Hydrogeology of the Piney Point-Nanjemoy, Aquia, and Upper Patapsco aquifers, Naval Air Station Patuxent River and Webster Outlying Field, St. Marys County, Maryland, 2000–06

Recent and projected population growth in southern Maryland continues to bring ground-water-quality and quantity issues to the forefront. Lithologic, borehole geophysical, water-level, and water-use data were compiled and interpreted to revise understanding of the hydrogeologic framework of the Piney Point-Nanjemoy, Aquia, and Upper Patapsco aquifers in southern Maryland, with emphasis on the Naval Air Station Patuxent River and Webster Outlying Field. Understanding of the hydrogeologic framework for the Upper Patapsco aquifer also has been revised based on the results of aquifer testing and water-quality sampling of two wells.

The Piney Point-Nanjemoy aquifer is 50 to 70 feet thick, with a top altitude of 213 to 260 feet below the North American Vertical Datum of 1988 and a hydraulic conductivity of 2 feet per day at Naval Air Station Patuxent River and Webster Outlying Field. Ground-water withdrawals from the Piney Point-Nanjemoy aquifer have been minimal since 1999 and water levels in the aquifer have not changed substantially since the 1950s. An overall decline of about 2.5 feet has been observed since 1997, however.

The Aquia aquifer is 100 to 145 feet thick, with a top altitude of approximately 450 feet below the North American Vertical Datum of 1988 and a hydraulic conductivity of 6 to 10 feet per day at Naval Air Station Patuxent River. The Aquia aquifer is approximately 50 feet thick, with a top altitude of 470 feet below sea level and a hydraulic conductivity of 6 to 10 feet per day at Webster Outlying Field. Water levels in the Aquia aquifer declined in response to increased withdrawals from the aquifer from the early 1940s through about 2000 at Naval Air Station Patuxent River and Webster Outlying Field, but have been generally stable from about 1999 through April 2006.

The Upper Patapsco aquifer at the Naval Air Station Patuxent River and Webster Outlying Field consists of layers of sand interbedded with layers of clay that total over 200 feet in thickness. The top of the aquifer near Naval Air Station Patuxent River and Webster Outlying Field ranges from approximately 620 to 680 feet below the North American Vertical Datum of 1988. The potentiometric surface of the Upper Patapsco aquifer near Naval Air Station Patuxent River and Webster Outlying Field has been declining at a rate of about 2 feet per year for the past several years. Aquifer testing indicates the transmissivity of the aquifer is about 2,100 to 3,900 feet squared per day. Water samples collected from the Upper Patapsco aquifer at Naval Air Station Patuxent River and Webster Outlying Field in 2000 and 2001 met all Federal drinking-water standards.