H. Ruanı and T.H. Illangasekare²

ıDepartment of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, Boulder, CO, 80309-0428, 306-492-6754 and ²Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, Boulder, CO, 80309-0428, 306-492-6644


In subsurface water systems, cationic contaminants such as plutonium are often adsorbed to soil particles because most soil particles are negatively charged. Those contaminants are mobilized only through the transport of colloids. In the vadose zone, soil interstitial water moves normally in fine pores rather than in coarse pores due to the capillary force. Moving in fine pores, colloids are easily coagulated, filtered and then are less mobile. However, preferential flow through continuous macropores such as earth worm holes have been observed and investigated for decades. Such preferential flow, if it occurs, will transport colloids much more easily and rapidly into the deep soil horizons and the ground water system.

Preferential flow through a vertical cylindrical macropore was simulated at the monolith scale using a two-dimensional finite element method. Preferential flow in the macropore is coupled with the flow in the soil matrix. Flow in the macropore is mainly controlled by surface ponding, flow loss to the soil matrix around the macropore or lateral infiltration, and the macropore size. Flow in the soil matrix is simulated by the Richards equation. A constant colloid concentration was assumed when the flow enters the macropore. The colloid transport in the macropore and in the soil matrix was then simulated using the computed flow field in both the macropore and the soil matrix. The colloid flux from the macropore is assumed to be controlled mainly by the combination of five factors. The first is the lateral infiltration rate through the macropore wall. The second factor is the ratio of the pore size in the soil matrix to the colloid size. A small ratio will cause the colloid coagulation in the soil matrix near the macropore and decrease the further transport of colloids and the lateral infiltration rate. The third factor is saturation of the soil matrix around the macropore. Water moves in larger pores when saturation is higher. The fourth factor is the flow velocity in the macropore. Higher velocity moves more colloids into a deeper part of the macropore. The last is the cumulative amount of colloids generated through a unit length of the macropore wall. This factor is similar to the second factor. The sensitivity analysis was performed for a hypothetical single macropore monolith, and quantitative results were obtained.


colloid transport, macropores, preferential flow

This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.