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An investigation into determining the source of elevated metal concentrations in seepage at Mount Polley Mine

University of British Columbia

Unusual water chemistry coinciding with the appearance of a precipitate were observed in two seepages from a rock storage facility at Mount Polley Mine. This rock storage facility was originally characterized as non-acid generating, meaning that the rock under surface weathering/oxidizing conditions will not produce acid. However, these two seepages were not behaving according to drainage predictions and resembled a product of neutralized acid rock drainage. Due to the localized nature of the affected seepages, the cause of this unusual water chemistry was hypothesized to be the leakage of an external source of acid into the RSF. The possible acid sources were proposed to be an experimental lined heap leach pad and excess stockpiled elemental sulfur that had been sourced to provide the acid required for the heap leach pad. An inspection revealed that the heap leach pad liner appeared to be intact. However, drainage from the sulfur stockpile was insufficiently contained. By using bacteria culture experiments, bacterial DNA extraction, and bulk characterizations of the elemental sulfur, it was concluded that the sulfur had been oxidized, leading to the generation of sulfuric acid that infiltrated the underlying rock. To determine the extent to which the rocks had reacted with the acid generated from the sulfur stockpile, the rocks underneath the sulfur stockpile were sampled. Visual inspection and further laboratory analyses revealed extensive alteration of these rocks under acidic conditions, rendering them similar to leached oxide ore sampled from the experimental heap leach pad. pH-controlled weathering fronts were evident in these rocks, progressing from leached near the contact with the sulfur pile (pH < 3), to active neutralization by silicates (pH 4 – 5), and finally to active neutralization by carbonates (pH > 7). These altered rocks have been re-classified as PAG due to the depletion of carbonates from exposure to sulfuric acid. The findings of this research will help the mine assess the long-term stability of the altered rocks and guide future development of remediation strategies.

Text

2019-12-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/72599

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/