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`~OLORADO DEPARTMENT OF PUBLI~EALTH AND THE ENVIRONMENT, Wat~uality Control Division ~tRationale -Page 4, Permit No. CO-0004 467 i While it is possible for a discharge ro 6e in compliance with mass-based linuts and yet still cause instream pollutant concentrations to exceeded, the relative discharge and receiving stream flows and pollutant concentrations involved in this permit are such that this will likely not occur. Based upon the above considerations, the permit will be amended to extend the stay of the flow limitation. This will allow the permittee additional time to establish: I) the maximum flow likely to be received by the the treatment system, and 2) the maximum flow which can be adequately treated. It will likely require at least one year to gather data needed to perform the analyses necessary to generate these analyses, due to the recent unusual precipitation pattern and due to the change in the WET requirements that are included in this amendment. Since the permit will expire within two years, the stay of the flow limit will be extended until the renewal permit is issued. Anew design flow will be established at that time. Still, in order to insure that stream standards are being mairuained during the interim two-year period, the amended permit will include a requirement to submit semi-annual reports which shall provide: I) a summary of the mass loading from the discharge, for all permit-limited metals, and a comparison of the mass loading against the wasteload allocations, and 2) a summary of monthly average ejfluen[ concentrations, for all permit-limited metals, an estimation of the resulting instream monthly average e,~Jluent concentrations immediately downstream of the discharge, and a comparison ojthe estimated instream concentrations against chronic stream standards. Each semi-annual report shall cover the previous six-month calendar period, from January-June, and from July-December, and will be due three months after the end of the six-month period. This is shown in the following table: Period to be Summarized Report Due July-December, 1995 March 31, 1996 January-June, 1996 September 30, 1996 July-December, 1996 March 31, 1997 January-June, 1997 September 30, 1997 /n order to implement these revisions, pages Ic(ii), Ic(iv) and le ojthe pernrit will be amended. C. Whole Effluent Toxicity The Division has developed new policies and permit language related to Whole E„~luenl Toxicity Testing. Also, in a letter dared October 12, 1994, the permittee requested that WET testing requiremems be amended to require single-species testing using Ceriodaohina dubia. The specific WET testing requirements that will be included in the permit, arul the Division's response to the permittee's request are discussed below.