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4 LC50 created in the accelerated test was 80.5%. Therefore although the effluent produced a toxic effect (and LC50 concentrations) in the cerio tests, it was not as toxic as predicted by the model. This could imply that other components are present in the effluent that mitigate the toxicity from the TDS. Daphnia magna Test The analysis for the Daphnia magna did not show an LC50 concentration produced in the 005 effluent. Although the analysis suggested that the Daphnia magna could have a survival effect in the 100% effluent, the actual test results showed no significant effect in any effluent concentration of the test. DISCUSSION By using the GRI Freshwater STR model it was determined that the 005 effluent contained quantities of dissolved solids that could cause a toxic effect to Cenodaphnia tested in the discharge. This toxicity was found in actual tests, although at slightly lower toxicity levels than the model predicted. The model detenined that there would be no LC50 concentration produced in tests with the Daphnia magna in the 005 discharge. The results of Daphnia magna tests run in the same discharge samples as the Cenodaphnia showed no significant toxicity in any effluent concentration. The PTI performed with the STR model indicates that TD5 was the cause of the toxicity seen in the Cenodaphnia tests conducted with the 005 effluent. It also suggests that tests conducted with the Daphnia magna would show a reduced toxic response to this discharge. Daphnia magna are known to be less sensitive to TDS levels than the Cenodaphnia. Daphnia magna is an EPA-approved alternative invertebrate species for acute WET tests when evidence suggests that TDS is the cause of a toxic response in the Cenodaphnia tests. Therefore since TDS is not considered a "toxicant" at levels where the Daphnia magna are not affected, it should be possible to substitute the Daphnia magna for the Cenodaphnia in the WET testing if the CDPHE approves the permit amendment. REFERENCES APHA/AWWANVPCF. 1985. Standard Methods for the Examination of Water and Wastewater. 16th Edition. American Public Heaith Association. 1268 pp. Hach Chemical Company. 1978. DR-EL/2,3 and 4 methods manual. Hach Chemical Company, Loveland, Colorado. Weber, C.I. 1993. Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to Freshwater and Marine Organisms (Fourth Edition). EPA 600/4-90/027F. 293 pp. J.E. Tietge, Mount, D.R., Gulley, D.D.. "GRI Freshwater STR Modei and Computer Program". Gas Research Institute, Chicago, III. 60631. December 1994.