This precautionary approach generally overestimates the exposure concentration by the amount that is not available and therefore overestimates the level of risk to biota in the environment. It is also well documented that chemicals that are irreversibly bound to solids are less degradable and less toxic than the total residue would predict. Even though it is a position that has been recognised by ECPA [1], and referenced by REACH [2] and OECD test guidance [3], there is no agreed guidance on how to determine what is available or not, and how it should be considered in the risk assessment. As a result, it continues to be debated from a scientific and regulatory point of view [4].

There is a need to define what is meant by ‘bound' in the context of chemical residues in soil, sediment and biosolids. Bound material may be unavailable and therefore depleted from the system. There is also a need to link extraction techniques (e.g. different solvents) to mechanism of binding and whether this is sufficient to define how much of the total chemical present is bioavailable.

Understanding the mechanisms of binding and what types of analytical methods are needed to identify such mechanisms would assist in performing better predictions of which chemical-solid-environment combinations may lead to non-extractable residues.

Guidance on the use of bound residues and bioavailability in environmental risk assessment (and PBT assessment) is lacking. This workshop addressed these concerns and identified future regulatory and research needs. Their findings were published as Workshop Report No. 17.

The workshop considered:  

• The environmental relevance of bound residues in exposure assessment and limitations of existing biodegradability tests;
• The technical challenges faced in the extraction and categorisation of bound residues;
• How residue data should be interpreted and analysed, especially data from higher tier biodegradability tests?
• When can it be concluded that bound residues are NOT a risk?
• When can they be considered removed and of no consequence?
• When bound residues are considered a potential risk, what needs to be done to address the remaining uncertainty from a fate perspective and from an ecotoxicity perspective?
  

1. ECPA. 2000. ECPA Position paper on soil non-extractable residues, ECPA, Brussels.
2. REACH. 2008. Guidance on information requirements and chemical safety assessment, Chapter R.7b: Endpoint specific guidance. European Chemicals Agency, Helsinki.
3. OECD. 2002. Aerobic and anaerobic transformation in aquatic sediment systems, Guideline for Testing of Chemicals No. 308, OECD, Paris.
4. Anders JR, Lukas WY, Hauke H. 2005. Special issue: Organic compounds in the soil environment: Formation, potential for re-mobilisation and environmental significance of bound residues. Environ Pollut 133(1):1-182.