Division 2.1 TREATMENT OF RESULTS

The amounts of test substance, transformation products, volatile substances (in % only), and non-extractable should be given as % of applied initial concentration and, where appropriate, as mgkg-¹ soil (based on soil dry weight) for each sampling interval. A mass balance should be given in percentage of the applied initial concentration for each sampling interval. A graphical presentation of the test substance concentrations against time will allow an estimation of its transformation half-life or DT₅₀. Major transformation products should be identified and their concentrations should also be plotted against time to show their rates of formation and decline. A major transformation product is any product representing ≥ 10% of applied dose at any time during the study.

The volatile products trapped give some indication of the volatility potential of a test substance and its transformation products from soil.

More accurate determinations of half-lives or DT₅₀ values and, if appropriate, DT₇₅ and DT₉₀ values should be obtained by applying appropriate kinetic model calculations. The half-life and DT₅₀ values should be reported together with the description of the model used, the order of kinetics and the determination coefficient (r²). First order kinetics is favoured unless r² < 0.7. If appropriate, the calculations should also be applied to the major transformation products. Examples of appropriate models are described in references 31 to 35.

In the case of rate studies carried out at various temperatures, the transformation rates should be described as a function of temperature within the experimental temperature range using the Arrhenius relationship of the form:

k = A·e^-B/T or $\mathrm{lnk=}\mathrm{ln\; A-}\frac{B}{T}$ ,

where ln A and B are regression constants from the intercept and slope, respectively, of a best fit line generated from linearly regressing ln k against 1/T, k is the rate constant at temperature T and T is the temperature in Kelvin. Care should be given to the limited temperature range in which the Arrehenius relationship will be valid in case transformation is governed by microbial action.