Completed Task Force

Guidance for assessing the impact of mixtures of chemicals in the aquatic environment


The potential risk from combinations of chemicals in the environment has recently moved up the scientific, regulatory and political agenda as a result of the increasing concern about the potential impact on the environment from a ‘cocktail effect’ and the perception that current risk assessment procedures are inadequate.

In this report, field based approaches for assessing impacts on the aquatic environment and develop guidance on suitable methods are reviewed; case studies are used to identify research needs, including how methods can be implemented and what diagnostic tools are required; and the value of retrospective assessment in assessing environmental capacity for future industrial development is considered. Finally, a framework is presented which will retrospectively allow the evaluation of the potential impact of chemicals or chemical mixtures in the environment (see figure 1).

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.

Retrospective causal analysis and in particular eco-epidemiological studies allow the contribution of chemical mixtures to be determined. These studies can show that mixture impacts may be spatially quantified in aquatic ecosystems, and there is a need to understand site-specific stressor combinations in order to define effective measures to improve ecological status. The limited datasets available show that whilst chemicals may be responsible for some environmental impacts, they are by no means the only or even the most important factor.

Two recommendations are made here. The first is to improve biological traits associated with non-perturbed sites. There is still a relative paucity of data regarding the life histories of most taxa (plant, invertebrate and vertebrate) associated with exceptional water quality. For example, the relative frequency of intersex of most fish species in the relative absence of chemical exposure is not known at present. Further, the relationships of intersex and population structure for most species are not known. In essence, basic ecological research is needed to understand reference-condition structure and function. Such research will provide the basis for refined predictions (both pro- and retrospective) for biological expectations per site and hence, a more accurate measure of biological impairment.

The second recommendation is to improve diagnostics that distinguish chemical factors from physical / chemical factors responsible for biological impairments. While many statistical and the best professional judgment approaches have been utilised to distinguish chemical effects versus other stressors with regard to diagnosing biological impairments – there still exists relatively few examples and well utilised approaches that could eventually become standard guidelines for stressor diagnostics. Issues regarding site, river reaches, catchments and regional scale assessments can require highly different methodologies, and therefore highly different diagnostics. Sufficient experience exists to begin the road toward guideline development. With appropriate diagnostic guidelines, appropriate interpretations of the importance of chemical mixtures compared to other factors can be made, therefore leading to better water quality management decisions.

Figure: Suggested approach to assessment of ecologic risk of mixtures of chemicals in the aquatic environment

Framework for environmental assessment

• Reference condition and ecological status evaluation

• Need to establish cause before management



• Causal Analysis – US EPA

• Eco-epidemiology

The findings of the task force have been published in October 2011 as Technical Report No. 111


ECETOC member company experts, an expert EU regulator and leading scientists reviewed recent developments in assessing the effects of mixtures at two day scoping meeting in October, 2008, involving. Key aims were to identify which concepts and data are available to support scientifically valid environmental assessments of mixtures, consider how mixtures are assessed in different legal settings and to identify research needs. The outcome from that meeting is summarised below.

The theory of mixture toxicity has become more sophisticated over recent years but the long held principles of concentration addition still seem to provide a generally reliable, rough and conservative, estimate of toxicity. This means we can usually predict the toxicity of mixtures, for risk assessment purposes, when we know roughly the properties (e.g. via summary parameters) or chemical components of a mixture. However, we do not tend to consider interaction of specific chemicals or chemical mixtures with other, unknown, chemicals present in the environment, i.e. we do not ask; what is the potential impact of all chemicals present in the environment? This leaves industry vulnerable to criticism, in particular, for not determining whether chemicals present in the environment, including those at concentrations below their respective PNECs, act additively to cause an overall effect. Current European monitoring programmes such as the NORMAN project continue to feed this type of criticism through highlighting the presence of hazardous chemicals. In addition, as the WFD is implemented and water quality is perceived to be below good ecological status, we can expect chemicals and mixtures of chemicals to be considered as suspected agents causing adverse effects. REACH does not address this question directly since it is driven by prospective risk assessment or simple hazard profiling, neither of which properly consider the combined action of unknown mixtures in the environment.

Considering that it is not possible to consider, or even predict, all the potential combinations of chemicals in the environment, prospective risk assessment of mixtures, as currently used in regulation of individual chemicals is not possible. However, a retrospective approach comparing actual with expected (or desired) biological quality, e.g. diversity and/or function, can provide integrated assessments of whether environmental mixtures really cause impacts. A key advantage of retrospective assessment is that actual ecosystems are assessed rather than extrapolates to them. A retrospective approach can also provide a reality check on the identification of priority concerns identified by the WFD. However, this is not a simple activity and requires development of methods to discriminate impacts of chemicals (or other stressors) from natural environmental variation.

Retrospective assessments can also inform the debate over the apparent loss of biodiversity – is chemicals management inadequate and therefore unsustainable?

Since both the chemical industry and the water industry have stakes in ensuring good water quality, this approach may facilitate future co-operation, i.e. a wider multi sector involvement in understanding the true impact of chemicals and the effectiveness of treatment infrastructure.

Terms of Reference

  • Review field based comparative approaches for assessing impacts on the aquatic environment and develop guidance on suitable methods.
  • Using case studies, identify research needs, including how methods can be implemented, what diagnostic tools are required.
  • Consider the value of retrospective assessment in assessing the capacity of aquatic communities for tolerating man made discharges.