Assessing the subsurface transport of plant protection products in wellhead protection areas via a quasi-3D groundwater model

Tracing the potential evolution (leaching / migration / transport) of plant protection products (PPPs) in wellhead protection areas (WHPAs) of wells supplying drinking water from agricultural areas, can provide relevant information on both agricultural practices and well management actions (e.g., where/when not to use PPPs or when not to pump water from the well) that need to be developed to preserve the quality of the subsurface drinking water resource. The spatial-temporal evolution of PPPs from the ground surface to the well screen in a shallow-unconfined aquifer is simplified assuming a vertical path in the vadose zone and a horizontal path in the saturated zone. Under this assumption, a quasi-3D model is developed by tacking advantage from the decoupling of vertical and horizontal paths. By this model, the travel times of PPPs coming from agricultural activities can be defined overcoming the simplistic two-dimensional description of WHPA. Moreover, the biochemical processes that principally affect the root zone can be accounted for according to recent field experiments and modelling (e.g., Mencaroni et al, 2022,2023, Costa et al. 2023). In the model, the whole surface of the 2D WHPA is subdivided into a regular cell-grid laying on the ground, where the size of each cell is consistent with the 2D model resolution. For each cell, the probability of a PPP particle spread on the topsoil to reach the groundwater table along the vertical, depends on the probability to be displaced from the ground surface, release probability, and on the soil infiltration capacity measured in the field. In the saturated zone, the particle transport develops along a horizontal path line, whose behavior depends on the hydraulic conductivity distribution, on the boundary conditions, and on the hydrological forcing. By this way, the agricultural plots that affect the quality of the groundwater supplied from a specific well can be defined. Here, a model application in the experimental site of Settolo, in Italy, is also presented. The site, located in a wine growing area of Valdobbiadene (Treviso), hosts a well supplying drinking water to 60,000 inhabitants and requires a careful analysis to drop the probability of groundwater PPPs pollution phenomena.