ELEVATED ARSENIC IN GROUNDWATER AT LANDFILL SITES IN NORTHERN CENTRAL MASSACHUSETTS

Elevated arsenic concentrations in groundwater have been repeatedly observed near landfill sites in North Central Massachusetts where arsenic may or may not have been an obvious component of the disposed waste material within the landfill. Landfill leachate generated by excess water percolating through the waste layers is often associated with pollutants in the downgradient subsurface water. The discharge of landfill leachates into the groundwater system also strongly influences the chemistry of aquifers in the proximity of the landfills.

The studied landfills are located within 2 municipalities along the New Hampshire border where elevated arsenic concentrations have been previously reported in the substrate soils overlying bedrock of the Merrimack Belt. The landfills are situated within the Wisconsinan-age deposits of Glacial Lake Nashua and its tributaries. Arsenic concentrations in groundwater collected from these sites range from 5.4 micrograms per liter (ug/L) upgradient of the landfills up to over 4,200 ug/L downgradient of the leachate plume. Iron concentrations have been observed reaching over 220,000 ug/L and manganese concentrations have been observed over 9,200 ug/L. High concentrations of dissolved metals are concurrent with low dissolved oxygen values, ORP values ranging from 339 millivolts to –64 millivolts, pH values ranging between 4.88 and 6.70, and specific conductance values ranging from 20 microsiemens per centimeter upgradient to 850 microsiemens per centimeter downgradient.

Arsenic concentrations have been observed to show poor correlations with concentrations of sodium and chloride, suggesting a contribution from sources outside the landfills. Both sodium and chloride are landfill geochemical signatures indicating the contribution of the landfill leachates to ground water. Very high concentrations of arsenic correlate with increasing alkalinity and decreasing sulfate concentrations, suggesting a dependence of arsenic concentration on increasingly reductive environments.

High arsenic, iron, and manganese concentrations under relatively anoxic conditions are likely a result of the reductive dissolution of iron and manganese hydroxide coatings on the surfaces of the substrate material.