RATES OF COLLOID MOBILIZATION IN AN UNSATURATED, SANDY SOIL

Measuring rates of colloid mobilization is important because rates of soil eluviation and of colloid-facilitated transport of contaminants may depend on how rapidly colloid mobilization occurs. Previous research suggests that colloid mobilization is maximized during unsteady-flow conditions and, therefore, we aimed to measure rates of colloid mobilization during transient, hydrological events. We collected three intact, soil cores (70 cm in length, 7.5 cm in diameter) from a site in Northampton County on the Eastern Shore of Virginia by hammering hollow, 1-m lengths of aluminum irrigation piping vertically into the soil surface and removing the cores from the ground. Soils at this site are well drained, originate from weathering of coarse-textured sediments, and have a clay content ranging from 2 to 10 % at depths of 0 to 0.50 m. In the laboratory, we suspended each core 40 cm above the bench top, secured the base of each core to a funnel containing DIW-rinsed, 0.15-mm-diameter sand, and connected each funnel to a fraction collector. We completed nine, successive ponding experiments on each core over three consecutive days. To initiate an experiment, we poured 660 mL of 0.1 mM NaCl, colloid-free solution against the 30-cm-high wall of aluminum casing extending above the soil surface, resulting in pond depths of 0.15 m. During each experiment, we measured volumetric moisture content and capillary-pressure head at three depths, and determined infiltration rates, outflow rates, colloid concentrations, colloid-mass fluxes, and colloid-mobilization rates. Maximum colloid concentrations were on the order of 100's of mg L^-1. Colloid concentrations declined in general from the first to the last experiment of the day, and maximum concentrations for the first experiment of the day were greater than those for the last experiment on the preceding day. Colloid-mass fluxes ranged from 0.001 to 0.18 mg min^-1 and colloid-mobilization rates ranged from on the order of 0.0001 to 0.1 mg L^-1 cm^-2. Our results suggest that increases in the initial volumetric moisture content over successive experiments decrease colloid mobilization. Although cores received a depth of water comparable to the total depth of water typically received at the field site on an annual basis, the supply of colloids was not depleted during the experiments.