Environmental quality standards for trace metals in the aquatic environment

The scientific basis for the regulation of metals that are potentially toxic to aquatic life is widely debated, especially with respect to bioavailability. Environmetal Quality Standards (EQSs) based on total metal concentrations and laboratory toxicity data for conditions unrepresentative of field situations, may both underprotect and overprotect. The setting of EQSs could be informed by knowledge about dose-response relationships in the field. The work described in this report was performed to attempt to establish dose-response relationships for macroinvertebrates and diatoms in streamwaters contaminated to different extents with trace metals (nickel, copper, zinc, cadmium, lead). Different possible variables were used to express metal concentrations and bioavailability. The results obtained were then used to evaluate different approaches to EQS setting. We carried out field studies of the ecology and chemistry of upland streams, influenced by past mining activity. We conducted field sampling and performed analyses at carefully-chosen field sites, and analysed the data to attempt to establish quantitative relationships between the chemical and ecological variables. The chosen sites were perceived to have several advantages, notably similarity in their physical characteristics, appreciable ranges of water and metal chemistries, and the absence of other pollutants. Therefore they made it possible to explore the effects of differences in streamwater chemistry on ecological response, with few confounding factors. The ecological part of the field work was conducted in Spring 2006, and comprised sampling and enumeration of benthic invertebrates and diatoms. Chemical characterisation of the streamwaters was achieved by repeated sampling during the 6 weeks before the ecological fieldwork. As well as carrying out conventional chemical analyses, we also used analytical speciation techniques (Diffusive Gradients in Thin films, DGT, and the Donnan Membrane Technique, DMT), and applied the WHAM chemical speciation model to calculate free metal ion activities and other variables. We collected separate samples of key invertebrate taxa and analysed them for metal body burdens, and we collected samples of stream bryophytes and analysed them for metals. In another supporting study, we collected water samples from the study sites and used them in laboratory toxicity experiments with an algal and a daphnid species. We carried out a literature review of metal mixture effects, to aid interpretation of field data from those sites (the majority) where more than one metal may exert a toxic effect. We also reviewed methods for setting EQSs. The chemical compositions of the study streams covered appreciable ranges of acidity, alkalinity, total ionic concentration and DOC concentration. The nutrient concentrations were sufficiently low for there to be negligible effects on either macroinvertebrates or diatoms. Suspended particulate matter concentrations were in the range 0 - 50 mg l-1. As expected because of the historical mining activities, most of the streamwaters were contaminated with trace metals, at levels exceeding EQS values. The study sites included several acid streamwaters, some of which were contaminated with trace metals, but all of which had high levels of aluminium. The results of DGT and DMT measurements, and of chemical speciation calculations, indicated that zinc and cadmium are present largely in inorganic forms in the streamwaters, whereas there was appreciable organic complexation of aluminium, copper and lead, and to a lesser extent nickel. The metal contents of both bryophytes and macroinvertebrates varied considerably with streamwater chemistry. The metal contents of both types of organism were approximately predictable using chemical speciation concepts, which is direct evidence that biota respond to the chemistry of the medium. Analysis of the relationships between ecological and chemical variables, and of the results of the laboratory toxicity measurements, led to the following conclusions. 1. Toxic metals were demonstrated to reduce species numbers of both macroinvertebrates and diatoms in the field. 2. Laboratory tests confirmed that 8 of the streamwaters in which field effects were observed contained metals at toxic concentrations. 3. The macroinvertebrate community provides an effective and sensitive tool for detecting metal toxicity. In the study sites, toxicity could be attributed definitely to Al and Zn, and there was some evidence of toxic effects due to H+ and Cu. 4. For diatoms, which were less sensitive than macroinvertebrates, the most likely toxicant was zinc, but cadmium may also have been active. 5. A function combining free metal ion concentrations and pH provided the best solution-based measure of metal toxicity towards macroinvertebrates, because it accounted best for aluminium effects. 6. Toxicity towards diatoms was expressed best, in terms of solution concentrations, by either total dissolved metal concentration or the metal concentration measured by DGT. 7. The field data permitted the formulation and partial parameterisation of the Toxicity Binding Model (TBM), which is based on Biotic Ligand Model principles, but applicable to mixtures of metals. The model permits clear toxicity thresholds to be identified, and quantifies the contributions of different metals to the overall toxicity. 8. Because of the dominant effect of zinc, among the trace metals, the available data did not permit conclusions to be drawn about the effect of chemical speciation on copper or lead toxicity. However, the finding that aluminium effects are best expressed in terms of the free ion, and the promising results from the TBM, imply that speciation-based measures provide the best way to describe the toxic effects of metals in the field. We considered the implications of the results of this work in the assessment and setting of Environmental Quality Standards for metals in freshwaters. The following points can be made. A. The results of the present study add significant weight to the use of bioavailability and chemical speciation in deriving EQSs. B. A full assessment of the effects of toxic metals in contemporary waters requires knowledge about past “pristine” conditions. In the contaminated streamwaters of the present study, dissolved concentrations of trace metals were considerably higher (by 10 to 1000 times) than those estimated for similar uncontaminated sites under pristine conditions. As well as solution concentrations, calculated loadings of biotic ligands with toxic metals can be compared, and when this is done with the TBM it appears that at present-day uncontaminated sites the loadings can be substantial, not much lower than threshold values. C. If bioavailability and chemical speciation are to be used in EQS setting, the biotic ligand approach is currently the best way forward. Although its application relies on calculations, the chemical speciation results can be tested with in situ analytical devices (DGT and DMT), lending robustness to the method. D. There may be regulatory advantages to confining EQS setting to individual metals. However, the results of the present study suggest a significant role of “natural” Al in field toxicity, and this implies that the issue of mixtures should be addressed. This can be done using biotic ligand concepts. E. By showing that metal toxicity operates in the field, with dose-response relationships, and in line with bioavailability and chemical speciation concepts, the study provides support for metals regulation through EQSs. It further shows that targeted fieldwork can provide the information necessary for EQS evaluation and modification.