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COLORADO DEAARTMENT OF PUBLIC HEALTH AND ENV/RONMENT, Water Quality Control Division <br />Rationale -Page 7, Permit No. CO-0032638 <br />The Permits Section evaluated the calculated WQBELs and has made a determination as to whether there is a <br />reasonable potential for the facility discharge to cause or contribute to an exceedance of a stream standard. If there <br />is a reasonable potential for the discharge to contribute to an exceedance, effluent limits are included in the permit. <br />For metals, a quantitative approach to reasonable potential evaluations is utilized as discussed in later paragraphs. <br />i. Metals -Using the assimilative capacities contained in [he WQA, an analysis must be performed [o determine <br />whether to include the calculated assimilative capacities as WQBELs in the permit. This reasonable potential <br />analysis is based on the Determination ofthe Requirement to Include Water Ouality standards-Based Limits in <br />CDPS Permits Based on Reasonable Potential dated December, 2002. This guidance document uti/tzes both <br />quantitative and qualitative approaches to establish reasonable potential depending ott the amount ofavailoble <br />data. <br />For total recoverable iron, dissolved manganese, and total mercut7; there was enough effluent data to conduct <br />a quantitative analysis. Thus, a reasonable potential analysis was conducted to determine if there was <br />reasonable potential for effluent discharges to cause or contribute to exceedances of ambient water quality <br />standards. This analysis was performed by using multipliers to establish the maximum estimated pollutant <br />concentration (MEPC), and comparing the MEPC to the applicable maximum allowable pollutant <br />concentration (MAPC) as calculated in Appendix A. The guidance specifies that where the comparison shows <br />that the MEPC exceeds the MAPC, limits must be established and where the MEPC is greater than half, <br />nvonitoring must be established Tables YI-6 and Y/-7 contain the data that were used and the results of the <br />reasonable potential evaluations. <br />Table VI-6 -Reasonable Potential Determination for Metals Effluent Limits for Outfall 004A <br />~~ ~ `N"~` Criteria `•,"••~" `~~ Detection MEPC 1VIAPC RP :' . <br />-~ nth Liinif Analysis, .Limit, ue/1 .. <br />Iran AQL Chrorac TR 35I0 1000 Yes <br />AQL Acute PD - ]7010 4738 Yes <br />Yes <br />Table YI-7 -- Reasonable Potential Determination for Metals Effluent Limits for Outfa11007A <br />900 <br />0.011 No <br />Based on the effluent data available, there was reasonable potential at Outfa[I 004A for exceedances of in- <br />stream standards for chronic total recoverable iron, and for chronic and acute dissolved manganese. <br />Reasonable potential at Outfa11007A was determined for chronic total recoverable iron. Previous monitoring <br />data shows that these limitations can be met and therefore they nre effective immediately. Note that an <br />amendment to the previous permit removed limitations for these parameters, which in the previous permit were <br />based upon water supply, and there was not a water supply intake downstream of this discharge. The <br />limitations in Shis permit are based upon aquatic life use. <br />According to Division procedure, in the instances where only chronic limitations or monitoring is prescribed <br />and no acute limitations or monitoring are included, or vice versa, the permit will include report, only, <br />requirements for the corresponding limit. <br />The data indicate that neither limits nor monitoring are required for total mercury, based on data for total <br />mercury that reflect analytical results have all been found at less thmr detectable levels. However, detecubn <br />levels only as low as 0.2 ug/1 have been achieved, versus total mercury limits as low as 0.0/ 0 ug/l. Until <br />