The ability of certain chemical substances to persist in the environment is an issue of global concern that requires careful consideration in environmental risk assessment. This is especially true when this ability is coupled with bioaccumulative (B) and toxicological (T) properties, i.e. when it is classed as a PBT or vPvB substance. However, assessing environmental persistence of chemical substances is not straightforward. Persistence cannot be directly measured; it can only be inferred from the continued presence of a substance (or of degradation products, when they are persistent) in the environment or the lack of observed degradation data in the environment.
Degradation rates, or half-lives, are determined in, or default rates assigned from, laboratory-based degradation tests. These tests can be simple screening tests (e.g. the OECD 301 and 310 ready biodegradability tests) or relatively complex higher tiered tests (e.g. the OECD 308 aerobic and anaerobic transformation in aquatic sediment systems, OECD 309 aerobic and anaerobic transformation in surface water and the OECD 303 aerobic sewage treatment). The degradation rates derived from these studies are compared to national and international half-life criteria for environmental persistence.
At a recent persistence workshop coordinated by ECETOC (2007), it was recognised that there were a number of research projects being undertaken to improve the predictability and reliability of biodegradation screening tests. It was also recognised that existing biodegradation methods were either not subject to any rigorous validation or that they were only validated with readily degradable chemicals (e.g. sodium benzoate and glucose). There was some concern that the potential impacts the research studies developing new approaches to biodegradation testing was being compromised as they were being conducted on a range of chemicals under different experimental conditions thus allowing little inter-test comparability. To maximise the impact of such studies in the future the ECETOC Workshop (2007) concluded that a reference set or validation set of chemicals was required to: (i) enable the outcome of research studies to be compared, and (ii) allow their degradative potential to be benchmarked.
This project aims to develop a list of reference chemicals covering a range of environmental persistence and non-persistence. This reference set could then be used to validate modifications to existing biodegradation test methods and to develop new test methods. The validation set would also help address concerns that some of the modifications or new methods could result in tests becoming too powerful or overly protective. The aim of the research is to establish a list of chemicals, with an agreed (by regulators and industry) set of properties and characterized set of degradation behaviour. The reference set of chemicals would be relevant to projects addressing:
- Compartment persistence / biotransformation
- Bioavailability and bound residues
- Determination of test method variability, e.g. via ring testing
- Field monitoring/exposure model validation
- Consider what criteria are needed to select a reference set of chemicals.
- Starting with existing data sources, e.g. ECETOC reports on persistence, identify a set of chemicals with agreed properties and biodegradation behaviour. This list would cover chemicals that biodegrade rapidly, e.g. readily biodegradable, as well as those that were very slow to biodegrade, e.g. not inherently biodegradable.
- Establish the properties for these chemicals, including physico-chemical as well as fate and partitioning properties.
- Agree the data with regulators and industry.
- Write up the results and publish the final outcome.
Short interim reports on progress are required at 3 to 6-monthly intervals. It is expected that the findings will be developed into a peer reviewed publication, following presentation at a suitable scientific conference.