Pesticide transport through the soil profile at the field scale is notoriously difficult to predict because of a lack of appropriate field-scale models and model parameters that take adequate account of heterogeneity in local flow rates and parameters. It is generally impossible to measure all the parameters required to describe that heterogeneity. A way of approximating pesticide transport at the field scale, given uncertainty in the representation of both local characteristics and field-scale distributions, is presented. The methodology allows prior estimates of field-scale distribution parameters to be conditioned on uncertain column-scale measurements. It is shown how this conditioning can drastically constrain steady-state flow predictions of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) transport to groundwater for a site in the Rhone Valley, Switzerland. Some of the simplifying assumptions of the current analysis, in particular, to allow for transient simulations, can be relaxed in the future as more computer power becomes available. The methodology, however, should remain valid.

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