Reports
Geohydrologic framework, ground-water quality and flow, and brackish-water intrusion in east-central Anne Arundel County, Maryland, with a section on Simulation of brackish-water intrusion in the Aquia aquifer in the Annapolis area using a solute-transport model
1996, Fleck, W.B., Andreasen, D.C., and Smith, B.S.
Report of Investigations 62
Abstract
This report focuses on the results of a study of the geohydrology of the geologic section that stratigraphically includes the Patapsco, Magothy, Matawan, Severn, Aquia, and Talbot Formations in east-central Anne Arundel County, Maryland. The principal aquifers within this Coastal Plain sequence of Holocene to Cretaceous sediments are the Aquia, Magothy, and Upper Patapsco. Results of aquifer tests at the Quiet Waters Park test site indicate that the horizontal hydraulic conductivity and the specific storage for the Aquia aquifer are about 4 feet per day and 4 x 10.6 foot-1, respectively. An average horizontal hydraulic conductivity of about 40 feet per day for the Magothy aquifer was calculated from specific-capacity measurements. The Upper Patapsco aquifer, which is about 200 to 300 feet thick, is equivalent to the uppermost sand unit in the Patapsco Formation.
Acidic precipitation that recharges the Aquia aquifer produces pH values as low as 4.4. This low-pH water, which also contains high dissolved-oxygen concentrations, reacts with the glauconitic and goethitic sand and weathered shell material ofthe Aquia aquifer to produce high dissolved iron and dissolved calcium and magnesium concentrations. The result is a calcium and magnesium bicarbonate-type water. The Magothy and Upper Patapsco aquifers lack significant quantities of shell material and, in contrast to the Aquia aquifer, contain an acidic iron sulfate-type water.
Brackish water that underlies freshwater within the Aquia and Monmouth aquifers wedges 200 to 500 feet inland along the shores of Annapolis Neck and the Mayo Peninsula. Four of eight test wells located along the shore and screened in the Aquia and Monmouth aquifers had chloride concentrations exceeding 5,900 milligrams per liter, indicating the presence of the brackishwater wedge. The nearshore brackish-water/freshwater interface is relatively sharp and steeply inclined; however, elevated chloride concentrations may indicate sources other than brackish-water intrusion. Three criteria were used to differentiate sources of elevated chloride concentrations: (1) the bromide:chloride ratio, (2) nitrogen concentrations, and (3) the theoretical location of the Ghyben-Herzberg interface. The water chemistry of the brackish-water wedge is affected by: (1) a chemically reducing, anoxic environment below the bottom of the Chesapeake Bay; (2) brackish-water dilution, and (3) cation exchange. These processes produce concentrations of iron and manganese as high as 240,000 and 2,500 micrograms per liter, respectively; increase concentrations of calcium, magnesium, sodium, potassium, and sulfate; and produce higher than predicted concentrations of calcium due to cation exchange with sodium.
A quasi-three-dimensional ground-water-flow model simulated hydraulic head changes for the transient time period 1900- 90. The model, which simulated the water-table and the Aquia, Monmouth, Magothy, and Upper Patapsco aquifers, was calibrated by matching hydrographs of 176 observation wells with simulated water levels. Analyses of rates of ground-water flow indicated that water that recharges the Magothy aquifer where the aquifer crops out under Round Bay requires about 300 to 1,700 years to travel to the point of discharge in the Upper Patapsco aquifer at the Arnold well field.
A two-dimensional solute-transport model for the Quiet Waters Park test site on Annapolis Neck simulated hydraulic heads and dissolved-solids concentrations as measured in 1990. Simulated equilibrium with respect to the 1990 conditions was approached after 600 years. Simulation results indicate that a small difference in well-screen depth with respect to the intelface of the brackish-water wedge makes a substantial difference in the freshwater life of a well. The extent of brackish-water intrusion is a function of the severity of the water-level declines in and near the site, which, in turn, is a function of the amount of freshwater diverted landward of the site and the proximity of pumped wells.
