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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Pnge 3, Permit No. CO-0032638 For hardnessdepetident metaLr standards, the numerical value ojthe aquatic life use based standard will depend upon the combined effluent/receiving stream hardness below tl:e discharge paint. The hardness values to be used are those which correspond to the lower 95th percent confidence limit of the mean hardness value at the per[odic low flow criteria, as determined from a regression analysis of sire-specific data. !f data jot such an analysis is not available, or [f a regression analysis is not appropriate, other methods may be used. Hardness data ojOak Creek, near Oak Creek, CO (Station Number 09238000), was extracted from the EPA STOREY System. The mean hardness of 120 mg/P as CaCOs will 6e used to calculate hardness dependent stnrrdnrds for both the unrtnmed tributaries to Trout Creek and Oak Creek The jollowing table summarizes th¢ metals standards for all rues. The most stringent of these will be used in calculating e,Qluent limitations. Table Ill-1 A9elals S[andards Summary -Oak Creek, Segment 6 and unnamed tributaries to Trout Creek, Segment 13a, of t/te Yampa River Sub-basin of Jhe Upper Colorado River Basin (al! concentrations are tig/P) -.... . Parameter. Aqutitic Lije Use Dunking Water. U3e :. ..,' :...:: Agrieulfurat:Use, ` ,' ,.. Dissolved Method'.:: : , i To(al Recoverable.Method==. ;':: ; 7'o/gl Recoverable'Method Acute Chronic . :. Acute:.. Glironie: :'' Chronic. Arsenic 360 I50 50 - 100 Cndmiurrr 4.82 1.31 10 10 Copper 21.06 13.82 - 1000 200 Iron - 1000* - 300** - Lend !28.67 5.04 50 - 100 Manganese - 1000* - SO** 200 Mercury - 0.01* 2.0 - - Nicke[ 1062.6 109.78 - - 200 Selenitun 135 17 - 10 20 Silver 2.79 0.10 50 - - Zi/ic 136.57 123.7 - 5000 2000 Aquatic Life Use staru(ards ar< hosed «pan the dissolved method of analysis, emepl far iron and manganese standards, which are bated upon the mm! recover'ab(e method, and the mcrnuy srardard, which is based upon the mm! method. Drinddng water Use standards art bnsrd upon the latat recoverable method of analysis, creeps far iron acrd manganese standards, which are based upon dm dissohed nralhod. 8. Receiving LYaJer Data rrnfi No gttnlity data for either Oak Creek or unnamed tributaries to Trout Creek were nvniln6le. Therefore, jot purposes of cnlculnring water quality-bused effluent lindtntions, background concentrations of pnrmneters of concern will be assumed m be zero. 2. Qunmity Anue and Chronic Low Fows.• Although the flows used to calculate acute and chron[c efJIuent limitntiats are the one day in three year low flow (1 E3) and the 30 day in three year low flow (30E3) respectively, these fIoi+•s are nor nvnilnble in the EPA STOREY database jot the receiving streams. Therefore, the low flows jar the unnnrned D~ihutnries to Trout Creek will be assumed to be 6.0 cfs.