Including spatial variability in Monte Carlo simulations of pesticide leaching

Pesticide leaching increases if spatial variability of pesticide properties is included in leaching assessments

A methodology is developed to quantify the uncertainty in a pesticide leaching assessment arising from the spatial variability of non-georeferenced parameters. A Monte Carlo analysis of atrazine leaching is performed in the Dyle river catchment (Belgium) with pesticide half-life (DT50) and topsoil organic matter (OM) content as uncertain input parameters. Atrazine DT50 is taken as a non-georeferenced parameter, so that DT50 values sampled from the input distribution are randomly allocated in the study area for every simulation. Organic matter content is a georeferenced parameter, so that a fixed uncertainty distribution is given at each location.

Spatially variable DT50 values are found to have a significant influence on the amount of simulated leaching. In the stochastic simulation, concentrations exist above the regulatory level of 0.1 µg per litre, but virtually no leaching occurs in the deterministic simulation.

It is axiomatic that substance parameters (DT50, sorption coefficient...) are spatially variable, but pesticide registration procedures currently ignore this fact. Including this spatial variability in future registration policies would have significant consequences on the amount and pattern of leaching simulated, especially if risk assessments are implemented in a spatially distributed way.

More information

• More information on PEARL and GeoPEARL can be found at the PEARL website
• More information on the use of models in European pesticide registration procedure can be found at the website of FOCUS (FOrum for the Co-ordination of pesticide fate models and their USe)