ENVIRONMENTAL IMPACTS OF THE FORMER NEW IDRIA MINE, THE SECOND LARGEST MERCURY MINE IN NORTH AMERICA, NEW IDRIA, CA

We are investigating water quality downstream from the former New Idria Mercury Mine in the California Coast Ranges. Mercury (Hg) was used extensively to extract gold and silver during the Gold Rush and abandoned mines continue to impact water quality throughout the state. The New Idria mine was the second largest mercury producer in North America and became a Superfund Site in 2011. Initial remediation was completed in 2015 and included rerouting acidic mine drainage away from mine waste. However, more than 40 acres of mine waste are exposed along San Carlos Creek, downstream from the mine. Previous studies from the 1990s demonstrate that transport of particles from the waste piles is the primary pathway for mercury to the local watershed. We sampled three locations over a ~5 km transect downstream from the mine and found Hg concentrations dropped from ~10,000 to 450 pM. Our data indicate that mercury in the stream is transported with the suspended particulate load, with unfiltered concentrations approximately 10-fold higher than filtered waters (~600 vs. 6,000 pM). We therefore attribute the mercury attenuation with distance to particles settling out of the water column. We also observed seasonal variability, with the highest Hg concentrations occurring in the wet season when stream discharge increases. Future work will include measuring the flux of Hg leaving the mine site during a storm event and evaluating sediment concentrations along a downstream transect.