Geochemical Characterization of an AMD-Affected Stream: Detection of Associated Trace Metal Contamination Using Element ‘Dilution Factors’

Abstract

This study characterized metal contamination in the Blesbokspruit River waters, near the coal-mining town of Emalahleni in Mpumalanga, South Africa, using enrichment factors (EFs) and contamination factors (CFs). We investigated the potential of Fe and Al ‘dilution factors’ (stream water divided by overbank sediment concentrations) under base flow hydrological conditions to detect AMD-related trace metal contamination in the study site. Waters of the Blesbokspruit River were characterized by acidic (< 3) to near neutral (≈7) pH, high EC (up to 2240 µS/cm), high sulfate (up to 1530 mg/L), ultra-high degrees of Fe contamination and minor to moderately severe enrichment of trace metals, all of which strongly indicate that the river is affected by AMD. Fe and Al dilution factors showed moderate to strong positive correlations with dissolved trace metals (Co, Ni, Zn, Pb, Cr, and Cd), but not with Cu, which is likely more associated with kaolinite in the study area. In addition, correlation analysis showed that dilution factors were better for detecting aqueous Co and Cd, and comparable for detecting aqueous Ni, relative to stream pH and aqueous Fe and Al. Dilution factors have an advantage over stream pH and aqueous Al and Fe because, once calculated, they can be used to monitor AMD-related trace metal contamination in streams during dry seasons by using only Al and Fe content in sediments. This can be beneficial when preservation methods or storage necessary for water samples are not available. However, the robustness of dilution factors during wet seasons requires further investigation.