An interdisciplinary approach to the prediction of pit lake water quality, Martha Mine pit lake, New Zealand

Abstract

Lakes resulting from open pit mining may be potential water resources or potential environmental problems, depending on their water quality. As the global abundance of pit mines and pit lakes increases, there is increasing pressure on the mining industry to create pit lakes that have environmental, social, and/or economical utility. This thesis uses an interdisciplinary approach involving mineralogy, physical limnology, and geochemistry to predict and improve the water quality of a proposed pit lake at the Martha gold mine, New Zealand. A mineral quantification method developed for this study measured the distributions and concentrations of wall rock minerals, and identified 8 relatively homogeneous wall rock regions, called mineral associations. Acid-base accounting using calcite and pyrite quantities identified 3 associations with acid-generating potential. Three physical limnology tools (relative depth, wedderburn number, and numerical modeling with DYRESM), predicted that the upper 2/3 of the lake will circulate annually during the winter turnover period, whereas the lower 1/3 will remain permanently isolated. Permanent stratification resulted from density differences between groundwater and river water inputs during lake filling, plus lake morphology. The geochemical model used the distribution of mineral associations to characterize the composition of pit wall runoff, and used the limnologic prediction to define the mixing frequency, mixing depth, and layer volumes. Initial modeling with the geochemical program PHREEQC indicated the lake will have a pH of 5, and Cu and Zn concentrations that exceed aquatic life protection guidelines. Sensitivity analyses showed that subaqueous water-rock reactions did not have a significant affect on lake pH, suggesting these reactions are less important geochemical factors in pyrite-bearing pit lakes. Surface adsorption onto ferrihydrite reduced concentrations of As, Pb, and Cu, suggesting these reactions are important geochemical factors in pit lakes. By covering the acid-generating mineral associations, lake pH increased above 6.5, allowing for future recreational use. Concentrations of Cu complied with aquatic life protection guidelines, however, Zn concentrations remained above these guidelines. This study demonstrates the value of interdisciplinary pit lake predictions in the design of closure plans for open pit mines. Such studies improve the ability of mining companies to sustainably develop mineral resources.