TOXICITY OF MINE-WASTE LEACHATES TO AQUATIC ORGANISMS AS A FUNCTION OF PH AND TRACE-METAL CONCENTRATIONS

Understanding processes that control the transport and bioavailability of metals originating from weathering of mine-waste material is critical for assessing impacts of mined sites. Formation of iron-rich precipitates is common at mined sites. These precipitates may attenuate trace metals as they migrate from their source through ground- and surface-water systems, thereby reducing the bioavailability of the metals. These attenuation reactions are pH dependent, and generally tend to produce lower dissolved trace-metal concentrations with increasing pH. We examined the acute toxicity of metals in mine-waste leachates to aquatic organisms as a function of pH and metal concentrations.

Water-based batch leachates were prepared using mine-waste material collected from eight hard-rock mine-waste dumps in Colorado and New Mexico. The leachates were pH-adjusted between ambient (pH 3-9) and pH 10 to test how pH affects both metal content and toxicity. The aim of these pH-adjusted leachates is to simulate changing geochemical conditions as trace metals are leached from waste material and migrate through ground- and surface-water systems. The pH-adjusted leachates were filtered, analyzed for dissolved metal concentrations, and used in toxicity tests.

Daphnia magna (water flea; static 48-hour acute toxicity tests) and Vibrio fischeri (Microtox Basic Test and 100% Test) were used to assess the acute toxicity of these pH-adjusted filtered leachates to aquatic organisms. The leachates were toxic to V. fischeri only at the lowest pH values (pH < 4), whereas D. magna showed a greater sensitivity to the leachates. Leachates with pH values less than about 6.5 and greater than about 8 were significantly more toxic to D. magna than were the samples with pH values near 7. Based on acute toxicity tests at low pH without metals present, some of the toxic effects appear to be due to pH, but additional toxic effects were observed at low pH in the presence of dissolved metals. Consequently, the toxic effects at low pH appear to be due to a combination of pH and dissolved-metal concentrations.