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Abstract

It has been widely documented that in many parts of New England, background concentrations of arsenic in groundwater may exceed existing and/or proposed standards. Consequently, the need to document that concentrations detected above the standard values are, in fact, representative of naturally occurring arsenic concentrations and not the result of an anthropogenic release of arsenic to the environment has become increasingly important in recent years. The study site is located in central Massachusetts, adjacent to a tributary to a public water supply reservoir. During the course of an investigation to evaluate the potential for subsurface discharge of non-contact cooling water, groundwater samples were collected from overburden and bedrock aquifers. Laboratory analysis indicated that elevated concentrations of arsenic were present in several samples. At some of those locations, detected concentrations in overburden and bedrock groundwater exceeded the reportable concentration for arsenic under the Massachusetts Contingency Plan of 0.050 mg/l (subsequently decreased to 0.010 mg/l), which triggered the need for further evaluation of subsurface conditions. In this study, multiple lines of evidence were used to support the position that elevated concentrations of arsenic were due to natural dissolution of arsenic-bearing minerals in the overburden and bedrock aquifers. These lines of evidence included: historical information that did not support any use or disposal of arsenic-bearing materials; available geologic mapping; field observations of overburden and bedrock encountered during well drilling; total concentrations of arsenic, iron, and manganese and microprobe analyses indicating the presence of arsenic-bearing minerals in overburden materials; and evaluation of geochemical characteristics (pH, dissolved oxygen, oxidation/reduction potential) of groundwater samples.

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