Exposure and effects of platinum and associated metals on the aquatic ecosystems within a platinum mining region

Abstract

Platinum group elements are precious metals that occur in nature only in trace concentrations. These metals are indispensable for humans due to their unique catalytic properties while they are also resistant to corrosion. Platinum group elements are used in various applications from automotive catalytic converters to medical applications and are emitted to the environment to such an extent that anthropogenic fluxes exceed natural fluxes. Due to its exceptional properties and innovative applications, the demand for these metals exceed the supply and the demand is predicted to keep increasing in the future. Three-quarters of the world’s platinum supply are from South Africa, and is situated in the Bushveld Igneous Complex. These intensive platinum mining activities have detrimental effects on aquatic ecosystems draining the mining regions. The Hex River is the main arterial drainage for this productive platinum mining region. However, mining activities are not the only stressors within this system, the city of Rustenburg and its industrial activities, as well as informal settlements also contribute metal and nutrient inputs to the Hex River. Therefore, the aims of the present study were to assess the Hex River system by means of water and sediment quality, as well as bioassessments (i.e. aquatic macroinvertebrates, fish, and associated parasites). Firstly, the primary source of platinum and associated metal pollution to the system was determined. Due to seasonal changes and altered physicochemical variables, the behaviour of metals in water and sediments within this system were assessed. Secondly, the macroinvertebrate community structure was determined and evaluated how the mining activities alter it from reference to impacted conditions. Thereafter, the bioaccumulation of metals by macroinvertebrates was also assessed to determine whether these metals are bioavailable. Thirdly, the fish health and associated human health risk associated with the consumption of these fish were determined and lastly, the metal accumulation in associated fish parasites were assessed. From the results it was confirmed that the intensive mining activities were the main source of Cr, Ni, Cu, As, and Pt contamination, while urban effluent contributed to Zn, Cd and Pb contamination in the Hex River. The metal behaviour was also influenced by several physicochemical variables and affected the bioavailability, uptake and toxicity of metals in the system. The intensive mining activities had a detrimental effect on macroinvertebrate community structures and decreased the species diversity from reference to impacted sites. Tolerant species thrived at the impacted sites and had positive relationships with the pollutants (Cl, SO₄, Cr, Ni, Pt) entering these sites. These metals were accumulated in several macroinvertebrate families, indicating that the metals were bioavailable to aquatic biota and the families Lymnaeidae, Baetidae, Tubificidae, and Chironomidae emerged as great biomonitoring organisms. The concentrations of platinum in the family Tubificidae were several times higher than freshwater clams from urban systems, indicating that platinum from mining activities are to a similar or even higher degree bioavailable to biota than platinum from auto-catalysts. Fish from this system also accumulated high metal concentrations and posed not only a threat to the fish health but also a health risk to humans that consume these fish. The fish from the impacted site were associated with a 10 – 90% higher human health risk compared to fish from the reference site and could potentially cause several carcinogenic and non-carcinogenic risks. The associated fish parasites accumulated higher metal concentrations than their fish host species and indicated which metals were bioavailable within the aquatic system, due to their lack of a digestive system. The parasites also accumulated non-essential metals more readily compared to the fish that accumulated essential metals more easily. It was also evident that platinum and associated metals from mining activities were as bioavailable to aquatic biota as platinum from auto-catalysts. The present study concluded that the Hex River is subjected to various stressors especially intensive mining activities, while urban and industrial effluent also contributed to metal and nutrient contamination. The metals that entered the Hex River were bioavailable to several aquatic biota species and posed a threat not only to the ecosystem health but also to humans utilizing fish and water from this catchment. The present study provided valuable data and information on the exposure and effects of platinum and associated metals on the aquatic ecosystems within a platinum mining region.