Reactivity of vadose-zone solids to S-metolachlor and its two main metabolites: case of a glaciofluvial aquifer

The vulnerability of groundwater to pesticides is governed in part by sorption mechanisms in the vadose zone, commonly studied in soil but less well-known in the geological solids. To alleviate this lack of knowledge, adsorption of the herbicide S-metolachlor (SMOC) and of two of its metabolites-metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA)-was studied with batch equilibrium method on seventeen surface soils and three geological solids of the vadose zone overlying a glaciofluvial aquifer. In grainsize terms, the latter three were sand for the first two samples and gravel for the third. Adsorption is ordered as follows: SMOC > > MESA > MOXA, except for one of the geological solids for which MESA adsorption was slightly higher than that of SMOC (K-d = 0.73 vs. 0.44 L kg(-1)). The low MOXA adsorption could only be quantified for the gravel sample (K-d = 0.74 L kg(-1)), which was also more reactive than all the other samples to MESA and SMOC (K-d = 2.08 and 28.8 L kg(-1), respectively). Statistical multivariate tests related the highest K-d values for SMOC with the soils and geological solids with the highest organic-carbon and clay-fraction contents. The highest K-d values for MESA were found in the samples containing high oxide concentrations. Our results shed a new light on the adsorption of SMOC, MESA and MOXA suggesting that during their transfer to groundwater, pesticides and metabolites can be adsorbed in the vadose zone on both soils and geological solids.