Assessing the ecotoxicity of goldmine tailings by utilising earthworms and soil mesofauna as bioindicators

Abstract

The mining industry plays a key role in the economic development of South Africa, which as a country, is one of the largest exporters of valuable resources such as, gold, platinum group metals (PGMs) and other metals and minerals. Unfortunately, mining produces large volumes of solid waste in the form of tailing disposal facilities (TDFs), which contain variety of heavy metals (e.g. Cr, Co, Ni, Cu, Zn, As, Pb and Cd) which are hazardous to the natural environment. Tailing disposal facilities are capable of altering the landscape, by taking up large areas of land and spreading of tailing material into the surrounding environment. Despite the negative effect associated with mining, the demand for valuable metals and minerals are still high. The aim of this study was to use soil metal analysis, earthworm bioassays, avoidance-behaviour tests and soil mesofaunal communities to assess the effects of a 78-year-old gold mine on the surrounding environment and inhabiting soil communities. Six soil samples were randomly taken from four different sites on a gold mine viz. two different TDFs and two grassy pasture sites. Sampling was done at four different times over the period of a year, to determine seasonal differences. The mesofauna were extracted and identified in order to determine the species diversity and abundance for each site. A list of mesofaunal species was constructed and divided into distinct functional groups based on the mesofauna feeding habits. Soil chemical analysis showed that the TDFs had the lowest pH levels and highest concentrations of heavy metals (especially chrome). Earthworms exposed to the TDF material showed significantly lower earthworm biomass than the control, with a very low cocoon production. Earthworms exposed to the two pasture sites had a higher biomass than the two TDF sites, while also being higher than the control. The northern pasture showed a lower cocoon production and juveniles per cocoon count than the control, while the southern pasture had a slightly higher cocoon production and juvenile per cocoon count than the control. Avoidance tests showed similar results, were earthworms generally preferred the pasture soils and control over the TDF material. Both the 100% BTDF and STDF exposures, had avoidance over the 80% threshold level. Mesofauna sampled from the TDFs site had the lowest species diversity and individual count compared to the two pasture site. Prostigmata, Cryptostigmata and Mesostigmata were the most dominant mite taxa within the TDFs, with Prostigmata being the most dominant group sampled. Prostigmatic mites might make good ecotoxicological bioindicators for future studies. Seasonal fluctuations influenced both the abundance and species with the highest individual count and species diversity recorded in Autumn. It can be concluded, that even though the gold mine was inactive for a long period of time, it still remains a highly contaminated area.