FIELD-SCALE COLLOID MOBILIZATION IN AN UNSATURATED, SANDY SOIL

Understanding colloid mobilization in unsaturated soils at the field scale is important because rates of soil formation and of colloid-facilitated contaminant transport may depend on the extent and rate of colloid release. Experiments on unsaturated, intact cores from a site on the Eastern shore of Virginia indicate that transient flow conditions may enhance colloid release. Air-water interfaces traveling through packed sand columns at ~0.007 cm s^-1, for example, mobilize colloids at concentrations of up to 25 mg L^-1, suggesting that scavenging of colloids by mobile air-water interfaces plays a key role in colloid release. Confirmatory data under field conditions are desired for testing of a mathematical model of the phenomena. Colloid mass flux data were collected during a sequence of field-scale infiltration experiments performed in a sandy-loam soil at the Eastern shore of Virginia. A 0.1-mM NaCl solution was ponded within a 1.5 x 0.5 x 0.5-m chamber to heights of 0.15 and 0.22 m. Infiltration rates of approximately 2.24 cm min^-1 were observed. Water was collected in four, zero-tension lysimeters (0.23-m in diameter) installed at a 0.50-m depth directly beneath the chamber. The moisture characteristic was evaluated in situ by installing both a TDR rod and a tensiometer at depths of 0.23, 0.33, 0.44, and 0.50 m. Colloid concentrations, analyzed with a spectrophotometer at a 400-nm wavelength, ranged from 1 to 49 mg L^-1. Maximum values of the colloid mass flux, ranging from 4 to 5 mg min^-1, coincide with increases in flow at the commencement of each infiltration event, suggesting that colloid release is enhanced under transient flow conditions at the field site. A one-dimensional mathematical model based on a combination of Richard's equation for flow and a simple interpretation using the capillary bundle approximation for colloid transport was used to interpret the data quantitatively.