The effects of Trojan Mine waste rock dump and Madziwa tailings dump on the environment

Abstract

The Mazowe Valley contains several of Zimbabwe‘s largest current and closed mining operations. It is densely populated and is also a major agricultural area. The urban and rural areas of Bindura, Goromonzi, Shamva, Marondera, Murehwa and Mutoko all draw water from within the Mazowe Valley. Irrigation of commercial crops is also a major water user. Madziwa and Trojan Nickel Mines, located 150 km north-east and 90 north of Harare respectively, are within the Mazowe Valley and are likely to impact negatively on water systems. Mining took place between 1966 and 2000 at Madziwa Mine and is ongoing at Trojan Mine. Nickel was and is still being produced from ore bodies consisting of mainly chalcopyrite, pyrrhotite, pentlandite and pyrite. Waste from underground mining at Trojan and from the mine‘s plant at Madziwa has been disposed on a waste rock dump and a tailings dam respectively. These are the focus of this study. Surface water samples were collected around the waste rock and both surface and groundwater samples were collected around the tailings dump to assess the impacts of the dumps on water chemistry. Water samples were analysed for metals, sulphate and carbonate. Surface waste rock samples and both surface and subsurface tailings samples were collected from the dumps to assess the geochemistry of the waste dumps. All samples were analysed for total elemental concentration. Mineralogical investigations and static tests were performed on some of the samples to assess acid mine drainage production potential of the dumps. Tailings geochemical data show concentration levels of heavy metals, including nickel (Ni), copper (Cu), chromium (Cr) and cobalt (Co) that are significantly higher than the level that is considered phytotoxic to plants [100ppm for Ni, Cr and Cu, and 40ppm for Co] and a deficiency in phosphorus (P), a plant nutrient. Static tests indicated that the tailings have a low neutralisation potential and will therefore form acidic drainage. This was confirmed by mineralogical studies, which revealed the weathering of iron sulphides and the presence of minerals with high neutralization potential (carbonates and ferromagnesian silicates) in small quantities (<10 wt %). Effluent from the dump was characterized by low pH values (2.3-6.6), high concentrations of iron (>6.2ppm), nickel (>0.3ppm) and sulphate (> 500ppm). Concentrations of metals generally decreased after the effluent had passed through natural wetlands. Chemical analysis of groundwater showed similarly high levels of sulphate. These findings show that acid mine drainage is seeping from the tailings dump and is dissolving and dispersing potentially toxic metals and salts into water systems. The geochemical data showed concentrations of nickel, copper and cobalt in the waste rock dump that are above the phytotoxic limit of plants (100ppm and 40ppm respectively). Static test results indicated that the waste rock dump would form near neutral to alkaline drainage. This was confirmed by mineralogical studies that revealed plenty of minerals with high neutralisation potential (e.g. calcite, serpentine and talc). Effluent emanating from the waste rock dump was characterized by near neutral to alkaline pH values (>6) with high concentrations of sulphate (>500 ppm) and some metals (e.g. Pb > 1.0 ppm and Ni > ppm). This confirms the static test results which indicated that the neutralization capacity of the waste dump exceeds its acid generation potential. Thus the neutral drainage is mobilizing potentially toxic elements out of the waste rock dump. Though the pond at the toe of the dump decreases the concentrations elements, they remain sufficient to increase levels of calcium, sulphate, arsenic, and nickel in the Pote River, which is located downstream of the mine. Environmental problems from the tailings and waste rock dumps persist at both mines despite efforts by mine operators to minimize their impacts in line with environmental regulations. However, lack of enforcement of the regulations by relevant authorities causes mine operators not to be sufficiently compliant. The waste rock dump and tailings dump have negative visual impacts on the environment. In addition, seepage from the dumps is causing the contents of metals and sulphates in surface and groundwater to exceed limits for safe effluent, drinking water, domestic use, irrigation and livestock agriculture, thereby putting the health of local communities, plants and animals at health risks. The impacts are more severe at Madziwa Mine.There is therefore need for enforcement of environmental legislation to check the challenges of acid mine drainage from mine waste dumps.