Soil ecological risk assessments of selected South African soils

Abstract

South Africa produces large amounts of solid mine waste, covering vast areas of land in the form of tailings dam facilities (TDF). Tailings material contains high levels of elements which poses potential risk for the environment and human health due to their potential toxic character. Agricultural practices such as tilling and chemical additions can also cause leaching of potentially harmful toxins into the surrounding environment. Both these soils are disturbed on a physical level, influencing the soil structure, chemical composition and soil biota present. This study analyzed in a comparative manner the chemical and physical properties of soils collected from gold mine tailings, agricultural areas and natural areas, and the consequences for soil life. Soil samples for the different land use types were taken in duplicate from the KwaZulu-Natal and North West provinces in South Africa. Topsoil layer formation was analyzed for all sites by sampling at depths of 0-5cm, 5-15cm and 15-25cm. Soil element content was assessed by means of metal indices. The physical and chemical characteristics of the soil were further analyzed by life cycle parameters of the oligochaete species Eisenia andrei and Enchytraeus doerjesi in gold mine tailings and agricultural soils. Ants were collected from all the land use types for species identification and analysis of element content. Soil layering regarding the vertical distribution of elements was observed in concentrations analyzed for the different land use types. Mining sites, especially in KwaZulu-Natal, indicated a decrease in element concentrations with increased depth. The opposite tendency was observed for the agricultural and natural soils, indicating increased concentrations with increased depth. Analysis of the elements indicated high levels of pollution in the mining sites which according to the metal indices were Co, Cd, and Ni. It was determined that tailings material from KwaZulu-Natal was more polluted than the tailings material from the mine in North West province, according to soil indices. Tailings material from both mining sites resulted in negative impacts on E. andrei and E. doerjesi, based on growth, hatching success, mortality and reproduction rate. A unique approach was taken during this study by exposing cocoons of E. andrei to soil samples. The negative effects of the mining samples on test species could be a result of the low organic matter, poor structure and high element content of the mining waste, compared to the agricultural and natural soil. Species numbers of ants were higher on the tailings material than agricultural and natural soils. Element analysis of ants collected, indicated high accumulation of elements such as Fe, Al, Cr, Cd, Pb, Cu and Zn in total body element content of one or more species, reflecting the combination of elements found in the different land use types. Accumulation levels were highest in Pheidole sp. compared to other species investigated. Ants are potential indicators of soil pollutants within a South African context. Risk assessment is necessary for analyzed soils to determine steps for sustainable re-use. A key system with regards to soil pollutant analyses should be incorporated in government policy for protection of South African soil.