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URS Operating Services.Inc. ARR for ESVClimac <br /> a — <br /> ,0 START,EPA Region VIII Revision: 0 <br /> ! <br /> Contract No.68-WS-0031 Date: 04/1998 <br /> Page I I of37 <br /> outfall. Zinc was detected at an elevated concentration at approximately four miles downstream of <br /> the CM NPDES outfall. <br /> In the 1994 SSI,elevated concentrations of arsenic,copper,silver,and zinc were identified in stream <br /> sediments with varying frequency from the CM NPDES outfall to approximately 3.1 miles <br /> downstream of the CM NPDES outfall. <br /> 6.3 ATTRIBUTION AND SURFACE WATER/SEDIMENT TARGETS <br /> Dissolved and total metals analyses indicate that aluminum,manganese,and molybdenum have been <br /> documented as elevated from the CM NPDES discharge for varying distances downstream. <br /> Concentrations of total manganese from surface water samples collected at the outfall,and at 1.1, <br /> 2.1,and 4.1 miles downstream were above the drinking water reference dose/cancer risk screening <br /> concentration(RD/SC)benchmark of 180 ppb. Concentrations of total molybdenum from samples <br /> collected from the CM NPDES outfall up to 10.1 miles downstream and at 12.1 miles downstream, <br /> were above the drinking water RD/SC benchmark of 180 ppb. Arsenic,copper,silver,and zinc were <br /> detected in sediments farther downstream of the CM NPDES outfall. Concentrations of total zinc <br /> from samples collected at 2.1,3.1,5.1,and 6.1 miles were above the ambient water quality criteria <br /> (AWQC)benchmark of 110 ppb(EPA 1995). These four elements were also identified in sediment <br /> sampling conducted for the 1994 SSI,potentially originating from an additional source or sources <br /> on Tenmile Creek downstream of the CM NPDES outfall. Additional sources of inorganic <br /> contamination(historic mines and railroad grades)are potentially present downstream of the site on <br /> Tenmile Creek'or along tributaries of Tenmile Creek,which could contribute to elevated detections <br /> in surface water and sediments on Tenmile Creek farther downstream of the CM NPDES discharge. <br /> Concentrations of dissolved manganese from surface water samples collected at the outfall, 1.1,and <br /> 2.1 miles were above the drinking water RD/SC benchmark of 180 ppb. Concentrations of dissolved <br /> molybdenum from samples collected at the CM NPDES outfall to 10.1 miles,and at 12,1 miles,were <br /> above the drinking water RD/SC benchmark of 180 ppb. The concentration of dissolved silver from <br /> the sample collected at.1.1 miles was above the AWQC benchmark of 2.3 ppb. The concentration <br /> of dissolved zinc from the sample collected at 2.1 miles was above the AWQC benchmark of 110 <br /> ppb(EPA 1995). <br /> 75-51119.00 <br /> F:\START\CI.IMAX\Fina1.ARR\CLIMAX.TXT.«pd:bas <br />