Leachate and Enzyme Bioassay Toxicity Assessment Tests at the Tip Top Mine, a Marginally Impacted Site

Abstract

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.

A Simple Scheme to Determine Potential Aquatic Metal Toxicity from Mining Wastes

“…Issues such as the degree of leachate dilution upon entering a receiving water, the actual water-rock ratio in the mine waste pile, and the toxicity-mitigating factors of hardness, alkalinity, and DOC must be considered. Consequently, our ongoing development of this assessment approach is incorporating a simple and inexpensive aquatic toxicity test that is based on a microbial enzyme bioassay (Blumenstein et al, 2005;Moehle et al, 2005Moehle et al, , 2006. Furthermore, the biotic ligand model, an acute toxicity model that relates water composition to metal toxicity (Santore et al, 2001), is being incorporated into this assessment approach (Smith et al, 2006b).…”

A Physical and Chemical Assessment of the Maude Monroe Mine Site

“…To assist in the assessment, element concentration pattern graphs (ECPG) are produced that compare concentrations of selected groups of elements from the three leachates and any water associated with the mining waste. Examples of how the leaching tests and the ECPG can be used in a site assessment are given in Bazin et al, 2002, Hageman et al, 2005, and Moehle et al 2006. The MWDT makes a distinction between leachates or waters with pH less than or greater than 5.…”