Surveillance of groundwater for residues of active substances and their metabolites is an important tier in the risk assessment scheme of plant protection products. Monitoring wells for sample collection can be placed at various distances to test fields treated with plant protection products depending on the purpose and design of a groundwater monitoring study. A common challenge during planning and evaluation of such studies is the need to demonstrate that wells and filter screens are appropriately placed in order to detect potential residues from treated fields. In case of edge-of-field monitoring, connectivity is mainly determined by the presence of hydraulic connectivity which depends on the well’s location, filter placement in the saturated zone, and other environmental factors and properties.  Two options were evaluated in the presented case study to assess hydraulic connectivity by means of exposure and transport models. The first option uses Darcy’s law to estimate well catchments which are likely to have a hydraulic connection to the well’s filter screen. It requires only few parameters which might be available as part of a monitoring study or could be estimated with sufficient accuracy. The second option uses process-based, dynamic models to simulate the transport and fate of substance residues in soil and groundwater. The influx of leachate and residues into the aquifer was simulated with FOCUS PEARL, a one dimensional model of the unsaturated zone. The aquifer was simulated with a two dimensional representation using the OpenGeoSys model. Its scenarios can be set up with a varying degree of complexity in order to accommodate different levels of data availability. Two dimensional aquifer models provide additional insight into dispersion behavior and influence of time-dependent groundwater recharge on substance transport. The presented modeling options could be used in a tiered or combined approach to assess hydraulic connectivity and support the interpretation of groundwater monitoring studies.

Assessing lateral hydraulic connectivity of edge-of-field groundwater monitoring wells using a tiered modeling approach

2019 Fall National Meeting

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