Legacy of mining: Mercury distribution and effects on aquatic biota in the Guadalupe River, Santa Clara County, California

Streams sediments in several subwatersheds of the Guadalupe River drainage are contaminated with mercury (Hg) at concentrations over hazardous waste criteria as a result of historic mining operations. Sediments, benthic macroinvertebrates, crayfish, and fish were collected at sites in four subwatersheds with different mining histories and analyzed for total mercury (T-Hg) and/or methylmercury (M-Hg). Benthic macroinvertebrate (BMI) communities were evaluated to determine whether impaired BMI metrics correlated with sediment T-Hg concentrations. Fish Index of Biotic Integrity (IBI) and Health Assessment Index (HAI) methodologies, combined with histopathological examination of fish livers and gonads, were used to evaluate the potential effects of Hg exposure on fish health. Habitat surveys were used to separate the confounding effects of habitat variability on BMI and fish assemblages from Hg effects.; Sediment T-Hg and M-Hg concentrations decreased exponentially with distance downstream from mine sites. Fish and invertebrate Hg concentrations at each site were correlated with sediment T-Hg and to T-Hg in other species. Numbers of BMI genera in the orders Ephemeroptera, Plecoptera, and Trichoptera present at each site and mean abundance of pollution intolerant BMI showed negative correlations with mean sediment T-Hg that accounted for 74% and 21% of variability, respectively. A negative correlation with sediment T-Hg explained about 2% of total BMI abundance. In fish, liver and gonad lesions occurred at 2--43% above background in the most exposed animals.; Productivity (age class) metrics for native fish species showed negative correlations with whole body T-Hg that explained about 10% of variability relative to other habitat variables. At the assemblage level, the overall IBI score showed a negative correlation with sediment Hg that explained less than 1% of variability.; These results strongly suggest that T-Hg influences BMI communities in the Guadalupe River watershed through an adverse effect on diversity and abundance of sensitive genera. The results further suggest that current sediment and fish T-Hg concentrations at the most contaminated locations affected by historic Hg mining are at or above thresholds for adverse effects on native fish populations. Near-term improvement absent active source control is unlikely.