Guidance for evaluating laboratory and field dissipation studies to obtain DegT50 values of plant protection products in soil

The European Commission asked the Panel to revise the Guidance Document on persistence in soil (SANCO/9188VI/1997 of 12 July 2000). Therefore the Panel started the development of a revised methodology for the assessment of exposure of soil organisms. This opinion provides guidance on how to derive the half-life for degradation in the top 30 cm of soil at reference temperature and moisture conditions (i.e. 20^oC and field capacity) from the results of field studies in which the plant protection product was sprayed onto the soil surface. This half-life is an important input parameter in model simulations of the exposure of organisms in soil for annual crops under conventional and reduced tillage and therefore this guidance is an important part of this revised methodology. The Panel proposes the splitting of field dissipation studies into two parts viz. before and after at least 10 mm of rain has fallen since application. The Panel recommends evaluating field dissipation studies with models capable of considering a biphasic decline and taking only the slow phase of this decline, taken to represent degradation in the soil matrix rather than loss processes from the soil surface, into account for estimating this half-life. If however, surface processes do not seem to occur the Panel proposes to use single first-order kinetics after eliminating data points before 10 mm of rain has fallen. The Panel proposes basing the relevant population of half-lives for a certain soil exposure scenario on the assumption that a half-life measured for any non-volcanic agricultural soil from temperate regions can be used to predict the half-life for any such soil within the EU. The aim is to estimate the geomean half-life of this relevant population. The Panel considers it necessary to include the uncertainty resulting from the sample size of the population in the estimation of this geomean. If the relevant population of half-lives for a certain exposure scenario consists of a mixture of values obtained in the laboratory and in the field, the Panel recommends rejecting the laboratory values only if the null hypothesis that laboratory and field half-lives are equal is rejected. The Panel considers that this guidance will also be useful to determine half-lives to be used in scenario calculations for the assessment of leaching to groundwater and surface water. Should the notifier want to use results of field dissipation studies for estimating the half-life in the top 30 cm of soil as an input parameter for exposure models, the Panel recommends incorporating the plant protection product to a depth of about 10 cm in soil immediately after application.