2009-09-25_PERMIT FILE - M2009076 (42)

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BIG FIVE TUNNEL: A GEOTECHNICAL ENGINEERING EVALUATION WAS CONDUCTED ON TWO MINE WASTE ROCK PILES (ONE ON THE NORTH SIDE OF CLEAR CREEK AND ONE ON THE SOUTH SIDE) CREATED FROM MATERIAL EXCAVATED FROM THE BIG FIVE TUNNEL. THE CONCLUSIONS OF THESE EVALUATIONS INCLUDED: • THE EASTERN PORTION OF THE WASTE ROCK PILE ON THE NORTH SIDE OF CLEAR CREEK AND THE WESTERN PORTION OF THE WASTE ROCK PILE ON THE SOUTH SIDE OF CLEAR CREEK ARE MARGINALLY STABLE TO UNSTABLE; A 100-YEAR FLOOD COULD ERODE THE TOE OF EXISTING SLOPES BACK SEVEN TO TEN FEET AND DECREASE SLOPE STABILITY. A 500-YEAR FLOOD COULD ERODE THE SLOPE BACK AN ADDITIONAL TWO FEET; AND, ALL SLOPES ARE SUBJECT TO EROSION IF LEFT EXPOSED. BASED ON THE RESULTS OF THE GEOTECHNICAL INVESTIGATIONS, VOLUMES OF MARGINALLY STABLE AND UNSTABLE MINE WASTE ROCK HAVE BEEN CALCULATED TO BE 2000 AND 6200 CY FOR THE NORTH AND SOUTH BANKS, RESPECTIVELY. IN ADDITION, THE VOLUMES OF WASTE ROCK WHICH COULD BECOME MARGINALLY STABLE OR UNSTABLE AFTER SEVEN TO TEN FEET EROSION BACK FROM THE TOE OF THE EXISTING WASTE PILES DUE TO A 100-YEAR FLOOD WERE ALSO DETERMINED. THE VOLUME WAS CALCULATED TO BE 7200 AND 7900 CY FOR THE NORTH AND SOUTH BANKS, RESPECTIVELY. QUARTZ HILL AND NATIONAL TUNNEL: THE WASTE ROCK PILE ADJACENT TO THE NATIONAL TUNNEL IS CONSIDERED STABLE. SOME SLOPES NEAR THE QUARTZ HILL TUNNEL ARE MARGINALLY STABLE AND SOME MINOR RECONTOURING IS RECOMMENDED. GREGORY INCLINE: IN 1987, THE TAILINGS PILE AT THE GREGORY INCLINE WAS RECONTOURED TO A 2:1 (HORIZONTAL TO VERTICAL) SLOPE AND A GABION-BASKET RETAINING WALL WAS CONSTRUCTED. THE GABION-BASKET WALL IS PARTIALLY IMMERSED IN CREEK WATER AND IS EXPOSED TO ACID DRAINAGE. WITHOUT MAINTENANCE, THE WALL HAS AN ESTIMATED LIFE OF FIVE YEARS. THE WALL IS SUPPORTING AN ESTIMATED 1,100 CUBIC YARDS OF TAILINGS WHICH WOULD OTHERWISE COLLAPSE INTO NORTH CLEAR CREEK. WE DETAILS ON THE GEOTECHNICAL EVALUATIONS CONDUCTED AT THE CLEAR CREEK/CENTRAL CITY SUPERFUND SITE CAN BE FOUND IN SECTION 8 OF THE REMEDIAL INVESTIGATION REPORT AND SECTION 5 OF THE ADDENDUM TO THE REMEDIAL INVESTIGATION REPORT. ESTIMATED IMPACT DUE TO CATASTROPHIC COLLAPSE: A COMPUTER MODEL WAS USED TO PREDICT WATER QUALITY IN CLEAR CREEK AND NORTH CLEAR CREEK IN THE ASSESSMENT OF HUMAN HEALTH AND ENVIRONMENTAL IMPACTS RESULTING FROM THE CATASTROPHIC FAILURE OF THE BIG FIVE TUNNEL AND GREGORY INCLINE WASTE ROCK AND TAILINGS PILES. THE COLLAPSE OF THE BIG FIVE WASTE ROCK PILES WAS MODELED UNDER REALISTIC, WORST CASE CONDITIONS. IN PARTICULAR, A TOTAL OF 8,200 CUBIC YARDS OF MATERIAL (BOTH NORTH AND SOUTH BANKS COLLAPSE) WAS INSTANTANEOUSLY INTRODUCED INTO CLEAR CREEK DURING NORMAL LOW FLOW (40 CUBIC FEET PER SECOND (CFS)). THE 8,200 CUBIC YARDS IS THE QUANTITY OF MATERIAL CURRENTLY UNSTABLE AND WHICH COULD COLLAPSE INTO CLEAR CREEK AT ANY TIME, INCLUDING DURING LOW FLOW CONDITIONS. ZINC AND ALUMINUM WERE SELECTED TO REPRESENT THE VARIETY OF GEOCHEMICAL BEHAVIOR EXPECTED FROM THE TWELVE IDENTIFIED CONTAMINANTS OF CONCERN. ARSENIC WAS NOT MODELED BECAUSE MEASURED CONCENTRATIONS IN THE STREAMS AND LEACHATES WERE BELOW DETECTION LIMITS. LEAD AND COPPER WERE NOT MODELED BECAUSE LEACHATE CONCENTRATIONS WERE LOW. A COLLAPSE OF THE BIG FIVE PILE INTO CLEAR CREEK WOULD RESULT IN A PEAK DISSOLVED CONCENTRATION OF 1,000 UG/L OF ALUMINUM AT THE POINT OF COLLAPSE. AFTER ONE DAY, THE CONTAMINATION WOULD REACH GOLDEN WHERE A MAXIMUM CONCENTRATION OF 730 UG/L OF ALUMINUM IS PREDICTED. THE ALUMINUM CONCENTRATION AT GOLDEN UNDER AMBIENT CONDITIONS IS 200 UG/L. A COLLAPSE OF THE BIG FIVE PILE WOULD RESULT IN A MAXIMUM DISSOLVED CONCENTRATION OF 1,100 UG/L OF ZINC AT THE COLLAPSE POINT, AN INCREASE FROM 400 UG/L OF ZINC UNDER AMBIENT CONDITIONS. THIS TRANSLATES INTO A MAXIMUM CONCENTRATION OF 960 UG/L OF ZINC AT GOLDEN AFTER TWO DAYS WHICH WOULD GRADUALLY DECREASE TO 400 0 / /L AFTER EIGHT DAYS. THE ZINC CONCENTRATION AT GOLDEN UNDER AMBIENT CONDITIONS IS ABOUT 300 L. THE MODELING RESULTS INDICATE THAT MAXIMUM CONCENTRATIONS OF ALUMINUM AND ZINC WOULD EXCEED AWQC IN ALL STREAM SEGMENTS DOWN TO GOLDEN. AT GOLDEN, THE AWQC WOULD BE EXCEEDED BY A FACTOR OF 24 AND 89, RESPECTIVELY FOR ALUMINUM AND ZINC. BASED ON THE MODELING OF ZINC AND ALUMINUM, IT IS ESTIMATED THAT CONCENTRATIONS OF SELECTED PARAMETERS IN CLEAR CREEK AT GOLDEN WOULD ALSO EXCEED MAXIMUM CONTAMINANT LEVELS (MCLS) ESTABLISHED UNDER THE SAFE DRINKING WATER