From point to field scale results: upscaling pointwise analysis of glyphosate vertical mobility through the spatial knowledge of the soil infiltration capacity

In the two-year period 2018-2020 the Centre of Hydrology ‘Dino Tonini’ of the University of Padova developed the UNI-Impresa research project SWAT (Subsurface Water quality and Agricultural pracTices monitoring) to study the interactions between agricultural practices, mainly those involved in the production of Prosecco, and the wellhead protection areas in the province of Treviso (Italy). Specific experimental activities, integrated by a modelling analysis of the collected data, were developed to understand the processes affecting the vertical evolution of a glyphosate-based pesticide in the unsaturated soil up to a depth of 0.70 m BGL. The pesticide, along with a non-reactive tracer (potassium bromide), was applied in November 2018 in two experimental sites (Settolo-Valdobbiadene and Colnù-Conegliano) organized nearby well-fields supplying public water systems. Its evolution subjected only to the natural hydrological forcing compared to the infiltration dynamics of the tracer was locally monitored by collecting and analyzing soil and water samples along six months. Both the application and the monitoring activities were carried out in each experimental site on two 25 m² parcels located at reciprocal distances of 30 m (Settolo) and 115 m (Colnù), obtaining a detailed information about the glyphosate vertical evolution. Each point-wise analysis highlights a strong tendency of the pesticide and its principal metabolite (AMPA) to be adsorbed to the soil matrix rather than to be dissolved in the infiltrated rainwater and carried toward the deeper layers of the soil. However, high concentrations of the pesticide spotted at the depth of -0.70 m suggest that preferential pathways and more intense precipitation events enhance the downward movement of the glyphosate, either dissolved in water or adsorbed to microscopic particles. Differences in the pesticide spatio-temporal evolution were observed between parcels belonging to the same site.  Despite the decay analyzed during the experiments is related to both the chemical-physical properties of the soil, the potential movement is dominated by the heterogeneity of the hydraulic properties of soil. Hence, the evaluation of the infiltration capacity was considered a low-cost proper method to extend the analysis to the field scale (~10² m characteristic length). In the experimental site of Colnù, the spatial variability of the soil infiltration capacity (mm/min) and dynamics has been assessed developing a series of tests using the double ring infiltrometer in 17 different positions within an area of 1.75 ha. The investigated area extends over two contiguous vineyards inside the wellhead protection area. Two tests positions correspond to the site parcels while the remaining were spatially distributed maintaining reciprocal distances ranging between 15 and 50 meters. The measured soil infiltration capacity shows a large spatial variability, up to two orders of magnitude. The geostatistical interpolation (kriging) of the achieved data gives a quantitative estimation of the soil vulnerability at the field scale based on the potentially infiltrating pesticide.