ARSENIC MOBILITY IN AN OVERBURDEN AQUIFER IN CENTRAL MASSACHUSETTS

Two public water-supply wells, located at the edge of a pond in north-central Massachusetts, have produced arsenic at approximately 20 to 30 µg/L for decades. Numerous postulated sources include the underlying bedrock, historical use of pesticides in local orchards, and discharge of wastes from a former tannery into the pond. Arsenic, as well as other metals, is present in the pond sediment and pore water at levels around 100 mg/kg and 100 µg/L, respectively. The production wells are screened from 40 to 60 ft below ground surface (bgs) in glacial sands and gravels. Local bedrock consists of fractured Paleozoic metaconglomerate containing graphite and sulfide minerals, including pyrite and cobaltite (CoAsS). During installation of monitoring wells for this study, soil and groundwater samples were collected along vertical profiles between the top of the overburden aquifer and bedrock at three locations. Reducing conditions were encountered in the upper ~45 ft of the aquifer. Throughout this interval, total arsenic and iron are strongly correlated and reach maxima of 189 µg/L and 21,900 µg/L, respectively. Below a redox boundary at the top of the producing horizon (~40 ft bgs), dissolved arsenic and iron levels drop sharply. Soil analyses show significant correlations between solid-phase iron and arsenic, aluminum, cobalt, copper, manganese, nickel, and zinc. These data and results obtained using PHREEQC to model arsenic adsorption are consistent with the reductive dissolution of iron oxides in the upper part of the aquifer and release of sorbed arsenic. Major-element chemistry from the close-interval groundwater sampling suggests that the pumping wells produce a mixture of waters from the upper and lower portions of the aquifer. The arsenic observed in the water-supply wells is tentatively attributed to the reductive-dissolution mechanism and mobilization of arsenic in the upper aquifer, and not to an anthropogenic source.