INTERPRETING NONSTEADY STATE TRACER BREAKTHROUGH EXPERIMENTS IN SAND AND CLAY SOILS USING A DUAL-POROSITY MODEL

The effects of preferential flow on Cl-36 transport in undisturbed sand and clay soil monolith lysimeters were quantified using a dual-porosity model (MACRO). A double tracer test with H-3 and Cl-36 was performed simultaneously to check the possible occurrence of sidewall flow in the lysimeters. In the dual-porosity model MACRO, simulations can be performed in both one and two flow domains. Run in one flow domain, the model reduces to numerical solutions of Richards' equation and the convection-dispersion equation. In the sandy soil, the occurrence of preferential flow was tested by simulating in one domain, assuming that a certain pore fraction takes no part in water flow and solute transport. For the clay soil, the one domain case was compared with two domain simulations accounting for macropore flow. The double-tracer tests showed that sidewall flow did not occur in either soil type. Simulations of water flow showed good agreement with observed seepage until late autumn, but were less good during winter because the model does not account for soil freezing and snowpack/snowmelt. Simulated water flows were similar in one and two domain simulations, presumably because water contents in the lysimeters were maintained close to held capacity during the experiment. The simulations indicated that preferential how occurred in the sandy soil, with the observed Cl-36 breakthrough curves, assuming an unwetted volumetric pore fraction of 20%, reproduced reasonably well. The rate of Cl-36 leaching was consequently increased by c. 25% compared with the simulation assuming no preferential flow. Macropore flow was clearly demonstrated in the clay soil. The two domain simulation matched the breakthough curve and accumulated Cl-36 leaching accurately when the soil water pressure head defining the boundary between pore domains was set to -50 cm. This implies that preferential Cl-36 transport was taking place in a wide range of pore sizes, including smaller mesopores. The one domain simulation failed to predict the pattern of breakthrough of Cl-36 in the clay soil, in that it seriously underestimated leaching at early times and overestimated leaching towards the end of the experiment. Accounting for preferential flow with the dual porosity model resulted in significantly improved estimates of solute transport, compared to the classical convective-dispersive treatment, for both nonstructured sands and structured clay soils.