Fate of glyphosate and AMPA in the vadose zone: dissipation, transport and adsorption

Broad contamination of systemic herbicide glyphosate –GLP– (N-(phosphonomethyl) glycine) and its metabolite aminomethylphosphonic acid (AMPA) in soil and water has become one of the main environmental issues worldwide, raising awareness of the potential harmful effects to human health and ecosystems. Physical, chemical, and biological soil properties contribute to the complex interaction between GLP and the environment, that makes any prediction of adsorption, transport, and degradation dynamics still challenging.

Within a wide project –SWAT– that tries to link GLP and AMPA dynamics through the vadose zone with groundwater contamination, the specific goals of this work are: 1. monitoring soil and water contamination of GLP and AMPA in agricultural lands; 2. identifying the driving factors leading to site-specific soil-water contaminant interactions.

Two experimental sites were located in northeastern Italy (Conegliano and Valdobbiadene municipalities) in the winegrowing terroir of the Prosecco wine production, recently included in the UNESCO’s World Heritage List. Each site was equipped with two soil-water monitoring stations (25 m² each), multi-sensor soil probes (temperature and water content) and suction lysimeters to monitor the full soil profile. Undisturbed soil cores were also collected and later analyzed for hydraulic, physical and chemical properties down to 70 cm. After GLP field contamination on November 2018 (0.188 g m^-2), soil and water were systematically sampled from each site, starting immediately after contamination and thereafter at each rain event for 6 months. Adsorption coefficients (K_f) were estimated in laboratory in order to get information about GLP sorption to soil particles at different soil layers along the full soil profile. Site-specific dissipation kinetics (DT₅₀) were also evaluated to better understand its decay rate.

First results revealed that GLP transport was highly site specific and locally affected by preferential flows when intense rainfall events occurred (12 mm h^-1 max rainfall intensity): GLP showed strong binding affinity to soil particles in the topsoil layer and it likely bypassed the porous matrix towards the deepest layers, where it was detected as in the surface one. The GLP dissipation dynamic was completed after 6 months of experimentation, whereas AMPA was still detected in the topsoil layer, attesting the full degradation after almost 300 days. Site-specific laboratory and field data will be integrated and further discussed to better understand the fate of glyphosate and AMPA in the vadose zone.