교육데이터 활용•지원 서비스

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