LINKING GEOSCIENCE WITH TOXICOLOGY TO PREDICT ACUTE METAL TOXICITY IN STREAMS

The chemical speciation of dissolved metals in water influences their biological uptake and effects. Geologic source materials (e.g., underlying rock type, mineral deposit type, soil, and sediment) can control key attributes of water chemistry and speciation that affect metal toxicity to aquatic biota. These attributes include concentrations of potentially toxic elements, alkalinity, pH, total dissolved solids, soluble major elements such as Na, Ca, and Mg, and dissolved organic carbon (DOC). The Biotic Ligand Model (BLM) is a computer program that predicts acute toxicological effects of metals on aquatic organisms. The BLM simultaneously considers several water-quality parameters (e.g., Ca, Mg, Na, K, sulfate, pH, alkalinity, DOC) to compute inorganic metal speciation, organic metal speciation, and metal-organism interactions. We found good agreement between results from traditional laboratory metal toxicity tests and BLM computations for waters prepared to simulate stream waters draining different underlying geologic materials. Geoscientists can use the BLM as a link between geochemical information and potential toxicological effects in streams. This can be accomplished by using known water-quality data or by using predictive approaches combined with reactive transport modeling. For example, information about the geology of a mined area, combined with watershed characteristics (e.g., bedrock geology, climate, sources of DOC), can be used to estimate the likely chemistry of both mine-drainage waters and receiving streams. This information can then be used to calculate concentrations of the likely dissolved constituents in the receiving streams, which is used by the BLM to compute potential acute toxicity of a particular metal to selected aquatic organisms. The BLM can be a predictive tool for ecological- and geology-based mineral-environmental risk assessments.