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Legacy Hg-Cu Contamination of Active Stream Sediments in the Gold Hill Mining District, North Carolina

The purpose of this study is to evaluate the longitudinal trends of mercury (Hg) and copper (Cu) in active channel sediments downstream from the Gold Hill mining district in the Piedmont of North Carolina. Mining for gold (Au) and Cu from 1844 to 1915 released both Hg (associated with Au processing) and Cu in a 254 km2 watershed. Multiple linear regression is used to quantify spatial and geochemical trends in 93 active channel samples collected from contaminated main stem and background tributary sites. Simple two-parameter regression models combining the effects of both watershed-scale dispersal processes (distance downstream) and reach-scale sediment transport (percent sand) explain 85 percent of the variance in Hg and 90 percent of the variance in Cu in active channel sediments. Contamination trends in two different sediment media, low bar and higher elevation bench deposits, were effectively similar when local grain size influence was accounted for in the two-parameter models. Background geochemistry models explain 84 percent of the variance of Hg and Cu in uncontaminated tributary samples using parameters related to grain-size, secondary geochemical substrates, and mineral weathering sources. More than 45 percent of the variance of Hg and 20 percent of Cu in contaminated sediment can be explained by background parameters. Geochemical signatures differ between Hg and Cu in active channel sediments due to variations in mining inputs, background geochemistry, and present-day pollution sources.