Adsorption, desorption and dissipation of metolachlor in surface and subsurface soils

Variations in soil properties with depth influence retention and degradation of pesticides. Understanding how soil properties within a profile affect pesticide retention and degradation will result in more accurate prediction by simulation models of pesticide fate and potential groundwater contamination. Metolachlor is more persistent than other acetanilide herbicides in the soil environment and has the potential to leach into groundwater. Reasonably, information is needed about the dissipation and eventual fate of metolachlor in subsoils. The objectives were to evaluate the adsorption and desorption characteristics and to determine the dissipation rates of metolachlor in both surface and subsurface soil samples.

Adsorption of metolachlor was greater in the high-organic-matter surface soil than in subsoils. Lower adsorption distribution coefficient (K(ads)) values with increasing depth indicated less adsorption at lower depths and greater leaching potential of metolachlor after passage through the surface horizon. Desorption of metolachlor showed hysteresis, indicated by the higher adsorption slope (1/n(ads)) compared with the desorption slope (1/n(des)). Soils that adsorbed more metolachlor also desorbed less metolachlor. Metolachlor dissipation rates generally decreased with increasing soil depth. The first-order dissipation rate was highest at the 0-50 cm depth (0.140 week(-1)) and lowest at the 350-425 cm depth (0.005 week(-1)). Degradation of the herbicide was significantly correlated with microbial activity in soils.

Metolachlor that has escaped degradation or binding to organic matter at the soil surface might leach into the subsurface soil where it will dissipate slowly and be subject to transport to groundwater.