DISTINGUISHING ANTHROPOGENIC SOURCES FROM NATURAL SOURCES OF DISSOLVED CONSTITUENTS IN A GLACIAL AQUIFER SYSTEM, WOODBURY, CONNECTICUT

A glacial aquifer system in Woodbury, Connecticut, was studied to identify factors that affect the ground-water quality in the contributing area to a public-supply well (PSW-1). Water samples were collected during 2002-2006 from PSW-1 and from 35 monitoring wells in glacial stratified deposits, glacial till, and fractured bedrock. The major chemical sources to ground water include (1) bedrock and glacial stratified deposits and till; (2) natural organic matter; (3) road salt (halite); (4) septic-system leachate; (5) organic solvent and gasoline station spills; and (6) surface-water leakage. Since these anthropogenic and natural sources can be difficult to distinguish, several constituents or conditions in ground water were used to identify them and their associated chemical processes, including the distribution of pH and concentrations of inorganic constituents and their ratios, recharge temperatures, ground-water age, dissolved organic carbon (DOC), volatile organic compounds (VOCs), and redox conditions.

Typically, ground water is derived from the glacial stratified deposits and is young (less than 7 years) and oxic, but localized reducing zones that result from abundances of organic matter can affect the mobilization of trace elements and the degradation of VOCs. Old ground water (more than 50 years) from fractured bedrock beneath the valley bottom that is manganese reducing to methanic constitutes as much as 6 percent of samples collected from wells screened at the bottom of the glacial aquifer. Dissolved arsenic and uranium concentrations generally are low, but water from a few wells screened in glacial deposits, likely derived from underlying Mesozoic rocks, contain arsenic concentrations up to 7 micrograms per liter. Concentrations of chloride, nitrate, DOC, and chloride/bromide ratios indicate that most samples result from mixing of ground water and road salt or septic-system leachate. Leachate from septic-tank drainfields can cause high nitrate concentrations in ground water and may provide up to 15 percent of the nitrogen in water from PSW-1. High concentrations of gasoline oxygenates and chlorinated solvents, and low concentrations of disinfection byproducts were detected in several wells, including PSW-1, and indicate an abundance of VOC sources from spills or septic-system leachate.