LABORATORY OBSERVATION OF FUNNEL FLOW MECHANISM AND ITS INFLUENCE ON SOLUTE TRANSPORT

Recent field experiments have strongly indicated that contaminants could move preferentially and rapidly in unsaturated soils. Without knowing a priori what the preferential flow mechanisms are, it is impossible to predict solute movement based on numerical modeling with limited measured soil properties. The objective of this study was to characterize the mechanism of funnel flow (i.e., how an inclined soil layer with abrupt textural discontinuity will trigger preferential flow) in laboratory experiments and to determine the influence of funnel flow on solute transport. To simulate soil heterogeneity encountered in the field, a coarser sand lens with 15-degrees inclination was packed in finer sand in a sand tank; three combinations of the coarser lens embedded in finer particles were tested. Water was applied at seven different rates in each combination to determine how the textural discontinuity between the two media would initiate the funnel flow, and to identify the critical water application rate at which water would enter the coarse sand lens. The water flow pattern, the influence of funnel flow on solute transport, and the effect of hysteresis on the funneling phenomenon were also examined. Results showed that funnel flow could occur along an inclined boundary between two soils if the following two conditions are both satisfied: (i) there is a macroscopic Haines' jump across the boundary, and (ii) water application rates are smaller than a certain critical rate. Under this condition, contaminants could be greatly congregated and accelerated by funnel flow.