GEOCHEMICAL ANALYSIS OF SEDIMENTS IN THE OMPOMPANOOSUC RIVER NEAR THE ELIZABETH MINE IN SOUTH STRAFFORD, VERMONT

The Elizabeth Mine, an abandoned copper mine in South Strafford, Vermont, is leaching highly acidic runoff into the West Branch of the Ompompanoosuc River from a forty-acre tailings pile. The mine is included in the massive sulfide deposits of the Vermont copper belt, which consists of stratabound and stratiform pyrrhotite, chalcopyrite and minor sphalerite and pyrite within metasedimentary and mafic metavolcanic rocks of Silurian to early Devonian age. Discovered in 1783, the mine originally produced copperas (iron sulfate) from pyrrhotite and later (1830-1958) produced copper from chalcopyrite. Previous studies indicate that the mine is a source of acid leachate, metals and silt to Copperas Brook and the Ompompanoosuc River. The main objective of this study is to assess temporal and spatial variations in metals concentration and speciation in sediments in the West Branch of the Ompompanoosuc River.

Bulk sediments (<1 mm fraction) were analyzed for trace and major element composition by fusion and inductively coupled argon plasma (ICAP) spectrometry. ICAP analysis of trace elements shows elevated levels (as compared to upstream, uncontaminated sites) of metals at the confluence of Copperas Brook-Ompompanoosuc River, including cadmium (~10x), chromium (~2x), copper (~10x), and zinc (~9x). Concentrations of both cobalt and titanium show elevated levels at all sites, ranging from ~0.5-5x and ~1.2-2.5x, respectively. Major elements such as iron oxide (~10x) and aluminum oxide (~1.4x) are also elevated in sediments from the Copperas-Ompompanoosuc confluence. Samples taken downstream show slightly elevated levels of Al₂O₃, Fe₂O₃, MgO and TiO₂, ranging from 1.2x to 3.4x. A surface sediment sample taken ~7.7 km downstream from the Copperas Brook-Ompompanoosuc River confluence contained trace metal concentrations higher than any site other than the Copperas-Ompompanoosuc confluence, indicating that Lords Brook, which drains an open pit to the south of the main tailings impoundment, is a significant source of metals to the Ompompanoosuc River. Further study to assess speciation will include additional ICAP analysis, x-ray diffraction (XRD), sequential chemical extraction and pH.