Microbiological impacts of acid mine drainage on urban agriculture in Soweto, South Africa

The late 17th century discovery of gold in Johannesburg was accompanied by skewed income distribution in the world's most unequal society and more prosaic curses like acid mine drainage (AMD). This acid water contaminates ecosystems such as patches of urban agriculture that communities rely on to supplement income and nutritional needs. We investigated the extent to which AMD contaminates streams, irrigation water and subsequently, soil and vegetation on small-scale farms. We targeted microbial communities as the first line of biotic casualties in AMD infiltrated substrates. Sites were all in Soweto, Johannesburg, the first was just adjacent to a mine dump, the second was in a midstream section and the last was downstream where locals cultivate fields. We tested contamination of river sediment, river water, cultivated and uncultivated soil, borehole water used for irrigation and the crops themselves. The Biolog EcoPlate (TM) microbial community profiling kit was used, it measures the ability of microbes to use various carbon sources. The Microtox (R) bio-assay was also used, it exploits a luminescent bacterium to indicate the level of biological toxicity in a medium. Both methods suggest severe contamination of environmental substrates close to the AMD source. The contamination attenuates downstream but was detected in soil, water and crops similar to 20 km away from the mine. Therefore, the impacts of mining are systemic in the catchment and authorities must appreciate the seriousness of the situation so that mitigation strategies can devised and implemented immediately.