The Former New Idria Mercury Mine: Water Quality Impacts from Acid Mine Drainage

Abstract

Mercury (Hg), which is mined from cinnabar (HgS), is an economic resource used extensively in California to extract gold and silver. Because Hg binds to carbon, fossil fuel combustion is a significant source of Hg globally. Although Hg is no longer mined in the US, abandoned mines continue to impact watersheds throughout California. The former New Idria Mine, located in the California Coast Range Mountains, was the second largest mercury producer in North America. It was designated a Superfund Site in 2011, with initial remediation completed in 2015. These cleanup efforts included redirecting acid mine drainage (AMD) at the site, but >40 acres of mining waste remain exposed. This is a concern because previous studies demonstrate this waste is the primary source of mercury to the local watershed. We conducted a preliminary sampling event in January 2019 to assess current water quality conditions in San Carlos Creek, which flows through the mine. Water upstream from the mine contained ~100 pM total mercury (HgT), similar to previously reported values. We observed extreme variability in HgT along a downstream transect, with concentrations dropping from approximately 10,000 to 450 pM at over approximately 5 km. We hypothesize this decrease is due to mercury settling out with iron precipitated from the AMD. Organic bioaccumulative monomethylmercury (MeHg) concentrations along the same transect varied from 1.0 to 3.2 pM. HgT in waters near the Mendota Wildlife Area (~100 km downstream) were <20 pM, suggesting current water quality impacts occur closer to the mine site. Our results indicate that remediation efforts to date have not reduced the flux of mercury from New Idria. Future work will include sediment analyses, wet and dry season sampling, and evaluating the importance of shallow groundwater as a mercury source.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)