University of California Water Resources Center

More than three million kilograms of mercury are estimated to have been lost into northwestern Sierra Nevada rivers during the course of gold mining in the Gold Rush period of the last century (1840s - 1880s). Mercury was used extensively in the gold recovery process to amalgamate fine gold particles. Gold mining has continued at a less intensive scale through the present, with a relative resurgence of dredging operations during the past decade. In this study, we investigated mercury levels in aquatic invertebrates and trout in the rivers of this region of the Sierra Nevada to determine the localized impacts of mining-derived mercury. These organisms were used as indicators of the bioavailable fraction of mercury, specifically that portion which can enter, transfer through, and be concentrated by the food web. The biota samples were used to determine relative "hot spots" of mercury contamination and to rank the various streams and rivers as to relative bioavailable mercury levels. Trout mercury was also investigated from a health perspective, to determine whether historic or current mining represented a human health concern. Thirty-five sites were sampled throughout the region during a two year period. A clear signature of mining-derived mercury was found, with notably elevated levels in the aquatic food webs of the upper forks of the Yuba River, the Middle Fork of the Feather River, the Bear River, and the North Fork of the Cosumnes River. Mercury was low throughout most of the American River watershed and in many tributaries away from the most intensively mined stretches of the various rivers. Areas appropriate for potential mitigation work are being further defined in ongoing work. Mercury concentrations in trout, while variable, were found to be uniformly below existing health standards, indicating the lack of direct health concerns within the region itself. Foothill reservoirs were found to operate as interceptors of mercury, with significantly lower levels found in biota below many reservoirs, as compared to upstream. Mercury concentrations in aquatic organisms increased in a predictable pattern with increasing trophic feeding level. Mercury in aquatic invertebrates can be used to determine relative mercury presence and bioavailability, to predict mercury levels in trout, and to integrate localized mercury conditions over the lifetime of the respective organisms. Because of the strong relationship with trophic feeding level, relative mercury concentrations may also be used to indicate the ecological feeding niche of individual organisms.