Leaching and degradation of herbicides and their transformation products in field experiments

A field sire equipped with suction lysimeters was installed in order to ass ess the leaching capacity of terbuthylazine, metolachlor and alachlor herbi cides from topsoil to groundwater and to study the mobility of their releva nt transformation products. Two different hydrological situations were simu lated: in the firn experiment a constant hydraulic head was applied to reac h the interstitial water saturation in the surficial soil layer; in the sec ond one an intermittent pluviometric condition was generated. KCI was used as a tracer to evaluate water infiltration velocity through the vertical pr ofile. Results showed significant differences both between the two experiments and inside each simulation. The constant hydraulic head accelerated infiltrati on rates while herbicide concentrations reached the maximum contaminations because soil adsorption capacity was underdeveloped. In both experiments, t he results showed two main process of chemical transport: in the first patt ern transport was mainly due to water infiltration through macropores; in t he second one the transport was driven by matrix flow. Metolachlor resulted to be the most mobile herbicide while desethylterbuthylazine revealed as t he most mobile transformation product. 2 OH-desethyltertbuthylazine was fou nd as the major hydroxy transformation products of terbuthylazine. No trace of alachlor and metolachlor metabolites was determined in thewater samples from suction lysimeters.