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~-'~ • `~ i COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 13. Permit No. CO-0024562. 5. Whole Effluent Toxicity (WET) Testing:(continued): c) Acute Biomonitoring: Based upon the potential toxicity of metals in the discharge, the Division believes there is reasonable potential for the discharge to interfere with attainment of applicable water quality classifications or standards. Because of this condition, the acute toxicity limit has been incorporated into the permit and becomes effective April 1, 1995. During the first three years, the permittee is required to conduct routine monitoring. If an unacceptable level of acute toxicity is identified, the permittee is required to conduct a toxicity incident response and identify a control program to eliminate that toxicity, as identified in Part I.B. of the permit. Once the acute toxicity limitation has become effective, the permittee is required to proceed with the automatic enforcement compliance schedule, as identified in Part I.B. of the permit, should a violation occur. d) Chronic Biomonitoring: The determination as to whether or not an individual facility must conduct chronic WET testing is dependent upoa two factors: the ratio of the chronic stream low flow (30E3) to the design flow of the wastewater treatment plant and the aquatic life classification of the receiving water. If the receiving water is classified aquatic life class 1, chronic WET testing is always required regardless of the dilution ratio. If the receiving water is classified aquatic life class 2 with all appropriate aquatic life-based numeric standards and if the chronic stream flow-to-design flow ratio is less than 19:1, chronic WET testing is required. If the receiving water has no aquatic life classification, chronic monitoring is not required. In this case the ratio was determined to be 1.7:1 for a facility design flow of 2.58 MCD and to be 1.2:1 for a facility design flow of 3.76 MGD. Therefore, chronic WET testing is required regardless of mining activity. Such testing shall be semi-annually, with two species for the first year, and with only the most sensitive species durictg years two and three. After the third year of monitoring, the chronic testing requireJnent ends unless applicable regulations are adopted and the permit is modified for such new requrements. The permit has already been condition for termination of chronic monitoring after 3 years, beginning in the 1st half of 1992 and lasting through June 30, 1995. A written request from ther permittee, however, will be required before relief from two species testing will be allowed.