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COLORADO DEPARTMENT OF PUBL~EALTH AND THE ENVIRONMENT, Wat~ualiry Control Division ~' <br />Ra[ionale -Page 3, Permit No. CO-00041467 ~ <br />B. Permit Revisions Related ro Administrative Srav of Flow Limitation <br />In a lever dated September 30, 1994, the permittee submitted hvo reports that the Division <br />required through an administrative stay of the flow limitation J.br point 004, which was <br />grained July 22, 1993. The information submitted included: a) a summary of e,Qluent <br />flow data, and b) an evaluation of the wasteload allocation that hat already been <br />performed in order to determine the limits currently listed in the permit. <br />An examination ojthe flows measured at point 004 show a high degree of variability, <br />ranging between 250 gpm and 1500 gpm. Some of the variability appears ro be due to <br />seasonal precipitation patterns, with an additional long-term component. While it is <br />possible that the long-term variability tray also be due precipitation patterns, on a <br />year-to-year bases, it is also possible that it is due to the re-rooting ojsurface flows, <br />modification of sub-surface drainage collection and conveyancer systems, and structural <br />modifications of the tailings impoundments at the site. <br />Due to the relatively short period of record available since the hydrologic modifications at <br />the minesite were completed, which was in the spring of 1994, it may not be possible to <br />detemtine what fraction of the apparent long-term trend should be attributed to the <br />hydrologic modifications, or to conclude whether or not the hydrologic conditions at the <br />site have established any measure of equilibrium. An additional factor that may <br />complicate the analysis of the flows is the unusual precipitation pattern which occurred <br />during the winter of 1994-1995 and spring of 1995, which resulted in unusually high <br />runof f during the spring and summer. <br />Without being able to determine that the hydrologic system has reached any measure of <br />equilibrium, it might be difficult to identify a discharge flow that would likely not be <br />exceeded in the future, and which could be used at an effluent flow limitation that would <br />not be violated. On the other hand, if the question of hydrologic equilibrium was <br />disregarded, the high flows experienced during 1995 might be useful in the specification of <br />such a flow. <br />Still, the specification of a design flow should not be based only upon what Jlow level will <br />likely not be exceeded. /[ is also necessary to consider who! flow level can still be <br />adequately treated. During the summer of 1995, when high flows were experienced, some <br />problems in maintaining effluent limitations were encountered. It is possible, through the <br />examination of flows and effluent quality, that the maximum flow that can be adequately <br />treated may be identified. <br />The analyses that will be required in order to identify a new design flow may require some <br />additional time. As explained below, it may be reasonable to ,grant the permitree an <br />extension of the stay of the flow limit in order to perform these analyses. <br />To begin with, the evaluation ojthe existing wasteload allocation that was submitted by <br />the permiaee included a comparison of the mass of metals discharged from point 004 <br />versus the mass discharge allowed through Jhe wasteload allocation. The comparison <br />shows that the mass actually discharged was less than what wcu' allowed for all metals. <br />