Workshop Report 28 – Estimating toxicity thresholds for aquatic ecological communities from sensitivity distributions

Abstract

WR 28 : Estimating toxicity thresholds for aquatic ecological communities from sensitivity distributions | 02/12/2014

IN A NUTSHELL

Predicting the toxicity of chemicals to aquatic communities is an integral element in environmental risk assessment. It is therefore a major component in environmental protection strategies and in the process of managing the safe use and disposal of chemicals. Hazard (toxicity) is most frequently predicted using concentration?effect data from single species toxicity tests which measure effects on individuals. However, the protection goals are generally wider i.e. populations, communities and ecosystems. Species sensitivity distributions, SSDs, describe the statistical distribution of species sensitivity to a toxicant and so can predict hazardous concentrations (HCps) affecting a certain percentage (p) of all the species in a community. Estimated HCps for environmental protection are usually the 5th percentile of the distribution and are used to derive a protective threshold concentration for an ecosystem.

ECETOC and the Environment Agency for England organised a three day workshop to discuss and review current statistical SSD models, when and how they should be used in regulatory applications and their ecological significance. A number of recommendations were made on how SSD methods could be further developed to improve the quality of decisions needed from both the prospective risk and retrospective impact assessment of chemicals.

The document is published as ECETOC Workshop Report No.28: Estimating toxicity thresholds for aquatic ecological communities from sensitivity distributions. 11-13 February 2014.

SUMMARY

This 3-day workshop organised by ECETOC and the Environment Agency for England and Wales took place in Amsterdam on the 11-13th February 2014.

The aims of the workshop were to review current thinking on when and how species sensitivity distributions (SSDs) should be used in environmental protection and management of chemicals and discuss how the methodology could be further developed to improve the quality of decisions. The workshop covered 3 specific areas:

a) What is the ecological relevance of an SSD?

b) What SSD statistical models are available for deriving toxic thresholds (HC5/PNEC) (hazardous concentration affecting p% of species / predicted no effect concentration) for aquatic communities?

c) What are the regulatory applications?

There were 41 attendees with experience in ecological risk assessment, ecology and statistics from academia, the chemical industry and the regulatory community. Appendix A provides a list of the attendees and Appendix B the meeting programme.

18 presentations were given during the workshop which covered the broader aspects of the use of SSDs in environmental protection and management, recent developments and specific case studies. In addition there were 3 syndicate sessions (with 4 discussion groups each) which focused on ecological, statistical and regulatory considerations.

There was general consensus that scientifically sound extrapolation approaches based on agreed but perhaps minimal and pertinent dataset SSDs for deriving toxicity threshold concentrations/PNECs should provide a more useful and transparent assessment of risks than a deterministic approach using generic factors applied to simple aquatic toxicity tests. The ability to understand the uncertainty was consider to be very important. For regulatory tools to be useful, they must not be overcautious (i.e. the tools become over protective and lead to unnecessary costs). When large datasets are available, the risk of being over protective is reduced and the use of SSDs becomes an option. When datasets are small, uncertainty is greater and consequently the more cautious deterministic approach may be more appropriate. The additional use of SSDs, (e.g. deriving expected impact magnitudes for focusing risk management of chemical contamination) was also acknowledged.

Continued validation against field and mesocosm data is required to ensure that thresholds expressed as an environmental quality standard (EQS) or PNEC have ecological relevance. The results of any extrapolation process (including SSDs) should always be critically assessed based on all available knowledge on the substance and related substances. It was agreed that SSD methodology is a valuable regulatory and management tool since it can give greater insight into the potential ecological effects than the assessment factor method, enabling better problem definitions and decisions. Use of SSD methodology should yield more generally applicable and acceptable results than those obtained from mesocosm-based methods.

The workshop considered the scientific and regulatory use of SSDs in chemical risk assessment and compared a range of accepted tools and their implementation. A novel approach, hSSD, to predict thresholds for defined species assemblages that uses knowledge of the general trends in how species sensitivity is related to their taxonomic distance was also discussed. A representative ETX model, the US Environmental Protection Agency (EPA) Web-ICE tool and the hSSD prototype were compared using case studies on the surfactant linear alkylbenzene sulfonate (LAS) and the pesticide, chlorpyrifos. The advantages and disadvantages of each approach were reported.

SSDs are used in both prospective risk- and retrospective impact assessment of chemicals. A prospective risk assessment needs to establish that there will be acceptable risk. In contrast, retrospective impact assessment uses diagnostic tools to identify the cause of existing adverse effects, using SSDs to quantify expected impacts. 3 distinct regulatory activities were identified:

1. The derivation of generic thresholds that need to be applied to many different locations, perhaps over very large geographical regions. These are assumed to offer sufficient protection everywhere, even in the most sensitive systems.

2. The derivation of scenario-specific thresholds that more closely reflect local conditions but which may not be transferable from one place to another.

3. Identifying the causes of biological impact (?diagnosis?) or expected impact magnitudes of existing (mixture) contamination, in order to inform the need and focus for any remedial action.

Looking to the future, although it was noted that expert judgement will always be needed, a compilation of current best practices, a review of the state of the science and answers to frequently asked questions would facilitate acceptance of SSDs by regulators and risk managers. This compendium of best practice should be a technical document aimed at users with knowledge of SSDs and ecosystems. A less technical document suited to a more general audience is also recommended.

Knowledge gaps were identified and a list of research topics were developed. Although not prioritised during the workshop, the report authors have sought an indication of priority for the work from the workshop participants.