Hydrogeochemical characteristics of surface waters and sediments impacted by acid mine drainage: Case of eMalahleni, Mpumalanga Province, South Africa

Abstract

Abstract This study focused on evaluating the hydrogeochemical characteristics of acid mine drainage (AMD), surface water and sediments as well as the associate ecological impacts. Samples were collected from seven distinct locations in Emalahleni, South Africa. Physicochemical parameters of water samples were measured in the field while chemical and mineralogical analysis was done in the laboratory. The results showed that AMD was characterised by low pH (2.69 to 3.30) and high EC (1863–3110 µS/cm), Fe (20–56 mg/L), Al (51–89 mg/L), Mn (9–21 mg/L) and other toxic metals which were above South African guidelines for wastewater discharge. The hydrogeochemical facie revealed the dominance of Ca-Mg-SO4-Cl water type. PHREEQC geochemical modelling, showed that alunite was oversaturated at the decant point while silicates containing minerals such as K-feldspar, k-mica and kaolinite were undersaturated, indicating their potential dissolution from surrounding geology. Sediments characterization showed the presence of goethite while at decant points and downstream of the wetland, precipitation of amorphous Fe(oxy)hydroxides and schwetmannite was observed. Geo-accumulation index (Igeo) indicated extreme pollution in sediments from S1 with (Cd, As and Zn), S4 (As) and S7 (Pb). The potential ecological risk index (RI) showed low ecological risk for the presence of toxic metals in S4 and S7 and very high risk at S1. Cd was observed to contribute to the highest levels of contamination at S1. Sediment quality indices showed that AMD was likely to cause impact to sediments. AMD from decant point contaminants sediments, river and dam water due to its low pH and continued oxidation of pyrite. To alleviate environmental impacts caused AMD, innovative treatment approaches are recommended.