Chemistry — More than a One-Line Item

Abstract

Research into water and sediment quality assessment is a multidisciplinary exercise involving chemists and biologists. During these exercises, it is not uncommon to hear my ecotoxicologist and ecologist colleagues refer to ‘the chemistry’ of waters or sediments, as if it was a one-line or even onenumber parameter categorizing the sample, that can be used in conjunction with their biological studies. This simplistic view fails to acknowledge that a sample’s (or system’s) chemistry is not just a single measurement of one or more chemical components, but embraces an understanding of the dynamics, persistence, reactivity, degradation, transformation, and speciation of these components and their products. Why is this important? Clearly concentration alone will be insufficient to describe or predict the effects of a contaminant on an ecosystem. Rather, the effects will be determined by the exposure dose which is controlled by the behaviour of the contaminant in that environment and its chemical speciation, as modified by the chemistry and physics of the associated water, sediment, or soil system. In many cases, we have a very simplistic understanding of the fundamental chemical processes that are taking place in environmental systems. For example, there are many cases (for example diffusion-limited or reaction-limited systems) in which chemical speciation will only play a minor role and contaminant mobility or reactivity will be much more important. Here, evaluations of total concentrations (potential supply) may be much more important than those that take chemical speciation into account. Environmental effects are increasingly being addressed from a risk perspective. This allows appropriate prioritization of management actions. The risk-assessment paradigm uses the chemistry and dose information as part of an ‘exposure characterization’, whereas the ‘effects characterization’ deals with the organism’s/ecosystem’s responses to this exposure. Simple approaches for screening risk assessment use measurements of total contaminant concentrations, but such crude chemistry has the potential to grossly overestimate the risk. Management, based on this low level of information, could waste money on expensive but unnecessary remediation or waste treatment. Full risk assessments require a more comprehensive evaluation of the chemistry that answer such questions as: • What is the chemical form of the contaminant (e.g. free, bound), and how is this altered by other parameters such as pH, Eh, etc? • What is the physical form of the contaminant? • What is the persistence of the compound in the environmental compartment? • What are its transformation products, dominant transformation pathways, and the rates of transformation?