Zimbabwean mine dumps and their impacts on river water quality – a reconnaissance study

Abstract

Zimbabwe has a substantial number of mines and 67 minerals have been mined in the country since 1900 but at present only 30 different minerals are being mined. Exploitation of a variety of ores, in rocks of diverse composition, provides the potential for a range of pollution problems. The severity and extent of contamination differs with the type of minerals mined. This paper presents part of the results of a broad study, carried out across Zimbabwe, which assessed the potential of different mine tailings dumps to cause environmental problems. The dumps considered in the study were divided into six dump types, namely: gold-mine dumps, base-metal mine dumps (dumps associated with the mining of nickel, zinc, copper and lead), minor-metals mine dumps (dumps associated with mining of antimony, arsenic, and selenium), platinum-group metal mine dumps, chromite and asbestos mine dumps, and sulphur (pyrite) mine dumps. The elemental chemistry of the dumps and physical characteristics (pH, total dissolved solids) of the dumps, tailings’ leachates, and stream waters around the dumps were used to assess the potential of the dumps to pollute water bodies. Samples were collected in both the dry and wet seasons. The dispersion and pollution patterns were derived from Eh-pH conditions around the dumps after considering the mobility of the elements present in these dumps under different Eh-pH conditions. In this paper potential to pollute is considered as the likelihood of the elements to disperse under the prevailing conditions at the dump. The concentrations of elements, type of elements and the potential dispersion and pollution patterns from each dump were used to characterise potential risk of water pollution associated with the different dump types. The results showed a slight increase in concentrations of most elements studied in downstream waters compared to upstream waters. The dump conditions varied from acidic to alkaline, and so the elements studied have different mobilities in different dumps. The elements that pose environmental risks differed from one dump type to another thus different dumps have different potentials to pollute the water bodies. From the study it emerged that the minor metals dumps show the worst pollution risk, followed by base metal dumps, gold-mine dumps, platinum group metals mine dumps, chromite asbestos mine dumps and sulphur mine dumps. The pH values of 79% of the waters sampled in streams both before and after the dumps were neutral, though the pH values of the leachates themselves was frequently very acidic (pH < 4). The low pH levels in leachate are associated with elevated metal and metalloid concentrations in the leachate and in adjacent streams. From this study, a decrease in stream water pH is only expected when there is severe contamination. However, most streams were sampled near the dumps, and results from such samples would not represent entire stream profiles. The general trend from the results is that pH increases downstream as the leachate and run-off from a dump are diluted. Although concentrations of elements are affected the pH for streams did not show significant changes as near the dumps the overall pH of the stream water was not affected. The dumps rarely dry up, and leachate continues to seep from dumps throughout the year, suggesting that AMD is a continuous process.