Superfund Information Systems: Site Profile

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The Central City, Clear Creek site consists of a 400-square-mile watershed extending from the Continental Divide east to near Golden, Colorado. The site is located in Gilpin and Clear Creek counties. The discovery of gold near Idaho Springs, Central City and Black Hawk in 1859 played a significant role in Colorado’s gold rush. For the next 20 years, the Black Hawk/Central City area was the leading mining center in Colorado primarily due to the construction of mills to process the gold and silver mined nearby. The silver crash in the 1890s led to a decline in area mining, but mining remained an important industry in Clear Creek and Gilpin Counties until about 1950. Only a limited number of mines are currently in operation.

The site consists of multiple mine waste piles, tailings impoundments, draining mine adits and impacted groundwater resources. Historic mining and milling activities resulted in the watershed becoming contaminated with heavy metals.

In 1992, limited stakes gaming began in Central City and Black Hawk. As property for the casinos was developed, many mine waste cleanup projects began. Construction of parking lots for the casinos capped or covered mine wastes. Cleanup is ongoing.

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What Has Been Done to Clean Up the Site?

The site is being addressed through federal, state and potentially responsible party (PRP) actions.

EPA has conducted five-year reviews at the site. These reviews ensure that the remedies put in place protect public health and the environment, and function as intended by site decision documents. The most recent review concluded that the remedy at the site remain protective of human health and the environment. EPA placed the site on the Superfund program’s National Priorities List (NPL) in September 1983.

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What Is the Current Site Status?

The site consists of four areas, referred to by EPA as operable units (OUs).

OU1 was designated to address acid mine drainage from five mine tunnels using passive treatment. The technology was later found to be infeasible due to acreage requirements for the reactors and the inability of the technology to efficiently remove metals from the waste stream. OU1 was amended by OU3. The amendment included active treatment of two of the five adit discharges. The other three mine discharges were transferred to OU4; OU1 focused on addressing sources of metals contamination within the North Fork of Clear Creek watershed.

OU2 addresses remediation of mill tailings and mine waste rock piles associated with the five discharging tunnels in OU1. The long-term remedy for the waste rock piles, selected in 1988, included slope stabilization at two of the piles, monitoring of the gabion wall at one of the piles and run-on control at all five piles. A 1999 update to the remedy included a combination of regrading, capping and construction of retaining walls, and runoff controls at two of the five piles. OU2 remedial actions are complete except for the Quartz Hill tailings impoundment, which was transferred to OU4. Operation and maintenance activities are ongoing at several of the OU2 waste piles.

OU3 was designated for a more comprehensive evaluation of the Clear Creek watershed, including active treatment of two of the five OU1 mine discharges. The long-term remedy for OU3, selected in 1991, included an alternative drinking water supply for residents, where required; passive treatment of the Burleigh discharge; chemical treatment of the Argo Tunnel discharge; reduction in the heavy metals load from Woods Creek; a groundwater collection system in the Idaho Springs area to address non-point source metals loading to surface water and capping or physical barriers; and institutional controls for select mine waste piles. Construction of the remedy is ongoing. The conceptual design for constructing a flow-through bulkhead in the Argo Tunnel is underway. The project will entail construction of a flow control structure to prevent large releases of contaminated water and sediment from overwhelming the Argo Tunnel water treatment plant.

OU4 focuses on sources of metals contamination to the North Fork of Clear Creek, a major tributary to Clear Creek, including waste rock and sediment controls on tributaries to the North Fork; the three remaining OU3 adit discharges that impact the North Fork; and the Quartz Hill tailings impoundment, located on Gregory Gulch, a tributary to the North Fork. The long-term remedy, selected in 2004, included treatment of various discharges, sediment control involving capping or removal of waste piles or other measures, construction of an on-site repository, and improvements to the North Fork of Clear Creek. Remedy construction is ongoing. Work will soon begin on the Quartz Hill tailings pile in Central City. The project will entail re-grading the tailings pile to a manageable slope for runoff from rain or snow melt. The final step will be to place a vegetative cover over the pile to prevent sediment erosion into the stormwater sewer. This project will be the final remedial component for OU4.

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