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COLORADO DEPARTMEM' OF HEALTH, Water Qun(ity Control Division Rationale -Page 6, Permit No. CO-0043648 lnor2anic Starulards Unionized Ammonia (NHS, Chronic = 0.02 mg/l Unionized Ammonia (NHjJ, Acute = 0.43/FT/FPH/2 mg/l Cyanide (free) = 0.005 mg/1 Su fate as HrS = 0.002 mg/l undrs Metal Criteria Standard. ur/! Arsenic (AsJ Acute and Chronic 50 Chromium, tri (C~ III) Acute and Chronic SO Iron (Fe) Chronic 1000 Manganese (MnJ Chronic 1000 Mercury (HgJ Lhronic 0.01 Analytical Method Dissolved Dissolved Total Recoverable Total Recoverable Total Cadmium (CdJ Acute TVS-trout TVS Cadmium (CdJ Chronic TVS TVS Chromium, hes (Cr VIJ Acute and Chronic TVS TVS Copper (CuJ Acute and Chronic TVS TVS Lead (Pb) Acute and Chronic TVS TVS Nickel (Ni) Acute and Chronic TVS TVS Selenium (SeJ Acute and Chronic TVS TVS Silver (Ag) Acute TVS TVS Zinc ~LnJ Acute and Chronic TVS TVS (7VS = Table Value Standards) Table Value Standards (IVSJ refers to Table //1 of the Basic Standards and Methodoloeies far Surface Water. For the metals listed with 7VS standards, three sets of standards may apply depending upon the receiving stream classfcation -those for protection of 1J aquatic lije use, 2J agricultural use and 3J drinking water use. Table Value Standards for the protection of aquatic life can be divided into two categories -those that are dependent upon the stream hardness, and those that are not. Standards for the protection of agricultural and water supply uses are all hardness independent. For this stream segment, aquatic fife use and agricultural use standards apply. For hardness-dependent metals standards, the numerical value ojthe aquatic life use based standard depends upon the combined e,Qluent/receiving strewn: hardness below the discharge point. The hardness valuer to be used are those which corresporul to the lower 95th percent confidence limit of the mean hardness value at the periodic lo~v flow criteria, as determined from a regression analysis ofsite-specific data. Ijdntn jar such an analysis is not available, or if n regression analysis is not appropriate, other merhodc may be used. /n this case, n regression analysis may nor be appropriate. Since under low flow conditions, the receiving stream will likely be totally comprised of spring water or resurfacing groundwater, it is likely that the hardness of the combined eflIuent (ij any discharge of pollutants occurs) and receiving stream flow will not vary over n wide range. Therefore, the mean hardness of Arequw Culch will be used to calculate metaLc standards. Hardness dale, along with data jot other water quality parameters, has been collected by CC&V ar several locariorts on Arequa Gulch. The data which is pertinent to the development of this permit is sumntnrized in section /I/. B. oJthis rationale.