Former Mercury Mining Activity Remediation-Repository Construction and Site Reclamation in the California Coast Range

Sediments containing mercury from historical mining operations in California's Coastal range have initiated multiple implementation programs to reclaim and mitigate the risk to human health and the environment. At a former historical mining district in Colusa County, California, approximately 22,000 cubic yards of mercury-impacted material was successfully removed and stabilized in an on-site repository. Common lithologies in the area include detrital serpentines, sandstone, and mudstone; and groundwater beneath the district is primarily geothermal. The implementation of integrated stormwater pollution prevention and construction quality assurance plans provided the framework for site stabilization and mitigation of mercury-laden sediment discharge to nearby waterbodies. Depending on excavation contour and slopes, certain best management practices (BMPs) were installed to promote the stabilization of sediment on the areas where mining-related waste rock was removed. Practices included facines, straw mats, hydroseed using local vegetation seed mixes, and hay bale dams; that were evaluated for performance before and during the rainy season. Hydroseed methods above and below straw matting for future potential seeding methods in the area were implemented. Success for stabilization will be measured by multiple site visits during the rainy season and the use of vegetative surveys before and after stabilization using a variety of methods (biological surveys, drone photography, and GPS).