The whole is less than the parts with metals mixture toxicity

What is it about?

Development of mineral or energy resources often necessitates some disturbance of mineralized rocks. This in turn can cause increased concentrations of metals in streams draining these mineralized areas. These real world aquatic environments may not have a lot in common with the standard laboratory toxicity test “environments” that are used to produce data for water quality criteria. The conventional approach applicable to mineralized areas, tests metal-by-metal, and species-by-species. The real world is never like that. Chemicals always co-occur in mixtures. For example, cadmium (Cd) and zinc (Zn) are an inseparable pair of elements in mineralized areas. If one occurs the other will too, with Zn concentrations routinely running about 200 times higher than Cd concentrations. Likewise, aquatic organisms always occur in mixtures of species too. Although aquatic insects dominate streams, they are seldom used in criteria because their complicated aquatic to aerial life cycle makes it difficult to rear them in laboratories. This situation led to our efforts to test natural aquatic insect communities with gradients of Cd and Zn alone and together. Individually, Cd was far more toxic to aquatic insects (particularly mayflies) than Zn, with effects at very low Cd concentrations of less than 1 part-per-billion. Yet in combination at typical ratios that occur in the environment, Cd and Zn were less toxic than was expected from adding up their individual effects. We believe that the reasons for this are while Cd is much more toxic than Zn, Zn is naturally about 200 times more abundant in water than Cd. Further, Zn has a higher chemical affinity to attach to the surface of the insect’s gills and is able to outcompete Cd from attaching to the gill. Even though Zn is itself moderately toxic, because it is more abundant it can block more highly toxic Cd from interfering with calcium uptake which is the likely mechanism of toxicity. Thus the overall toxicity of the Cd+Zn mixture was reduced, relative to the sum of the expected damage that each alone could produce. We used a geochemical modeling approach to allow us to predict metal mixture toxicity to different types of waters than the ones we specifically tested. This can help environmental managers in the mining industry or government agencies to plan for real world metal toxicity scenarios (e.g., metal mixtures) and mitigate the potential effects of effluents to aquatic life in ways that metal-by-metal aquatic life criteria cannot.

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Why is it important?

This and related work can help environmental managers in the mining industry or government agencies to plan for real world metal toxicity scenarios (e.g., metal mixtures) and mitigate the potential effects of effluents to aquatic life in ways that metal-by-metal aquatic life criteria cannot.

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