Impact of the Prosecco production on the wellhead protection areas: analysis of the glyphosate vulnerability across the vadose zone through the study of the spatial variability of the topsoil hydraulic and chemical properties

Most of the Prosecco production in Italy is carried out in the province of Treviso where the aquifers are widely exploited for drinking water supply. In the period 2018-2020 the Centre of Hydrology ‘Dino Tonini’ of the University of Padova developed the research project SWAT to acquire information on the possible interactions occurring between the plant protection products (PPPs) used in vineyards and the soil of the wellhead protection areas. To analyze the evolution of Glyphosate (GLP) up to a depth of -0.70 m, an experimental activity was carried out in two sites (Settolo-Valdobbiadene and Colnù-Conegliano) organized nearby well-fields supplying the local water systems. Two underground monitoring stations were installed in each site at the reciprocal distances of 30 m (Settolo) and 115 m (Colnù) to collect data about the quality of the infiltrating rainfall water. In November 2018, a GLP solution was applied on a 25 m2 parcel above each station and the evolution of the herbicide, subjected to the hydrological forcing only, was monitored in the following six months. The results show that GLP was mostly adsorbed to the topsoil acting like a barrier to infiltration. However, high GLP concentrations were observed at -0.70 m in the site of Settolo, indicating possible infiltration events through soil preferential pathways. The different results observed between parcels of the same site, suggested a dependency of this vertical mobility on the heterogeneity of the hydraulic properties of the soil. To extend the experimental evidence to the scale of the wellhead protection area, two series of tests are under development in each site using a double ring infiltrometer and a 30 cm deep soil sampler to evaluate the spatial variability of the infiltration capacity, the soil organic content (TOC) and the soluble phosphorous concentration (PO43-), both correlated to the GLP dissipation. The preliminary results highlight a large spatial variability for the soil infiltration capacity, reaching differences up to two orders of magnitude for the Colnù site. The geostatistical analysis of the information on topsoil properties, will give a quantitative estimation of the GLP potentially infiltrating and hence the vulnerability of the two wellhead protection areas considered suggesting a possible approach to evaluate the risk in large areas.