Centennial M1 Property Clean-Up Project Remedial Action Plan

Centennial M1 Property Clean-Up Project Remedial Action Plan

This remedial action involves excavation and on-site consolidation of approximately 133,800 cubic yards of contaminated soil that has metal concentrations (primarily arsenic, lead, mercury, thallium and nickel) above commercial/industrial use cleanup levels. In addition, stabilization of approximately 4,200 cubic yards of soil is required to chemically stabilize constituents of concern from a small area of the Site by mixing this material with Portland cement to reduce its water solubility prior to consolidation. The consolidation area is approximately 5.6-acres in size and will be capped with four to five feet of clean soil that will be obtained from the Site so soil import will not be necessary. The capping and regrading of excavated areas will utilize approximately 129,100 cubic yards of soil from designated borrow areas on the Site. Following the consolidation and capping, erosion control measures and groundwater monitoring wells will be installed to ensure the effectiveness of the remedy. The purpose of the project is to remediate the soil to levels that no longer present a risk to human receptors under a commercial/industrial use scenario. Rise Grass Valley Inc. (Rise) entered into a Voluntary Cleanup Agreement (VCA; Docket No. HSA-FY18/19-014) with DTSC for the voluntary cleanup of soil contamination on the Centennial M1 Property (site). A Centennial M1 Property Remedial Action Plan (RAP; 2020) was prepared that describes the proposed procedures for conducting remedial activities related to the Remedial Action Objectives (RAO’s). The 56-acre site is located at 10344 Centennial Drive near the city limits of Grass Valley (City) in unincorporated Nevada County (County), California. This site was historically used for disposal of mine waste (tailings and waste rock) from a former underground hardrock (lode) gold mine, the Idaho-Maryland Mine (IMM). Mine waste is present on approximately two-thirds of the site. Site investigation has identified that some of the mill tailings, waste rock, and affected soil at the site that contain lead, arsenic, mercury, and other metals at concentrations exceeding background soil metals concentrations and regulatory benchmark concentrations.

Grass Valley

Nevada

Citywide, Unincorporated

California Air Resources Board (ARB), California Department of Conservation (DOC), California Department of Forestry and Fire Protection (CAL FIRE), California Department of Parks and Recreation, California Department of Transportation, District 3 (DOT), California Department of Transportation, Division of Aeronautics (DOT), California Department of Transportation, Division of Transportation Planning (DOT), California Department of Water Resources (DWR), California Governor's Office of Emergency Services (OES), California Highway Patrol (CHP), California Native American Heritage Commission (NAHC), California Natural Resources Agency, Central Valley Flood Protection Board, Department of Toxic Substances Control, Office of Historic Preservation, Sierra Nevada Conservancy, State Water Resources Control Board, Division of Drinking Water, California Department of Fish and Wildlife, North Central Region 2 (CDFW), California Regional Water Quality Control Board, Central Valley Sacramento Region 5 (RWQCB)

California Department of Fish and Wildlife, North Central Region 2 (CDFW), California Regional Water Quality Control Board, Central Valley Sacramento Region 5 (RWQCB)

Other (Voluntary Cleanup Agreement)

Remedial Action Plan

Aesthetics, Agriculture and Forestry Resources, Air Quality, Biological Resources, Cultural Resources, Drainage/Absorption, Energy, Flood Plain/Flooding, Geology/Soils, Greenhouse Gas Emissions, Hazards & Hazardous Materials, Hydrology/Water Quality, Land Use/Planning, Mineral Resources, Noise, Population/Housing, Public Services, Recreation, Sewer Capacity, Solid Waste, Transportation, Tribal Cultural Resources, Utilities/Service Systems, Vegetation, Wetland/Riparian, Wildfire