Reports
Hydrostratigraphic framework of the Piney Point-Nanjemoy aquifer and Aquia aquifer in Calvert and St. Mary's Counties, Maryland
1996, Harry J. Hansen
Open File Report 96-02-8
Abstract
Aquifers and confining beds underlying Calvert and St. Mary's Counties, Maryland were initially identified by correlation with lithostratigraphic units, including the Miocene Chesapeake Group, the Eocene Piney Point and Nanjemoy Formations, the Paleocene Marlboro Clay, and the Paleocene Aquia and Brightseat Formations. Although both formations and aquifers are mappable rock bodies, their boundary discontinuities (contacts) are different. A dual nomenclatural system can be ambiguous because geologically significant, sand-on-sand contacts are not defining boundaries in a hydraulic sense.
The following aquifer-confining bed sequence illustrates the need for stratigraphers and hydrogeologists to define their mapping units carefully in areas where a dual nomenclatural system is used:
Upper Confining Bed -- This unit consists of the undivided Chesapeake Group (St. Mary's, Choptank, and Calvert Formations), except for the basal beds of the Calvert Formation.
Piney Point-Nanjemoy Aquifer -- Although the Piney Point aquifer and Nanjemoy aquifer can be differentiated on geophysical logs, they usually function as a single hydrologic unit, consisting of a basal sand of the Calvert Formation; unnamed, thin and patchy sands tentatively assigned to the Oligocene(?); the Piney Point Formation; and the upper, sandy facies of the Nanjemoy Formation.
Middle Confining Bed -- This unit consists of fine-grained sediments in the lower part of the Nanjemoy Formation and the Marlboro Clay.
Aquia Aquifer -- In Calvert and St. Mary's Counties, Md., the Aquia aquifer correlates with a major sand facies of the Aquia Formation, except for fine-grained sediments at the base of the unit that may be undifferentiated from the underlying confining bed.
Lower Confining Bed -- The Paleocene Brightseat Formation overlies the truncated edges of various upper and lower Cretaceous units, creating a locally complex hydrostratigraphic unit that include fine-grained sediments occurring both above and below a regional unconformity.
