Reports
Hydrogeology, water quality, and water-supply potential of the Aquia and Magothy aquifers in southern Anne Arundel County, Maryland
2002, Andreasen, D.C.
Report of Investigations 74
Key Results
Water use in Southern Anne Arundel County consists primarily of self-supplied domestic use from the Aquia aquifer. The deeper Magothy aquifer is utilized to a lesser extent as a source for mobile home parks and irrigation. A total of approximately 1.8 and 0.22 million gallons per day were pumped from the Aquia and Magothy aquifers in 2000, respectively. Total water demand in Southern Anne Arundel County may increase from about 2.0 million gallons per day in 2000 to a total of 2.8 million gallons per day by 2020 to support a projected population of 32,750. The Aquia and Magothy aquifers are the most likely sources for future withdrawals, although deeper aquifers in the Potomac Group are also available. The natural water quality of the Aquia and Magothy aquifers is generally acceptable for self-supplied domestic use; however, the Magothy aquifer contains iron concentrations at levels requiring treatment. A ground-water-flow model of the Aquia and Magothy aquifers was developed to determine: (1) the effect of projected (2020) water use on water levels in the Aquia and Magothy aquifers; (2) maximum yield of the Aquia and Magothy aquifers; and (3) effect on water levels from pumpage in the Aquia aquifer in Calvert and St. Mary's Counties. The model consisted of three layers representing the water-table aquifer, the Aquia aquifer, and the Magothy aquifer. The model was calibrated using water levels measured during 2000, and by comparing simulated and measured water levels for observation wells with long-term record.
What are the hydrogeologic characteristics of the Aquia and Magothy aquifers in Southern Anne Arundel County? (pgs. 9 to 32)
- The Aquia aquifer is a medium to coarse sand consisting of clear and white quartz and dark-green, brown, and black glauconite. Shell beds and iron- or calcite-cemented layers are common. The Magothy aquifer consists of medium to coarse, well sorted, white sand interbedded with black and gray, lignitic clay.
- The top of the Aquia aquifer ranges from about 100 feet above sea level in its outcrop area northwest of the study area to as much as 250 feet below sea level in the southern part of the study area. By comparison, the top of the Magothy aquifer ranges from 15O to 500 feet below sea level at those same locations.
- Transmissivity of the Aquia and Magothy aquifers ranges from 930 to 2,680 feet squared per day and 450 to 4,570 feet squared per day, respectively.
- Water levels have declined at rates ranging from 0.2 to 1.4 feet per year in the Aquia aquifer and 0.7 to 0.9 feet per year in the Magothy aquifer since the 1970s.
- The available drawdown in the Aquia aquifer in 2000 ranged from 0 feet through the central part of Southern Anne Arundel County to 150 feet at Rose Haven. Water levels exceeded the management level within a 2-mile-wide band located in the central part of Southern Anne Arundel County. By comparison, available drawdown in the Magothy aquifer in 2000 ranged from approximately 125 feet in the Davidsonville area to 360 feet at Rose Haven.
What is the natural water quality of the Aquia and Magothy aquifers? (pgs. 38 to 41)
- Water produced from the Aquia and Magothy aquifers is a calcium bicarbonate type. The pH of the Aquia aquifer ranges from 7.3 to 8.0, while the pH of the Magothy water is about neutral (pH=7).
- Iron concentrations in the Aquia and Magothy aquifers range from 0.15 to 4.5 and 3.6 to 7.6 milligrams per liter, respectively. Iron concentrations greater than about 0.3 milligrams per liter may require treatment to increase potability.
What is the simulated effect of pumping an additional 0.8 million gallons per day from the Aquia and Magothy aquifers to support a projected 2020 population of 32,750 in Southern Anne Arundel County? (pgs. 59 to 67)
- If withdrawn from the Aquia aquifer, the increased withdrawals combined with increased withdrawals to the south will cause water levels in the Aquia aquifer to decline by as much as 22 feet. The Aquia aquifer can supply the projected 2020 water demand without depleting the available drawdown in most of Southern Anne Arundel County. However, water levels exceed the management level (as currently defined) in a band as much as 3.5 miles wide extending from Waysons Corner to Rhode River. Constraining the use of the Aquia aquifer in Southern Anne Arundel County and Calvert County will reduce drawdown in Southern Anne Arundel County.
- If withdrawn from the Magothy aquifer, the increased withdrawals combined with regional withdrawals will cause water levels in the Magothy aquifer to decline about 20 feet by 2020. Available drawdown ranges from 100 to 350 feet. The Magothy aquifer can supply the projected increase in water demand through either individual residential wells or public-supply wells without a significant reduction in available drawdown. However, greater drilling depths, treatment costs for the removal of iron, and the practicality and expense of centralized public-water systems are important considerations related to its use. Water levels in the Aquia aquifer will not be affected by the increased pumpage from the Magothy aquifer.
Will water levels in the Aquia and Magothy aquifers in Southern Anne Arundel County stabilize if withdrawals in these aquifers are held constant at the 2000 amount? (pg. 67)
- When Aquia withdrawals in the model area and in areas farther to the south are held constant at the 2000 level ( 1.8 million gallons per day in Southern Anne Arundel County), simulated water levels in the Aquia aquifer stabilize with respect to those withdrawals in less than 1 year.
- Simulated water levels in the Magothy aquifer stabilize within about 3 months when withdrawals in the Magothy aquifer are held constant at the 2000 level (0.22 million gallons per day in Southern Anne Arundel County).
What is the total simulated water-supply potential of the Aquia and Magothy aquifers in Southern Anne Arundel County? (pgs. 67 to 73)
- Available drawdown in the Aquia aquifer in most areas can sustain an increase in withdrawals. In addition, recharge from the outcrop area could be induced by the increase in withdrawals. However, increasing withdrawals from the Aquia aquifer will cause water levels to exceed management levels in the central part of Southern Anne Arundel County. As of 2000, water levels in the Aquia aquifer have exceeded the management level within a 2-mile-wide band located in the central part of Southern Anne Arundel County. Therefore, as defined by the present management guideline, the Aquia aquifer in Southern Anne Arundel County has reached its maximum allowable yield.
- The maximum simulated yield of the Magothy aquifer is approximately 38 million gallons per day based solely on the available drawdown in the Magothy aquifer. When pumped continuously at this rate at 26 hypothetical wells for the period 2000 to 2020, water levels in the Magothy aquifer are as deep as 280 feet below sea level in Southern Anne Arundel County. The vertical hydraulic gradient that develops between the Magothy and Aquia aquifers under this pumping scenario causes some leakage of water from the Aquia aquifer downward to the Magothy aquifer. Simulated water levels in the Aquia aquifer decline by as much as 22 feet from 2000 levels. Lowering pumpage in the Magothy aquifer to 7 million gallons per day reduces drawdown in the Aquia aquifer to less than 4 feet. The drawdown caused by pumping the Magothy aquifer at 7 million gallons per day can be offset by reducing pumpage in the Aquia aquifer to a rate 25 percent below the 2000 level (or approximately 1.4 million gallons per day). Reducing pumpage in the Aquia aquifer further will allow a greater amount to be pumped from the Magothy aquifer.
What is the simulated effect of projected 2020 withdrawals from the Aquia aquifer in Calvert and St. Mary's Counties on water levels in Southern Anne Arundel County? (pgs. 74 to 75)
- Withdrawals from the Aquia aquifer are projected to increase to 8.2 million gallons per day in Calvert County and 11.3 million gallons per day in St. Mary's County by 2020. Pumpage values, derived from population projections made by the Maryland Office of Planning for Calvert County and the St. Mary's County Metropolitan Commission for St. Mary's County, represent high growth estimates for each county. Simulated withdrawals in Calvert and St. Mary's Counties cause water levels in the Aquia aquifer to exceed the management level in a band as much as 3 miles wide, extending from Waysons Corner to Rhode River.
- Constraining withdrawals in Calvert County will reduce the amount of future drawdown in Southern Anne Arundel County. Decreasing withdrawals in Calvert County to 6.1 million gallons per day based on a 2020 population projection by the Calvert County Department of Planning and Zoning reduces the width of the area where the management level was exceeded by about 1 mile.
