EPA has compiled a significant volume of soil, sediment, surface water and biota geodata, collected either directly, by our contractors and in many cases by our Federal, State and Tribal agency partners (grantees) to complete the remedial investigation (RI) and Feasibility Study at the Operable Unit (OU) 2 (i.e., the river, the floodplain and biota). EPA’s goal for the OU2 area is to select a remedy in 2020 that reduces, or controls risks to human health and the environment. Specifically, this RI involves the investigation and evaluation of the heavy metals (mercury, arsenic and lead) contamination and associated drainages in the Carson River Basin which has resulted from contaminant migration from the Carson River Mercury Mines Site.

The Human Health and Ecological Risk Assessments, contained within the RI, evaluated past and current releases and their impact to human health and the environment. The primary concern is the effect of mercury contamination from the mining operations on the residences near the former stamp mills, the fisheries, waterfowl, water plants and recreational users of the Carson River (“River”) Basin. The OU1 area Superfund Site consists of mining wastes and releases from approximately 236 abandoned gold and silver mines from three basic source areas: the main stem of the River starting at New Empire (Morgan Mill) to Dayton, the Comstock Lode in Gold, Six and Sevenmile Canyons and several mills around Washoe Lake. Mercury from the former mill sites, has contaminated soils at and near the former mill sites, sediments in the drainages and the floodplain, the river banks, the fish and wildlife over more than a 130-mile length of the River extending downstream to the Lahontan Valley and ultimately at its terminus at the USFWS Stillwater and Fallons National Wildlife Refuges.

Contamination from the Site is a legacy of the Comstock mining era of the late 1860s, when mercury was imported to the area for processing of gold and silver ore. Ore mined from the Comstock Lode was transported to the individual mill sites, where it was crushed and mixed with mercury to amalgamate the precious metals before refining. The mills were located in the drainages in and around Virginia City, Silver City, Gold Hill, Dayton, Six Mile Canyon, Gold Canyon, and along main stem of the Carson River between New Empire and Dayton. An estimated 7,500 tons of mercury were discharged into the tributaries and River, primarily in the form of mercury-contaminated tailings.