Reports
Hydrogeology and estimation of ground-water contributing areas of the Perryman well field, Harford County, Maryland
1997, Drummond, D.D. and Johnston, R.P.B.
Report of Investigations 63
Abstract
The protection of public ground-water supplies from chemical contamination is a major priority of federal, state, and county governments. As part of a wellhead protection program, the hydrogeology of the Perryman area in Harford County, Maryland was studied in order to estimate the extent of contributing areas of the wells in the Penyman well field. The Perryman well field consists of eight production wells screened between 45 and 192 ft below land surface, which pumped an average of 2.2 million gallons per day of ground water in 1994. Water from several of these wells has had nitrate concentrations exceeding the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (10 mg/L as N), and beginning in 1992, analyses of water from two of the wells showed concentrations of TCE (trichloroethene) that exceeded the MCL (5 ug/L). Agricultural application of ferti lizer and discharges from on-site septic systems are poss ible sources of nitrate; and military, commercial, and industrial activities are possible sources of TCE. TCE-contaminated soil at the Army Fire Training Area (AFTA) on Aberdeen Proving Ground, is a possible source of TCE.
The Perryman area is underlain by fluvial clay, silt, sand, and gravel sediments of Lower Cretaceous and Quaternary ages. The sediments create a system of irregularly shaped aquifers and confining units which produce complex ground-water flow paths. These sediments were divided into three aquifers (designated aquifers 1,2, and 3) and two intervening confining units (designated confining units 1 and 2). Aquifer 1 is a water-table aquifer, and ranges in thickness from 0 to 85 ft. Aquifers 2 and 3 are semiconfined to confined aquifers, and range in thickness from 0 to 105 ft, and 0 to 100 ft respectively. These aquifers are underlain by relatively low-permeability bedrock of Paleozoic age. The natural flow gradient is from the central part of the study area outward toward the tidal estuaries (Chesapeake Bay, Swan Creek, and Bush River).
A ground-water flow model was developed to simulate hydraulic heads and flow for present (1994) conditions, and for projected- pumpage scenarios. A particle-tracking program was used to estimate: 1) the contributing areas of wells in the well field, 2) traveltime of water entering the wells, 3) migration of the TCE plume, and 4) TCE concentrations in water from wells in the Perryman well field. Simulations indicate that contributing areas for the wells in the Perryman well field extend about one-half mile to the southwest and about two miles northeast of the well field. Traveltime for water entering the wells ranges from a few years to more than 500 years, but most of the contributing areas are within the 0-20 year traveltime zone.
Simulations indicate that if 1994 pumpage were continued for 20 years, heads would not change appreciably from 1994 heads, and that TCE concentrations at the contaminated wells would decrease due to a decline in pumpage prior to 1994 and the removal of TCE-contaminated soil from the AFTA. Under these conditions, the main part of the TCE plume would migrate to the southeast toward the Chesapeake Bay. Simulations in which pumpage is increased for 20 years show a concomitant increase in TCE concentrations at the contaminated wells, and simulations in which pumpage is decreased for 20 years show a decrease in TCE concentrations. A simulation in which the contaminated wells are shut off for 20 years indicates that the TCE plume would migrate westward toward a previously uncontaminated well.
