Formation of soil macropores and preferential migration of linear alkylbenzene sulfonate (LAS) in soils

Effects of freeze-thaw and wet-dry cycles on the formation of soil macropor es and preferential flow under simulative conditions in the laboratory, and preferential migration of linear alkylbenzene sulfonate (LAS) in paddy and soybean soils in field lysimeters were studied. Results showed that both f reeze-thaw and wet-dry cycles could cause macropores and preferential flow in soils and that the former was stronger than the latter. The preferential breakthrough of LAS through 1.5 m soil monolith was 23 days for both paddy and soybean soils. However, their leachate volumes at preferential breakth rough were much different, i.e. 69 mm for paddy soil and only 9 mm for soyb ean soil. The leachate volumes at equilibrium governed by soil adsorption/d esorption processes were very similar for soils cropped to paddy rice (185 mm) and soybean (178 mm) though their breakthrough times were much differen t, i.e. 44 days for the former and 58 days for the latter. The preferential breakthrough of LAS was 21 and 35 days ahead of equilibrium breakthrough f or paddy and soybean soils, respectively. The peak LAS concentrations resul ted from preferential breakthrough accounted for 20-30% of those at equilib rium in soybean soil. This ratio was as high as 30-86% in paddy soil. Prefe rential migration of LAS could not be observed in repacked soil monolith wi thout macropores. (C) 1999 Elsevier Science Ltd. All rights reserved.