ARSENIC SPECIATION AND GROUNDWATER CHEMISTRY NEAR AND WITHIN A LANDFILL LEACHATE PLUME IN CENTRAL MASSACHUSETTS

A contaminant groundwater plume beneath a capped landfill in Devens, North-Central Massachusetts contains dissolved arsenic concentrations exceeding 15,000 ppb in some locations. The landfill includes solid waste materials deposited over the course of over 100 years and closed in the early 1990’s. Beneath the waste material is a several meter thick layer of peat believed to be a part of the historic marshland, a 30 to 100 ft. thick sequence of glacial lake deposits, and a 5 to 10 ft. layer of till overlying the bedrock. The primary source of arsenic has not yet been conclusively determined and likely involves a combination of the landfill waste material, the peat, and/or the underlying overburden sequence. Arsenic mobilization is likely assisted by reducing conditions created by the decomposition of organic materials within the landfill section. Another possibility is an arsenic bearing groundwater discharging from the underlying bedrock from the oxidation of naturally occurring sulfides. We are presently testing the possibility that arsenic speciation could aid in the better identification of the arsenic source(s) and its transport within the underlying aquifer. Typical arsenic species in natural waters include inorganic arsenite [As(III), As(OH)₃], arsenate [As(V), AsO(OH)₃], and methylated arsenic, monomethylarsonic acid [MMA(V), CH₃AsO(OH)₂], an dimethylarsinic acid [DMA(V), (CH₃)₂AsO(OH)]. Arsenic speciation data along with groundwater parameters such as pH, redox conditions, and other dissolved components may provide further insights about the arsenic sources, the process of its mobilization, and how it is transported through the aquifer.