Standardized biodegradability tests: Extrapolation to aerobic environments

Assessing the risk of chemicals for the environment requires an estimation of degradation rates in order to evaluate a realistic exposure concentration for organisms. As QSARʹs (quantitative structure activity relationships) for degradation are far from generally applicable, methods are proposed to extrapolate results obtained from standardized degradation tests to the most relevant environmental compartments. The design of the OECD laboratory test methods is such that, at least at the screening level, almost general applicability is achieved. Inevitably, tests conditions do not reflect biodegradation in the real world. However, if data are available on biodegradation it appears that, for most chemicals, references are made to one of those standardized OECD methods. A systematical procedure to translate the result of an OECD test (or a combination of test results) to transformation rates under realistic circumstances is still lacking. In this report a methodology is presented to evaluate standardized test results in terms of rate constants for degradation in a biological waste water treatment plant, surface water and aerobic soil. The extrapolation of a positive result in OECD screening tests relies a.o. on the assumption that the half-life is inversely proportional to the total concentration of micro-organisms. A first attempt was made to validate this extrapolation by comparing results of reported field studies on biodegradation of 20 chemicals with positive results in OECD Ready Biodegradability tests.