PARENT SESSION
MA4 The Use of Ecological Models and Concepts in Aquatic Higher Tier Risk Assessment.
9:00 AM to 12:30 PM, Monday, 07 May 2001
Session Chair: Paul van den Brink
Room 4

(028) Proposed new decision tree for pesticide leaching to groundwater in the Netherlands.

Verschoor, Anja¹, Boesten, Jos², Brouwer, Willem³, Leistra, Minze², van der Linden, Ton¹, Linders, Jan¹, Pol, Werner⁴, ^{1 2 3 4}

ABSTRACT- According to the Dutch law pesticide concentrations in groundwater should not exceed the value of 0.1 g/l. Ultimately pesticides should fulfil this criterion at a depth of 10 m below soil surface. The question rises whether one single value above this limit leads to rejection of the registration. If not, how many exceedings are allowed? In the current Leaching Decision Tree the criterion is operational only in the first tier by a 90% vulnerable scenario. To evaluate higher tier computatuions and experiments, the criterion should be operational also at higher tiers and furthermore be consistent throughout the decision tree. The following explicit definition of the criterion is proposed: the concentration in groundwater should be less than 0.1 g/l in at least 90% of the application area for at least 50% of the time. This definition should be incorporated in all tiers of the leaching evaluation process. The proposed decision tree for pesticide leaching to groundwater consists of three tiers: 1. Simple estimation procedures of concentrations leaching to groundwater at 1-2 m depth 2. Refined estimation procedures of concentrations leaching to groundwater at 1-2 m depth 3. Estimation of concentrations in groundwater at 10 m depth. Tier 1 assumes a fixed crop and a limited number of application times. The first step of Tier 2 consists of refined scenario calculations with PEARL including the most appropriate crop rotation, application times and dosages. The 90th percentile concentrations are calculated with a PEARL-GIS tool that includes simulations for many (>100) soil profiles. Field or lysimeter studies may result in a correction factor to be applied to this 90th percentile. Alternatively, additional transformation rate studies with relevant Dutch soils in the laboratory may result in revised DT50 values which are used as input for the PEARL-GIS tool. If these steps do not solve the problem, monitoring studies for shallow groundwater may be performed. Tier 3 considers pesticide transformation in the water-saturated zone and, if appropriate, monitoring in deep groundwater.

Key words: decision tree leaching, leaching criterion, risk assessment, pesticides