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Oxbow Mining W WTF Water Quality Assessment CO-0000132 <br />The upstream background pollutant concentrations used in the mass-balance equation will vary based <br />on the regulatory definition of existing ambient water quality. For most pollutants, existing quality is <br />determined to be the 85`h percentile. For metals in the total recoverable form, existing quality is <br />determined to be the 50`~ percentile. For pathogens such as fecal coliform, existing quality is <br />determined to be the geometric mean. <br />For non-conservative pazameters and ammonia, the mass-balance equation is not as applicable and <br />thus other approaches aze considered where appropriate. Note that conservative pollutants aze <br />pollutants that are modeled as if mass is conserved and there is no degradation, whereas non- <br />conservativepollutants degrade and sometimes aze created within a receiving stream depending on <br />stream conditions. Amore detailed discussion of the technical analysis for these pazameters is <br />provided in the pages that follow. <br />Pollutants Evaluated <br />The following parameters were identified by the WQCD as pollutants to be evaluated for this <br />facility: k ,A <br />• Metals (outfa11015 only) o~~'~ \ ~,Y <br />• Chlorine (outfa11018 only) ~ t~,p^^ ~+ ~~ ~~ <br />During assessment of the facility, neazby facilities, and receiving stream water quality, no additional <br />pazameters were identified as pollutants of concern. <br />Oxbow Mining, LLC WWTF: The Oxbow Mining LLC WWTF is located in the SW 1/4, SE 1/4, <br />Section 8, T13S, R90W; latitude 38° 55' 45" north, longitude 107° 28' 00" west; at 3737 Highway <br />133, Gunnison County in Gunnison County. The current design capacity of outfall 015 is not yet <br />determined. Because the WTP backwash frequency is variable, a worst case scenario of 6 <br />backwashes per day at 30 minutes per backwash with a 270 gpm flow rate equals 0.049 MGD or <br />0.075 cfs. Wastewater treatment is accomplished using sedimentation. The technical analyses that <br />follow include assessments of the assimilative capacity. <br />Nearby Sources <br />An assessment of neazby facilities based on EPA's Permit Compliance System (PCS) database found <br />30 dischargers in the Gunnison County area, including the subject facility. Twenty-five facilities <br />were discharging to another watershed. Other facilities conducted construction-related operations <br />(e.g., sand and gravel) and thus had no pollutants of concern in common with the Oxbow Mining, <br />LLC W WTF. Because Delta County is approximately 2 miles downstream on the North Fork of the <br />Gunnison River, Delta County dischazgers were also assessed. PCS found 31 dischargers in the <br />Delta County area. Twenty facilities were dischazging to another watershed. Other facilities <br />conducted construction-related operations (e.g., sand and gravel) and thus had no pollutants of <br />concern in common with the Oxbow Mining, LLC WWTF. The neazest dischargers were: <br />• Mountain Coal Company (CO-0038776): The Mountain Coal Company West Elk Mine is <br />located in the NE 1/4, Section 16, T13S, R90W; 38° 55' 30" north latitude,107° 2T 00" west <br />longitude; approximately one mile east of Somerset, CO, on Highway 133 in Gunnison <br />County. Mine water discharges to the North Fork of the Gunnison River, Sylvester Gulch, <br />West Sylvester Gulch, the Dry Fork of Minnesota Creek, and Lone Pine Gulch. Because <br />Appendix A Page 7 of 10 Draft <br />