Effect of flow rate on the adsorption and desorption of glyphosate, simazine and atrazine in columns of sandy soils

Summary Adsorption and desorption of the herbicides glyphosate [ N ‐phosphonomethyl‐aminoaceticacid], simazine [6‐chloro‐ N , N ′‐diethyl‐1,3,5‐triazine‐2,4‐diamine] and atrazine [6‐chloro‐ N 2 ‐ethyl‐ N 4 ‐isopropyl‐1,3,5‐triazine‐2,4‐diamine] were studied in four sandy soils from Western Australia. Distribution coefficients ( K d s) were calculated from breakthrough curves (BTCs) resulting from leaching step changes in concentrations through small saturated columns of soil at flow rates ranging from 0.3 to 30 m day –1 . A comparison was made with K d s obtained after batch equilibrating solutions of the herbicides with the same soils. The K d s of herbicides in soils decreased with increasing flow rate and most strongly for glyphosate in soils rich in clay content. Resulting increases in mobility of about 40–50% were estimated for simazine and atrazine and > 50% for glyphosate at flow rates of 3 m day –1 . Adsorption and desorption rates were estimated by fitting numerically simulated BTCs to experimental BTCs. Best fits were obtained with a time‐dependent Freundlich adsorption equation. The resulting coefficient for time dependency in the equation suggests that the rates of adsorption and desorption are controlled mainly by diffusion in an adsorbing layer on or in soil particles.