Mobility in Sandy Soils of Four Pesticides with Different Water Solubility

Abstract

The mobility of four pesticides 2,3,6-TBA (2,3,6-trichlorobenzoic acid), dichlorprop ((±)-2-(2,4-dichloro-phenoxy)-propionic acid), atrazin (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and dichlobenil (2,6-dichlorobenzonitril) was measured in column and batch experiments. The soils used were classified as sands and loamy sands with organic matter contents ranging from 0.0 to 3.65%. Systematic deviations were generally found in column experiments between experimental and fitted effluent curves. Thus the analytical model equation used did not describe all aspects of the transport process well. Violation of the assumption of a linear adsorption isotherm is proposed as one cause of these discrepancies. The estimated retardation factors from the column experiments were in the range 0.92–1.27, 1.01–1.41, 1.34–4.77 and 4.22–55.6 for 2,3,6-TBA, dichlorprop, atrazin and dichlobenil, respectively. The column experiments were found to give reproducible estimates of the transport parameters, but the estimated degradation rates showed substantial variation. The linear form of the Freundlich isotherm (K_d -value) obtained from the column experiments was compared to the more or less non-linear form obtained in batch experiments by calculating theoretical effluent curves for the two types of adsorption isotherms. The agreement was poor for dichlorprop in topsoils, but reasonable to good for all other soils and pesticides. In the case of dichlobenil in topsoil, with a Freundlich isotherm far from linear, the chosen c ₀-level strongly influenced the shape and position of the calculated effluent curve.

Key Words:

• column experiment
• adsorption
• batch experiment