ARSENIC IN POND BOTTOM SEDIMENTS: AN INDICATOR OF ELEVATED ARSENIC IN GROUNDWATER, CENTRAL MASSACHUSETTS

Many localities in a northeasterly trending zone in Central Massachusetts have been previously noted for significantly elevated arsenic levels in overburden geologic materials. Overburden arsenic data cluster in the range of 20 to 50 ppm of extractable As, but can reach values well in excess of 500 ppm. Although, the distribution pattern and source of arsenic is still a subject of ongoing studies, the ground waters percolating through overburden strata (and bedrock as well) often show also elevated arsenic, commonly in excess of the 10 ppb limit set by the EPA (typically 20 to 500 ppb). The mechanism of arsenic uptake to ground water (which is not the focus of this paper) appears to be either (1) presence of a reducing environment in the overburden; and/or (2) oxidation of As bearing sulfides in the bedrock. In this paper we present data on arsenic concentration in bottom sediments from ponds proximally located near the eastern margin of the elevated arsenic zone.

Samples of bottom sediments from a series of ponds reveal a pattern that correlates with the hydrologic regime of the area; three distinct groups of As concentrations are observed. One group of samples from an area not affected by the contaminated groundwater flow has As in the dried sediments in the 20 to 50 ppm range (same as the surrounding overburden). A second group of samples has a range between 80 and 150 ppm. This group corresponds to a discharge zone of groundwater contaminated by As in the range of 30 to 190 ppb. The last group is a sequence of samples with a steep progressive range from 200 to 7000 ppm. This group is correlated with a historic and ongoing contribution of highly contaminated ground water containing dissolved arsenic from 300 to 2000 ppb. Our study confirms cycling of arsenic from the overburden: (1) arsenic transfer from the overburden (solid phase) to ground water (dissolved), (2) ground water transport of arsenic into the surface waters and ponds; and (3) arsenic removal and accumulation in the pond bottom sediments (solid phase). During the discharge of arsenic contaminated ground water into the oxidized waters of the ponds, hydrous ferric oxide precipitates and concurrently absorbs dissolved arsenic species all accumulating in the pond bottom sediments. Arsenic accumulation over time can reach proportions that may pose a significant risk to the environment.