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COLORADO DEPARTMENT OF PUBLIC HEALTH AND THE ENVIRONMENT, Water Quality Control Division Rationale -Page !5, Permit No. CO-0045225 !n this case, while !here are no other pollutaru sources that have been identified, there are severs! special considerations that mrest be taken into account, as discussed below. 3. Sire-Specific TMDL Considerations a) Delineation oJTMDL Hydrologic System Boundary For Cripple Creek, there is a monitoring station located above Squaw Gulch, which can be used to assess upstream pollutant loadings. There are no other known storm-related surface runoff pollutant sources like (he /ronclad Overburden Storage Area that would contribute runoff loadings to Cripple Creek either above or below Squaw Gulch, from its headwaters down fo the confluence with Fourmile Creek. At that point, it is likely that pollutam loadings from the /ronclad Overburden Storage Area would be very small in comparison (o the TMDL jot Fourmile Creek, having already been constrained by the TMDL jot Cripple Creek. And, due to so many other factors that must be considered -especially ivi(hin the context of precipitation induced loadings - it would be impractical to assess the relationship between pollutant loading from the overburden storage area and the water qua[iry in Fourmile Creek, or to translate a load nllacation based upon a Fourmile Creek TMDL up to the overburden storage area. As a result oJthe above considerations, (he hydrologic rys(em that must be examined in (he TMDL analyses can be limited to Cripple Creek from the upstream monitoring station CCOl, to immediately below the confluence with Squaw Gulch. !n the case of analyses based upon the protection oJSquaw Gulch water quality standards, the TMDL is very simple. Since Squaw Gulch is an imermittent stream, the Release Concentrations associa[ed with the protection oJSquaw Gulch water quality standards are equal to the standards themselves. Assuming that metals standards for Squaw Gulch are removed by the WQCC, this will apply to pH only. Since it is likely that metals standards jot Squaw Gulch will be removed, protection of Cripple Creek standards will be the underlying goal of the TMDL analyses. The remainder of (his section describes how these Cripple Creek TMDL analyses were performed. b) Cripple Creek TMDL -Upstream Loading (CtQt ) !t is highly unlikely that the receiving stream wilt be experiencing tow flow conditions when a regenerative release occurs. Such releases are likely to occur either as a result oJan extremely large precipitation event or a long series of moderately large evems. Either situation would likely not occur during a particularly dry period, and there would likely be significant recharging of the alluvial aquifers of the strearns in the area that would continue to provide dilution flows for a period of time after the storm event 77(erejore, instead ojusing low flows, the average flow of the receiving stream ivas used for the upstream flow Qt , as it should be a reasonable estimate of the most expected value for upstream dilution. For upstream pollutant concentrations, Colorado TMDL guidance sperifies that median values be used for parameters such as metals. Therefore, median values at station CCOI were used. Ir should be noted that in the legal agreements between the permittee and the Division, calculations were done using average concentrations instead of median concentrations, so there is a slight difference behveen the release concemrations originally calculated, and [hose appearing in this permit. The speciflc upstream average Jlow and median pollutant concentrations that will be used are listed in section II1 of this rationale, and are also presented in section 4 below.