This report is an update of a literature review conducted for MEND in 1993 on the development and application of tools to predict the effects of transition metals in sediment, water and aquatic food. Several of the tools discussed in 1993 have now been validated, and in some cases adopted by regulatory agencies. These tools can be used for mine effluent receiving waters to assess the potential for metal toxicity in sediment, to derive site specific water quality guidelines or to assess the risk for secondary exposure. These approaches include the acid volatile sulphide – simultaneous extracted metals (AVS – SEM) approach to predict the absence of toxicity in anoxic sediments, the biotic ligand model (BLM) to predict waterborne metal toxicity and the dynamic multipathway bioaccumulation model (DYMBAM) that predicts metal bioaccumulation in aquatic organisms.
Predicting metal bioavailability and toxicity in sediments remains a challenge. For sediments lacking oxygen, an approach based on the assumption that metals bound to sulphides are not bioavailable was successfully developed and applied to predict absence of toxicity for several metals (cadmium, copper, lead, nickel and zinc). The models presented in 1993 that predict free metal ion concentrations based on adsorption equilibrium with iron hydroxides have been used in several studies. However, this approach has not been fully validated or adopted by regulatory agencies. The AVS – SEM approach was further developed to use the difference between AVS and SEM rather than the ratio of the two as originally proposed, and to include normalization for the organic matter content of the sediment. The approach is based on the assumption that sedimentary metal in the presence of excess sulphides and in the presence of organic matter will have low bioavailability. The AVS – SEM approach has been used successfully by regulatory agencies, e.g. United States Environmental Protection Agency. However, this approach is restricted to a relatively small number of metals and only predicts the absence of toxicity.
For metals in the water column, the BLM approach allows prediction of acute toxicity of several metals within a factor of 2 to 3, as well as chronic toxicity for invertebrates. However, the capacity to predict metal speciation in natural waters and the extension of the BLM from acute to chronic toxicity still requires research. The BLM approach is based on the free ion activity model (FIAM) that was discussed in the 1993 MEND literature review. The approach uses geochemical equilibrium models to estimate the amount of metal bound to a biotic ligand, e.g., fish gills, for a specified water chemistry. Metal toxicity is then estimated based on the given water chemistry. Significant progress has been made since 1993 in the capacity to predict metal speciation in natural waters using geochemical models. However, available studies suggest that these models do not always provide accurate estimates. Therefore, efforts should be made to use, if available, corrected default parameters and to validate results with actual measurements. Nevertheless, the BLM approach normally allows the prediction of metal toxicity within a factor of 3 or less. This is a major improvement on the current approach since metal toxicity based on total concentrations can vary by several orders of magnitude as a function of different water chemistries. Application of the BLM to acute metal toxicity to fish (rainbow trout and fathead minnows) has been extremely successful and is based on a solid mechanistic understanding of the effects of metals on fish regulation of ion concentrations in internal fluids. Extension of the BLM to other organisms and to chronic toxicity has also been moderately successful. However, the underlying mechanisms of toxicity are not fully understood and the acute BLM framework does not always directly apply. Nevertheless, the BLM approach has been adopted to different extents by regulatory agencies such as the United States and the European Union, and accounts for metal bioavailability in acute and chronic water quality guidelines and ecological risk assessments.
Food has been shown to be an important route of metal exposure and data from laboratory feeding experiments are currently integrated in the DYMBAM modelling approach that considers metal uptake from water and diet. There is no consensus in the literature on the relative importance of waterborne versus dietary metals for biological uptake and toxicity. Toxicity has most often been associated with waterborne exposure. For metal uptake however, dietary exposure can be significant and in some cases can be the major route of exposure for freshwater invertebrates (for example Daphnia magna, Sialis velata or Chaoborus punctipennis). The DYMBAM approach explicitly considers both routes of exposure and has been used successfully to predict metal bioaccumulation in laboratory and field exposures. Generally, the approach cannot predict metal toxicity, as metal toxicity is not directly related to total body metal accumulation.
Based on the above, future research efforts should focus on the chronic toxicity and particulate metal. Major progress have been achieved regarding our capacity to predict the acute effects of waterborne metals but additional research is required to expand the approach for chronic effects, especially for fish and plants. In addition, our ability to predict the effects of metal in sediment and metal in suspended particulate matter is still limited. Research should aim towards an improved assessment of the metal speciation in the exposure media, and a better understanding of the physiological effects and adaptation to trace metals.
