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INTRODUCTION <br />Somerset Mining Company conducted Preliminary Toxicity <br />Investigations (PTI) on mine effluent from outfall 011 in 1992, to <br />address high concentrations of sodium bi-carbonate in the effluent, <br />it's toxicity to CeriodaAhnia dubia, and a reduction in toxicity <br />with effluent pH adjustment. The mine effluent is composed mostly <br />of spring water entering the mine prior to dewatering. Comparative <br />toxicity tests indicated that the spring water was also acutely <br />toxic to the organisms and contained high concentrations of <br />carbonates, levels higher than those determined present in the <br />effluent. Therefore, it was suspected that toxicity was introduced <br />via high alkaline spring water entering the mine at the B-Seam. <br />outfall 011 is discharged into the Gunnison River. The Somerset <br />Mining Company permit to discharge states an Instream Waste <br />Concentration (IWC) of 1% effluent for the outfall. Therefore, <br />based on the above preliminary investigation test results, stream <br />bio-assessment studies indicating no significant instream <br />biological impact, and high dilution, the facility was granted test <br />modification to lower the effluent pH to drive off COz prior to WET <br />testing utilizing C. dubia. <br />The most recent WET test conducted using the modified effluent pH <br />procedure did not remove acute toxicity to C. dubia in the 100% <br />effluent sample. Therefore, as outlined in the correspondence to <br />Somerset Mining Company from Ms. Kathleen Sullivan of the Colorado <br />Department of Health dated April l0, 1995, the facility conducted <br />the following Toxicity Identification Evaluation to further <br />characterize the toxicant(s) and/or confirm TDS toxicity. <br />2 <br />COMMERCIALTESTING & ENGIN EE RING CO. <br />F<68 L~~ <br />Original WatermerkeA Far Vour Protection .,.. <br />