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INTRODUCTION Biomonitoring provides an effective method for testing the toxicity of effluents from municipal and industrial discharges. Among the advantages of biomonitoring is the ability to test complex effluents containing a broad range of contaminants. The biomonitoring methods generate data that cannot be obtained solely from chemical analyses. At the same time, chemical analyses often go hand in hand with the biomonitoring tests. Biomonitoring tests were conducted for Mountain Coal Company during November, 1998. In accordance with accepted Environmental Protection Agency (EPA) and State of Colorado procedures, acute tests were performed using Daphnia magna and the fathead minnow (Pimephales prome/as). The Daphnia magna test failed the WET test guidelines by producing an LC50 concentration. Accelerated testing was required to determine persistence of the toxicity. This report details the results of the first accelerated test. MATERIALS AND METHODS Sample Collection A sample of the Lone Pine Pipeline (#013) discharge was collected in one-gallon plastic containers at 09:15 on November 24, 1998. The effluent was packed in an ice chest and shipped to the lab the next day. The Chain of Custody forth showing collection and lab arival times is included in Appendix 1. In the lab, the samples were refrigerated at 4°C until used. Test Organisms Daphnia magna, a freshwater crustacean, was used as the invertebrate test organism for the accelerated testing. The Daphnia magna are cultured in the laboratory in batch cultures held at approximately 23 °C. The daphnids were less than 24 hours old at the start of the test. All in- houseorganisms are tested at least monthly in a reference toxicant test with sodium chloride to confirm their overall health. Test Procedures Upon receipt at the lab water samples were analyzed for alkalinity, hardness, conductivity, dissolved oxygen, chlorine, ammonia and pH. Alkalinity and hardness were determined titrimetrically according to methods described in Hach Chemical Co. (1978). Ammonia was measured by an Orion ion specific electrode according to procedures in APHA/AWWANVPCF (1985). A total of twenty organisms at each concentration were used for the acute tests. Daphnia magna were exposed in 30 ml plastic disposable beakers. Twenty ml of the effluent mixture was added to each beaker. Exposure medium was replaced after each 24 hour period and organisms surviving at each concentration were counted and recorded. Routine measurements were made of temperature, pH, and dissolved oxygen in each concentration, before and after the water change. Acute tests were performed according to the procedures outlined in Weber (1993). Daphnia magna tests were conducted over 48 hours. The test was run with exposure concentrations of 100%, 50%, 25%, 12.5%, and 1.9% effluent. Water used for the control and as the dilution water was moderately-hard laboratory reconstituted freshwater.