Reports
Summary of hydrogeologic data from a deep (2,678 ft.) well at Lexington Park, St. Mary's County, Maryland
1984, Hansen, H.J. and Wilson, J.M.
Open File Report 84-02-1
Abstract
Test well St.M.-Df 84 was drilled in Lexington Park, Maryland to a total depth of 2,678 feet below land surface. The purpose of the test well was to evaluate the ground-water potential of the Coastal Plain sediments occurring below the Aquia aquifer, which currently is the primary source of water for high-capacity wells in the area. Eight predominantly sandy intervals, ranging from 24 to 67 feet in thickness , were identified between the base of the Aquia aquifer at 637 feet and the base of fresh water at approximately 1,797 feet.
The shallowest of the pre-Aquia aquifers, which occurs between 830 and 910 feet, was chosen for aquifer testing. Forty-five feet of well screen interset with 35 feet of well casing were installed in this sandy interval. The aquifer material is typically a gray, medium-grained, moderately well-sorted quartzose sand. Aquifer parameters and water samples were obtained from a 24-hour pumping test. Biostratigraphic data were obtained from core samples.
Lithologic and paleontological evidence indicates the tested aquifer consists of deltaic to marginal marine beds of Early Paleocene age. It is tentatively correlated with a portion of the Mattaponi Formation of Cederstrom (1957), an important aquifer underlying the northern neck of Virginia. For the purposes of this report, the tested interval is called the Mattaponi(?) aquifer.
An evaluation of the pumping test data shows that the Mattaponi(?) aquifer at Lexington Park has a transmissivity value between 17,290 gpd/ft and 20,160 gpd/ft, and a hydraulic conductivity between 258 gpd/ft2 and 300 gpd/ft2. The storage coefficient of the aquifer, estimated from its barometric efficiency, is 0.000084. The static water level of the Mattaponi(?) aquifer at Lexington Park is about 7.4 feet below sea level.
Water from the Mattaponi(?) aquifer is of good quality and it can be used for most purposes with minimum treatment. It is alkaline (pH of 8.4); soft (hardness of 3 mg/l); and relatively low in dissolved solids (concentration of 186 mg/l). The temperature of the water at the discharge pipe was 69°F.
All of the Coastal Plain sediments below the Mattaponi(?) Formation are placed in the Potomac Group. The upper part of the Potomac Group, which contains seven sandy intervals with thicknesses varying between 24 and 64 feet, is assigned to the Patapsco Formation. In the absence of pumping test data, geophysical logs were used to estimate aquifer properties of the Patapsco sands. Effective porosities are in the 3 to 37 percent range and clay volumes estimated to be about 10 to 14 percent. These log-derived values are similar to comparable Mattaponi(?) values and at least broadly imply that their hydraulic conductivities may be similar. Estimates based on specific conductance values derived from an induction log suggest that the Patapsco aquifers contain ground water with total dissolved solids (believed to be chiefly sodium bicarbonate) in the 375 mg/l to 660 mg/l range. A relatively high discharge temperature can be expected, perhaps ranging from 73°F (at 1,075 feet) to 89°F (at 1,750 feet).
The sands of the lower part of the Potomac Group, assigned to the Patuxent and “Waste Gate” Formations, contain brackish water with estimated total dissolved solids in the 1,585 mg/l to 2,645 mg/l range. Additionally, these sands, which occur between about 2,003 feet and 2,620 feet, are generally thin (25 feet or less) and exhibit significantly lower effective porosities (22 to 29 percent) than the shallower Patapsco and Mattaponi(?) aquifers. Higher than expected bottom hole temperatures were recorded during logging runs at 1,926 feet (93°F) and 2,678 feet (121°F).
The top of the basement rock was logged at 2,623 feet. Cores of the rock were identified as diabase, which is a greenish black, medium grained, crystalline rock rich in plagioclase and pyroxene minerals. The core yielded a potassium-argon radiometric date of 169 ± 8 million years (Middle Jurassic).
