Reports
The geohydrology and water-supply potential of the Lower Patapsco aquifer and Patuxent aquifers in the Indian Head-Bryans Road area, Charles County, Maryland
1999, Andreasen, D.C.
Report of Investigations 69
Key Results
The Indian Head-Bryans Road area, in northwestern Charles County, relies primarily on ground water pumped from the lower Patapsco aquifer. Approximately 1. 8 million gallons per day were withdrawn in 1997. Increasing population in the area may require an additional 5 million gallons per day by 2020. The upper Patuxent aquifer, lying beneath the lower Patapsco, is a potential source for the projected increase in water demand. The lower Patapsco and upper Patuxent aquifers are the primary sources of ground water in the Indian Head-Bryans Road area. (pgs. 9 to 32)
- The aquifers consist of fine to coarse sand separated by layers of dense clay.
- Transmissivity-a measure of the ability of the aquifer to transmit water- ranges from 190 to 1,100 feet squared per day in the lower Patapsco aquifer and 152 to 2,600 feet squared per day in the upper Patuxent aquifer. The lower Patuxent aquifer is less transmissive (80 feet squared per day) at the one site tested.
- The productivity of the lower Patapsco aquifer is constrained by less available drawdown in "updip" areas along the Potomac River. At the 1997 pumping rate (1.8 million gallons per day) and pumping-well distribution, water levels in the lower Patapsco aquifer have reached the 80-percent management level along the Potomac River shoreline in the central part of the Indian Head Peninsula.
- The Arundel Clay provides an effective hydraulic seal between the two aquifers in most places.
- The lower Patapsco and upper Patuxent aquifers exhibit lateral hydraulic continuity with the Waldorf-La Plata area. Lateral hydraulic continuity of the upper Patuxent aquifer is apparently disrupted by a flow barrier, the location of which is estimated to be northwest of Bryans Road.
Ground-water-flow model simulations which simulate conditions from 1998 to 2020 indicate that a total of 4 million gallons per day is available for future use from the combined lower Patapsco and upper Patuxent aquifers in northwestern Charles County. (pgs. 62 to 68)
- The lower Patapsco aquifer is capable of producing an additional 0.6 million gallons per day beyond the 1997 rate of approximately 2 million gallons per day (2.6 million gallons per day total) provided that 0.5 million gallons per day of existing production along the Potomac River is shifted to areas southeast of the Bryans Road-Indian Head area.
- The upper Patuxent aquifer is capable of supplying 3.4 million gallons per day before drawdowns reach the 80-percent management level along the Potomac River shoreline.
Projected 2020 water use in the lower Patapsco and upper Patuxent aquifers will cause significant water-level declines. (pgs. 60 to 62 and 73 to 76)
- Water levels in the lower Patapsco aquifer may decline by as much as 60 feet in the vicinity of Bryans Road and 20 feet at Indian Head between 1997 and 2020 because of increased pump age from the lower Patapsco aquifer outside the Indian Head-Bryans Road area. Water-level declines exceed the 80-percent management level along the Potomac River in the central part of the Indian Head peninsula. The 80-percent management level is also exceeded in a small area northwest of Bryans Road along the Potomac River.
- The lowest water levels in the upper Patuxent aquifer simulated as model-cell averages are 330 feet below sea level based on withdrawals from the upper Patuxent aquifer of 2.1 million gallons per day, and 540 feet below sea level based on withdrawals of 4.0 million gallons per day.
- The upper Patuxent aquifer begins to de-water within a narrow band located along the Potomac River shoreline near Chapman's Landing and in some pumping wells when pumped at a rate of 4.7 million gallons per day; model-cell water levels are as low as 650 feet below sea level.
Pumping from the upper Patuxent aquifer will cause minimal head decline in the lower Patapsco aquifer. (pg.76)
- Drawdown in 2020 in the lower Patapsco aquifer caused by 3.4 million gallons per day withdrawn from the upper Patuxent aquifer is less than 18 feet. Water from the upper and lower Patuxent aquifers is a sodium bicarbonate type of good quality. (pg. 37)
- Dissolved concentrations of all major inorganic constituents were within the recommended limits set by the U.S . Environmental Protection Agency. Dissolved solids, pH (field), and specific conductance (laboratory) range from 214 to 378 milligrams per liter, 7.7 to 7.9, and 322 to 627 microsiemens per centimeter, respectively.
