An investigation into arsenic speciation in a wetland impacted by acid mine drainage

The formation of acid mine drainage (AMD) and release of toxic contaminants, such as arsenic (As), is a serious environmental problem encountered worldwide. In this study, we investigate the crucial role the Klip River wetland system plays in attenuating As arising from gold mining activities within the Witwatersrand Basin in Johannesburg, South Africa. Mining operations in the region commenced over 130 years ago and have been associated with the widespread pollution of water resources by AMD. We investigated As concentrations, bioavailability and speciation in a peat core from the Klip River wetland as well as in samples from the main tributaries and tailing storage facilities (TSFs) in the upper catchment. Total As concentrations in tributary and TSFs samples ranged between 10.1-89.9 mg kg-1 and 77.4-106 mg kg-1, respectively, with concentrations in the wetland varying between 1.91-73.8 mg kg-1. In general, As bioavailability was low in both catchment (19%) and wetland (4%) samples, with elemental associations suggesting the majority is bound in an immobile form to organic matter and sulfide. As(v) was the predominant species detected in all samples (0.0901-16.6 mg kg-1), with As(iii), MMA and DMA present in lower concentrations. Strong correlations between As and S suggest that speciation and methylation are dependent on both chemical and microbial activity. The study highlights the vital role that wetlands can play in sequestering As in the environment.