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Test Design Guidelines for conducting the acute biomonitoring tests are contained in Peltier and 'Neber (1985). In summary, quadruplicate 30 ml plastic disposable beakers were used at each concentration far the daphnid tests. Each vessel contained 20 ml of exposure medium in which five neonates/young were placed, for a total of 20 organisms per concentration. The exposure medium was changed in each container after 24 hours and the test was terminated after 48 hours. Surviving organisms were counted daily and measurements of temperature, pH, and dissolved oxygen recorded. Total alkalinity, hardness, conductivity, total residual chlorine, and ammonia were measured in the effluent and control at the start of the test. The fathead minnow tests were conducted in 260 ml plastic disposable cups containing 200 ml of the exposure medium. Five organisms in each of four replicates were used at each concentration. The test was terminated after 96 hours with the same procedures being followed as above. Deviations from Established Procedures Within 24 hours after starting the test, dissolved oxygen concentrations in the 10090 effluent concentration dropped to 2.7 mg/I which is below a 409a saturation level. To assess the effects of the dissolved oxygen levels compared to toxicant concentrations, a second test was begun on August 15 with all test concentrations being aerated. RESULTS A significant acute toxicity was noted to both the daphnids and the fathead minnows in the effluent sample. In the D. magna test, survival in the daphnid test ranged from 0-100% in the various exposure concentrations. The 1009'0, 75%, and 50% concentrations had zero survivors, respectively. This led to an LC50 for this effluent of 35.4%. The Ceriodaphnia test produced a similar result with an LC50 of 33.89'° and no survivors in the 100, 75, and 50% exposures. In the August 14 fathead minnow test, survival ranged from 09a in the full strength effluent to 100% at concentrations of 5090 or less. Control survival was 100%. This produced an LC50 of 71.39x. The August 15 sample had a greater mortality overall which resulted in an LC50 of 36.99'°. This indicates that the low oxygen levels in the August 14 test did not significantly increase toxicity in that test. DISCUSSION The toxicity was generally high overall and was probably due to a high conductivity which ranged over 19,000 micromhos per cm. This would be sufficiently high to affect both the daphnids and the fathead minnows.