Background

The Potomac Group is a geologic unit that includes important aquifers in the Coastal Plain of the Mid-Atlantic region. Potomac aquifers are heavily used in northeastern and southern Maryland, as well as northern Delaware and southern New Jersey. The geology of the Potomac Group is complex. The unit consists of a thick sequence of multicolored silts, clays, sands and gravelly sands. Individual aquifer sands are separated by clays and commonly thin, pinch out, or grade into silt and clay, resulting in a number of discontinuous bodies. Uncertainties in regional correlation exist that affect estimates of recharge to, and the interconnections between, the water-producing sands. Water availability is an increasingly critical issue in Maryland. Population growth and periodic drought conditions have highlighted the need for a better understanding of the Potomac aquifers, particularly in Cecil County, Maryland.

Objective

The objective of this project is to improve our understanding of the Potomac Group in Cecil County, Maryland, particularly toward the northern extent of the unit where Potomac aquifers are the main source of groundwater and where development is causing concern in water resource management. An accurate geologic framework for the Potomac strata is necessary to provide reasonable assessments of groundwater resources and availability in this area.

Methods

A combination of new drilling and existing information was used to improve upon the regional correlations of the Potomac group strata. Multiple lines of investigation and data collection appear key to acquiring a representative view of each borehole and the Potomac Group locally. Cores provide lithologic information to compare with geophysical logs and the material for further analyses. Geophysical logs provide continuous information for a borehole and can fill in information on lithologies where core recovery is low. Analyses of fossil pollen and spores provide some data to tie particular core intervals to particular ages and a stratigraphic framework.

Therefore, in compiling existing information, efforts were made to acquire data from boreholes and/or wells that were drilled to basement (penetrating through the entire thickness of Potomac sediments locally); have lithologic descriptions based upon cores or extensive cuttings; were geophysically logged; and/or were subsampled and analyzed for fossil pollen and spores.

New drilling for the project focused on a couple select areas where: (1) the Potomac Group outcrops or subcrops below the surficial aquifer, (2) coring had the potential to fill known data gaps and (3) the coring could extend through the Potomac Group to bedrock. These areas included the peninsula known as Elk Neck and the area around Elkton, Maryland.

Three new boreholes (CE Cd 91, CE Be 155 and CE Bf 156) were drilled from the ground surface through the Potomac Group sediments to the underlying bedrock. In an effort to recover as much core as possible continuous coring techniques were used. Core was described and photographed. Geophysical logging of the boreholes included (natural) gamma, spontaneous potential, single point resistance, and resistivity (16-in and 64-in normals). Subsamples of the core were analyzed for fossil pollen to help with correlations of the layers.

Project Status and Preliminary Findings

Preliminary results indicate that the oldest portions of the Potomac Group are absent in the Elkton area. Depth to weathered bedrock or saprolite is approximately 171 feet below the land surface in the vicinity of the two Elkton boreholes. Lithologies encountered at these two Elkton boreholes, located only 2.5 miles apart, differed considerably which emphasizes the difficulty of correlating Potomac strata over fairly short distances.

On Elk Neck the Potomac Group is considerably thicker and includes older portions of the unit (not thought to be present there by some previous investigators). At borehole CE Cd 91 (Black Hill Ranger Station site), Potomac sediments extended to depths of approximately 490 feet below the land surface. Saprolite at this location was approximately 80 feet thick extending to weathered bedrock at about 570 feet below the land surface.

The data from the new boreholes are being analyzed and compared with data from previous investigations. Data and results are being compiled into a report that will be released online in the near future.

Acknowledgements and Funding

A portion of this work was funded in part by the USGS National Cooperative Geologic Mapping Program, Award No. G11AC20406 (fiscal year 2011). Drilling on the Elk Neck Peninsula was funded in part by the Maryland Department of the Environment.

The drilling could not have been conducted without the assistance of the Town of Elkton, Maryland, and the staff of Black Hill Ranger Station, Maryland Department of Natural Resources, Forest Service. The MGS is grateful to the Delaware Geological Survey for their cooperation and assistance with many aspects of this project.