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402User Guide for Thresholds and ClassificationsAppendix 20B: Additional guidance – degradation in soil20B.1 Methods for calculating DT50/90 valuesGeneral recommendationsThese recommendations are from the European Commission (EC, 2000). For the calculations of DT 50 andDT 90 values in laboratory as well as field dissipation studies the following recommendations are given.For a sound regression analysis in calculating DT 50 , at least five sampling times are required, includingzero time.Care should be taken when using time points from near the end of soil degradation/field dissipationstudies for the calculation of DT 50/90 values, when the concentration of the remaining active substance islow (< 2–5% initial concentration), especially when concentrations are approaching the limit ofquantification for the method of analysis for non-radiolabelled studies.Experience shows that DT 50 can usually be calculated from first-order kinetics, and this is the preferredmethod. The determination coefficient r2 should be in a range between 0.85 and 1.0. In practice therewill be many cases where r2 will be lower than 0.85. In such situations is advisable to distinguishwhether a DT 50 is needed for modelling purposes or as a trigger value for further (field) studies. Sincemost models can handle only first-order kinetics, for pragmatic reasons the determination coefficient r2 ≥In order to trigger further studies a DT 50 value can be calculated according tothe best fit. If the use of first-order kinetics to calculate degradation rates results in a determinationcoefficient of r2 < 0.7, then other methods can be tested and used.As a first option, the approximation of two degradation or dissipation rates to first-order kinetics (one forthe initial part and one for the later part of the degradation or dissipation process) should be tested,which may be shown up by a hinge point in the curve. Rather simple statistical methods are available instandard statistical software to show this. A hinge point can arise as a result of a change in thecontribution to degradation of various processes over a period. For example, a hinge point may becaused by a significant decline of microbial activity or bioavailability in the soil or by adaptation.Therefore, the hinge point does not represent an instantaneous change in the degradation process but isthe product of the limitations of sampling intensity, and does not reflect gradual changes in processesand possible bioavailability.The results of the fit give the first-order rate coefficient as one of the two regression coefficients. TheDT 50 and DT 90 are calculated using the formulae:DT 50 = ln 2 and DT 90 = ln 10kkWhen there is a hinge point in the degradation curve, the calculation of the DT 90 is less simple, thecomplication may be taken into account on a case-by-case basis.January 2012 EPA0109
403User Guide for Thresholds and ClassificationsMathematical models that fit the data may be used (for example, Gustafson and Holden (1990);ModelMaker (no date); TopFit).From the shape of the curve of concentration against time, one can decide whether a lag phase has to betaken into account. A lag phase may be assumed where at least three measurement points are more or lesson a horizontal line. The length of the lag phase has to be reported. The DT50 is then calculated by leavingout the experimental results within the lag phase. At least five sampling times (including zero time) must beavailable after excluding the three points of the lag phase.Special aspects of laboratory studiesThe following aspects should be taken into account when considering soil degradation studies in thelaboratory.Often the DT 90lab is difficult or impossible to obtain for persistent compounds, because of the obviousproblems with extrapolation beyond the end of study periods and the general problem with extremelylong study durations making statistical analysis of the data very inaccurate. When first-order kinetics isapplicable, then mathematically the DT 90lab can be estimated as three times DT 50lab . In addition to thesepoints, loss of microbial activity of the soil might result in a decrease in the rate of degradation afterapproximately 2 to 4 months of incubation.Effect of temperature on the degradation rate, where relevant.The Arrhenius equation is a validated relationship that can be used to describe temperature effects ontransformation. As a guide, the DT 50 approximately doubles for each 10°C decrease in temperature. AQ10 value of 2.20 could reasonably be used to extrapolate DT 50 data derived at 20°C to expected valuesat 10°C. A Q10 value can also be calculated, if degradation studies have been carried out at differenttemperatures. In every case, the method used for calculating the compound-specific Q10 value shouldbe clearly described.(See EFSA (2006) for detailed guidance on using Q10 values for pesticides.)The methods described are also used for metabolites, breakdown or reaction products, where they arerelevant from the toxicological, ecotoxicological, or environmental point of view, if separate studies withthese substances are available.ReferencesEC 2000. Guidance Document on Persistence in Soil. European Commission.http://ec.europa.eu/food/plant/protection/evaluation/guidance/wrkdoc11_en.pdf Retrieved 8 October 2007.EFSA (European Food Safety Authority) 2006. Opinion of the scientific panel on plant protection productsand their residues (PPR) related to the default Q10 value used to describe the temperature effect ontransformation rates of pesticides in soil. Question number: EFSA-Q-2005-058. EFSA Journal 322: 1–40.http://www.efsa.europa.eu/EFSA/efsa_locale-1178620753812_1178620770752.htmJanuary 2012 EPA0109
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