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storms and periods of melting snow. <br /> CDPHE and EPA have studied the 400-square mile upper Clear Creek basin. Because of the <br /> immense scale of this project, CDPHE and EPA have chosen to refer to the Clear Creek basin as <br /> the Central City/Clear Creek Superfund Study Area. Over the years, EPA and CDPHE have <br /> divided the Study Area into four operable units (OUs). Each of the OUs was designated to <br /> address heavy metals contamination associated with historic mining activities in the Clear Creek <br /> drainage basin. The Records of Decision(RODS) for each OU identify several discrete areas <br /> within the Study Area where response actions have been or will be performed to reduce metal <br /> contamination. The RODS also identify a variety of response actions including treatment of <br /> tunnel discharges, capping of mine waste piles, collection and treatment of groundwater, and <br /> construction of drainage and sediment controls as the means of controlling the release of <br /> hazardous substances into the environment. The Golden Gilpin Mill Site is an area originally <br /> identified for clean up in the Operable Unit 3 ROD. Figure 1 shows the location of the Golden <br /> Gilpin Mill Site in Black Hawk, Gilpin County, Colorado. <br /> The Golden Gilpin Mill Site (Site) is located along the banks of North Clear Creek <br /> approximately 0.25 mile north of the City of Black Hawk. Physical features at the Golden <br /> Gilpin Mill Site include the mill building and the tailings pond. The site covers approximately <br /> two acres,with the tailings ponds covering about one acre to the south of the mill building. <br /> Mining and other equipment is stored to the north of the mill building. The general arrangement <br /> of these Site features is shown in Figure 2. The mine waste pile has been previously sampled <br /> during Site investigations. Contaminants in the waste rock include cadmium, copper, lead, zinc <br /> and other trace metals. Contaminants from the waste pile and other nearby sources contribute to <br /> the water quality degradation of North Clear Creek and Clear Creek. A summary of analytical <br /> data characterizing waste material at the Golden Gilpin Mill Site can be found in the Clear Creek <br /> Phase II Remedial Investigation Report(CDM 1990). <br /> II. OPERABLE UNIT BACKGROUND <br /> The Central City/Clear Creek Superfund Site was nominated for listing on the National Priorities <br /> List(NPL) in 1982 and added to the NPL in September 1983. There are currently four operable <br /> units associated with the Site. A Record of Decision for Operable Unit 3 (OU3) was finalized in <br /> September of 1991, selecting consolidation and capping as the preferred remedy for the Golden <br /> Gilpin Mill Site. The Superfund Site was administratively restructured in 2006 to organize all <br /> remaining remedial design and remedial action tasks under OU4. The Golden Gilpin Mill Site <br /> remedial action was conducted with OU4 remedial action funding. <br /> The selected remedy for OU3 addressed a portion of the principal threats at the Site by treating <br /> highly toxic and highly mobile liquid wastes (i.e., acid mine discharges)that present significant <br /> risk to the environment. The selected remedy also addressed low-level threat wastes through <br /> management of source materials such as mine tailings and waste rock material. Principal <br /> components of the selected remedy include: <br /> 0 Treatment of tunnel discharges, <br />