The Homestake Mining Company implemented a remediation project at the site of a mercury mining district dating back to the 1860s and operating as late as the 1940s along Sulphur Creek in Colusa County, California. At issue were mercury laden sedimentsand mine waste materials located immediately adjacent to the creek which were at risk of being transported by erosion into Sulphur Creek and on downstream into the Bear Creek drainage. The project involved the removal of approximately 17 cubic meters (22,000 cubic yards) of mercury-impacted mine waste and placement of the material in an on-site repository which was then stabilized against erosion. The site also contained abandoned mining equipment including retorts that offered the possibility of the presence of elemental mercury. A temporary, lined inspection area was provided to permit inspection of mine equipment and determine if some materials would require off-site disposal as hazardous waste. Mine waste removal sites were stabilized using various best management practices (BMPs) and some mine waste deposits were stabilized in place. Erosion control techniques used in the project included facines, straw mats, coir log barriers, straw bale dams, hydroseeding with native grass species, grass filters, and a self deploying riprap barrier. Project design also included contingencies for potential use of in-stream structures including rock vortex weirs for the stabilization of high gradient stream beds and a sediment control dam for the collection offine grained mine waste below areas where excavation and removal were simply impractical. Equipment access across the active perennial stream channel of Sulphur Creek was accomplished using temporary bridges that were removed at the end of the project. The crossing of an ephemeral channel was accomplished using a temporary ford which was also removed at the end of the project. All roads not present prior to project design and other facilities required for access or construction had to be reclaimed or "erased", leaving the final site as undisturbed as possible. Success of the stabilization methods are being monitored with multiple site visits during the rainy season and vegetative surveys before and after stabilization (biological surveys, drone photography, and GPS).
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