Arsenic Contamination in the Water Supply of Milltown, Montana

Abstract

In 1981, four community supply wells in Milltown, Montana were found to contain arsenic levels ranging from 220 to The wells produced water from an unconfined aquifer composed of coarse sand, gravel and boulders, underlain by bedrock, adjacent to a small hydroelectric reservoir. Historical assessment showed that the contamination was long term. The reservoir sediments, that originated partially from large scale mining and milling operations up stream, were identified as the source of aquifer contamination. Reservoir water flowing through contaminated sediment produces arsenic-rich pore water that migrates vertically into the underlying highly conductive pre-dam alluvial floodplain sediments. This sulphur-limited, anoxic environment results in ground water arsenic concentrations up to ca. at the base of the fine-grained reservoir sediments. Arsenic is released at a rate of 1.6 MT/y from a study section covering of the reservoir. The groundwater flowing north from beneath this section, and into the adjacent aquifer, contains arsenic at a median concentration of 740 and discharges arsenic at approximately 24.6 MT/y. Pre-dam floodplain groundwater originating from beneath other portions of the reservoir is suggested as one source of the additional arsenic flux. Ground water transport into the adjacent Milltown area results in a reduction of arsenic concentrations with distance. Based on relationships with chloride and sodium, mechanical dispersion does not account for the observed reduction in arsenic concentrations. Analyses of Fe(II)/Fe(Total), As (III)/As(Total), As/Fe and As/sulphate suggest co-precipitation/precipitation of arsenic with metal sulfides and/or co-precipitation/adsorption with iron oxyhydroxides dominate arsenic reduction during transport. Estimates of available arsenic within the reservoir sediments suggest only 15 to 25 y of discharge would remain if no additional arsenic was added. However, arsenic concentrations are not declining in wells with long term records and arsenic bound in contaminated sediment from upstream is delivered at an average of 590 to 330 Chapter 12 740 MT annually. Current USEPA remediation alternatives range from leaving the existing system in place to total dam and reservoir sediment removal. However, it is not clear how well the consequences of such actions are understood.