A digital map of the thickness of the surficial unconfined aquifer, including from the land surface and unsaturated zone to the bottom of sediments of geologic units identified as part of the surficial aquifer, was produced to improve understanding of the hydrologic system in the Maryland and Delaware portions of the Delmarva Peninsula. The map is intended to be used in conjunction with other environmental coverages (such land use, wetlands, and soil characteristics) to provide a subsurface hydrogeologic component to studies of nitrate transport that have historically relied on maps of surficial features. It could also be used to study the transport of other water soluble chemicals. The map was made using the best currently available data, which was of varying scales. It was created by overlaying a high resolution land surface and bathymetry digital elevation model (DEM) on a digital representation of the base of the surficial aquifer, part of hydrogeologic framework, as defined by Andreasen and others (2013). Thickness was calculated as the difference between the top of land surface and the bottom of the surficial aquifer sediments, which include sediments from geologic formations of late-Miocene through Quaternary age. Geologic formations with predominantly sandy surficial sediments that comprise the surficial aquifer on the Delmarva Peninsula include the Parsonsburg Sand, Sinepuxent Formation (Fm.), and parts of the Omar Fm. north of Indian River Bay in Delaware, the Columbia Fm., Beaverdam Fm., and Pennsauken Fm. (Ator and others 2005; Owens and Denney, 1986; Mixon, 1985; Bachman and Wilson, 1984). Formations with mixed texture and sandy stratigraphy including the Scotts Corner Fm. and Lynch Heights Fm. in Delaware are also considered part of the surficial aquifer (Ramsey, 1997). Subcropping aquifers and confining beds underlie the surficial aquifer throughout the Peninsula and may increase or limit its thickness, respectively (Andreasen and others, 2013). Stream incision through the surficial aquifer into older fine-textured sediments is more common in the northern part of the Peninsula where confined aquifers and their confining beds subcrop beneath the surficial aquifer. The potential for nitrate transport is greatest where relatively coarse sediments of the unconfined surficial aquifer (such as sand and gravel), are present beneath uplands and streams. Where these sediments are truncated and the streambed is incised into underlying fine-textured sediments, the potential for nitrate transport is much less and typically limited to stream-bank seeps that flow across the floodplain.
In parts of south-central Maryland and southern Delaware the surficial aquifer sediments are complex with surficial sandy sediments generally less than 20 ft thick (indicated as 19 ft on the map). They include the Parsonsburg Sand and some surficial sandy facies of the Omar Fm. underlain by predominantly fine-textured sediments of the Walston Silt and Omar Fm. (Denney and others, 1979; Owens and Denney, 1979). Even though the surficial aquifer is relatively thin in this area, extensive ditching of flat poorly drained farmland allows seasonal transport of nitrate from groundwater to streams when the water table is above the base of the ditches (Lindsey and others, 2003). Geologic units of the Coastal Lowlands that surround the Peninsula are relatively thin in many areas and are primarily composed of fine-grained estuarine deposits with some coarse-textured sediments, in particular remnant beach-ridge and dune deposits (Ator and others, 2005). The Kent Island Fm. (Owens and Denney, 1986), which is part of the Coastal Lowlands on the western side of the Peninsula, has predominantly fine-grained sediments and is not included in the surficial aquifer in Maryland, as defined by Bachman and Wilson (1984); the surficial aquifer is shown to have 0 ft thickness on the map in the area mapped as Kent Island Fm. Also shown on the map as 0 ft thickness are areas in the northern most portion of the peninsula in New Castle and Cecil counties where surficial aquifer sediments are not present and other areas such as stream valleys where surficial aquifer sediments are also not present. Nitrate transport through groundwater to surface water is limited in the areas with fine-grained sediments at or near the land surface that promote denitrification in groundwater (Ator and others, 2005). Where extensive tidal marshes overly the Coastal Lowlands they also limit nitrate transport to surface waters. Available sub-regional or county-scale geologic maps produced by the Delaware and Maryland State Geologic Surveys should be consulted when using this product (www.dgs.udel.edu; www.mgs.md.gov). Local-scale maps will be particularly important in understanding areas such as where the surficial aquifer is completely truncated or very thin and overlies confining beds or confined aquifers, in the Coastal Lowlands, and in south-central Maryland and Delaware. References: Andreasen, D.C., Staley, A.W., and Achmad, Grufon, 2013. Maryland Coastal Plain Aquifer Information system: Hydrogeological Framework: Maryland Department of Natural Resources Resource Assessment Service Maryland Geological Survey Open-File Report No. 12-02-20,121 p. Ator, S.W., Denver, J.M., Krantz D.E., Newell, W.L., and Martucci, S.K., 2005. A surficial hydrogeologic framework for the Mid-Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 1680, 44 p., 4 plates. Bachman, L.J. and Wilson, J.M., 1984. The Columbia Aquifer of the Eastern Shore of Maryland: Maryland Geological Survey Report of Investigations No. 40, 144 p. Denney, C.S., Owens, J.P. and Sirkin, L.A., 1979. The Parsonsburg Sand in the Central Delmarva Peninsula, Maryland and Delaware: U.S. Geological Survey Professional Paper 1067-B, 16 p. Lindsey, B.D., Phillips, S.W., Donnelly, C.A., Speiran, G.K., Plummer, L.N., Böhlke, J.K., Focazio, M.J., Burton, W.C., and Busenberg, Eurybiades, 2003. Residence times and nitrate transport in ground water discharging to streams in the Chesapeake Bay watershed: U.S. Geological Survey Water-Resources Investigations Report 03-4035, 201 p. Mixon, R.B., 1985. Stratigraphic and geomorphic framework of the upper most Cenozoic deposits in the southern Delmarva Peninsula, Virginia and Maryland: U.S. Geological Survey Professional Paper 1067-G, 53 p. Owens, J.P. and Denney, C.S., 1979. Upper Cenozoic Deposits of the Central Delmarva Peninsula, Maryland and Delaware: U.S. Geological Survey Professional Paper 1067-A, 28 p. --------, 1986. Geologic map of Dorchester County, Maryland: Maryland Geological Survey, 1 sheet, scale 1:62,500. Ramsey, K.W., 1997. Geology of the Milford and Mispillion River Quadrangles, Delaware: Delaware Geological Survey Report of Investigations No. 55, 40 p.