Integrating field data and modeling: Dimensional perspectives on pesticide transport

In a European pesticide regulatory context, 1D models are recommended for leaching risk assessments to align with common European evaluation methodologies. However, it is well-known that real-world flow and transport of potential contaminants are rarely confined to 1D due to the ubiquitous heterogeneous geology.

In this study, we use a detailed field monitoring setup established through the Danish Pesticide Leaching Assessment Program to investigate the performance of tracer- and pesticide simulations in 1D, 2D, and 3D numerical models. Our approach involves hydrological monitoring and groundwater sampling for analyses of tracers and pesticides from a heavily instrumented agricultural field site on a glacial outwash plain.

Although the site is characterized as being homogeneously sandy, the monitoring reveals substantial spatial differences in both the compound detections and concentration magnitudes. This raises multiple questions about how to represent 3D field measurements (from different depths and different locations) in lower-dimensional models, including:

• Model simplifications: How do the assumptions inherent in 1D modeling affect the accuracy of leaching risk assessments for pesticides, particularly when accounting for spatial variability in hydrology and geology?
• Upscaling/downscaling: What are the implications of scaling field measurements to a 1D framework, and can such simplifications still adequately capture the critical transport processes observed in 2D and 3D environments?
• Regulatory implications: How might the insights gained from 2D and 3D simulations challenge or strengthen the current regulatory reliance on 1D modeling in Europe?

To address these questions, we use data from bromide tracer experiments and monitoring of the degradation product DMS following cyazofamid field applications. These data are represented in 1D, 2D, and 3D numerical modeling frameworks to evaluate the raised concerns.