The decision tree for the evaluation of leaching from soils

The Dutch decision tree on leaching from soil has been re-designed to be more in line with EU guidelines on the assessment of the leaching potential of substances. EU procedures aim at protecting groundwater against contamination with pesticides above the level of 0.1ug/L, with special regard to the groundwater as a source for drinking water. The Netherlands has adopted the following operational criterion: The long-term median concentration of an active ingredient or its relevant metabolites at 10 m depth should not exceed the value of 0.1 ug/L under at least 90% of the area of potential use of the substance. Like the old decision tree, the new decision tree consists of three tiers:

Details In the first tier, the FOCUS Kremsmünster scenario, is used to identify substances with a negligible leaching risk
Details The second tier involves calculations with the GeoPEARL model, which calculates the potential of a substance to reach the uppermost groundwater under the area of use. If the minimal information from the dossier is not sufficient to demonstrate acceptable leaching potential, the applicant may use information from additional laboratory and field studies (including lysimeter studies)
Details The third tier evaluates the behaviour of the substance in the saturated part of the soil, up to a depth of 10 m below soil surface
Details Because drinking water abstraction areas are generally in more vulnerable regions, special attention is given to the protection of drinking water abstraction areas

Report can be downloaded here

An overview of the new decision tree for the evaluation of pesticide leaching from soils is given in RIVM report 601450019.



Tier 1 of the decision tree

In tier 1, the FOCUS Kremsmünster scenario is used to distinghuish substances with a small or negligible risk of leaching from substances with some leaching potential on the basis of basic dossier information. The evaluation in the first tier is without regard to the area of use. Tier 1 can only be applied to substances that meet the following criteria:
Details the susbstance should not be volatile;
Details the substance should not show pH dependent sorption behaviour;
Details the average degradation half-live should be above 10 days and the sorption coefficient above 10 L/kg;

Only leaching estimates obtained with FOCUS PEARL 3.3.3. are accepted. Refer to section 2.3.1. of RIVM report 601450019 for details. The backgrounds of FOCUS PEARL 3.3.3 are described in Alterra report 013). The user's guide can be found in RIVM report 711401008.
Details Download FOCUS PEARL 3.3.3


Tier 2 of the decision tree

The second tier of the decision tree evaluates in greater detail whether a substance, which according to the evaluation in the first tier has some potential to leach to groundwater, indeed has a potential to leach to groundwater above the threshold level. The second tier is described in section 2.3.2. of RIVM report 601450019, and can roughly be divided into two parts:
Details a part using the GeoPEARL model. GeoPEARL is a spatially-distributed model, calculating the leaching potential of a substance for the area of potential use. A detailed description of the concepts of GeoPEARL is given in RIVM report 716601007, a user-manual is provided in RIVM report 716601008. Tier 2 often starts with GeoPEARL calculations, using input from the basic dossier. In a second step, the applicant may use laboratory, lysimeter and field data to refine the GeoPEARL simulations. Interpretation of field and lysimeter experiments may also show that the leaching of the substance cannot be simulated with the (Geo)PEARL model. In these cases, the ratio of the calculated leaching and measured leaching can be used to adjust the 90th percentile of the leaching concentration obtained by GeoPEARL. See RIVM report 601506007 for details.
Details a part in which monitoring data of the uppermost groundwater are considered. This latter part is described in detail in RIVM report 601450015. The result of the evaluation of the monitoring data may overrule the evaluation based on GeoPEARL, provided that all criteria, stated in that report, are met;
Details Download GEOPEARL 3.3.3


Tier 3 of the decision tree

Tier 3 considers the behaviour of the substance in the water-saturated zone of the soil, i.e. the zone between 1 and 10 m below soil surface. A substance enters Tier 3 when the target concentration as calculated with GeoPEARL at the end of tier 2 was above 0.1 ug/L or when monitoring in the uppermost groundwater showed a concentration above 0.1 ug/L. Tier 3 is divided into two major parts:
Details a part which considers behaviour studies with soil materials from the water-saturated zone. The applicant can perform transformation and sorption studies in materials obtained from the saturated zone between 1 and 10 m depth and show that under all chemical conditions, oxic to methanogenic, transformation is fast enough to reduce the concentration below the level of 0.1 ug/L. For details, please refer to RIVM report 715801005.
Details a part which considers monitoring data obtained from a depth of 10 m or more below soil surface. The procedure for the monitoring and the interpretation of the results is described in detail in RIVM report 601450015.


Special consideration for drinking water abstraction areas

Groundwater protection areas are, on average, more vulnerable to pesticide leaching than other agricultural areas (see Alterra report 1041). For selected hypothetical substances, the calculated GeoPEARL target concentration was considerably higher for the ensemble of groundwater protection areas than for the agricultural area, dependent on the crop for which the calculations were run. Therefore, a safety factor is introduced for these areas: a plant protection product cannot be used in groundwater protection areas if the calculated target concentration for the area of use is above 0.01 ug/L. Additional information on the behaviour in groundwater protection areas might, however, demonstrate the possible safe use and overrule the safety factor.
Details  Download the report on drinking water abstraction areas (Alterra report 1041)