Utilizing earthworm and microbial assays to assess the ecotoxicity of chromium mine wastes

Abstract

Mining plays an important role in the South African economy which results in environmental impacts. This holds a potential hazard for ecosystems surrounding mining areas and also for public health in the surrounding communities. The aim of this study was to use soil enzymatic analyses and earthworm (Eisenia andrei) responses viz. growth, reproduction, lysosomal membrane stability and tissue metal concentrations to determine the effect caused by chromium mine waste on the activity of soil microbial community and soil invertebrates. Results indicated that chromium mining did have an ecotoxic effect on enzymatic activity, as the material which exceeded the Cr benchmark for microorganisms showed the least amount of enzymatic activity. Significant differences in enzymatic activity were observed between the different samples. Earthworm biomass increases were low in the mining material exposed worms and might have been correlated with the low enzymatic activities in the materials. Biomass was however not considered a sensitive endpoint. Lysosomal membrane stability, measured as NRRT, proved to be a sensitive endpoint, showing the same pattern from day 7 up to day 28. Hatching success of cocoons was not considered a sensitive endpoint, due to the low cocoon production in the mining material exposed worms. Since mine waste materials often contain complex mixtures of metals that might be toxic on their own or in combination with other factors, it is difficult to attribute any observed effect to any of the specific metals analyzed. The metal concentrations were however compared to benchmarks in order to determine which of the metals could have had a toxic effect on the soil organisms. The only benchmark exceeded, was the PNEC for microorganisms, for Cr in the unrehabilitated silt (TDF1) material. None of the other benchmarks were exceeded, indicating that perhaps granular composition of the materials might have had a greater influence than the metals.