2011-08-08_REVISION - M1977493 (36)

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• • Water Quality Monitoring Plan Climax Molybdenum Company Version: R1 Permit No. M -1977 -493 Abandoned mine lands located outside of the Climax site represent other potential sources of contamination in the upper Arkansas River Watershed. 3.1.4 Hydrogeology Within the Arkansas River Watershed, groundwater occurs very close to the ground surface within layers of alluvial sands and gravels, and glacially derived silts and clays. These soils overlay the Paleozoic Minturn Formation (west of the Mosquito Fault) and the Precambrian Silver Plume Granite (east of the Mosquito Fault). The thickness of the alluvial and glacial soils ranges between 10 and 40 feet in the Arkansas drainage. Hydraulic properties of the bedrock and soils in the drainage have not been measured, however, the results of a hydrogeologic evaluation on the flooding of the underground workings (Titan, 1994) determined that the bedrock on the east side of the Mosquito Fault is of low permeability and not capable of transmitting significant quantities of groundwater. This evaluation also determined that the seams of clay and fault gouge associated with the Mosquito Fault create a hydraulic barrier to east -west groundwater flow between the bedrock units. Demonstration of the Mosquito Fault as a hydraulic barrier to potential process water impacts to the Arkansas was discussed with Allen Sorenson of DRMS during a site inspection and in verbal and written correspondence in the fall of 1994. Subsequent geophysical evaluations were performed to establish the criteria to maintain water levels 19 feet below the apex of the Mosquito Fault. The Mosquito Fault is located approximately 350 ft west of the 5 Shaft. A pumping station installed in 5 -Shaft maintains the water level at an elevation (11,080 ft msl) that does not allow the flow of impacted water to the Arkansas River Valley. The level of water in 5- Shaft is kept below the apex of the Mosquito Fault to prevent mine water from potentially impacting the Arkansas River. Climax also monitors a well located approximately 60 feet from the 5 -shaft for changes in water level. Groundwater flow direction in the Arkansas River Watershed is generally westward, paralleling the flow of the Arkansas River. As in the Tenmile drainage, the groundwater flow is controlled by the bedrock topography. 3.1.5 Monitoring Sites 3.1.5.1 POC Monitoring Site The groundwater quality in the Arkansas River Valley has historically been monitored by well ARwell located downgradient of the Arkansas Pond, the source of industrial /potable water for the mill site (Figure 2). ARwell was constructed in September of 1993, and the well log is included in Appendix B. ARwell was completed in alluvium and is therefore representative of alluvial groundwater conditions in the valley below Climax facilities. The groundwater in the vicinity of ARwell discharges to the Arkansas River. Water quality samples have been collected at ARwell on a quarterly basis since1994 Samples from the ARwell were also collected on a monthly basis beginning in June 1993 and ending October 1994. The analytical sampling results from the quarterly sampling at ARwell are summarized and presented in Appendix A. As illustrated by the analytical results collected since 1994, the water quality at the ARwell demonstrates low to non - detectable concentrations of primary constituents, with the exception of manganese. The observed manganese concentrations have ranged from 0.057 mg /L to 1.0 EPP — Appendix C August, 2011 5