VIOLATION UPON WHICH THE ACTION IS BASED. THE CALIFORNIA GULCH NATIONAL PRIORITIES LIST SITE NEAR LEADVILLE, COLORADO, IS CHARACTERIZED BY SURFACE WATER, GROUND WATER, AND SOILS CONTAMINATION RESULTING FROM MINING AND MINERAL DEFELOPMENT ACTIVITIES OVER THE PAST 125 YEARS. HAZARDOUS SUBSTANCES, INCLUDING ARSENIC, CADMIUM, LEAD AND ZINC ARE RELEASED FROM MINES DRAINING INTO THE YAK TUNNEL AND HUNDREDS OF WASTE PILES SCATTERED THROUGHOUT THE 11.5 SQUARE MILE SITE. EPA HAS INCURRED AND IS INCURRING RESPONSE COSTS CONSISTENT WITH THE NATIONAL CONTINGENCY PLAN TO CONDUCT A REMEDIAL INVESTIGATION AND FEASIBILITY STUDY FOR THE SITE AND TO UNDERTAKE A SMALL REMOVAL ACTION. EPA WILL INCUR FUTURE RESPONSE COSTS IN UNDERTAKING OR OVERSEEING THE DESIGN AND IMPLEMENTATION OF A REMEDY FOR THE SITE.