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Cotter Corporation Water Quality Assessment CO0036251 <br />JD -9 Mine — Outfall 002 <br />The ratio of the low flow of the Dolores River to the JD -9 Mine's WWTF design flow is 7:1. <br />Mixing Zones <br />The amount of the available assimilative capacity (dilution) that may be used by the permittee for the <br />purposes of calculating the WQBELs may be limited in a permitting action based upon a mixing zone <br />analysis or other factor. These other factors that may reduce the amount of assimilative capacity <br />available in a permit are: presence of other dischargers in the vicinity; the presence of a water diversion <br />downstream of the discharge (in the mixing zone); the need to provide a zone of passage for aquatic life; <br />the likelihood of bioaccumulation of toxins in fish or wildlife; habitat considerations such as fish <br />spawning or nursery areas; the presence of threatened and endangered species; potential for human <br />exposure through drinking water or recreation; the possibility that aquatic life will be attracted to the <br />effluent plume; the potential for adverse effects on groundwater; and the toxicity or persistence of the <br />substance discharged. <br />Unless a facility has performed a mixing zone study during the course of the previous permit, and a <br />decision has been made regarding the amount of the assimilative capacity that can be used by the facility, <br />the Division assumes that the full assimilative capacity can be allocated. Note that the review of mixing <br />study considerations, exemptions and perhaps performing a new mixing study (due to changes in low <br />flow, change in facility design flow, channel geomorphology or other reason) is evaluated in every <br />permit and permit renewal. <br />If a mixing zone study has been performed and a decision regarding the amount of available assimilative <br />capacity has been made, the Division may calculate the water quality based effluent limitations <br />(WQBELs) based on this available capacity. In addition, the amount of assimilative capacity may be <br />reduced by T &E implications. <br />For this facility, 100% of the available assimilative capacity may be used as the facility has not had to <br />perform a mixing zone study, and the discharge is not to a T &E stream segment, and is not expected to <br />have an influence on any of the other factors listed above. <br />Ambient Water Quality <br />The Division evaluates ambient water quality based on a variety of statistical methods as prescribed in <br />Section 31.8(2)(a)(i) and 31.8(2)(b)(i)(B) of the Colorado Department of Public Health and <br />Environment Water Quality Control Commission Regulation No. 31, and as outlined in the Division's <br />Policy for Characterizing Ambient Water Quality for Use in Determining Water Quality Standards Based <br />Effluent Limits (WQP -19). The ambient water quality was not assessed for the unnamed tributaries of <br />East Paradox Creek and Bull Canyon and the East Paradox Creek and Bull Canyon, because the <br />background in- stream low flow conditions are zero, and because no ambient water quality data are <br />available for the unnamed tributaries, the East Paradox Creek and Bull Canyon upstream of the Cotter <br />Corporation Mines' WWTF discharge. <br />Appendix A (wQA V 7.0) <br />Page 14 of 23 Last Revised 06/10/10 /AO <br />