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COLORADO DEPARTMENT OF HEnLTH, Water Qttnlih~ Control Division <br />Rationale -Page 6, Permit No. CO-0043648 <br />In this case, a regression analysis may not be appropriate. Since under low flow conditions, the receiving <br />stream will probably be totally comprised ojspring water or resurfacing groundwater, it is likely that the <br />hardness of the combined effluent (if any discharge of pollutants occurs) and receiving stream flow will not vary <br />over a wide range. Therefore, the mean hardness ojArequa Gulch will be used to calculate metals standards. <br />Hardness data, along tivith data for other water quality parameters, Itas been collected 6y CC&V at several <br />locariorts on Aregua Gulcl:. Tl:e da[a which is pertinent to [he developmeru of this permit is summarized in <br />section ///. B. of this rationale. <br />The mean hardness at rite upper station - AG 1.0, is 389 mg/l, while the mean hardness at rite lower station - <br />AG 1.5, is 843 mg/l. These Jardness values approach or exceed the mnxintum hardness recommended by [he <br />EPA for use in standards calculations, wl:iclt is 400 mg/l. And, examining the most recent data collected at <br />bor/t stations, it appears likely that rite hardness at point AG 1.5 is likely ro remain above 400 ntg/l in the future. <br />Therefore, for standards calcu[ariort purposes, the value of 400 mg/l will be used. The following tables <br />summarize the metals standards for both stream segntenrs of interest. <br />Table II/-1 Metals Standards Summary -Squaw Cu[ch, Segment 20 of the Upper Arkansas Sub-basin of <br /> the Arkansas River Basin, All Concentrations Are ug/f <br /> Stream Standard, ug/l <br />Metal Acttte Chronic Analytical Method <br />Arsenic - 100 To[a! Recoverable <br />Cadmium 19 3.4 Dissolved <br />Chromium 111 - 100 Dissolved <br />Chromium V/ 16 11 Dissolved <br />Copper 65 39 Dissolved <br />Iron - 1000 Total Recoverable <br />Lead 899 28 Dissolved <br />Manganese - 1000 Dissolved <br />Mercury - 0.01 Total <br />Nickel 2645 274 Dissolved <br />Selenium 135 17 Dissolved <br />Silver 22 0.8 Dissolved <br />Zinc 379 343 Dissolved <br />Table III-2 Metals Standards Summary - Arequa Gulch, Segment 22 of the Upper Arkansas Sub-basin of <br /> the Arkansas River Basin, All Concentrations Are ug/l <br /> Stream Standard, ug/l. <br />Metal Acute : Chronic Analytical Method <br /> Standards That Have No Tetnporary Modifications <br />Arsenic - 100 Dissolved <br />Chromium l!/ 5,405 644 Dissolved <br />Cltrontium V/ 16 11 Dissolved <br />Copper 65 39 Dissolved <br />Lead 899 28 Dissolved <br />Mercury - 0.01 Total <br />Nickel 2645 274 Dissolved <br />Selenium 135 17 Dissolved <br />Silver 22 0.8 Dissolved <br />(Eff 3/2/98) <br />