Stability, transformations, and fate of residual mercury at the inoperative New Idria mercury mine, New Idria, California [electronic resource]
Keywords: Geological and Environmental Sciences.

Mercury mining in the California Coast Range has resulted in over a thousand mercury mines of various sizes that have yet to be remediated and are potential point sources of Hg pollution. The New Idria mine, located in San Benito County, California, was the second largest mercury mine in North America and was not remediated following its closure in 1972. Only within the past two years has it been designated as an EPA Superfund site and is currently undergoing remediation. This thesis focuses on the stability of mercury remaining at the site, the types of mercury phases present at the site as well as downstream from it, and the transport of mercury from the site. The stability of cinnabar (HgS), the primary mercury-bearing phase at the New Idria mine and other mercury mines in the California Coast Range, was investigated in the presence of microorganisms found in the acid mine drainage (AMD) system at the site. Geomicrobiological and geochemical studies of the identity and effects of these microorganisms on the solubility of HgS identified Thiomonas species and showed that the AMD bacterial consortium significantly enhances the solubility of cinnabar and metacinnabar, the two common HgS crystalline phases present. These two phases are very insoluble (Ksp = 10^-54 and 10^-52, respectively) in the absence of these bacteria and are often considered to be relatively nonbioavailable. The phases of mercury present in the waste piles and downstream sediments were analyzed using a combination of sequential chemical extractions and synchrotron-based techniques, including a new low-temperature extended x-ray absorption fine structure (EXAFS) spectroscopy method developed in this project. This new method allows, for the first time, quantification of the amount of elemental mercury present in complex mine waste samples and associated sediments that contain a variety of Hg-bearing phases. When coupled with field- and lab-based evasion studies of Hg vapor from mine wastes, this new approach showed that high mercury evasion rates into the atmosphere from the New Idria and other Hg mine sites in the California Coast Range can be positively correlated with high levels of elemental Hg in the mine wastes. Another new discovery from this study is that freshwater diatoms in the New Idria drainage system are one of the major sinks for mercury. Selective chemical extraction and EXAFS studies of the species of mercury associated with diatom frustules showed that these frustules can stably sequester mercury in low bioavailability forms. These studies also showed that the abundant iron-(oxy)hydroxide (ferrihydrite) nanoparticles present in the New Idria drainage system have very little associated mercury, which contradicts long-held assumptions that sorption of Hg(II) on these nanoparticles is a major Hg sequestration and transport mechanism in the New Idria drainage system. A general conclusion from this study is that the majority of mercury in the New Idria drainage system is in relatively stable, low bioavailability forms. The understanding gained from this study of the stability, forms, and transport of mercury at the New Idria site can be extrapolated to other similar inoperative mercury mine sites in the California Coast Range and should aid in their future remediation efforts.