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METHODS A two species acute bioassay was conducted with effluent from the West Fork of Sylvester Gulch, outfa11017. An effluent grab sample was collected on August 1, 2000, stored on ice, and delivered to Commercial Testing and Engineering, Co. (CT&E) on August 2, 2000. The effluent sample was received at a temperature of 20.0 ° C and assigned CT&E sample tracking number 72-433054. The bioassay was initiated approximately 26 hours after sample collection. Testing was conducted in accordance with the most recent protowls specified by the Water Quality Control Division of the Colorado Department of Health (1998), outlined in Table 1. Basic characteristics of the effluent and dilutions were measured as follows: Characteristic Method/ErEuipment EPA Method Alkalinity Ammonia Chlorine, Total Residual Conductivity Dissolved Oxygen Hazdness pH Temperature Titrimetric (APHAI, 1989) Lachat Analysis Method LaMotte STC-CI Colorimeter Orion Model 160 Cond. Meter Orion Model 850 DO Meter Titrimetric (APHA, 1989) Fisher Model 925 pH Meter IVISTZ calibrated thermometer EPA 310.1 & 130.2 EPA 350.1 EPA accepted EPA 120.1 EPA 360.1 EPA 310.1 & 130.2 EPA 150.1 EPA accepted The tests exposed daphnids, Daphnia magna, to five concentrations of effluent, 100%, 75%, 50%, 25%, and 1.9%, prepared using reconstituted laboratory water at 160 mg/L CaC03. Additionally, the tests exposed fathead minnows, Pimephales promelas, to five concentrations of effluent, 100%, 75%, 50%, 25%, and 1.9%, prepazed using reconstituted laboratory water at 115 mg/L CaC03. Control replicates were exposed to 100% laboratory water. The Binomial and Probit methods determined the LCso's, where applicable. Significant differences in mortality were analyzed using Steel's Many-One Rank tests. American Public Health Association s National Institute of Standards and Technology COMMERCIAL TESTING & ENGINEERING CO. F 466 L1E Odginal Watermarked For Your Protection