WATER QUALITY DOWNSTREAM FROM THE SECOND LARGEST MERCURY MINE IN NORTH AMERICA: NEW IDRIA, CA

We evaluated water quality downstream from the former New Idria Mercury Mine in the California Coast Ranges. Mercury was used extensively to extract gold and silver during the Gold Rush and abandoned mines continue to impact watersheds throughout the state. New Idria was the second largest mercury producer in North America and was designated a Superfund Site in 2011. Studies published in 2000 demonstrate that mining waste piles, rather than acid mine drainage, are the primary source of mercury to the local watershed. However, no long-term investigation to evaluate seasonal trends in the flux of mercury released from New Idria have been conducted. This preliminary investigation is a first step towards addressing that data gap, and will provide a baseline of pre-remediation conditions downstream from the site. We sampled water along an ~8 km transect downstream from the mine and found unfiltered mercury concentrations dropped more 20-fold over that distance, with ~90% of the mercury associated with unfiltered water compared to filtered (< 0.22 µm) water. These results, in combination with suspended particulate matter (SPM) data, demonstrate that mercury in this system is particle-bound and settles out of the water column with iron flocculant related to acid mine drainage. This unconsolidated material can be remobilized, as demonstrated by seasonal variability in water quality, with the highest Hg concentrations occurring in the wet season when stream discharge increases. Future work will include additional storm event sampling and evaluating mercury concentrations in downstream sediment. We are also investigating water quality in the Mendota Wildlife Refuge, located ~50 km from New Idria.