Chemical speciation dynamics and toxicity assessment in aquatic systems

The key to risk assessment of contaminant effects in the environment (water, sediments, soil) is the ability to document cause-and-effect relationships. In ecotoxicological research, biotic responses are related to quantified contaminant concentrations, which in most cases are still expressed in terms of "total elemental concentration" and not in terms of "elemental species." However, it becomes evident that the abundance and distribution of pollutants in the environment, their bioavailability, and their toxicity to aquatic and terrestrial organisms (including humans) can often be better understood in terms of "elemental species." The persistence, mobility, chemical reactivity, sorption dynamics, and so on of contaminants in soil and water are governed by a range of changing physicochemical parameters (pH, temperature, organic matter, suspended solids, etc.), which finally dictate the effects at the organism level. Examples are given to demonstrate that knowledge of the nature and concentration of elemental species of pollutants is crucial in assessing the impact of contaminants on aquatic ecosystems. The experimental approach to evaluate chemical speciation dynamics in relation to toxic effects is illustrated in a case study on the acute toxicity of aluminum in mixing zones at the confluence of rivers with different pH values. This study under nonequilibrium ecosystem conditions has provided new insights into the mechanism of toxicity of aluminum to freshwater organisms. In conclusion, an integrative approach by environmental chemists and ecotoxicologists is recommended to evaluate environmental pollution. Studies on the assessment of the impact of changing physicochemical parameters on the transformation kinetics and chemical speciation of pollutants, which finally determine toxicity and bioconcentration in organisms, deserve more attention in environmental toxicology.