Laboratory method for determining immobile soil water content and mass exchange coefficient

Preferential flow in soil can enhance the leaching of agricultural chemicals. In a number of studies it has been shown that the mobile-immobile solute transport model (MIM) is a useful tool to characterize preferential flow. In the present study, a new laboratory method for determining the MIM parameters θ_m and θ_im (mobile and immobile water content), as well as a (mass transfer coefficient), is developed. The computations are uncomplicated and the method requires only simple equipment. It is applied to short, undisturbed soil columns. Measured values ranged from 0.11 to 0,27 for θ_im θ^-1 and from 0.015 h ^-1 to 0.034 h^-1 for α for an Iowan soil (Nicollet silt loam). For two sandy Eutric Gleysols from Germany, low values for θ_im θ^-1 from 0.04 to 0.07 and from 0.001 h^-1 to 0.008 h^-1 for α were determined. Although the new method is a flow-interruption technique, values for the Nicollet silt loam compare well with those from conventional leaching experiments. Values for the Eutric Gleysols agree with the observation that these soils were poorly structured. Because the new method does not assume negligible dispersion, it is applicable to a wider range of soils and boundary conditions than comparable approaches. We conclude that the new method provides parameter values that are suited to describe non-equilibrium solute transport.