Mercury-contaminated sites are a continuing challenge to delineate and adequately remediate. Every state within the United States currently has at least one water body-specific or statewide fish consumption advisory due to elevated fish tissue mercury found in their surface waters. Mercury was mined extensively in the United States’ Coast Range from the 1850’s to 1970’s and used in gold mine sites in the Sierra Nevada and elsewhere. In California alone there are thousands of potentially mercury impacted mine sites, with the vast majority of them unaddressed. Even today, mercury ore processing facilities and artisanal gold mining sites globally release a variety of mercury species, each with unique fate and transport dynamics that present challenges to site investigations and remediation. Legacy industrial sites that used elemental or other forms of mercury are often faced with similarly difficult remediation scenarios.
Mercury Session A: Site Investigation
This session focuses on recent and ongoing site investigations that add to the growing body of knowledge on mercury fate and transport at contaminated sites. Challenges associated with mercury contaminated sites arise from the large variety of chemical species and conditions that manifest from mining and other industrial operations. Gold and silver mills, and many industrial facilities typically received elemental (liquid) mercury as a feed stock to their processes. Mercury mine sites typically processed mercury ore with a resulting variety of sources including left behind waste rock, crushed ore, calcines, and fallout from inefficient mercury vapor collection. These sites are also located in areas of naturally elevated geogenic sources of mercury. Additional mercury in atmosphere deposition on the landscape can be difficult to differentiate. Release of mercury from these contaminated and background sources ultimately impacts downstream water bodies, transported by streams and rivers, and deposited within lakes, reservoirs, and estuaries.
In aquatic ecosystems, ionic mercury can be biochemically transformed into methylmercury, the organic form of the metal that partitions into biotic tissue and biomagnified. The mechanisms that control methylation and bioaccumulation rates are complex and site specific. Many sites that have been remediated continue to have persistent elevated mercury in fish, years after cleanup. There is a clear and present need to develop and implement more comprehensive investigation techniques and strategies to better inform conceptual site models that feed into more successful remedial applications.
Mercury Session B: Site Remediation
This session focuses on recent and ongoing mercury site remediation success stories. Remediation and reclamation of mercury-contaminated sites (including waterbodies) are challenging because of remote site conditions, complex mercury speciation/transformation processes, and multiple routes of mercury transport and exposure. Commensurate with that complexity, remediation projects need to be multi-faceted—addressing site hydrology, soil erosion/stability, leachability, loss/degradation, vegetation, downstream impacts and more. Stabilization of mercury source areas is often the initial step to remediation of diffuse contaminated media.
Remediation products can be applied to mercury -contaminated soils to effectively reduce leachability below soluble leachate limits, allowing on-site material to be reclassified from a hazardous waste to a non-hazardous waste. Such treatments can allow more economic land disposal options. Treatability studies can be performed on contaminated soils and waste materials to quantify treatability of mercury as well as chemically similar contaminates such as PCBs. Such treatments can lead to reductions in leachate concentrations and cumulative mercury loading.