A Study of Environmental Assessment of Acid Mine Drainage in Ngwenya, Swaziland

Abstract

Acid mine drainage (AMD) is an industrial pollution of prime concern for the whole world. It is the driving force for an array of water pollutants especially heavy metals. Heavy metals (Cr, Mn, Fe, Ni, Co, Cu, Zn, Pb, Cd) were investigated in this study. Water and sediment samples were collected from the Ngwenya Iron Ore Mine Quarry Dam to investigate the occurrence of AMD phenomenon in the old mine. The main Fe compounds found in the Ngwenya Mine ore are haematite and pyrite. The Quarry Dam is located in one of the mine pits and it has neither visible inlet nor outlet. Physico-chemical parameters (pH, EC, ORP, T) were determined in the water by Electrochemical methods using the WTW 340i probes. Anions in the water were quantified by Ion Chromatography (IC) whereas heavy metals in water and sediments were quantified by Flame Atomic Absorption Spectrometry (FAAS). The BCR-Sequential Extraction procedure was used to speciate the heavy metals in the sediment samples. The mean pH of 4.34 in the Quarry Dam water is comparable to the pH of other AMD polluted water. The high mean EC of 4.522 mS/cm depicted that the water was laden with ions which are dissolved from the ore by the AMD. The sulfate ion, a well-known indicator for AMD pollution in water, is the second dominant anion where the order is NO3-> SO42-> Cl-. The heavy metal levels in water and sediments were in the order Co > Ni > Cr >Zn > Mn > Cu > Fe > Cd > Pb and Fe > Mn > Cr > Ni > Pb > Co > Cd > Cu > Zn respectively. The heavy metals are all bioavailable, hence they are potential health risks to both biota and residents in the vicinity of the mine. The pH of the water in the Quarry dam is comparable to those obtained for some other AMD polluted water. It is inferred that AMD is being produced in the old mine. It is recommended that an Environmental Impact Assessment of AMD must be carried out before the mine is re-opened.