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Central Appalachia WWTF Water Quality Assessment CO-003 <br />low flow criteria, representative regional data shall be used to perform the regression <br />analysis. Where a regression analysis is not appropriate, asite-specific method should <br />be used. <br />Hazdness data for the unnamed tributaries to East Salt Creek or East Salt Creek neaz the point of <br />discharge of the Central Appalachia WWTF were unavailable. Data for USGS Station 09153330, <br />West Salt Creek Neaz Cazbonera, CO, were available. The mean hazdness was computed to be 968 <br />mg/1 based on sampling data from this station for the period of record from June 1979 through <br />September 1981. However, as required by the Basic Standards and Methodologies for Surface <br />Waters, Regulation 31, hazdness is capped at 400 mgl1. Thus, a hazdness value of400 mg/1 and the <br />formulas contained in the TVS were used to calculate the in-stream water quality standazds for <br />metals with the results shown in Table A-3. <br />Ambient Water Quality <br />The WQCD 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 Basic Standards and Methodologies for Surface <br />Waters, Regulation 31. The ambient water quality was not assessed for the unnamed tributaries to <br />East Salt Creek because the background in-stream low flow condition is zero, and because no <br />ambient water quality data aze available for the unnamed tributaries upstream of the Central <br />Appalachia WWTF discharge. <br />III. Water Quantity <br />The Colorado Regulations specify the use of low flow conditions when establishing water quality <br />based effluent limitations, specifically the acute and chronic low flows. The acute low flow, referred <br />to as 1 E3, represents the one-day low flow recurring in athree-yeaz interval. The chronic low flow, <br />30E3, represents the 30-day average low flow recurring in a three-yeaz interval. <br />Low Flow Analysis <br />Although there is periodic flow in the unnamed tributaries to East Salt Creek upstream ofthe Central <br />Appalachia WWTF, the lE3 and 30E3 monthly low flows are set at zero based on information <br />provided by local Water Commissioners. For this analysis, low flows aze summarized inTable A-4. <br />Table A-4 <br />Low Flows for unnamed tributaries to East Salt Creek at the Central Appalachia <br />WWTF <br />Acute <br />30E3 <br />Appendix A Page 5 of 9 Draft <br />