Heavy metal contamination and its spatial distribution from mining wastesin receiving environments of Mazowe District, Zimbabwe

Abstract

The purpose of this study was to investigate heavy metal contamination and its spatial distribution from mining wastes (especially artisanal/informal mining) and the impact of agriculture on heavy metal pollution in receiving environments of the Mazowe district, Zimbabwe. Agriculture and mining have considerably contributed to heavy metal and nutrient concentration into the world’s freshwater systems. This study aimed to improve the understanding of the contribution of agriculture and informal mining to heavy metal and nutrients pollution of Mazowe River. Also, the research aimed to establish the contribution of sulphates and phosphorous on heavy metal mobility in agricultural and mining areas. The research was conducted in Mazowe District in Mashonaland Central Province of Zimbabwe. Collection of the samples was carried out at five locations along Mazowe River, from its outlet at Mazowe dam to about 25 km downstream. The samples were preserved using WHO approved preservation methods and transported to a licensed laboratory in Harare for testing and analysis of heavy metals (Hg, Pb, Cr and Cd), extractable sulphates and phosphorus at Zim Labs laboratory, in Harare - Zimbabwe. Heavy metal analysis was done using the flame Atomic Absorption Spectrometer after wet digestion. The results indicated an average concentration of heavy metals in the river sediments ranging from 0.01mg/kg (Hg) to 26.68 mg/kg (Pb) during the wet season. During the dry season the levels of Hg, Pb, Cr and Cd were less than 0.01mg/kg. There were low detectable levels of heavy metals in the water samples for both wet and dry seasons. Significant differences in mean levels of phosphates and extractable sulphate pollutants were observed in the wet and dry season. The average phosphorus levels detected in the sediment samples during the wet season was 14.93mg/kg which was 93.7% higher than that of water samples which was at an average of 0.94 mg/l. The sampling points were small due to budget constraints and limited access to some areas because of property rights’ issues and national key points along Mazowe River. Thus, it would imperatively be unjustified to generalise the effects of the research outcome to other locations within Mazowe River Catchment or locations along Mazowe River downstream from the study area, without considering other land use patterns. The results showed that the presence of heavy metals in the sediment samples of Mazowe River could be highly attributed to informal mining along the river while extractable sulphates and phosphorous in both water and sediment samples could be attributed to agricultural activities. These findings may assist the environmental management and mining authorities to regulate the activities of artisanal mining to combat heavy metal contamination of rivers from mining activities