EFFECTS OF DEGREE OF WATER SATURATION ON DISPERSIVITY AND IMMOBILE WATER IN SANDY SOIL COLUMNS

Three natural nonaggregated soil samples, with similar grain-size dist ributions, have been used to determine the dispersive behavior of poro us media under steady, saturated and unsaturated flow conditions. Trit ium was used as a tracer and was found to have no sorption on the soli d matrix. Generated breakthrough curves (BTCs) for the unsaturated exp eriments were symmetrical with no evidence of tailing. The unsaturated experiments for two of the soils were adequately described by conside ring all the water in the pore volume as mobile. However, about 10% of the pore water, independent of the degree of saturation, was found to be immobile in the case of the third soil during unsaturated flow, Fo r this soil, there was no mass transfer between the two water regions, indicating that the immobile water is essentially isolated from the f lowing water fraction. For all three soils, dispersivity under unsatur ated conditions was found to be higher, independent of the degree of w ater saturation, than the value determined for the saturated experimen ts. This is inconsistent with what would be expected from the simple b undle-of-capillary-tubes model and does not agree well with a more sop histicated conceptualization of the porous medium. The data, however, clearly indicate a wider range in pore-water velocities when these soi ls are desaturated. (C) Elsevier Science B.V.