Testing of leachability and persistence of sixteen pesticides in three agricultural soils of a semiarid Mediterranean region

Leaching, the movement of water and chemicals into deeper soil layers and groundwater is a subject of worldwide interest because a high percentage of drinking water is extracted from groundwater. The objective of this study was to evaluate the potential leaching and persistence of sixteen pesticides (one fungicide, three nematicides/insecticides, and twelve herbicides) for three Mediterranean agricultural soils with similar texture (clay loam) but different organic matter content (1.2-3.1%). Adsorption was studied in batch experiments and leaching was tested using disturbed soil columns (40 cm length × 4 cm i.d.). Degradation studies were carried out during 120 days under laboratory conditions. Mobility experiments showed that pesticides can be grouped according to their potential leaching. Thus, pesticides showing medium leachability were included in group 1 (referred as G1) while those with high leachability were termed as G2. The differences observed in the leachability can be attributed to the different organic carbon (OC) content in the soils (0.7-1.8%). Values of log KOC were higher in the order: soil C > soil B > soil A, which agrees with the OC content in each soil. The calculated half-lives ranged from 4.2 days for carbofuran in soil A to 330 days for prometon in soil C. As a general rule, when higher OC content in the soil the greater persistence of the pesticide was observed as a consequence of the increased adsorption. The first order kinetics model satisfactorily explains the disappearance of the studied pesticides in the soil. (Author)