TR 111 – Development of guidance for assessing the impact of mixtures of chemicals in the aquatic environment


TR 111 : Development of guidance for assessing the impact of mixtures of chemicals in the aquatic environment | October 2011

ISSN-0773-8072-111 (print)
ISSN-2079-1526-111 (online)


Activities in Europe continue to highlight the presence of chemical combinations in the environment. Furthermore, there is increasing concern about the potential impact on the environment of mixtures of chemicals and the perception that current risk assessment procedures are inadequate. The toxicity of chemical mixtures is relatively well understood through the concepts of concentration addition and independent action, with synergism being acknowledged as only a rare occurrence. It is generally accepted that concentration addition should be used as a default first tier in environmental risk assessment of mixtures. Prospective risk assessments are generally done at the level of single substances, some of which are in fact mixtures themselves, or known mixtures of substances in chemical products. The assessment factors employed in the different conservative risk assessment processes under which these are regulated may cover the potential for any combined effects from exposure to multiple substances.

The problem is that in the real world, predicting the chemicals to which an environment is exposed is difficult and often impossible. A framework is presented here which will retrospectively allow the evaluation of the potential impact of chemicals or chemical mixtures in the environment.

Initially, this involves identification of an impact which may be obvious or may be more subtle requiring the comparison of the site to a reference site. Methods to identify impacts include traditional taxonomic based indices, such as the River Invertebrate Prediction and Classification System (RIVPACS) to the trait-based methods developed more recently. These methods determine the ?ecological status? of a site by comparison with a reference condition.

Well established methods such as Whole Effluent Toxicity testing and Direct Toxicity Assessment have been successfully employed to indicate the potential for environmental impacts from chemicals. Toxicity Identification Evaluation and Effects Directed Analysis can subsequently be used to identify the toxic components within effluents or environmental samples allowing effective management.

Where there is no obvious cause of environmental impacts, causal analysis needs to be employed to identify probable cause as these are many, ranging from habitat, including a number of natural perturbances, to anthropogenic stressors of which toxic chemicals are one. Examples of eco-epidemiological studies from US and Europe illustrate how these studies should be conducted using various statistical and “Weight of Evidence? (WoE) methods to identify probable cause, which is necessary before employing risk management options to mitigate or remediate effects from chemicals.