Reports
Hydrogeology, digital simulation, and geochemistry of the Aquia and Piney Point-Nanjemoy aquifer system in Southern Maryland
1983, Chapelle, F.H. and Drummond, D.D.
Report of Investigations 38
Abstract
The hydrogeology and ground-water geochemistry of the Aquia and Piney Point-Nanjemoy aquifers in Southern Maryland have been investigated. This study was made in order to evaluate the availability and chemical quality of water from this aquifer system.
The Aquia aquifer is Paleocene in age and is a medium- to fine- grained quartz sand. It is highly glauconitic and contains abundant carbonate shell material. The transmissivity of the Aquia ranges from 200 to 2,000 feet squared per day and tends to increase to the northeast along strike. The Aquia is overlain by the Marlboro Clay and lower Nanjemoy Formation which together act as a confining bed. This confining bed ranges from 100 to 250 feet thick and exhibits vertical hydraulic conductivities that range from 10-7 to 10-10 feet per second. In 1980, approximately 6 million gallons of water per day was produced from the Aquia aquifer in Southern Maryland. Several cones of depression ranging from 20 to 80 feet below sea level have developed in response to this pumping stress.
The Piney Point-Nanjemoy aquifer is Eocene in age and is a coarse- to fine-grained glauconitic quartz sand which contains abundant carbonate shell material. The transmissivity of the Piney Point-Nanjemoy aquifer ranges from 100 to 500 feet squared per day. The Piney Point-Nanjemoy is overlain by Chesapeake Group sediments which act as a confining bed. This confining bed ranges from 150 to 250 feet thick and exhibits vertical hydraulic conductivities which range from 10-7 to 10-10 feet per second. In 1980, approximately 2 million gallons of water per day was produced from the Piney Point-Nanjemoy aquifer in Southern Maryland.
Hydrogeologic information including water levels, altitude of top, thickness, and transmissivity of the Aquia and Piney Point-Nanjemoy aquifer system is presented as a series of maps. Hydrogeologic information such as thickness, vertical hydraulic conductivity, and specific storage of confining beds is also presented. This information combined with geologic cross sections obtained during bridge construction across the Chesapeake Bay and the Patuxent River demonstrate that Pleistocene river channels have in places truncated aquifers and confining beds. This erosional truncation is a major control on the natural head distribution of the Aquia and Piney Point-Nanjemoy aquifer system.
A quasi three-dimensional digital model of this aquifer system was constructed. The Aquia and Piney PointNanjemoy aquifers were modeled as confined aquifers separated by semipermeable confining material. Recharge to these aquifers was considered to occur by leakage from the overlying Pleistocene water-table aquifer. The pumping history of the aquifer system was simulated from 1890 to 1980 during model calibration. The model was calibrated by adjusting the vertical hydraulic conductivity of confining beds and matching calculated water levels to measured water levels. Recharge rates to the aquifer system during the simulated pumping history varied from 0.22 to 0.57 inches per year. Simulations with the calibrated model indicate that the aquifer system can sustain the 1980 rates of pumpage to the year 2000 with water-level declines of about 0.5 feet per year in the Aquia aquifer. Simulations that increase pumpage according to projected population growth predict water-level declines on the order of 1.5 feet per year in the Aquia aquifer to the year 2000. A simulation which assumes that all holders of Gound-water Appropriation Permits pump their maximum allocation for 10 years predicts water-level declines of 8 feet per year near Lexington Park and Piney Point in southern St. Marys County, Maryland.
The principles of equilibrium chemistry were utilized to investigate the ground-water geochemistry of the Aquia and Piney Point-Nanjemoy aquifers. Trends of calcium, magnesium, sodium, and bicarbonate concentrations plotted versus distance along the flowpath delineate three regions in the aquifers where different patterns of concentration changes occur. Chemical models were constructed and tested for each region to account for the observed changes in water chemistry. Mass balance calculations based on the verified models suggest that observed calcite cementation of the Aquia aquifer is post-depositional and has resulted from the reaction of ground water with aquifer material.
Downloads and Data
Report of Investigations 38 (pdf, 78 MB)Plate 1 (pdf, 2.3 MB)Plate 2 (pdf, 2.2 MB)Plate 3 (pdf, 2.2 MB)Plate 4 (pdf, 2.3 MB)Plate 5 (pdf, 1.9 MB)Plate 6 (pdf, 1.5 MB)Plate 7 (pdf, 1.4 MB)Plate 8 (pdf, 1.4 MB)Plate 9 (pdf, 2.0 MB)Plate 10 (pdf, 1.7 MB)Plate 11 (pdf, 1.8 MB)Plate 12 (pdf, 0.99 MB)Plate 13 (pdf, 0.99 MB)