Effects of increased withdrawals from the Aquia aquifer on the Mayo Peninsula, Anne Arundel County, Maryland with an evaluation of water quality
2018, Gemperline, J.M., VanDerwerker, T.J., and Andreasen, D.C.
Report of Investigations 84
The Mayo Peninsula in east-central Anne Arundel County, Maryland relies primarily on the Aquia aquifer for drinking-water supply. The deeper Magothy aquifer is also utilized but to a much lesser extent. The lifting of a building moratorium in 2017 on the peninsula associated with increased capacity for treating sewage effluent may result in the construction of up to approximately 630 homes (an approximate 20 percent increase over the number of housing units in 2010). The potential effects of increased domestic water withdrawals to support the possible growth on water levels and on the brackish-water interface in the Aquia aquifer were evaluated. In addition, water quality of the Aquia aquifer, both natural and affected by anthropogenic contaminants, that might constrain the use of the aquifer for domestic potable supply was also evaluated.
EFFECTS OF INCREASED WITHDRAWALS FROM THE AQUIA AQUIFER
The potential increase of between 0.093 and 0.158 million gallons per day resulting from the possible new development on the Mayo Peninsula was evaluated using a one-layer numerical groundwater-flow model (MODFLOW) to estimate the effects on water levels in the Aquia aquifer and on the brackish-water/freshwater interface. The two rates are based on estimated household water-use rates of 147 and 250 gallons per day, respectively.
- Additional withdrawal from the Aquia aquifer resulting from new development will likely result in drawdown (lowering of the water table) of less than 0.25 feet for most of the peninsula.
- Potential drawdown from new development on the peninsula is greatest near Glebe Bay, where drawdown could reach approximately 2 feet.
- Where potential drawdown is greatest, the brackish-water/freshwater interface could rise by approximately 55 feet to more than 65 feet and pose a risk for existing wells near the shoreline.
- The effects of the potential increased withdrawals from the Aquia aquifer could be reduced by shifting pumpage to the deeper Magothy aquifer in areas of greatest drawdown.
EVALUATION OF AQUIA AQUIFER WATER QUALITY
Water quality of the Aquia aquifer on the Mayo Peninsula is generally good; however, in areas near the shoreline (less than 600 feet), chloride concentrations are commonly elevated resulting from brackish-water intrusion. Additionally, anthropogenic contamination from the surface may affect the potability of water from the Aquia. A total of 24 wells were sampled during this study to characterize the water quality of the Aquia aquifer; one well screened in the Magothy aquifer was also sampled for comparison.
- Three wells with chloride concentrations between 284 and 436 milligrams per liter and chloride/bromide mass ratios between 249 and 393 were determined to be affected by brackish-water intrusion. All of these wells are located near the shoreline (less than 600 feet).
- Eight wells with chloride concentrations between 81 and 221 milligrams per liter and a chloride/bromide mass ratio greater than 400 were determined to be affected by surface-based chloride sources.
- One well exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level for arsenic (0.01 milligrams per liter) and three wells exceeded the Maximum Contaminant Level for cadmium (0.005 milligrams per liter). Three wells exceeded Secondary Maximum Contaminant Level for chloride (250 milligrams per liter); seven wells exceeded the Secondary Maximum Contaminant Level for manganese (0.05 milligrams per liter); 17 wells exceeded the Secondary Maximum Contaminant Level for iron (0.3 milligrams per liter); and seven wells were outside the acceptable range for pH (6.5-8.5). Nine wells exceeded the Drinking Water Advisory for sodium (20 milligrams per liter).
- The uppermost portion of the Aquia aquifer is weathered (maximum depth of approximately 75 feet below land surface) and produces water that may be acidic (pH less than 6.5). Cadmium and chloride concentrations tend to be greatest in that portion of the aquifer.
- The deeper Magothy aquifer, an alternative drinking water supply on the peninsula, is less susceptible to surface contamination and brackish-water intrusion; however, iron concentrations can range from 14.0 – 26.0 milligrams per liter, requiring treatment for domestic supply.