Union Mine Landfill, El Dorado County, California

The Union Mine Landfill property was the site of extensive historic underground and surface gold mining. Mine shafts, tunnels, cuts, and mill sites were developed in the mid-1800s through early-1900s. Several decades following cessation of the mining activities, the property was developed as a local refuse dump site, and later as a landfill. Landfill operations were conducted in the central area of the property where numerous shafts (main shafts and air shafts), tunnels, cuts, and drifts were located. The area underlying the landfill was also occupied with extensive underground workings (to depths of approximately 2,000 feet below the surface), which provided the potential for surface water and groundwater contamination. Groundwater from the flooded underground workings flowed from abandoned tunnels, and surfaced near filled shafts as springs around the southern and eastern perimeter of the landfill.

Stratus personnel conducted field mapping of historic mine tunnels and shafts, and historical document research, to locate surface and underground mine workings under the active portions of the landfill and in the peripheral area, supporting remedial design for landfill expansion.

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Landfill expansion and development challenges included capping and sealing mine shafts, and sealing off mine tunnel drainage to prevent further contamination of surface water in preparation for construction of new landfill cells.

Stratus personnel were also responsible for providing construction quality assurance (CQA) for the Class III old landfill area of the Union Mine Landfill which consisted of geotechnical monitoring, observation and testing of a 13.6-acre portion of the landfill. Stratus personnel provided construction oversight and documentation for the preparation of the subgrade, the placement of a geotextile cushion layer, installation of a 60-mil LLDPE liner, and a geocomposite drainage net. In addition, staff observed and documented the placement of a 1-foot thick vegetative cover and the construction of surface water features. Stratus personnel were also responsible for certifying that the landfill was closed in accordance with the approved CQA Plan specifications and applicable codes. During installation of this 3-layer landfill cover, Stratus personnel observed density/moisture testing on the foundation material, and as part of quality assurance Stratus personnel observed and documented the survey of the top of the certified foundation layer prior to the placement of the geosynthetics, the top of the certified vegetative layer to confirm a 1 foot thickness, and the completed surface drainage features of the final cover.

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Landfill gas monitoring and reporting services are currently being provided by Stratus in compliance with AB-32 and Title 17 California Code of Regulations Sections 95460 to 95476.  Our scope of services includes: landfill gas extraction well monitoring; perimeter probe testing; flare leak testing; structure/building monitoring for landfill gas; surface emission monitoring; and monthly and annual reporting.

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