THE EFFECT OF CAPILLARY-PRESSURE HEAD ON COLLOID MASS FLUX FROM VARIABLY SATURATED SOIL

Colloids may transport contaminants through unsaturated soils to the groundwater.  Both transient hydrological events and sustained, steady water fluxes are associated with colloid release.  We examined the controls on colloid mobilization from an intact, unsaturated soil core (20 cm diameter, 23 cm length) during steady flow conditions.  We collected the core from an agricultural field in the Shenandoah Valley near Harrisonburg, VA where soils contain 26% clay (predominantly illite and kaolinite) and infiltration occurs only at capillary-pressure heads (Y) above -20 cm.  We measured the colloid-mass flux from the base of the core during consecutive, 1.5-month periods (P₁, P₂, and P₃) distinguished by capillary-pressure heads (Y_B) of -18.5, -11.5, and -18.5 cm, respectively.  Mass fluxes generally were less than 0.3 mg h^-1 but approached 1 mg h^-1 for brief intervals.  Mean mass fluxes were 0.0886, 0.197, and 0.171 mg h^-1 for P₁, P₂, and P₃, respectively.  Fluxes peaked immediately afterY_B increased from -18.5 cm to -11.5 cm.  The decrease in Y_B from -11.5 cm to -18.5 cm also induced an ephemeral increase in mass flux.  Increases in fluxes following each change in Y_B suggest that flow transients associated with capillary-pressure changes enhanced colloid mobilization, but that a quasi-steady mobilization rate was attained shortly thereafter.  Intervention analysis, used to measure changes in mass flux, showed a significant increase in mass flux of 0.079 mg h^-1 for the increase inY_B from -18.5 cm to -11.5 cm and a significant decrease in mass flux of 0.050 mg h^-1 for the decrease inY_B from -11.5 cm to -18.5 cm.  Relative errors in parameter estimates of the intervention function generally were less than 10%.  We used a constant volumetric flow rate but differing Y_B and observed higher colloid concentrations at higher Y_B.  We hypothesized that either shear mobilization due to increased porewater velocities or the number of pores through which water flows limited mass fluxes.  If a velocity mechanism were of overpowering importance, a lower Y_B would dictate higher effluent concentrations because the total flow would occur through fewer pores resulting in higher porewater velocities.  Thus, colloid release most likely is controlled by a diffusion-limited colloid supply mechanism dependent upon the number of pores contributing to flow.