Geochemical Characterization of Mine Waste From the Pike Hill Superfund Site in Vermont, Usa

Abstract

The Pike Hill mines Superfund site consists of a group of mines that worked copper-rich Besshi-type massive sulfide deposits. The site contains the abandoned Smith, Eureka, and Union mines and was listed in 2004 as a Superfund site due to aquatic ecosystem impacts. This study is part of a larger project that includes mine waste characterization, surface-water geochemical studies, and documentation of downstream impacts on biota. It is intended to be a precursor to a formal remedial investigation by the U.S. Environmental Protection Agency (USEPA). The goal of this paper is to provide a relative comparison of the various waste piles through characterization of bulk geochemistry, mineralogy, paste pH, acid-base accounting, and metal leachability. In addition, results were compared to similar studies of mine waste from the nearby Ely and Elizabeth Superfund sites. Mine-waste samples at the Pike Hill mines include flotation-mill tailings and waste rock that is composed of fine-grained to bouldersized host rock and mineralized rock. The waste is primarily composed of silicates, oxides, sulfates, and sulfides, including pyrrhotite, pyrite, chalcopyrite, and sphalerite. Samples locally contain native sulfur and calcite; efflorescent sulfate salts have been observed on waste piles and adit walls. Composite minewaste samples contain concentrations of Cd, Cu, and Fe that exceed USEPA Preliminary Remediation Goals (PRGs). The concentrations of Se are elevated relative to the average composition of eastern U.S. soils. All mine-waste samples, except the processed flotation-mill tailings, which contain calcite, have paste pH values of 4 or less and negative net-neutralization potentials indicating the samples are acid generating. Twenty-four-hour leachate tests, which use a solution that approximates eastern U.S. precipitation, indicate that potentially toxic trace elements and acidity can be released under simulated weathering conditions. Mine waste at Pike Hill mines is chemically and mineralogically similar to that at the Elizabeth and Ely mines. Also, metals are leached and acid produced in comparable concentrations. Based on the results of this study, mine waste at the Pike Hill Superfund site contaminates soils, is acid generating, and may release potentially toxic metals to streams.