Hydrogeology, water-supply potential, and water quality of the Coastal Plain aquifers of Harford County, Maryland
1993, Drummond, D.D. and Blomquist, J.D.
Report of Investigations 58
A study was conducted to investigate the hydrogeology of the Coastal Plain aquifers of Harford County, Maryland. The study refined the hydrogeologic framework, assessed the potential for the aquifer system to meet projected pumpage requirements, and documented chemical quality of ground water.
The Coastal Plain aquifer system of Harford County comprises a wedge-shaped body of sediments which dips and thickens to the southeast and is underlain by hard crystalline bedrock. The sediments comprise three lithostratigraphic units: the Talbot Formation of Pleistocene age, the upland gravels of Late Tertiary age, and the Potomac Group of Early Cretaceous age. The Talbot Formation consists of a shallow silty clay facies and a deeper gravelly sand facies interbedded with dark clay layers. The upland gravels form isolated patches of intercalated sand and gravel and thin lenses of silty clay, which cap some of the higher hills near the Fall Line. The Potomac Group consists of sand and gravel units intercalated with multicolored clay lenses. The lithology ofthe bedrock underlying the Coastal Plain sediments is poorly documented but probably consists of Piedmont schistose, gneissic, gabbroid, and amphibolide rocks observed in outcrops northwest of the Fall Line.
The Coastal Plain sequence was divided into four aquifers and three intervening confining units. The aquifer boundaries do not generally coincide with the stratigraphic unit boundaries. The aquifers were designated as aquifers 1, 2, 3, and 4, and the confining units were designated as 1, 2, and 3. This aquifer division is somewhat arbitrary and is poorly defined in places.
Base-flow measurements were made at several points along each of the seven major streams that drain the study area to estimate the amount of ground water that discharges into the surface-water system. Discharge ranged from 0 to 79.4 cubic feet per second, and groundwater specific discharge ranged from - 4.7 X 10-3 to 11.7 x 10-3 feet per day, with a weighted average of 1.4 x 10-3 feet per day.
Almost all recharge to the regional ground-water system comes from precipitation, although small amounts could also be derived from losing reaches of streams and from brackish-water intrusion from the estuaries. The recharge water then flows through the aquifer system in directions and at rates that are controlled by aquifer properties, confining-unit properties, and head gradients.
A quasi-three-dimensional, finite-difference ground-water flow model was developed to simulate the response of water levels to projected pumpage in Coastal Plain aquifers. The effects of projected pumpage on water levels was evaluated by entering various pumpage scenarios into the model and simulating water levels until the year 2000 under average annual recharge conditions. Additional simulations were made with recharge reduced by 35 percent for 1 year (year 2001) and 3 years (year 2001 through 2003) to evaluate the effect on water levels of a drought similar to the drought in the mid-1960's.
A simulation in which pumping at the 1989 levels was continued until the year 2000 showed no significant additional 'drawdotvn from the 1989 potentiometric surfaces. A simulation in which 1989 pumpage was increased by 20 percent showed additional drawdowns of 4 feet in aquifer 2 at the Aberdeen well field and 4 feet in aquifer 3 at the Perryman well field. The greatest simulated drawdowns occurred in the "safe yield" simulation, in which pumping at the Perryman well field was increased until simulated pumping-water levels were reduced to 80 percent of available drawdown. Total "safe-yield" pumpage at the Perryman well field was 9.2 and 8.3 million gallons per day for average recharge and 1-year drought conditions, or 3.0 and 2.7 times the 1989 pumpage. Other alternative pumpage scenarios were simulated, such as the addition of a well field at Trimble Road, the addition of a well field at a powerplant near Perryman, the addition of a well field at a bleach-bottling plant near Perryman, average and maximum ground-waterappropriation permit amounts, and additional wells at the Perryman well field. The drought simulations indicate that water levels in aquifers 1, 2, and 3 generally would be about 5 feet deeper for a 1-year drought than for average recharge conditions, and about 14, 11, and 8 feet deeper, respectively, for a 3-year drought. Water levels in aquifer 4 were only slightly deeper in the drought simulations than under average recharge conditions.Particle-tracking analysis and examination of head gradients and fluxes were used to evaluate the potential for brackish-water intrusion and contaminant migration to pumping centers as a result of projected amounts of pumpage. The analysis indicated that the most probable projected pumping scenarios are not likely to result in brackish-water intrusion but could result in contaminant migration from Aberdeen Proving Ground.
Ground water in the Coastal Plain aquifers is mildly acidic and contains low total-dissolved-solids concentrations. The chemical facies of ground water are variable throughout the Coastal Plain. Water from deep confined aquifers is the chemically stable sodium bicarbonate type. Elevated concentrations of chloride and nitrate in ground water in much ofthe unconfined aquifer indicate that these contaminants are derived from anthropogenic sources. Other regional water-quality problems include the presence of iron and manganese in concentrations exceeding USEPA (U.S. Environmental Protection Agency) secondary maximum contaminant levels. Analyses of carbon-14 and tritium in ground water indicate that residence time in aquifer 2 is low (less than 50 years) and residence time in the deeper confined system is greater than 43 years.
Water-quality analyses from four Coastal Plain streams generally reflect the composition of ground water. Water from James Run and Winters Run have chemical facies similar to facies in ground water in the Piedmont. Water from Fosters Branch and Haha Branch is similar to the less mineralized water in the shallow Coastal Plain aquifers. Mass-balance calculations of upstream and downstream analyses in Cranberry Run were used to estimate the ionic concentration of ground-water discharge. The estimated discharge concentrations are averages of water concentrations from the unconfined aquifer, and indicate anthropogenic sources of chloride and nitrate in the Aberdeen and Perryman area of the Coastal Plain.