Journal of Applied Geophysics 42Ž. 1999 1±22 Saltwater intrusion into the freshwater aquifer in the eastern shore of Virginia: a reconnaissance electrical resistivity survey Ali A. Nowroozi ), Stephen B. Horrocks, Peter Henderson Department of Ocean, Earth and Atmospheric Sciences, Old Dominion UniÕersity, Norfolk, VA 23529-0496, USA Received 28 November 1998; accepted 15 January 1999 Abstract Contrasts between apparent high resistivity of the saturated freshwater zones and apparent low resistivity of the saturated saltwater zone are recorded on 111 Schlumberger sounding field curves. The field measurements are inverted to subsurface layers. Generally, resistivity decreases with depth from the high value of the unsaturated zone near the surface to the low values of the saltwater saturated zone at depth of 30 to 130 m. Contour maps are constructed for variations of resistivity at depths of 3, 5, 10, 20, 30, 40, 50, 60, 70, 100 and 130 m. Resistivity contours of less than 30 V m, are indicated at depths of 5±20 m near the narrow bands of Chesapeake Bay and the Atlantic Ocean in the coastal regions of Accomack County. Within the area covered by the cities of Onancock, Accomac, and Wachapreaque, low-resistivity contours of less than 30 V m are observed from 30 to 60 m depth. In 70 to 130 m depth range, a major part of Accomack County is covered by low-resistivity contours of 10 to 30 V m which connect the Chesapeake Bay to the Atlantic Ocean. Vertical profiles of the contour maps indicate the shape of the saltwater plumes. The interface appears to be as shallow as 30 m where intrusion has occurred, and extends downward to a depth of 130 m where the saltwater of the Atlantic ocean and Chesapeake Bay are connected. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Intrusion of saltwater; Freshwater aquifers; Resistivity survey; Eastern shore; Delmarva Peninsula 1. Introduction these sources,Ž Horsley Witten Hegemann, 1992. In the coastal areas, withdrawal of a There are no running streams in the Eastern large volume of groundwater may allow saltwa- Shore of Virginia; therefore, water needed for ter intrusion into the freshwater aquifers. This the economic development of this area is drawn potential saltwater contamination poses a threat from the groundwater aquifers. More than 15 to the sustainable development and economic million gal per day of water are exploited from well being of the Eastern Shore. Measurements from water quality research stations indicate r ) Corresponding author. Department of Geological Sci- that in several areas the salt freshwater inter- ences, Old Dominion University, Norfolk, VA 23529-0496, face is relatively shallow and water withdrawals USA. Fax: q1-804-683-5194; E-mail: [email protected] are from depths close to the saltrfreshwater 0926-9851r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0926-9851Ž. 99 00004-X 2 A.A. Nowroozi et al.rJournal of Applied Geophysics 42() 1999 1±22 interface. An excessive level of water with- zone, Frohlich et al.Ž. 1994 reported that a layer drawal from these regions may cause intrusion with resistivity of 230 V m or less is indicative of saltwater into freshwater aquifersŽ Fennema of saltwater pollution in the freshwater lenses. and Newton, 1982. Furthermore, ground water SabetŽ. 1975 estimated a range of 20 V mto modeling by RichardsonŽ. 1992 indicates that, several hundred ohm meters for the resistivity based on certain simulation parameters, some of clean sand and gravelŽ not containing silt or saltwater contamination in a few areas of the shale. which is saturated with freshwater in Eastern Shore are possible. In addition, the southeastern region of Virginia and the northern Ground Water Supply Protection and Manage- part of North Carolina. He also reported that the ment Plan for the Eastern Shore of Virginia resistivity of the same sand containing silt, clay Ž.Horsley Witten Hegemann, 1992 recom- or brackish water is much lower and concluded mended further study to quantify the limits of that freshwater is unlikely to be produced from salinityŽ e.g., greater than 250 mg chloride per horizons of resistivity less than 10 V m. Fur- liter. for this area. This information is essential thermore, he reported that the resistivity of for water allocations and establishment of the freshwater-bearing horizonsŽ water containing limitations that may be required to be set on less than 1000 mgrl of dissolved solids. varies water withdrawals from some regions. between 19 and 25 V m, but that the resistivity The large differences between the resistivity determined by electrical soundings is not neces- of saltwater saturated zones and the freshwater sarily the same as that obtained from sampling saturated zones have been used by a number of wells. investigators for determination of saltwater in- trusion in many coastal areasŽ Van Dam and Meulenkamp, 1967; De Breuk and De Moor, 2. Regional geology 1969; Zohdy, 1969; Sabet, 1975; Respond, 1990; Ginsberg and Levanton, 1976; Urish and The Eastern Shore of Virginia consists of Frohlich, 1990; and Frohlich et al., 1994. In Northampton and Accomack Counties. In the addition, GoodellŽ. 1986 and Flanzenbaum east, a series of salt marshes and barrier islands Ž.1986 used electromagnetic methods to deter- separate the region from the Atlantic Ocean, mine saltwater intrusion into a freshwater aquifer and in the west are pocket beaches and shallow in the Eastern Shore and Southeastern Virginia. tidal creeks of the Chesapeake Bay. This area is Van Dam and MeulenkampŽ. 1967 deter- approximately 70 miles long and 6 to 10 miles mined salinity of ground water in the western wide trending approximately N±S; it is nearly part of the Netherlands using the resistivity flat with the maximum elevation of about 50 ft method. Their interpreted resistivities were in Accomack and about 40 ft in Northampton closely related to the salinity of groundwater. In County. The soil consists of 8 to 10 in. of sandy some areas the estimated depth of the interface loam topsoil and 30 in. of sandy clay soil. The was checked by subsequent drilling; errors up to regional geology has been discussed extensively 15% or more were encountered in areas of by Cushing et al.Ž. 1973 , Robbins et al. Ž. 1975 , insufficient data on resistivity, geology or hy- Shideler et al.Ž. 1984 , Mixon Ž. 1985 , Coleman drogeology. They consider 40, 12, and 3 V m and MixonŽ. 1988 , Coleman et al. Ž. 1990 , Foyle as fresh, brackish, and saline water, respec- and OertelŽ. 1992 , and Foyle Ž. 1994 . The East- tively. ZohdyŽ. 1969 used the resistivity method ern Shore geology consists of sediments dipping to show that the static water table was at a depth northeast with a thickness of 3000 ft in the west of about 300 ft in El Paso, TX, while the of the Peninsula. The thickness of layers in- saltrfreshwater interface varied from this depth creases to about 7500 ft in the east of the to about 1700 ft. In the New England coastal PeninsulaŽ. Meng and Harsh, 1988 . The sedi- A.A. Nowroozi et al.rJournal of Applied Geophysics 42() 1999 1±22 3 mentary layers overlie a hard-rock basement of the surficial sediments are very thick where that also dips northeast. The sediments have a sediments fill the large valley cuts across the complicated depositional historyŽ. Foyle, 1994 peninsulaŽ. Foster, 1990 . and vary in age from Early Cretaceous to re- Fig. 1 presents the geology of eastern Vir- cent. Robbins et al.Ž. 1975 estimated that about ginia based on MixonŽ. 1985 . The surface geo- 70% of the sediments is from Early to Late logical units are sedimentary deposits of coastal Cretaceous age deposited in a fluvial environ- barrier±lagoon complexes and estuarine mar- ment. In addition, about 30% of the sediments shes, Qs, Qm, and Qsm,Ž. Holocene ; fluvial and are of Tertiary age deposited in a marine envi- estuarine deposits of Onancock lowland, Qp and ronmentŽ.Ž. Cushing et al., 1985 . Mixon 1985 QkŽ. Holocene and Pleistocene ; marginal-marine concluded that the Tertiary sediments are over- deposits bordering the eastern part of central lain by a veneer of Quaternary sediments de- upland, Qw and QjŽ. Pleistocene ; and posited in several different environments. Some marginal-marine, estuarine and fluvial deposits Fig. 1. The surficial geology of Eastern Shore of Virginia. This map is modified fromŽ. Mixon, 1985 . 4 A.A. Nowroozi et al.rJournal of Applied Geophysics 42() 1999 1±22 of central upland and Franktown plain, Qno, occurring from June through SeptemberŽ Ball, Qnb, and QaŽ. Pleistocene . The composition of 1977. Freshwater derived from the rainfall seeps strata varies considerably, but mostly consists of through the shallower porous strata and forces sand, shell fragments, mud, peat, gravel, silts, the brackish and salty water away from the and silty clays. These strata collectively form land. The sources of freshwater include the the shallower Columbia aquifer. Below the shallower unconfined Columbia aquifer of Pleis- Columbia aquifer lies the Yorktown formation tocene to Holocene age, as well as a series of of middle Pliocene age. The Yorktown forma- confined aquifers associated with the Yorktown tion is composed of glauconitic sand, some formation of Miocene to Pliocene ageŽ Meng sandy gravel, and silty clay with shell fragments and Harsh, 1988. which also constitute an important freshwater The composition of the Quaternary Columbia aquifer of this areaŽ. Mixon, 1985 . sediments ranges from very fine silty sand to very coarse and gravelly clean sand inter-be- dded with clay and silt. The Yorktown sedi- ments consist mainly of a shelly formation of 3. Hydrogeology Pliocene ageŽ. Mixon, 1985 which is exposed on the southwestern bank of the James River The average precipitation for the land area is and extends toward the Chesapeake and into the roughly 43 in.