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Minrec, Incorporated Water Quality Assessment CO- 0048233 <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 available <br />in a permit are: presence of other dischargers in the vicinity; the presence of a water diversion downstream <br />of the discharge (in the mixing zone); the need to provide a zone of passage for aquatic life; the likelihood <br />of bioaccumulation of toxins in fish or wildlife; habitat considerations such as fish spawning or nursery <br />areas; the presence of threatened and endangered species; potential for human exposure through drinking <br />water or recreation; the possibility that aquatic life will be attracted to the effluent plume; the potential for <br />adverse effects on groundwater; and the toxicity or persistence of the 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 flow, <br />change in facility design flow, channel geomorphology or other reason) is evaluated in every permit and <br />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 (WQBELs) <br />based on this available capacity. In addition, the amount of assimilative capacity may be reduced by T &E <br />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 Environment <br />Water Quality Control Commission Regulation No. 31, and as outlined in the Division's Policy for <br />Characterizing Ambient Water Quality for Use in Determining Water Quality Standards Based Effluent <br />Limits (WQP -19). Ambient water quality is evaluated in this WQA analysis for use in determining <br />assimilative capacities and in completing antidegradation reviews for pollutants of concern, where <br />applicable. <br />To conduct an assessment of the ambient water quality upstream of the Minrec facility, data were obtained <br />primarily from sampling conducted by Minrec, Inc. at a location upstream of their discharge point. Data <br />for total recoverable iron, chloride, and sulfate were available for a period of record from 1993 through <br />2000. For total recoverable and dissolved arsenic, dissolved copper, dissolved cadmium, dissolved <br />manganese, total mercury, dissolved lead, dissolved silver, and dissolved zinc for which Minrec had not <br />conducted sampling, data from Division Station 133 (Thompson Creek @ Hwy 133) was used. This <br />location is approximately 6 miles downstream from the facility discharge location, and after the confluence <br />of North Thompson Creek with Thompson Creek. <br />Appendix A (WQa v 7.0) Page 10 of 25 Last Revised Sept 14, 2009 /ES <br />