Influence of Natural Sources on Mercury in Water, Sediment and Aquatic Biota in Seven Tributary Streams of the East Fork of the Upper Carson River, California

The East Fork of the Upper Carson River (EFUCR) drainagebasin contains the Leviathan mine site, a known source of acidmine drainage, and numerous small prospects and mineralizedareas, some of which are enriched in mercury. In 1999 aninvestigation was undertaken to characterize total mercuryconcentrations in water, sediment, and whole aquaticmacroinvertebrates from seven tributaries of the EFUCR watershedon a seasonal time step. In the fall, methyl mercury in water,sediment and Stoneflys was determined for three sites. Both total and dissolved mercury in water at all sites, notimpacted by acid mine drainage, exhibited a seasonal trend withthe lowest concentrations occurring in the winter, higherconcentrations in the summer, and the highest concentrationsrecorded in the fall. Winter samples were taken during a periodof ice melt. It is hypothesized that the high fall concentrations were due to elevated mercury concentrations ingroundwater, which was recharging the stream under baseflow conditions and had interacted with substrate naturally enrichedin mercury. Mercury concentrations in Leviathan Creek wereelevated when acid mine drainage was being discharged directlyinto the Creek. Mercury concentrations in individual stoneflyscollected from specific stream reaches were similar, and methylmercury and total mercury concentrations in macroinvertebratetissue were roughly equivalent. Comparison of mercury bodyburden of macroinvertebrates between tributaries showed thatmacroinvertebrates in the Cottonwood Creek watershed had highermercury concentrations relative to the other sites. Thistributary had higher sediment mercury concentrations than allsimilar tributaries not impacted by acid mine drainage.