effect of vertically decreasing macropore fractions on simulations of non-equilibrium solute transport

The effects of a depth-decreasing macropore fraction and the corresponding decrease in the diffusive mass-transfer coefficient for solute exchange between soil matrix and macropore regions were simulated for a hypothetical, one-dimensional, mobile-immobile, transport domain. These effects are illustrated using an expression for a Damkholer number, and with numerical simulations with a dual-porosity module within the HYDRUS-2D code. The depth-wise change in the macroporosity was represented with an exponentially decreasing macropore fraction, based on a pore-network model. Five simulations were run, four with various steady infiltration rates (two at water saturation and two at different levels of unsaturation) and one with transient infiltration rates. Deviations between the predicted solute breakthrough curves (BTCs) for simulations using depth-variable parameters and those using spatially averaged parameters were minor at the lowest infiltration rate, indicating little effect of non-equilibrium conditions. However, at higher infiltration rates, the BTC peak height and peak timing varied by as much as a factor of two, indicating significant influence of the variability of the macropore fractions with depth. Using depth-averaged values to represent the variable macropore fraction will closely approximate the BTC of a spatially variable case for both steady-state and transient infiltration; however, the concentration profile within the transport domain will not be accurately predicted. A sensitivity analysis conducted for the saturated hydraulic conductivity, length of the soil ped, and dispersivity parameters shows that changes in the prediction error between the spatially variable and surface-valued simulations are most sensitive to these parameters as the soil approaches water saturation.