The Tip Top Mine in Gamble Gulch, Colorado is a high mountain site where the stream above the mine is pristine and below the influx of acid rock drainage, the aquatic ecosystem appears to be impacted. An aquatic toxicity assessment study was carried out to determine the impact of contaminants on the stream and to test the leaching methods and simple toxicity tests that have been developed at the Colorado School of Mines. All tests show that the stream water above the adit inflow is pristine. However, the stream below shows concentrations of Al, Cu and Zn that are slightly above acute aquatic toxicity limits. Leaching tests on stream sediment samples taken below the adit entrance show concentrations of contaminants that are on the borderline of being toxic. Physical and chemical assessments of the mine waste piles on the site show that they are not impairing the immediate vicinity or the stream. It appears that the cause of any aquatic toxicity is the adit water entering the gulch or the heavy oxyhydroxide precipitates lining the streambed below the mine site.
Over the past three years, a decision tree has been developed to rank mine waste sites for potential environmental impacts. This approach relies on simple leach tests to determine the chemical composition and toxicity of water in contact with mining wastes. When the pH of the leachate solutions is less than 5, the toxicity of the water is certain. However, when the pH of the leachate solutions is greater than 5, lower concentrations of toxic metals make toxicity assessment uncertain and a simple "in-vitro" test is necessary. These methods were used to evaluate a mine site that is marginally impacted. The Tip Top Mine in Gamble Gulch, Colorado is a high mountain site where the stream upstream of the mine is pristine and downstream of the influx of acid rock drainage, the aquatic ecosystem is marginally impacted. Aquatic toxicity assessments, made using a microbial enzyme bioassay, were conducted to determine the impact of contaminants on the stream. All tests show that the stream water upstream of the adit inflow is unimpacted. However, the stream downstream of the inflow shows concentrations of Al, Cu and Zn that are only slightly higher than acute aquatic toxicity limits. Leaching tests on stream sediment samples taken at the adit entrance show concentrations of contaminants that are also higher than toxicity limits. Simple enzyme bioassay tests, using metals sensitive bacteria, were conducted to establish the toxic response of the sediment leachate. The preliminary results show that leachate water upstream of the adit is not toxic and downstream, the leachate solution is marginally toxic. Duplicate leach tests and enzyme bioassay tests were conducted to determine the reproducibility of these approaches.
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