Assessing Remediation Efforts at the former New Idria Mercury Mine, California

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. In fact, the two largest Hg producers in North America were located in the California Coast Range Mountains: the New Almaden Mine near San Jose and the New Idria Mine, ~170 km SE of San Jose. Both are Superfund Sites, with extensive remediation completed at New Almaden. Cleanup at New Idria has been more challenging, due in part to its remote location. Our study is designed to evaluate water and sediment quality downstream from New Idria following initial remediation activities, which were completed in 2015. The cinnabar deposit at New Idria is located within a metamorphosed region of the Coast Ranges. The ore occurs in altered shale and sandstone which sit stratigraphically above a serpentine diapir. Fractures created preferential pathways for hydrothermal fluids and cinnabar was deposited in fracture veins. Pyrite (FeS2) associated with this cinnabar enabled the generation of acid mine drainage (AMD). Initial remediation actions rerouted the AMD, but > 40 acres of mining waste remain exposed. Only one peer reviewed study conducted in the 1990s has quantified water quality impacts from the New Idria Mine. This work suggests the waste piles are the primary source of Hg to San Carlos Creek, which flows through the site. To our knowledge, no published studies have explored Hg transport beyond this creek. We conducted a preliminary sampling event in Jan 2019 to assess the effectiveness of the remediation efforts. Water upstream from the mine contained ~100 pM Hg, similar to previously reported values. About 0.3 km downstream from the site, concentrations jumped about 100-fold, exceeding 10,000 pM, then dropped to ~5,500 pM downstream 2.5 km, presumably due to Hg settling out with iron precipitated from the AMD. Hg in waters near the Mendota Wildlife Area (~100 km downstream) were < 20 pM, indicating Hg from the mine is not currently reaching this habitat. Our results indicate that remediation efforts to date have not reduced the flux of Hg from New Idria. Future work will include quantifying the concentration of bioaccumulative organic Hg in our field area and evaluating the importance of groundwater as a Hg source.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019