypothetical there is no potential for downward vertical leakage increases in ground-water withdrawals were devel of saltwater through the upper Yorktown-Eastover oped in cooperation with Accomack County, confining unit above the freshwater part of the upper Northampton County, and the VWCB. The results Yorktown-Eastover aquifer. of the simulations of the scenarios are examined to The slow movement of the saltwater- increase our understanding of the response of the freshwater interface was investigated using a tran ground-water-flow system to additional stress. The sient simulation that continued 1988 withdrawals for simulations are not intended to predict exact ground- 1,000 years. The model-simulated interface did not water conditions in the future; however, the results reach an equilibrium position for 1988 withdrawals provide information that could be useful in evaluat by the end of the 1,000-year simulation period. The ing the ground-water resource and its ability to meet simulated position of the saltwater-freshwater inter future water needs. face toe for the 1,000-year run is shown along with the 1988 interface in figures 43^5. The position of Southern Northampton County Scenario the interface toe is shown because it is the most landward extension of the saltwater-freshwater inter The southern part of Northampton County is face. The locations of greatest interface movement experiencing rapid growth. Protection of the ground- in each aquifer correspond to the areas of greatest water resource in this area is of concern because pumpage. Although the transient simulation from most of the expected development is in coastal areas 1940 to 1988 shows no movement of the interface that could be susceptible to saltwater intrusion. In toe from the prepumping steady-state simulation, this scenario, withdrawals are increased in the continuing 1988 withdrawals for 1,000 years causes southern part of Northampton County, and currently landward movement of the interface toe along most permitted users as well as potential projected users of the coast in all three aquifers. The interface toe at are included. The scenario consists of two separate the southern end of Northampton County in the model simulations that illustrate the effects of upper Yorktown-Eastover aquifer (fig. 43) moved increased withdrawals with two different well distri approximately 1 mi landward on the bay side of the butions. In simulation 1, withdrawals are increased peninsula and 0.5 mi landward on the ocean side of to permitted levels at existing well locations, and the peninsula. The interface toe also moved land additional withdrawal wells are placed in the vicin ward approximately 1 mi in the upper Yorktown- ity of expected growth areas (fig. 46). In simulation Eastover aquifer near the town of Chincoteague. 2, withdrawals are increased by the same amount Maximum landward movement of the interface toe but are distributed evenly throughout hypothetical is approximately 1.5 mi in the middle Yorktown- well fields in the center of the peninsula. Eastover aquifer (fig. 44), also near the town of Chincoteague. The interface toe moves landward a Simulation 1 maximum of approximately 1 mi in the lower Results from the transient simulation of 1988 Yorktown-Eastover aquifer (fig. 45) southwest of conditions were used as initial conditions for a Chincoteague. The results of this simulation support 50-year transient simulation to examine the effects previous findings that movement of the saltwater- of increased withdrawals in the southern part of freshwater interface is slow and takes place over Northampton County. A summary of locations and long periods of time. It is important to remember, rates of hypothetical withdrawals and aquifers pene however, that a sharp-interface model provides no trated for the southern part of Northampton County information on the rate of movement of dilute salt in simulation 1 is presented in table 17. Total with water in the transition zone. drawal for the area is 3.761 Mgal/d, which repre sents an increase of 3.213 Mgal/d over 1988 with Application of Ground-Water-Flow Model drawal. Withdrawals from existing wells in the southern part of the peninsula are increased to their The prepumping, steady-state-model analysis permitted levels. Additional withdrawal wells were and the historical transient-model analysis indicate located by the VWCB according to preliminary or that the model conceptualization is a reasonable rep expected permit applications (fig. 46). Approxi resentation of the ground-water-flow system of the mately 57, 29, and 14 percent of the additional Analysis of the Ground-Water-Flow System 71 76 C 45' 30' 75° 15' 38 C 45' 30' SIMULATED 1988 INTERFACE TOE 37° 15' SIMULATED INTERFACE TOE AFTER SIMULATION OF 1988 \VTTIIDRAWALS 10 20 KILOMETERS Figure 43. Simulated position of the saltwater-freshwater interface toe for a 1,000-year transient run using 1988 with drawals in the upper Yorktown-Eastover aquifer. 72 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75 C 15' 38 C 45' 30' EXPLANATION SIMULATED 1988 INTERFACE TOE 37° 15' SIMULATED INTERFACE TOE AFTER 1,000-YEAR SIMULATION OF 1988 WITHDRAWALS 10 20 MILES i i I 20 KILOMETERS Figure 44. Simulated position of the saltwater-freshwater interface toe for a 1,000-year transient run using 1988 with drawals in the middle Yorktown-Eastover aquifer. Analysis of the Ground-Water-Flow System 73 76° 45' 30' 75° 15' 38° 45' 30' EXPLANATION SIMULATED 1988 INTERFACE TOE 37° 15' SIMULATED INTERFACE TOE AFTER 1,000-YEAR SIMULATION OF 1988 WITHDRAWALS 0 10 20 MILES 20 KILOMETERS Figure 45. Simulated position of the saltwater-freshwater interface toe for a 1,000-year transient run using 1988 with drawals in the lower Yorktown-Eastover aquifer. 74 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 10 MILES 10 KILOMETERS Figure 46. Location of hypothetical withdrawals for the southern Northampton County scenario, simulation 1. Table 17. Withdrawals for southern Northampton County scenario, simulation 1 [Mgal/d, million gallons per day; latitude and longitude are reported in degrees, arc minutes, arc seconds] Map Withdrawal number1 Latitude Longitude (Mgal/d) Aquifer penetrated 1 37 13 36 0760019 0.047 Upper Yorktown-Eastover 2 37 13 14 0760021 .047 Upper Yorktown-Eastover 3 37 13 53 0755923 .093 Upper Yorktown-Eastover 4 37 13 54 075 59 08 .093 Upper Yorktown-Eastover 5 371435 0760034 .200 Upper Yorktown-Eastover 6 371456 0760052 .200 Upper Yorktown-Eastover 7 371455 0760008 .200 Upper Yorktown-Eastover 8 371520 0760031 .200 Upper Yorktown-Eastover 9 37 15 28 0755956 .200 Upper Yorktown-Eastover 10 371720 0760051 .004 Upper Yorktown-Eastover 11 37 17 20 075 58 10 .190 Upper Yorktown-Eastover 12 371746 0755728 1.600 Upper, middle, and lower Yorktown-Eastover 13 37 17 15 075 55 12 .150 Middle Yorktown-Eastover 14 371711 0755524 .152 Middle Yorktown-Eastover 15 371540 0760121 .125 Upper and middle Yorktown-Eastover 16 371605 0760019 .260 Upper and middle Yorktown-Eastover 'Locations shown on figure 46. pumpage comes from the upper, middle, and lower water budget presented in table 16 is for the con Yorktown-Eastover aquifers, respectively. Pumpage fined freshwater-flow system; the withdrawal for the rest of the model area is held constant at the amounts are slightly less than the total ground-water average pumping rate for the final pumping period withdrawals for the simulations (table 18) because a (pumping period 12) in the historic transient simula small part of the withdrawals are from the saltwater- tion. Total withdrawals for the entire model area for flow system. The increase in freshwater withdrawal simulation 1 are greatest in the upper Yorktown- of 3.19 Mgal/d in the southern part of Northampton Eastover aquifer and least in the lower Yorktown- County causes a 1.82 Mgal/d increase in the amount Eastover aquifer (table 18). of recharge to the confined aquifer system over The hypothetical increased pumpage in the 1988 conditions. The amount of natural discharge southern part of Northampton County results in from the confined aquifers is reduced by 1.12 water-level declines of greater than 15 ft in each of Mgal/d from 1988 conditions. the confined freshwater aquifers (figs. 47^9). The The 50-year simulation of increased pumpage maximum water-level decline for the upper in southern Northampton County results in slight Yorktown-Eastover aquifer is 16.2 ft near the town landward movement of the simulated saltwater- of Cape Charles. Maximum water-level declines of freshwater interface on the Chesapeake Bay side of 38.8 and 48.7 ft occur east of the town of Cheriton the peninsula off Cape Charles (figs. 47^9). The for the middle and lower Yorktown-Eastover aqui interface toe in the upper Yorktown-Eastover aquifer fers, respectively (table 19). The predicted declines moves inland from the 1988 interface toe position are in addition to declines caused by ground-water along approximately 12 mi of the western coastline withdrawals in 1988. Simulated water levels in southern Northampton County (fig. 47); maxi throughout the model area remain above the tops of mum landward movement is approximately 1 mi. the aquifers, indicating that dewatering would not The simulated position of the interface toe in the occur at this rate and distribution of withdrawal. middle Yorktown-Eastover aquifer does not change The majority of the water for the increased in response to the hypothetical increase in withdraw ground-water withdrawal is provided by an increase als. Slight landward movement of the saltwater- in the amount of recharge entering the confined sys freshwater interface toe occurs in the lower tem and a decrease in the amount of discharge leav Yorktown-Eastover aquifer (fig. 49). The simulated ing the confined system (table 16). The simulated 1988 interface toe position is onshore at this loca- 76 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 18. Withdrawal by aquifer for model scenarios [Values in millions of gallons per day] Scenario Pumping Aquifer period 12 Southern Southern Northeastern Permitted (1987-88) Northampton Northampton Accomack withdrawal simulation 1 simulation 2 simulations 1&2 Upper Yorktown-Eastover 1.888 3.801 2.951 2J31 4.446 Middle Yorktown-Eastover 2.103 2.915 3.088 3.431 6.959 Lower Yorktown-Eastover 1.070 1.558 2.235 1.201 2.419 Total 5.061 8.274 8.274 7.163 13.824 Table 19. Maximum water-level decline from 1988 flow conditions for model scenarios Aquifer Decline Grid Grid Approximate (feet) row column area! location Southern Northampton County simulation 1 Upper Yorktown-Eastover 16.2 85 24 Town of Cape Charles Middle Yorktown-Eastover 38.8 80 29 East of Cheriton Lower Yorktown-Eastover 48.7 80 26 Town of Cheriton Southern Northampton County simulation 2 Upper Yorktown-Eastover 8.0 68 26 Town of Nassawadox Middle Yorktown-Eastover 22.0 76 26 To\vnofEastviHe Lower Yorktown-Eastover 22.4 76 26 Town of Eastville Northeastern Accomack County simulation 1 Upper Yorktown-Eastover 17.2 19 34 TownofHallwood Middle Yorktown-Eastover 29.7 19 34 Town of Hall wood Lower Yorktown-Eastover 26.4 19 34 TownofHallwood Northeastern Accomack Countv simulation 2 Upper Yorktown-Eastover 15.8 19 34 TownofHallwood Middle Yorktown-Eastover 27.9 19 34 TownofHallwood Lower Yorktown-Eastover 24.6 19 34 Town of Hall wood Permitted withdrawal Upper Yorktown-Eastover 29.1 56 28 Town of Exmore Middle Yorktown-Eastover 95.3 56 28 Town of Exmore Lower Yorktown-Eastover 68.0 34 32 Town of Accomac tion on the southern tip of the peninsula. The hypo cause drawdowns of greater than 5 ft to extend off thetical increase in withdrawals causes the western shore. The area of reversed flow indicates a poten boundary of the 1988 interface position to move tial for downward vertical leakage of saltwater from approximately 0.5 mi in the lower Yorktown- the Chesapeake Bay and nearshore estuaries into the Eastover aquifer. freshwater part of the upper Yorktown-Eastover Although saltwater intrusion due to horizontal aquifer. The amount of saltwater that is introduced movement of the saltwater-freshwater interface takes into the freshwater system vertically through the place over long periods of time, saltwater intrusion confining unit is probably relatively small; however, due to induced downward vertical leakage can occur salt concentrations could be high and could signifi rapidly as a result of large changes in head gradient. cantly affect the quality of the water withdrawn. Simulated water levels show offshore water-level This area is further complicated because of the declines that cause a reversal of ground-water flow present-day channel in the Chesapeake Bay (figs. 3 from 1988 conditions (fig. 47). Simulated water- and 11). The upper Yorktown-Eastover confining level declines in the upper Yorktown-Eastover aqui unit probably has been eroded, and rates of saltwa fer show that the increased withdrawals on the coast ter intrusion could be increased because of a direct Analysis of the Ground-Water-Flow System 77 76 C 45' 30' 75° 15' 38" 45' 30' EXPLANATION AREA OF REVERSED SALTWATER FLOW 5 LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Literal, 37° 15' in feet, is variable SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM SOUTHERN NORTHAMPTON, SIMULATION 1 WITHDRAWALS 10 20 MILES ' ' ' 10 20 KILOMETERS Figure 47. Water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 1. 78 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75° 15' 38 C 45' 30' EXPLANATION LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS--Interval, in feet, is variable SIMULATED 1988 INTERFACE TOE 37° 15' 10 20 MILES MM- ' ' I 10 20 KILOMETERS Figure 48. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the middle Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 1. Analysis of the Ground-Water-Flow System 79 76' 45' 30' 75° 15' 38" 45' 30' EXPLANATION LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval 5 feet 37° 15' SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM SOUTHERN NORTHAMPTON, SIMULATION 1 WITHDRAWALS 10 20 MILES I I Figure 49. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the lower Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 1. 80 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia connection between the upper Yorktown-Eastover Table 20. Location of southern Northampton scenario aquifer and the saltwater in the Chesapeake Bay. withdrawals, simulation 2 [Mgal/d, million gallons per day] The model results indicate that the hypothetical increase in pumpage in southern Northampton Grid location Withdrawal County at current well locations could create water- Aquifer ______(Mgal/d) quality problems. Heavy pumpage along the coast in Row Column the upper Yorktown-Eastover aquifer could cause water levels to decline offshore and induce leakage 64 26 0376 Upper, middle, and lower Yorktown-Eastover of saltwater from the Chesapeake Bay into the fresh 66 26 376 Upper, middle, and lower Yorktown-Eastover 68 26 376 Upper, middle, and lower Yorktown-Eastover water part of the aquifer. The amount of water- 70 26 376 Upper, middle, and lower Yorktown-Eastover quality degradation that could result from this proc 72 26 376 Upper, middle, and lower Yorktown-Eastover ess cannot be quantified by this study. Model results 74 26 376 Upper, middle, and lower Yorktown-Eastover 76 26 376 Upper, middle, and lower Yorktown-Eastover indicate that this is a potential concern and needs to 78 26 376 Upper, middle, and lower Yorktown-Eastover be considered in future studies. 80 26 376 Upper, middle, and lower Yorktown-Eastover 82 26 376 Upper, middle, and lower Yorktown-Eastover Simulation 2 In the second simulation, withdrawals are throughout the model area remain above the top of increased in southern Northampton County by the the aquifers. same amount as simulation 1; however, the with The simulated ground-water budgets for the drawals are removed from the coast and redistrib freshwater-flow system indicate recharge to the con uted throughout Northampton County to hypothetical fined system increases by 1.52 Mgal/d over 1988 well fields in the center of the peninsula. The total conditions, whereas natural discharge from the con withdrawal for the area is divided equally among 10 fined aquifers decreases by 1.35 Mgal/d from the hypothetical well fields and is distributed equally simulated 1988 discharge rate (table 16). The among all three confined aquifers (table 20). Pump- change in flow through the system is a result of the age for the rest of the model area is held constant at 3.21 Mgal/d increase in freshwater withdrawals over the average pumping rate for the final pumping period in the historic transient simulation (1987-88). 1988 rates. Although the total ground-water with As in the previous simulation, the results of the drawal for simulation 1 is identical to simulation 2, transient simulation of 1988 conditions are used as the freshwater withdrawal is slightly less. The with initial conditions, and the simulation is continued drawal locations in simulation 1 are near the coast for a period of 50 years. Withdrawals by aquifer are and result in more withdrawal from the saltwater- presented in table 18 for all of the model scenarios. flow system. A comparison of recharge and dis Modeled water-level declines from simulated charge for the two southern Northampton simula 1988 water levels are shown in figures 50-52. tions shows that the same withdrawal amount and a Declines are centered in the middle of the peninsula, different areal distribution can produce a change in and the maximum water-level declines are 8.0, the flux through the system. Slightly less water 22.0, and 22A ft for the upper, middle, and lower (0.30 Mgal/d) enters the confined system in simula Yorktown-Eastover aquifers, respectively (table 19). tion 2, and slightly less water (0.23 Mgal/d) is Since pumpage is no longer concentrated in the discharged naturally from the confined system. The Cape Charles area, water-level declines are smaller decrease in flow through the system in simulation 2 in each aquifer for simulation 2 than they are in is balanced by a small increase in the amount of simulation 1. Water-level declines are greatest in the water that is released from aquifer storage lower Yorktown-Eastover aquifer because transmis- (table 16). sivities are smaller there than in the middle or upper Slight landward movement of the simulated Yorktown-Eastover aquifers. The placement of the saltwater-freshwater interface from the 1988 position wells in the center of the peninsula causes the occurs during the 50-year transient simulation in the water-level contours to follow the shape of the upper Yorktown-Eastover aquifer (fig. 50). The peninsula, and less drawdown occurs in offshore interface position for the middle and lower areas. As in simulation 1, the simulated water levels Yorktown-Eastover aquifers does not change from Analysis of the Ground-Water-Flow System 81 38° - 45' - EXPLANATION LLNE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SLMULATED 1988 WATER LEVELS-Interval, 37" 15' - in feet, is variable SLMULATED 1988 INTERFACE TOE SLMULATED INTERFACE TOE RESULTING FROM SOUTHERN NORTHAMPTON, SIMULATION 2 WITHDRAWALS 20 KILOMETERS Figure 50. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe in the upper Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 2. 82 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75° 15' 38" 45' 30' 37" 15' LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable SIMULATED 198 8 INTERFACE TOE 1C) 20 MILES i I Figure 51. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe in the middle Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 2. Analysis of the Ground-Water-Flow System 83 76 C 45' 30' 75° 15' 38 C 45' 30' 37° 15' LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable SIMULATED 1988 INTERFACE TOE 10 20 MILES j_I i 20 KILOMETERS I______I Figure 52. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe in the lower Yorktown-Eastover aquifer, southern Northampton County scenario, simulation 2. 84 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia the 1988 position. The simulated interface toe 2, a constant-head boundary (an unlimited supply of moves landward a maximum of approximately 0.5 water across the boundary) is used to represent a mi in the upper Yorktown-Eastover aquifer. less severe case, or minimum water-level decline. Although the position of the interface does not move The response of the actual ground-water system as much as it did in simulation 1, this model result would most likely fall somewhere between the two illustrates the sensitivity of the fresh-ground-water cases. system in southern Northampton County. The addi The initial conditions for both 50-year tran tional withdrawal of 3.21 Mgal/d results in some sient simulations of increased withdrawals in north movement of the saltwater-freshwater interface in eastern Accomack County are provided by the the uppermost confined aquifer, even with well results of the transient simulation of 1988 condi fields placed as far away from the interface as possi tions. Withdrawal locations are near the shore of the ble. The peninsula in this area is narrow (4-5 mi peninsula (fig. 53); the total hypothetical withdrawal wide), which limits its ability to sustain large for the area is 3.5 Mgal/d (table 21), an increase of ground-water withdrawals. 2.05 Mgal/d over 1988 withdrawals. Approximately Simulated water levels show that saltwater 31, 63, and 6 percent of the total withdrawals come intrusion into the uppermost confined aquifer from the upper, middle, and lower Yorktown- through downward vertical leakage does not occur Eastover aquifers, respectively (table 18). Pumpage when withdrawals are distributed equally to all three for the rest of the model area was held constant at aquifers and placed in the center of the peninsula. the average pumping rate for the final pumping Freshwater is flowing from the upper Yorktown- period in the historic transient simulation (1987-88). Eastover aquifer; water-level declines offshore are The withdrawals for these scenarios are concentrated not large enough to reverse the hydraulic gradient in the upper two aquifers because the area is too far and induce saltwater leakage through the confining east to obtain good-quality water from the lower unit. Yorktown-Eastover aquifer. Many of the withdraw als in this area come from the unconfined aquifer, Northeastern Accomack County Scenario which is not included in this model; therefore, the Chincoteague Island is a popular tourist loca pumpage for simulations 1 and 2 is lower than the tion in northeastern Accomack County that requires total projected increase for the northeastern part of a large supply of freshwater in the summer months. Accomack County. This area is on the easternmost boundary of the Eastern Shore's freshwater-flow system and has a Simulation 1: No-Flow Boundary high potential for water-quality degradation by salt The results of simulation 1 show that modeled water intrusion. The northeastern part of Accomack water levels decline from simulated 1988 water lev County has several major ground-water users, and in els throughout much of the northern model area this scenario, the response of the ground-water-flow (figs. 54-56). The maximum water-level declines system to increased withdrawals is examined. are 17.2, 29.7, and 26.4 ft for the upper, middle, Two simulations are included in this scenario and lower Yorktown-Eastover aquifers, respectively in order to evaluate model-boundary effects. The scenario consists of large increases in withdrawals (table 19). The location of the maximum water-level over calibrated 1988 conditions in the northeastern decline is near the town of Hallwood for all three corner of the model grid. At this level of with aquifers. Simulated water levels are above the tops drawal, the effects of the increased stress extend to of the aquifers, indicating that the dewatering of the northern and eastern boundary of the model; the confined aquifers is not a concern for this therefore, the results of the simulation are affected simulation. by the model-boundary conditions. The boundary The simulated ground-water budget for the effects were quantified by simulating two different freshwater-flow system is presented in table 16. The types of boundary conditions. In simulation 1, a increase in freshwater withdrawals of 2.05 Mgal/d no-flow boundary (no water available across the over 1988 amounts results in a 1.07 Mgal/d increase boundary) is used to represent the most severe case, in flow into the confined system and a 0.53 Mgal/d namely, maximum water-level decline. In simulation decrease in natural flow out of the confined system. Analysis of the Ground-Water-Flow System 85 10 MILES 10 KILOMETERS- Figure 53. Location of hypothetical withdrawals in the northeastern Accomack County scenarios. Table 21 . Hypothetical withdrawals for the northeastern the constant-head simulation (simulation 2) than in Accomack County scenario the no-flow simulation (simulation 1). Although the [Mgal/d, million gallons per day; latitude and longitude are reported in boundary conditions influence water levels in the degrees, arc minutes, arc seconds] north and northeastern part of the model area, Map Latitude Longitude Withdrawal Yorktown-Eastover water-level declines in the Virginia part of the East number 1 (Mgal/d) aquifer penetrated ern Shore are similar for both simulations, indicat 1 375626 0752844 0.314 Upper, middle ing that the boundary conditions do not greatly 2 375626 0752723 1.217 Upper, middle affect results in the study area. The maximum 3 375134 0753041 .128 Upper water-level declines are 15.8, 27.9, and 24.6 ft in 4 375911 0752528 .055 Upper, middle the upper, middle, and lower Yorktown-Eastover 5 375256 0753324 1.800 Upper, middle, and lower aquifers, respectively (table 19). The location of the 'Locations shown on figure 53. maximum water-level declines is near the town of Hallwood for all three aquifers. The maximum Simulation of the increase in withdrawals in water-level declines in simulation 2 differ from the northeastern part of Accomack County with a those in simulation 1 by less than 2 ft in all three no-flow boundary condition results in a slight aquifers. As in simulation 1, the water levels are landward movement of the simulated saltwater- above the tops of the aquifers throughout the model freshwater interface in the upper and middle area. The amount of ground-water flow through the Yorktown-Eastover aquifers (figs. 54-56). The system in simulation 2 is affected by the constant- interface toe moves approximately 0.5 mi landward head boundary condition (table 16). The flow into in the upper and middle Yorktown-Eastover aqui the confined system for simulation 2 decreases by fers. The interface position in the lower Yorktown- 0.27 Mgal/d over simulated 1988 conditions, even Eastover aquifer does not change from the simulated though withdrawals are increased by 2.05 Mgal/d. 1988 position during this 50-year simulation. The boundary nodes are supplying the water needed Simulated water levels indicate several areas for the increase in withdrawal. A comparison of the in northern Accomack County where offshore water- results of the two simulations in the northeastern level declines resulting from the hypothetical part of Accomack County further indicates the increase in ground-water withdrawal have caused a effects of the different boundary conditions. The reversal in ground-water flow from 1988 conditions flow into the confined system through the uppermost (fig. 54). There is a potential for downward vertical confining unit in simulation 2 is 1.34 Mgal/d less leakage of saltwater into the freshwater part of the than the flow into the confined system for simula upper Yorktown-Eastover aquifer as a result of the tion 1. The pumpage in both simulations is identi increase in withdrawals in northeastern Accomack cal. In simulation 1 (no-flow boundary), the source County. of the water withdrawn is increased recharge and decreased discharge, whereas in simulation 2 Simulation 2: Constant-Head Boundary (constant-head boundary), much of the water with Simulation 2 is identical to simulation 1 drawn is derived from flow from the boundary except that the northern and northeastern grid nodes. boundaries in simulation 2 are represented by a The simulated position of the saltwater- constant-head boundary instead of a no-flow bound freshwater interface toe for simulation 2 (figs. ary. The water levels for the boundary nodes are 57_59) is similar to the interface-toe position for held constant at the simulated 1988 values of the simulation 1 (figs. 54-56). Changing the boundary nearest nodes. This type of boundary condition pro conditions from a no-flow to a constant-head bound vides an unlimited source of water; therefore, the ary in this situation does not affect the ground- results indicate smaller head declines in simulation 2 water-flow system enough to cause a substantial dif than in simulation 1 from an increase in pumpage. ference in the movement of the saltwater-freshwater The water-level declines for the constant-head interface during the 50-year simulation. The simulation are presented in figures 57-59. Compari saltwater-freshwater interface for simulation 2 in the son with figures 54-56 shows water-level declines upper Yorktown-Eastover aquifer does not move north and northeast of the pumping center are less in landward for as long a distance along the coast as it Analysis of the Ground-Water-Flow System 87 76° 45 EXPLANATION AREA OF REVERSED SALTWATER FLOW LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable 37" 15' - SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM NORTHEASTERN ACCOMACK COUNTY, SIMULATION I WITHDRAWALS Figure 54. Water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 1. 88 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76" 45' 30' 75" 15' 38" 45' 30' EXPLANATION LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval. in feet, is variable SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM NORTHEASTERN ACCOMACK COUNTY. SIMULATION 37" 15' I WITHDRAWALS 20 MILES ?0 KILOMETERS Figure 55. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the middle Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 1. Analysis of the Ground-Water-Flow System 89 76 C 45' 30' 75° 15' 38'- 45' 30' EXPLANATION LINE OF EQUAL MODELED WATER-LEVEL DECLINE 37° 15' FROM SIMULATED 1988"WATER LEVELS-Interval, in feet, is variable SIMULATED 1988 INTERFACE TOE 20 MILES 20 KILOMETERS Figure 56. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the lower Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 1. 90 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° 45' 30' 75° 15' 45' 30' EXPLANATION AREA OF REVERSED SALTWATER FLOW LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable 37" 15' SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM NORTHEASTERN ACCOMACK COUNTY, SIMULATION 2 WITHDRAWALS 20 MILLS 20 KILOMCTLRS Figure 57. Water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 2. Analysis of the Ground-Water-Flow System 91 76 C 45' 30' 75° 15' 38 C 45' 30' EXPLANATION LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-lnterval, in feet, is variable SIMULATED 1988 INTERFACE TOE 37° 15' SIMULATED INTERFACE TOE RESULTING FROM NORTHEASTERN ACCOMACK COUNTY, SIMULATION 2 WITHDRAWALS 10 20 MILES i i I 20 KILOMETERS Figure 58. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the middle Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 2. 92 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 75° 15' EXPLANATION 37° 15" - UNE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS--Interval, in feet, is variable SIMULATED 1988 INTERFACE TOE 20 KILOMETERS Figure 59. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater inter face toe in the lower Yorktown-Eastover aquifer, northeastern Accomack County scenario, simulation 2. Analysis of the Ground-Water-Flow System 93 Table 22. Permitted withdrawals as of January 1, 1990 [Latitude and longitude are reported in degrees, arc minutes, arc seconds; Mgal/d, million gallons per day] Map Facility Latitude Longitude Permitted number 1 withdrawal (MgaVd) 1 Accomack Nursing Home 374528 0753721 0.029 2 American Original 373045 0754828 .250 3 Bayshore Concrete 371540 0760121 .125 4 Byrd Foods 374530 0754010 .600 5 C&D Seafood 371711 0755524 .152 6 Custis Enterprises 372150 0755522 .441 7 Dicanio 37 13 36 0760019 .047 8 Dicanio 37 13 14 0760021 .047 9 Dicanio 37 13 53 0755923 .093 10 Dicanio 37 13 54 0755908 .093 11 Eastern Shore Seafood 375121 0753337 300 12 Exmore Foods 373201 075 49 16 2.001 13 Holly Farms 375256 0753324 1.800 14 JWTaylor Packing 375239 075 35 27 .549 15 KMCFood 371746 0755728 1.600 16 NASA, Wallops Island 375134 0753041 .127 17 NASA, main base 375626 0752844 .263 18 New Church Energy 375823 0753213 336 19 Peaceful Beach 373105 0755650 229 20 Perdue 374429 0753920 2.639 21 H. Alien Smith 37 17 15 0755512 .150 22 Town of Cape Charles 371605 0760019 260 23 Town of Chincoteague 375626 0752723 1.340 24 Town of Exmore 373231 0754914 320 'Locations shown on figure 60. does in simulation 1. The maximum amount of eastern boundaries, saltwater intrusion through landward movement is approximately 0.5 mi in each downward vertical leakage is possible, given the simulation. The movement of the interface in the hypothetical projected increase in ground-water middle Yorktown-Eastover aquifer is identical for withdrawal in northeastern Accomack County. both the no-flow and the constant-head simulations (figs. 55 and 58). The location of the saltwater- Permitted-Withdrawal Scenario freshwater interface toe in simulation 2 remains The final scenario presented in this report unchanged from 1988 conditions for the lower examines the ground-water-flow system's response Yorktown-Eastover aquifer. to currently (1990) permitted withdrawals. In 1976, Simulated water levels indicate several areas the State of Virginia established Accomack and of reversed ground-water flow (fig. 57) where there Northampton Counties as a Ground-Water Manage is potential for induced downward vertical leakage ment Area. Thus, all nonagricultural ground-water of saltwater into the freshwater parts of the upper users withdrawing more than 300,000 gal/month Yorktown-Eastover aquifer. A comparison with the must obtain a permit from the VWCB. As of 1990, area of reversed flow from simulation 1 (fig. 54) most of the permitted ground-water users on the shows that simulation 2 identifies a smaller area of Eastern Shore were withdrawing less water than potential induced saltwater leakage. The area of their permits allowed. In this scenario, ground-water reversed ground-water flow is smaller for simulation conditions are simulated that would result from 2 than simulation 1 because the water-level decline increasing withdrawal on the Eastern Shore to 1990 is reduced as a result of the unlimited supply of permitted levels. water from the constant-head boundary. The results Permitted withdrawal amounts as of Janu of simulation 2 indicate that, even when an infinite ary 1, 1990, ranged from 0.029 to 2.639 Mgal/d amount of water is allowed through the northern and (table 22). Permitted withdrawals are widely spread 94 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia over the northern and southern ends of the peninsula not be simulated by the sharp-interface model. The (fig. 60). Withdrawals for ground-water users that chloride concentrations in the mixing zone probably do not have permits were continued at 1988 rates. fluctuate more rapidly than the position of the sharp Pumpage for the permitted scenario (13.824 Mgal/d) interface. represents a 173 percent increase (8.763 Mgal/d) Water-level declines caused by pumpage in over 1988 withdrawals (table 18). Simulated 1988 nearshore and coastal areas indicate several areas of conditions were used as initial conditions for a 100- reversed ground-water flow from the Atlantic Ocean year transient simulation of 1990 permitted with and Chesapeake Bay to the freshwater parts of the drawals. Although water levels respond quickly to upper Yorktown-Eastover aquifer (fig. 61). The changes in stress, the movement of the saltwater- areas of reversed flow indicate a potential for verti freshwater interface takes place over long periods of cal leakage of saltwater into the freshwater parts of time. The simulation was carried out to 100 years to the upper Yorktown-Eastover aquifer. The rate of provide insight into the long-term effects of vertical leakage of saltwater is highly dependent on increased withdrawals on the movement of the the vertical hydraulic conductivity of the upper saltwater-freshwater interface. Yorktown-Eastover confining unit in the vicinity of Modeled water-level decline from simulated the flow reversal. 1988 water levels is shown in figures 61-63. Water- level declines exceed 25 ft in the upper Yorktown- Discussion of Model Results Eastover aquifers and 65 ft in the middle and lower Yorktown-Eastover aquifers. A maximum water- The model results from the three scenarios of level decline of approximately 95 ft occurs in the increased ground-water withdrawals provide infor middle Yorktown-Eastover aquifer near the town of mation on the regional response of the ground-water Exmore (table 19). Water levels remain above the system to additional stress and its ability to meet tops of the aquifers, indicating from a regional per future water needs. The simulations are not intended spective that dewatering would be minimal at to predict exact ground-water conditions in the permitted-withdrawal levels. future; however, a comparison of model results pro The permitted-withdrawal scenario involves a vides useful information for the evaluation of alter greater increase in withdrawals over 1988 pumpage native withdrawal scenarios. than any of the previous scenarios; therefore, the The distribution of ground-water withdrawals changes in the flow into and out of the confined directly affects the ability of the ground-water sys system are the most dramatic (table 16). The tem to sustain increased withdrawals without incur increase in freshwater withdrawals of 8.65 Mgal/d ring saltwater intrusion. An increase in ground-water over 1988 amounts results in an increase in flow withdrawals lowers ground-water levels around the into the confined-aquifer system by 4.33 Mgal/d and pumping centers. Ground-water flow is diverted to a decrease in natural flow out of the confined- the major pumping centers; water from adjacent aquifer system by 2.99 Mgal/d. parts of the aquifer and from adjacent aquifers or The position of the simulated saltwater- confining units replaces the water withdrawn. Large freshwater interface for the 100-year transient water-level declines could necessitate lowering of permitted-withdrawal scenario is shown in figures pump intakes, could increase the rate of movement 61-63. Interface movement coincides with the areas of the offshore interface between saltwater and of greatest water-level decline due to increased freshwater, and could induce leakage of poor-quality pumpage. Maximum inland movement of the water from adjacent aquifers or surface-water bod saltwater-freshwater interface toe is approximately ies. Excessive head declines and detrimental effects Imi in the upper Yorktown-Eastover aquifer near on water quality can be minimized with proper well the town of Cape Charles and in the middle placement. Withdrawal wells can be placed in areas Yorktown-Eastover aquifer near the town of Chin- that would minimize interference with other major coteague. Maximum inland movement of the inter ground-water users. face toe is approximately 1 mi in the lower Any increase in withdrawals from the confined Yorktown-Eastover aquifer near the town of Hall- freshwater aquifers on the Eastern Shore increases wood. The water-quality effects on the width of the the amount of recharge to and decreases the amount mixing zone between saltwater and freshwater can of natural discharge from the confined-aquifer system Analysis of the Ground-Water-Flow System 95 76° 45' 30' 75° 15' 38' 45' 30' EXPLANATION 37° 15' .12 LOCATION OF PERMITTED WITHDRAWAL AND IDENTIFIER 10 20 MILES i i I 10 20 KILOMETERS Figure 60. Location of permitted withdrawals. 96 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76" 45 45' - 30' - EXPLANATION Illlll AREA OF REVERSED SALTWATER FLOW 75 ____ LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in - feet, is variable 37" 15' - SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM NORTHEASTERN ACCOMACK COUNTY, SIMULATION 2 WITHDRAWALS 10 20 MILLS ?0 KILOMETERS Figure 61. Water-level decline from simulated 1988 water levels, simulated position of the saltwater-freshwater interface toe, and area of reversed saltwater flow in the upper Yorktown-Eastover aquifer, permitted-withdrawal scenario. Analysis of the Ground-Water-Flow System 97 76 C 45' 30' 75° 15' 38° 45' 30' EXPLANATION 5 LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable 37° 15' SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM PERMITTED-WITHDRAWAL SCENARIO 20 MILES Fishermans Island 10 20 KILOMETERS Figure 62. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe in the middle Yorktown-Eastover aquifer, permitted-withdrawal scenario. 98 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' 38 C 45' 30' EXPLANATION 5_____ LINE OF EQUAL MODELED WATER-LEVEL DECLINE FROM SIMULATED 1988 WATER LEVELS-Interval, in feet, is variable 37° 15' SIMULATED 1988 INTERFACE TOE SIMULATED INTERFACE TOE RESULTING FROM PERMITTED-W1THDRAWAL SCENARIO 10 20 MILES i_I _I 20 KILOMETERS Figure 63. Water-level decline from simulated 1988 water levels and simulated position of the saltwater-freshwater interface toe in the lower Yorktown-Eastover aquifer, permitted-withdrawal scenario. Analysis of the Ground-Water-Flow System 99 A reduction in freshwater discharge to the Chesa ing constant heads (that represent the water table) peake Bay and Atlantic Ocean changes the equilib resulted in a slight increase in the heads in the simu rium between the freshwater and the surrounding lated confined aquifers. Correspondingly, decreasing saltwater. The interface between freshwater and salt the overlying constant heads resulted in lower heads water begins to move inland as saltwater replaces in the simulated confined aquifers. The hydraulic the withdrawn freshwater. A reduction in freshwater property that dominates flow through the system is discharge also could affect salinity levels at the vertical leakance of the uppermost confining freshwater-discharge sites in nearshore inlets, bays, unit, and as a result, the model is more sensitive to and estuaries. changes in confining-unit vertical leakance than it is Model results indicate that water-level declines to changes in the overlying constant heads. The in close proximity to the simulated location of the results of the sensitivity analysis of the calibrated saltwater-freshwater interface have the most dra model to changes in horizontal hydraulic conductiv matic effect on the rate of interface movement. ity of aquifers and leakance of confining units are Large water-level declines in the center of the pen presented in this section. Sensitivity simulations insula have a minimal effect on the rate of move were conducted by increasing or decreasing an indi ment of the saltwater-freshwater interface; however, vidual parameter while all other characteristics small water-level declines in coastal areas adjacent remained unchanged. The larger the resulting to the interface position cause a noticeable increase changes in water levels are, the more sensitive the in the rate of interface movement. model is to that parameter, and the smaller the Two potential pathways for saltwater intrusion change, the less sensitive the model is. into the freshwater aquifers of the Eastern Shore Withdrawal conditions from simulation 1 of were examined in this report. Model simulations the southern Northampton County scenario were show saltwater intrusion through lateral movement used to examine the model's sensitivity to large of the saltwater-freshwater interface and through increases in withdrawals. Variations in hydraulic downward vertical leakage of saltwater in areas characteristics were compared by simulating a where a saltwater source overlies the freshwater part 50-percent increase and decrease in hydraulic con of the uppermost confined aquifer. Model simula ductivity and vertical leakance. Water-level differ tions indicate that lateral movement of the saltwater- ences that resulted from changing the calibrated freshwater interface is slow and takes place over hydraulic parameters are shown in figures 64-67. long periods of time. However, a reversal of the The maximum water-level changes for each aquifer ground-water-flow direction can take place in short for each sensitivity run (table 23) show that the timeframes and could result in induced vertical leak model is most sensitive near major pumping areas. age of saltwater through the confining unit into the Generally, the water levels simulated by the model freshwater part of an aquifer. Areas of reversed flow are more sensitive to decreases than they are to of saltwater into freshwater areas are seen in sce increases in hydraulic conductivity and vertical leak nario results where heavy withdrawals are present in ance. The responses of the saltwater-freshwater coastal areas and water-level declines extend interface to changes in hydraulic conductivity and offshore. vertical leakance were slow and not sensitive over the 50-year simulation period. Increasing hydraulic Sensitivity Analysis conductivity and decreasing vertical leakance result in an interface position that is slightly closer to the Model-sensitivity analyses were conducted to shore in a few locations than the calibrated-scenario examine the response of the calibrated model to interface position. Decreasing hydraulic conductivity changes in boundary conditions and estimated and increasing vertical leakance result in an inter hydraulic characteristics. The model sensitivity to face position that is slightly farther offshore in a few the northern no-flow boundary condition is illus locations than the calibrated scenario. trated in the section of this report describing simula tion 2 of the northeastern Accomack County sce Model Limitations nario. Model runs also were conducted to determine the sensitivity of the model to changes in the overly The ground-water-flow model developed for ing constant-head boundary. Increasing the overly the Eastern Shore is a tool that was used to assist in 100 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76 C 45' 30' 75° 15' 38 C 45' 30' 37° 15' LINE OF EQUAL DIFFERENCE BETWEEN SENSITIVITY RUN AND CALIBRATED SOUTHERN NORTHAMPTON COUNTY SCENARIO, SIMULATION 1--Interval, in feet, is variable 10 20 MILES 20 KILOMETERS Figure 64. Difference in simulated water levels resulting from a 50-percent increase in horizontal hydraulic conductivity for the southern Northampton County scenario simulation 1, upper Yorktown-Eastover aquifer. Analysis of the Ground-Water-Flow System 101 76° 45' 30' 75° 15' 45' 30' EXPLANATION 37° 15' LINE OF EQUAL DIFFERENCE BETWEEN SENSITIVITY RUN AND CALIBRATED SOUTHERN NORTHAMPTON COUNTY SCENARIO, SIMULATION 1--Interval, in feet, is variable 10 20 MILES i_I _I 10 20 KILOMETERS Figure 65. Difference in simulated water levels resulting from a 50-percent decrease in horizontal hydraulic conductivity for the southern Northampton County scenario simulation 1, upper Yorktown-Eastover aquifer. 102 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia 76° Jenkins Bridge Hallwood ._ /"^Tangier Island 37° 15' - EXPLANATION LINE OF EQUAL DIFFERENCE BETWEEN SENSITIVITY RUN AND CALIBRATED SOUTHERN NORTHAMPTON COUNTY SCENARIO, SIMULATION 1--Interval, in feet, is variable 20 KILOMETERS Figure 66. Difference in simulated water levels resulting from a 50-percent increase in confining unit leakance for the southern Northampton County scenario simulation 1, upper Yorktown-Eastover aquifer. Analysis of the Ground-Water-Flow System 103 76 C 45' 30' 75° 15' 38 C 45' 30' 37° 15' EXPLANATION LINE OF EQUAL DIFFERENCE BETWEEN SENSITIVITY RUN AND CALIBRATED SOUTHERN NORTHAMPTON COUNTY SCENARIO, SIMULATION 1--Interval, in feet, is variable 0 10 20 MILES I.W.MM- ' ' ' Island 20 KILOMETERS I______I Figure 67. Difference in simulated water levels resulting from a 50-percent decrease in confining unit leakance for the southern Northampton County scenario simulation 1, upper Yorktown-Eastover aquifer. 104 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia Table 23. Maximum water-level changes resulting from sensitivity runs Maximum Grid location of increase Maximum Grid location of decrease water-level ______water-level ______increase Row Column decline Row Column (feet) (feet) 50-percent increase in hydraulic conductivity Upper Yorktown-Eastover 4.0 3 27 -3.5 43 28 Middle Yorktown-Eastover 10.5 80 29 -33 45 28 Lower Yorktown-Eastover 14.6 80 26 -32 45 29 50-percent decrease in hvdraulic conductivity Upper Yorktown-Eastover 5.8 43 28 -10.7 3 27 Middle Yorktown-Eastover 5.4 43 29 -24.5 80 29 Lower Yorktown-Eastover 5.5 45 29 -39.5 80 26 50-oercent increase in confining unit leakance Upper Yorktown-Eastover 3.6 30 31 -1.5 58 42 Middle Yorktown-Eastover 7.6 80 29 -23 60 43 Lower Yorktown-Eastover 5.6 80 26 -3.0 57 40 50-percent decrease in confining unit leakance Upper Yorktown-Eastover 1.0 51 39 -6.7 29 33 Middle Yorktown-Eastover .9 66 37 -13.9 80 29 Lower Yorktown-Eastover 1.0 63 37 -10.0 80 26 the analysis of the ground-water-flow system. The system, but flow in the unconfined aquifer was not model is an approximate representation of a com simulated. The deep, saltwater aquifers (approxi plex physical system. The hydrogeologic character mately 300 ft below land surface) also are not simu istics of a conceptualized three-dimensional system lated by the model. As of 1990, no water was being of aquifers and confining units are integrated in the pumped from the deep aquifers in the study area. model. Stresses can be applied to this quasi-three- The saltwater-freshwater interface is repre dimensional representation of the system, and the sented in the model as a sharp interface. There are relative effects of those stresses on the water levels, no offshore data for the Eastern Shore; therefore, the water budget, and the saltwater-freshwater inter the actual position of the saltwater-freshwater inter face can be examined. face and the width of the transition zone are The model was developed to simulate regional unknown. Saltwater and freshwater are simulated as effects of hydrologic stresses on the ground-water- immiscible fluids, and mixing due to hydrodynamic flow system. The large spatial and temporal scale of dispersion is neglected. Leakage between the salt the model makes it unsuitable for the analysis of water and freshwater zone is restricted by the local effects, short-term effects, and small-scale model. Saltwater is not allowed to leak into the withdrawals. A small-scale analysis of the flow freshwater zone. The leakage of freshwater is dis system would require spatial and temporal refine tributed between the saltwater and freshwater zones ment of the aquifer and confining-unit characteristics based on the amounts of each type of water in the and hydrologic stresses. node receiving the leakage. The approach is The model simulates ground-water flow in the designed to reproduce the general response of the Eastern Shore's freshwater-bearing confined aqui interface and does not provide information concern fers, from which the majority of withdrawals are ing the nature of the transition zone between salt made. The water table in the unconfined aquifer was water and freshwater. Vertical leakage of saltwater specified in the model as a constant-head boundary into freshwater is not directly simulated; evidence of to simulate the regional recharge-discharge relation vertical saltwater intrusion from overlying salty- between the unconfined aquifer and the confined surface-water bodies is provided by examination of Analysis of the Ground-Water-Flow System 105 the water-level gradients and areas of reversed fers, the upper, middle, and lower Yorktown- ground-water flow. The model is not able to simu Eastover aquifers. Maps delineating the tops of the late upconing of saltwater as a result of pumpage. aquifers arM confining units were developed from This approach is considered an initial step in the correlation of lithologic and geophysical logs, water- process of characterizing the interactions between quality analyses, and water-level data. saltwater and freshwater around the Eastern Shore. Prior to 1940, ground-water withdrawals on Data concerning the dispersive properties of the sed the Eastern Shore were minimal, and the ground- iments and a solute-transport-modeling approach to water system was in a state of long-term dynamic the saltwater-freshwater interface are needed to fully equilibrium. Water from precipitation falling on the characterize the water quality in the transition zone peninsula recharged the Columbia aquifer and between saltwater and freshwater. flowed from the topographic highs near the center of The location of the historical and present-day the peninsula to discharge into streams, estuaries, saltwater-freshwater interface is not known. The the Chesapeake Bay, and the Atlantic Ocean. Some model simulates the location of the interface by sim water flowed vertically through the upper Yorktown- ulating saltwater and freshwater flow and balancing Eastover confining unit to recharge the confined- pressures along the interface. The historic pre- aquifer system. Water-level measurements made stressed position of the interface is assumed, for the after withdrawals began on the peninsula indicated purposes of this report, to be in equilibrium with lowering of water levels and creation of cones of present-day sea levels. However, the interface posi depression around major pumping centers. tion may not have reached an equilibrium position Annual ground-water-withdrawal data for the and may still be responding to long-term sea-level model area were compiled by aquifer for commer fluctuations. cial, industrial, and municipal withdrawals. Prior to 1965, there were few large users of ground water on SUMMARY the Eastern Shore. By 1970, increased population along with commercial and industrial growth greatly The Eastern Shore of Virginia is totally increased the demand for the ground water. Ground- dependent on ground water for its freshwater sup water use, excluding domestic and irrigation, was ply. Increased pumpage due to intensifying agricul estimated to be about 5.04 Mgal/d in 1988. The tural, industrial, commercial, and urban develop upper, middle, and lower Yorktown-Eastover aqui ment could limit the continued use of this resource. fers supplied 36, 42, and 22 percent of the 5.039 Ground-water withdrawal has caused lowering of Mgal/d withdrawal, respectively. Major pumping water levels and has created cones of depression centers on the Eastern Shore were located near the around areas of heavy ground-water use. The water- towns of Chincoteague, Hall wood, Accomac, level decline has resulted in well interference in sev Exmore, Oyster, Cheriton, and Cape Charles. eral localities. Continued water-level decline could Data on chloride concentrations were compiled result in additional well interference among the by aquifer to provide information on the distribution ground-water users and intrusion of saltwater into of chlorides in the study area. Chloride concentra the freshwater parts of aquifers. tions in each aquifer are typically lower in the mid This report describes the hydrogeology and dle of the peninsula than they are along the coast. ground-water flow system of the Eastern Shore. A Chloride concentrations increase with depth and are model that includes the ability to track the move higher in the lower Yorktown-Eastover aquifer than ment of the saltwater-freshwater interface was used in the middle and upper Yorktown-Eastover aqui to aid in the hydrologic analysis of the effects of fers. The elevated chloride concentrations (greater withdrawals on the ground-water-flow system. than 250 mg/L) found in the lower Yorktown- The sediment of the Eastern Shore forms a Eastover aquifer across the peninsula near Exmore, layered sequence of aquifers and intervening confin Va., could be a result of different hydraulic proper ing units. This report focuses on the aquifers and ties related to the presence of an ancient Pleistocene confining units (approximately the upper 300 ft) that river channel. Chloride concentrations in the lower make up the fresh-ground-water system. The aqui Yorktown-Eastover aquifer are stratified, and con fers that contain freshwater are the unconfined centrations are lower near the top than near the bot Columbia aquifer and the upper three confined aqui tom of the aquifer. 106 Hydrogeology and Analysis of the Ground-Water-Flow System of the Eastern Shore, Virginia A model was developed for the Eastern Shore Bear, Jacob, 1979, Hydraulics of ground water: New to simulate changes in ground-water-flow conditions York, McGraw-Hill, 569 p. that result from changes in hydrologic stresses. Sim Bond, L.D., and Bredehoeft, J.D., 1987, Origins of sea- ulation included ground-water flow both prior to water intrusion in a coastal aquifer a case study of ground-water pumpage and throughout the history of the Pajaro Valley: Journal of Hydrology, v. 92, pumpage. The maximum simulated water-level p. 363-388. Cederstrom, D.J., 1945, Geology and ground-water decline since prepumping conditions was 53 ft in the resources of the Coastal Plain in southeastern Vir lower Yorktown-Eastover aquifer near the town of ginia: Virginia Geological Survey Bulletin 63, Accomac, Va. Simulated water-level gradients indi 384 p. cated a change in the direction of ground-water flow Colman, S.M., Halka, J.P., Hobbs, C.H., Mixon, R.B., from prepumping conditions. Prepumping flow was and Foster, D.S., 1990, Ancient channels of the from topographic highs in the center of the penin Susquehanna River beneath the Chesapeake Bay and sula to the Chesapeake Bay and Atlantic Ocean. the Delmarva Peninsula: Geological Society of Simulated 1988 conditions show ground-water flow America Bulletin, v. 102, p. 1268-1279. is being diverted toward the major pumping centers. Colman, S.M., and Mixon, R.B., 1988, The record of major Quaternary sea-level fluctuations in a large Ground-water pumpage is supplied by an increase in Coastal Plain estuary, Chesapeake Bay, eastern vertical leakage to the confined-aquifer system from United States: Palaeogeography, Palaeoclimatology, the unconfined aquifer and a decrease in vertical and Palaeoecology, v. 68, p. 99-116. leakage from the confined-aquifer system to the Cooper, H.H., Jr., 1959, A hypothesis concerning the unconfined aquifer. The simulated position of the dynamic balance of freshwater and saltwater in a interface between saltwater and freshwater did not coastal aquifer: Journal of Geophysical Research, change in response to historic pumpage. v. 64, no. 4, p. 461-467. Three scenarios predicted ground-water condi Cooper, H.H., Jr., and Jacob, C.E., 1946, A generalized tions that result from increasing withdrawals in graphical method for evaluating formation constants southern Northampton County, in northeastern and summarizing well-field history: American Accomack County, and throughout the peninsula at Geophysical Union Transactions, v. 27, no. 4, p. 526-534. 1990 permitted rates. Simulation results indicate that Gushing, E.M., Kantrowitz, I.H., and Taylor, K.R., water levels continue to decline as withdrawals 1973, Water resources of the Delmarva Peninsula: increase and could result in well interference among U.S. Geological Survey Professional Paper 822, major ground-water users and in a reduction in 58 p. freshwater discharge to the Chesapeake Bay and Essaid, H.I., 1986, A comparison of the coupled Atlantic Ocean. The water-level declines associated freshwater-saltwater flow and the Ghyben-Herzberg with the increased withdrawals could cause slight sharp interface approaches to modeling of transient movement of the saltwater-freshwater interface over behavior in coastal aquifer systems: Journal of a 50-year simulation period. The potential for Hydrology, v. 86, p. 169-196. induced vertical leakage of saltwater from overlying 1990a, The computer model, SHARP, a quasi- salty-surface-water sources into the freshwater parts three-dimensional finite-difference model to simulate of the upper Yorktown-Eastover aquifer is indicated freshwater and saltwater flow in layered coastal aqui fer systems: U.S. Geological Survey Water- by areas of reversed ground-water gradients caused Resources Investigations Report 90-4130, 181 p. by offshore water-level declines. -1990b, A multilayered sharp interface model of coupled freshwater and saltwater flow in coastal sys REFERENCES CITED tems model development and application: Water Resources Research, v. 26, no. 7, p. 1431-1454. 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