User Guide to Thresholds and Classification - Environmental ...

389User Guide for Thresholds and ClassificationsEC 50 = 7.47 mg active/kg dry soil.20.1.2. Degradability in soilThe HSNO Act classification criterion (in Schedule 6 of the Hazardous Substances (Classification)Regulations 2001) includes consideration of the half-life of the substance in soil, where:Soil DT 50 is the half-life in soil, which is the time required to reduce the original concentration of thesubstance in the soil by 50%.Unlike the HSNO Act criteria for rapid degradation in aquatic systems, the regulations have no further detailsto assist with interpretation of the above criterion. The EPA policy is to consider only degradation (abiotic andbiotic) when determining the applicability of a DT50 value for use in hazard classification. Other processessuch as dissipation, volatilisation, or leaching are not relevant for the classification of the substance, but areused for risk assessment.The most commonly used guidelines for testing soil degradation are OECD 307 and United StatesEnvironmental Protection Agency (USEPA) Office of Prevention, Pesticides and Toxic Substances (OPPTS)835.3300 (see Appendix 20A for acceptable test methods). Generally, freshly sampled representative soilsare characterised with regard to common soil properties (for example, texture, pH, and organic carboncontent) and incubated under static soil moisture and temperature conditions in the dark.The use of 14C-labelled material is preferred. During incubation soil samples are taken and analysed foractive substance, metabolites, volatile components, and bound residues. The time taken for degradation of50% and 90% of the active substance and major metabolites is derived from the formation and declinecurves. The degradation pathway must be reported for one soil. The rate of degradation is also investigatedin a minimum of three additional soils at 20°C. Further evaluation of the rate of degradation is alsoundertaken at 10°C.Multiple DT 50 valuesWhere degradation data are available from several acceptable studies and a single value study is needed formodelling or a trigger value, usually an average of the kinetic parameters is sufficient. However, in somecircumstances, such as when degradation rates are strong functions of soil properties such as pH, averagingis not appropriate.The geometric mean should normally be used as the average of degradation parameters because it providesthe best representation of the average of different first-order degradation curves over the entire period. Usingthe geometric mean also has the advantage that the same result is obtained from averaging first-orderdegradation rates and averaging the corresponding half-lives (FOCUS, 2006).Metals: bioavailablity in soilAs noted in chapter 19, the property of degradation has limited relevance to metals and inorganic metalcompounds. Numerous interactions with the soil matrix will reduce the bioavailability of a metal to soilorganism. Poorly soluble metal compounds may release toxic species over time, resulting in theJanuary 2012 EPA0109