2009-09-25_PERMIT FILE - M2009076 (41)

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• increases in Clear Creek through Idaho Springs, even though the ARGO Tunnel Water Treatment Plant, operating since April 1998, removes the largest portion of the loading. Treatment of the ARGO Tunnel discharge addressed the largest single point-source discharge of contaminants to Clear Creek, Sampling points in Clear Creek bracketing the various source areas through Idaho Springs and downstream of Idaho Springs and the ARGO Tunnel Water Treatment Facility show that the loading from Virginia Canyon and the Big Five Tunnel continue to cause exceedances of stream standards in that reach of Clear Creek and downstream of Idaho Springs. These sample points are located upstream of the Big Five Tunnel and Idaho Springs, downstream of the Big Five Tunnel and upstream of Virginia Canyon, downstream of Virginia Canyon and upstream of the ARGO Tunnel, and downstream of the ARGO Tunnel and Idaho Springs. Despite some dilution from cleaner water from Chicago Creek in this stretch, concentrations of contaminants of concern still increase due to contribution from Virginia Canyon and the Big Five Tunnel. Virginia Canyon ground water and surface water is scheduled to be collected and conveyed to the ARGO Water Treatment Facility pursuant to the OU 3 ROD, with construction to begin in July 2005. As indicated above, recent sampling data show that the Big Five discharge contributes to exceedances of stream standards (i.e., for zinc) in Clear Creek for a good portion of the year. This typically occurs in the winter months during low flow conditions in Clear Creek. Data summaries and analyses showing increases in metals concentrations in Clear Creek resulting from releases from the Big Five Tunnel and other sources are included in the November 2001 Clear Creek Surface Water Investigation, Analytical Results Report, CDPHE, October 2001, and Upper Clear Creek • Watershed Trace Metals Data Assessment, Clear Creek/Central City Superfund.InvestigativeArea, TDS Consulting, Inc., January 2002. The addendum to the latter report issued by 'IDS Consulting in June 2003 continues to show the increases in metals concentrations from the Big Five Tunnel discharge, and the resulting exceedances of stream standards for Clear Creek: The modifications considered by this ESD are warranted by the following conditions: 1. An interim waiver was utilized in the OU 3 ROD for the Big Five Tunnel to allow reevaluation of its designation as a priority discharge under Section 304(1) of the Clean Water Act. The Big Five discharge was not removed from the Section 344(1) list, and final action is required. 2. Acid mine drainage from the Big Five Tunnel continues to exceed stream standards and adversely impact Clear Creek through point-source and diffuse non-point-source discharge. 3. Acid mine drainage from the Big Five Tunnel has continued to sustain a pond that poses a potential public health hazard adjacent to the mine portal in Idaho Springs. Although no action is planned to address the potential for blowouts from the tunnel, action is necessary to remove the pond and thus prevent periodic discharges from pond overflows due to heavy precipitation events or seasonal increases in flow from the tunnel that cause direct discharges of acid mine drainage and sediments with hazardous substances to Clear Creek. • 4. At the time of the OU 3 ROD, the determination was made that "treatment does not attain a level of benefit which is proportional to the cost" for some discharges. 5