Influenza di alcune proprietà chimiche e idrauliche sui processi di adsorbimento e movimento di glifosate in suoli di alta e bassa pianura veneta Influence of some chemical and hydraulic proprieties on the process of adsorption and movement of glyphosate in soils of the upper and lower Veneto plain

Among the ecosystem services delivered by the soil, the soil filtering against potential contaminants of the water resource -such as plant protection products- is of primary importance. Nevertheless, the multitude of exogenous substances on the market and their different physicochemical characteristics, as well as the strong site-specific variability of the soils, makes it difficult to accurately predict the environmental fate of such molecules that are released into the environment. With this thesis work, some hydraulic and chemical properties of different agricultural soils of the Veneto region were analyzed, with the aim of understanding the dynamics that regulate adsorption and transport phenomena of glyphosate, and determine the susceptibility of the territory to subsurface water contamination. Two soils were sampled in Colnù (Conegliano) and Settolo (Valdobbiadene) sites in the northern Veneto plain, and compared with a Cambisol soil, typical of the low-lying venetian plain and located at the Experimental Farm of the University of Padua in Legnaro (Padua), which usually has a shallow water table around 1 m depth. Chemical analyses at different depths were performed to determine Al and Fe oxide and hydroxide contents, whose quantification is crucial to better understand the adsorption of glyphosate to the soil. Also, the adsorption coefficients were experimentally determined and described according to Freundlich isotherms. As for the soil hydraulic properties, undisturbed samples at different depths were analyzed by determining the saturated water conductivity and the water retention curve. Finally, the HYDRUS-1D hydrological model was fed with the experimental data previously obtained and used to simulate the environmental fate of glyphosate across the soil in each site-specific context. Each simulation involved the application of glyphosate in pre-sowing conditions on a maize crop, repeated in three consecutive years. Results showed that Fe and Al oxides and hydroxides increased with the depth in Legnaro soil, where the extraction method with oxalate returned the greatest quantity of both Fe and Al oxides. In contrast, results from Colnù and Settolo did not follow the same trend as per Legnaro. In particular, the oxalate extraction method results in extracting greater quantities of oxides and hydroxides only in the Conegliano sites; in Valdobbiadene the oxides most extracted are those chelated to the organic matter. extracted with pyrophosphate. As regards the Kf, the highest values were found in Conegliano. Correlation analysis showed that the content of fine soil particles –silt and clay– and the SOM-chelated oxides determined an increase in Kf. Numerical simulations highlighted the pivotal role of a shallow water table in promoting the movement of glyphosate which, despite its affinity for the soil phase, partly moved towards the deep layers favored by high soil moisture and water conductivity. On the other hand, in high plain soils, or where the aquifer is deep, glyphosate tends to slow down its movement, to be adsorbed to soil particles, and therefore degrade before reaching the aquifer. In-depth site-specific investigations are required to provide to policy makers and local authorities useful 4 tools to define more precisely the areas more or less susceptible to possible environmental contamination, both for the surface and sub-surface waters, so as to identify safeguard and protection actions.