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

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CLEAR CREEK/CENTRAL CITY SUPERFUND SITE EXPLANATION OF SIGNIFICANT DIFFERENCES BIG FIVE TUNNEL DISCHARGE May 2005 INTRODUCTION AND STATEMENT OF PURPOSE This document explains the significant difference between the remedy for the Big Five Tunnel discharge selected in the Clear Creek/Central City Superfund Site (SITE) Operable Unit 3 (OU 3) Record of Decision (ROD), signed September 30,199 1, and the planned remedy for the Big Five Tunnel discharge, located at the west end of Idaho Springs, Clear Creek County, Colorado. The Colorado Department of Public Health and Environment (CDPHE) is the lead agency for the SITE and is conducting the Remedial Action at the Big Five Tunnel under a Cooperative Agreement with the United States Environmental Protection Agency (EPA). EPA is assisting as the support agency and maintains the lead for enforcement at the SITE. Under Section 117 ofthe Comprehensive Environmental Response, Compensation, and Liability Act • of 1984 (CERCLA or Superfund), as amended, 42 U.S.C. §9601 et seq., EPA is required to publish an Explanation of Significant Differences (ESD) when significant, but not fundamental changes are proposed to the'previously selected site remedy. The National Oil and Hazardous Substances Pollution Contingency Plan (NCP), §300.435(c)(2)(1), sets forth the criteria for issuing an ESD and requires that an ESD be published if a remedial action is taken which differs significantly in either scope, performance, or cost from the remedy selected in the ROD for the SITE. The purpose of this ESD is to include in the OU 3 remedy the collection of the Big Five Tunnel- discharge, the conveyance of the discharge to the ARGO water treatment facility for treatment, and the isolation and capping of contaminated pond sediments at the outfall of the Big Five Tunnel. In summary, the circumstances that have led to the need for this ESD include the following: • 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 3040) list, and final action is required. • Acid mine drainage from the Big Five Tunnel continues to exceed stream standards and adversely impact Clear Creek through diffuse non-point-source discharge and periodic overflows of the pond created by the tunnel outfalI. • • 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 within the city of Idaho Springs. Periodic discharges from pond overflows due to heavy