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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 17. Permit No. CO-0027154 VI. TERMS AND CONDITIONS OF PERMIT A. Effluent Limitations (continued) 4. Discussion of Limitations Salinity (continued): A review of the previously determined total dissolved solids limitations for the three continuous outfalls (001, 005 and 007) was made. Due to the intermittent nature of discharge from the surface runoff only outfalls (outfalls 002 - 004, 006 - 010, 016 and 017), these outfalls would contribute relatively insignificant TDS loadings. The actual annual loading from these outfalls varies significantly in proportion to volume water generated as the result of precipitation. The previous limitation of 5,000 mg/1 (converted to equivalent conductivity limitations for each outfall - see next paragraph) is still appropriate for outfalls 001, 005 and 007, based on the potential downstream uses of this water. In 1990, the permittee submitted data to support the substitution of electrical conductivity for total dissolved solids monitoring. The Division reviewed these data and approved this substitution. The permittee was granted approval to use electrical conductivity for salinity monitoring on August 28, 1990. The data were reviewed again at the time of this permit renewal and concurrence with the earlier approval has occurred. This renewal permit will continue to approve electrical conductivity for salinity monitoring for all outfalls and appropriate limitations, expressed in umho/cm, are included for outfalls 001, 005 and 007. 5. Whole Effluent Toxicit~,(WET) Testing: For this facility, acute and chronic WET testing are required for outfalls 001, 005 and 007 only. (See Parts I.A and I.B of the permit.) Purpose of WET Testin¢: Section 6.9.7 of the Regulations for the State Discharge Permit System (6.1.0), passed by the Water Quality Control Commission (WQCC), has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities, WET testing is being utilized as a means to ensure that there are no discharges "in amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life' as required by Section 3.1.11 (1)(d) of the Basic Standards and Methodologies. Chemical analysis of effluent has provided only a partial evaluation of the potential impact a discharge could have on the receiving stream. Also, chemical analysis cannot evaluate the synergistic or antagonistic effect of compounds. There are also compounds for which an accurate or reproducable method of chemical analysis has not yet been developed, as well as compounds which are just beginning to be evaluated for toxic effects. WET testing will provide a more comprehensive means of evaluating the toxicity of a discharge than could otherwise currently be accomplished.