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New methodology for evaluating the degradation of pesticides in soil

This revision will consist of a tiered exposure assessment for organisms in soil based on scenarios for analytical and numerical models

The Scientific Panel on Plant Protection Products and their Residues (PPR Panel) of EFSA was asked in November 2007 by EFSA to prepare a revision of the Guidance Document on persistence in soil (SANCO/9188VI/1997 of 12 July 2000). This revision will consist of a tiered exposure assessment for organisms in soil (for annual crops under conventional and reduced tillage) based on scenarios for analytical and numerical models (EFSA, 2010b). In this exposure assessment, degradation parameters derived from field dissipation and soil accumulation studies are important input parameters for the numerical models. Therefore this opinion aims to provide guidance on best practice for using the results of standard field studies and soil accumulation studies in which plant protection products have been sprayed on the soil surface.

The half-life for degradation in the top 30 cm of soil at 20oC and pF = 2 is an important input parameter for numerical models that simulate exposure of organisms in soil. For soil under conventional or reduced tillage, the main use of this half-life is to simulate the degradation rate for soil depths between 1 and 30 cm. When deriving such a half-life from field dissipation and soil accumulation studies, appropriate measures have to be taken to ensure that the value obtained is not influenced strongly by processes in the top millimetres of soil (such as volatilisation and photodegradation).

Based on current knowledge and data commonly available in dossiers of plant protection products, it is impossible to estimate with enough certainty photodegradation rates of plant protection products in the top millimetres in soil. Studies with sieved soils in the laboratory demonstrate that photodegradation is limited to the top 2 mm of soil. Furthermore there are uncertainties assessing volatilisation for surface-applied compounds.

Current numerical models used for simulating behaviour of plant protection products in soil in the context of the EU regulatory exposure assessment are unable to describe satisfactorily the daily fluctuations of the soil temperature and of the volume fraction of water in the top millimetres of soil.

The parameters describing the relationship between the degradation rate coefficient in soil and the soil temperature (i.e. the Arrhenius activation energy) or the volume fraction of water in soil (i.e. the exponent B) show a considerable variation between soils and plant protection products. This uncertainty results in a considerable uncertainty in the degradation half-life within the top 30 cm of soil obtained from field studies by inverse modelling assuming default values of the Arrhenius activation energy and the exponent B.

To guarantee that the residues describe the degradation in the soil matrix rather than at the soil surface 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.

Assessment of degradation half-lives in the top 30 cm of soil derived from field dissipation studies can be based on inverse modelling using the approach of normalised decline curves proposed by FOCUS (2006). The normalised decline curves can be either described with the DFOP (double first-order kinetics in parallel) or Hockey-Stick models.

The Panel considers soil accumulation studies with only two or three soil samplings per year not suitable for estimating the degradation half-life in the top 30 cm of soil because the fraction of the dosage that penetrates to soil depths deeper than a few millimetres cannot be estimated with sufficient accuracy.

Once appropriate degradation half-lives from laboratory and field studies are available, the estimation of the half-life to be used as input for the required exposure scenario consists of two more steps: (i) assess the relevant population of half-life values for the required exposure scenario, and (ii) estimate reliably the required statistical attribute (certain percentile or some mean value) based on this population. The Panel proposes to base the relevant population of half-lives 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. This assumption is a working hypothesis that has to be underpinned further. The type of attribute has to be consistent with the scenario-selection procedure which was based on taking the geomean half-life assuming a log-normal distribution. So the Panel recommends taking the geomean half-life. The estimation of the geomean half-life of the population has to consider the uncertainty resulting from the limited number of samples in the sample population.

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 excluding the laboratory values only if the null hypothesis that laboratory and field values are equal is rejected. If the relevant population of half-lives for a certain exposure scenario consists of less than four values based on field studies, the Panel recommends using both laboratory and field values for estimating the geomean (even if this null hypothesis is rejected).

The Panel considers the guidance proposals for estimating half-lives also useful for assessment of leaching to groundwater and surface water because the main use of the half-lives in these groundwater and surface water scenarios is the same as for the soil exposure assessment considered in this opinion (i.e. simulating the degradation rate for soil depths between 1 and 30 cm).

Some uncertainty in the estimation of the half-lives has been addressed, but the Panel recognizes that further uncertainties exist and recommends that further work be done to evaluate their combined impact on the reliability of the exposure assessment.

However, the Panel is of the opinion that the provision it has made for these uncertainties within the proposed procedures, together with the improved handling of processes in the top millimetres of soil, will mean that the DegT50 of parent substances will be underestimated only for a small proportion of the substances.

The Panel recommends compiling a database for all substances listed in Annex I of all relevant and reliable half-lives of agricultural top soils within the temperate regions at 20oC and pF = 2 to test the assumption that this half-life does not vary systematically between geographical zones in the temperate regions for non-volcanic soils.

Should the notifier wish 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 into the soil immediately after application.

The Panel recommends research be conducted to further improve the reliability of mechanistic models for simulating loss processes at the soil surface especially for photodegradation and volatilisation.

The Panel recommends including in future exposure assessment methodologies the uncertainty resulting from the use of the sample geomean to estimate the geomean of the statistical population, and intends to develop approaches for this in a forthcoming guidance.

by T N
03 january 2011, Technical Area > Science News

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