A screening model for nonaqueous phase liquid transport in the vadose zone using Green-Ampt and kinematic wave theory

In this paper, a screening model for flow of a nonaqueous phase liquid (NAPL) and associated chemical transport in the vadose zone is developed. The model is based on kinematic approximation of the governing equations for both the NAPL and a partitionable chemical constituent. The resulting governing equation is a first-order, quasi-linear hyperbolic equation to which the generalized method of characteristics can be applied. This approach generally neglects the contribution to the NAPL flux from capillary pressure gradients. During infiltration under ponded conditions, or when the NAPL flux exceeds the maximum effective conductivity of the soil, the effect of capillary suction is included in the model through the usage of the Green-Ampt model. All of the resulting model equations are in the form of ordinary differential equations which are solved numerically by a variable time step Runge-Kutta technique. Results from a simple column experiment were used to evaluate the vadose zone flow model assumptions. Independently measured parameters allow simulation without calibration of the model results. The match of the model to the data suggests that the model captures the qualitative behavior of the experimental system and is capable of an acceptable degree of quantitative agreement.

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