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<br /> <br />COIARADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale, Page 4 Permlt No. CO-0026981 <br />AMENDMENT N0. 1 (Continued): <br />The discharge design flow values were derived from weekly discharge flow <br />readings that were recorded during previous times which are assumed to <br />correlate to Q7_10 low flow occurrences. From the USGS gauging station data <br />for the Uncompahgre River near Ridgeway, seasonal low stream flows occurred <br />during the weeks of February 7 through 13, 1981; May 13 through 19, 1981; and <br />October 6 through 12, 1980/September 24 through 30, 1981. The corresponding <br />weekly average facility flow during these low flow weeks were 676 gpm (or 1.5 <br />cfs), 574.5 gpm (or 1.3 cfs), and 1627/1481 gpm (or 3.6/3.3 cfs; average = 3.5 <br />cfs). <br />Ambient concentrations in Canyon Creek above the discharge were also <br />redetermined for the metal parameters which have permit limitations. The <br />ambient data, as well as the data for determining the revised ambient water <br />quality standards for both segments, was based upon a water quality stream <br />study done for the permittee for the period from March 19, 1987 through <br />November 10, 1987. The individual data for the five sample locations in the <br />two segments are included in the document Recommended Stteam Classification <br />and Water Quality Standards for Segment 9 Canyon Creek Basin, prepared for <br />Denver Mining Finance Company by FS~gineering Science (received by the Division <br />in Mazch 1988), which is included in the permit file for this facility. The <br />ambient data upstream of the discharge were determined from samples at <br />Sneffles Creek and at Imogene Creek, which are the two tributaries that form <br />Canyon Creek. There were a total of 26 samples used in determining the mean <br />values for most of the parameters (for copper, lead, and zinc). The mean of <br />each of the seasonal samples from the two sources was determined with respect <br />to flow weighting considerations, with 80% of the sample weighting for the <br />Sneffles Creek samples and 20% of the sample weighting for the Imogene Creek <br />samples. Outlier values (including the April 29, 1987 sample) were not <br />included in determining the mean concentration. The mean ambient upstream <br />concentrations, as well as the mass balance calculations for determining the <br />revised permit limits, are summarized in Appendix A. <br />The revised permit limits are summarized on the next page in Table 3. <br />