Validation of artificial mussels as indicators of platinum group element exposure in the Hex River System
Abstract
Platinum group elements (PGE) occur naturally in the environment in very low concentrations. However, these elements have increased over the last couple of decades, since they have been increasingly used in a number of applications. Mining activities in South Africa have been reported to contribute prominently to PGE emissions in the environment. The knowledge of (PGE) contamination in freshwater systems is important for the risk assessment for aquatic health and water resource management. From this study it could be seen that laboratory bioaccumulation studies alone are not sufficient since organisms in natural environments are affected by different environmental conditions that influence the bioavailability of metals to organisms. PGE concentrations in living organisms from field studies give valuable information on the metals that are associated with the mining activities and the bioavailable PGE fraction and the effective PGE concentrations inside of the organisms. The use of the artificial mussel (AM) have been successfully applied for a wide range of metals in marine, and more recently, fresh water systems, but never before used for determining PGE concentrations. The design of the AM was validated and optimised in the laboratory for uptake of PGEs. An optimum loading and elution protocol was developed for platinum (Pt). It was demonstrated that the Chelex® beads in the AMs accumulate Pt in a dose-response relationship and reflect the bioavailable environmental concentrations. The AMs were exposed to environmental PGEs alongside an established bioindicator species. This obtained information on the bioaccumulation and effects of the biota and how the bioaccumulation correlates with the measured uptake by the AMs when exposed for the same period. Both the AMs and the transplanted organisms were exposed in plastic baskets at each site. Metals that were analysed during this study include As, Cd, Co, Cr, Ni, Pb, Pd, Pt, V and Zn. When the AMs were exposed in the tailings dam and the two impoundments different PGE concentrations could be found. The AMs accumulated significantly higher Pd concentrations in Bospoort Dam during the second survey, while these concentrations were similar in the two impoundments during the first survey. For Pt it could be seen that these concentrations were similar in both impoundments during the second survey and Bospoort Dam contained slightly higher Pt concentrations during the first survey. The water concentrations reflect that there is a Pd concentration gradient along the Hex River, while the Pt concentrations were similar in the two impoundments, and the tailings dam contained significantly higher PGE concentrations. The accumulation patterns within in the AMs and the transplanted clams indicated different uptake patterns. Clams will take up metals from the water both in the dissolved form and as complexes bound to particulate matter. Thus the physical and chemical conditions at the different sites and during the different surveys influence the form of the metal available for uptake by the clams. On the other hand the AMs showed great accumulation patterns for all metals. The AMs correlated well with the concentrations found in the water column, which indicated that Bospoort Dam had higher metal concentrations, while the transplanted clams indicated that Olifantsnek Dam had higher concentrations. It was found that the clams from the reference site were exposed to high metal concentrations at the source site prior to deployment. It is therefore highly likely that the responses following deployment in the study sites were masked by the pre-exposed conditions. Mechanisms against oxidative stress were observed with increased CAT activities (non-significant increases) when exposed to conditions in Olifantsnek Dam. These responses in turn caused a depletion in energy reserves and damage to lipid and protein content of the clams. Metallothionein induction (based on total metals) were higher in Bospoort Dam, while the Pt-MT were higher in the reference clams and to a degree in Olifantsnek Dam, which correlated with Pt concentrations found in the clam tissues. This study provided some insight into PGE concentrations that can be found in PGE mining areas in South Africa, in areas that are not impacted (Olifantsnek Dam) by these activities to areas that are heavily impacted (tailings dam)