ARSENIC EQUILIBRIA IN GROUND WATER AT LANDFILLS, CENTRAL MASSACHUSETTS

Central Massachusetts is a region with above normal range of natural arsenic concentrations (10 to 1000 mg/kg) in the overburden overlying bedrock sequences of the Merrimack Belt. Hydrochemical characteristics of ground water near landfills within this zone indicate a substantial mobilization of arsenic beneath and immediately down gradient from these sites. Arsenic levels, typically less than 30 mg/L up gradient, range from 50 to 700 mg/L down gradient occasionally exceeding 5000 mg/L. High arsenic concentrations correlate with iron concentrations of up to 220,000 mg/L and manganese concentrations of up to 9,250 mg/L. ORP ranges from 339 mV to –64 mV and pH values range between 4.88 and 6.70. Our study of 57 ground water samples from five landfill sites indicates that the source of arsenic is most likely due to a reductive dissolution of HFO coatings and release of sorbed arsenic complexes. All samples were analyzed for major and minor dissolved constituents as well as characterized by water quality parameters in the field. Chemical parameters analyzed as part of this study include alkalinity, chloride, fluoride, nitrite, nitrate, bromide, sulfate, ortho-phosphate, sodium, ammonium, potassium, magnesium, calcium, lithium, iron, manganese, arsenic, DOC, DIC, temperature, pH, ORP, conductance, dissolved oxygen, and turbidity. Our objective for this study is to evaluate the role of hydrochemical constraints, if any, on the levels of arsenic present in the down gradient ground water. The Geochemist’s Workbench program is applied to model As, Fe, and Mn using the measured constituents in the liquid solutions. Eh - pH modeling of arsenic in the up gradient section of ground water points to the presence of singly and doubly dissociated pentavalent arsenic (H₂AsO₄- and HAsO₄--) whereas in the contaminated down gradient ground water the arsenic is in the form of undissociated H₃AsO₃. No arsenic bearing solids in equilibrium with ground water have been identified. However, there is an evidence for precipitation of solids containing iron and manganese. In the case of manganese the model indicates saturation with respect to rhodochrosite and for iron saturation with respect to siderite and pyrite. Arsenic variation diagrams suggest that the solid phases of Mn or Fe appear to have no effect on the dissolved arsenic.