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

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HUMAN UPTAKE OF METALS THROUGH INHALATION OR INGESTION. EXPOSURE TO HUMANS ER BOTH CURRENT LAND USE CONDITIONS AND POTENTIAL FUTURE USE SCENARIOS, THE PRINCIPAL ENTIAL PATHWAYS BY WHICH HUMAN RECEPTORS COULD BE EXPOSED TO SITE CONTAMINANTS FROM THE TAILINGS AND WASTE ROCK PILES IS THROUGH INHALATION OR INGESTION OF MATERIAL FROM THE PILES. IMPACTS RESULTING FROM INGESTION OF SURFACE WATER WERE EVALUATED AS PART OF OPERABLE UNIT NO. ONE. EXPOSURE SCENARIOS FOR AVERAGE AND MAXIMUM PLAUSIBLE CASES WERE DEVELOPED FOR BOTH THE INHALATION AND INGESTION POTENTIAL EXPOSURE PATHWAYS. BASED ON ESTIMATES OF EXPOSURE AND A QUANTITATIVE DESCRIPTION OF EACH CONTAMINANT'S TOXICITY, HUMAN HEALTH RISKS WERE THEN ASSESSED. THE MAJOR CONCLUSIONS OF THIS ASSESSMENT ARE PRESENTED IN TABLE 1 AND CAN BE SUMMARIZED AS FOLLOWS: INHALATION OF WIND-ENTRAINED DUST FROM THE GREGORY TAILINGS PILE RESULTS IN UPPERBOUND LIFETIME EXCESS CANCER RISKS OF 7 X 10-6 AND 3 X 10-5 FOR THE AVERAGE AND MAXIMUM PLAUSIBLE CASES, RESPECTIVELY, PRIMARILY FROM EXPOSURE TO ARSENIC. INHALATION OF WIND-ENTRAINED DUST FROM THE ARGO TAILING PILE RESULTS IN UPPERBOUND LIFETIME EXCESS CANCER RISKS OF 2 X 10-5 AND 3 X 10-5, FOR THE AVERAGE AND MAXIMUM PLAUSIBLE CASES, RESPECTIVELY. RISKS FROM INHALATION OF DUST FROM THE OTHER TAILINGS AND WASTE ROCK PILES ARE SIMILAR. INGESTION OF ARSENIC-CONTAMINATED MATERIAL FROM THE TAILINGS AND WASTE ROCK PILES UNDER CURRENT USE, OR THE EPISODIC EXPOSURE SCENARIO, POSES AN UPPERBOUND LIFETIME EXCESS CANCER RISK OF 2 X 10-5 FOR THE AVERAGE CASE AND 1 X 10-4 UNDER MAXIMUM PLAUSIBLE CONDITIONS. • INGESTION OF ARSENIC-CONTAMINATED MATERIAL FROM THE TAILINGS AND WASTE ROCK PILES • UNDER THE POTENTIAL FUTURE USE RESIDENTIAL SCENARIO POSES AN UPPERBOUND LIFETIME EXCESS CANCER RISK OF 1 X 10-4 UNDER AVERAGE CONDITIONS AND 9 X 10-4 UNDER MAXIMUM PLAUSIBLE CONDITIONS. THE RISKS FOR INDIVIDUAL SITES ARE PROVIDED IN TABLE 1 AND A MORE DETAILED DISCUSSION OF THESE EXPOSURE PATHWAYS AND THE RESULTING RISKS CAN BE FOUND IN THE PUBLIC HEALTH EVALUATION, SECTION 10 OF THE REMEDIAL INVESTIGATION REPORT. IMPACTS ON AQUATIC LIFE DUE TO CATASTROPHIC SLOPE FAILURE AND RUNOFF DURING STORM EVENTS ARE DISCUSSED IN THE FOLLOWING PARAGRAPHS. EXPOSURES TO AQUATIC LIFE THE EXPOSURE OF AQUATIC LIFE IN CLEAR CREEK AND NORTH CLEAR CREEK WERE CONSIDERED. THE PRINCIPAL POTENTIAL PATHWAYS BY WHICH THE TAILINGS AND WASTE ROCK PILES IMPACT AQUATIC LIFE ARE: DEGRADATION OF SURFACE WATER QUALITY CAUSED BY COLLAPSE OF THE PILES INTO THE CREEKS; AND DEGRADATION OF SURFACE WATER QUALITY CAUSED BY RUNOFF FROM THE PILES. CATASTROPHIC SLOPE FAILURE: THE FAILURE OF A SLOPE RESULTS IN MOVEMENT OF A MASS OF WASTE ROCK AND/OR TAILINGS INTO THE ADJACENT CREEK. IF THESE MATERIALS ARE REACTIVE OR TOXIC AND ARE INTRODUCED INTO A WATER OR AIR STREAM, WHERE EXPOSURE TO ENVIRONMENTAL OR HUMAN RECEPTORS CAN OCCUR, THEN A HAZARD EXISTS. THE POTENTIAL FOR CATASTROPHIC SLOPE FAILURE WAS ASSESSED FOR EACH SITE. ARGO TUNNEL: A GEOTECHNICAL EVALUATION WAS PERFORMED ON THE WASTE ROCK/TAILINGS PILE AT THE 0GO TUNNEL AND MILL. THE EVALUATION CONCLUDED THAT THE TOE OF THE SLOPE BEHIND THE COMMERCIAL ILDINGS HAD BEEN CUT AWAY AND THE SLOPE WAS MARGINALLY STABLE AND INCREASED WEAKNESS COULD RESULT FROM FURTHER ACTIVITIES OR DISTURBANCE. A SLOPE FAILURE WOULD IMPACT THE BUILDINGS BUT MATERIALS WOULD PROBABLY NOT REACH CLEAR CREEK. THE VOLUME OF SUCH A FAILURE WAS ESTIMATED AT 11,200 CUBIC YARDS (CY).