Comparison of Lysimeter Results and Leaching Model Calculations for Regulatory Risk Assessment

: Experimental results from 34 outdoor lysimeter studies have been compared with leaching model simulations in order to evaluate their role in the tiered regulatory decision making scheme for the protection of groundwater. 63 compounds (28 active substances and 35 metabolites) were evaluated, resulting in a total of 73 model/lysimeter comparisons. In the majority (84%) of comparisons the model predictions and lysimeter results were in agreement, resulting in the same regulatory decisions being made. In the remaining 16% of cases, model calculations predicted concentrations in excess of 0.1 μg L -1 whereas the lysimeters gave concentrations below 0.1 μg L -1 (thus resulting in different regulatory decisions). In some cases, these differences could be attributed to conservative model input parameterisation but in other cases it may be that the experimental duration was insufficient for breakthrough to be reached. Groundwater monitoring of metabolites (after 7–20 years of parent substances applications) for which modelling indicated a potential risk showed concentrations below 0.1 μg L -1 in shallow roundwater. In no circumstances did lysimeter studies show concentrations above 0.1 μg L -1 whilst model predictions were below 0.1 μg L -1 . This, along with the groundwater monitoring data, therefore supports the tiered nature of the current regulatory assessment scheme. Recommendations are made for further optimising the design of future outdoor leaching studies.