Quantitative indices to characterize the extent of preferential flow in soils

Mathematical calculations or physical measurements to evaluate the extent of preferential solute transport or quantitatively distinguish the contribution of different transport processes are lacking in literature. Displacement studies on leaching of bromide and two pesticides were conducted under unsaturated steady state flow conditions in 24 undisturbed soil columns collected each from the two sites differing in soil structure and organic carbon content. The breakthrough curves (BTCs), developed using the leachate data of all individual columns, exhibited different shapes including some with early breakthrough and increased tailing which qualitatively indicates the presence of preferential flow. Transport parameters of bromide BTCs fitted to the convection-dispersion equation and a non-equilibrium two-region model were determined using the program CXTFIT and also by time moment method. Using these parameters and the time moments of BTCs of non-preferential and preferential flow columns (PFCs), the contribution of mechanisms other than hydrodynamic dispersion and two-region effect to the overall variance of PFCs was evaluated. The results indicated that the respective values of 0.5 and 1.0 for the first normalized moment (μ1′) and skewness (S) of bromide BTCs could be used as reference indices to denote the extent of preferential flow. An integrated parameter, PFSP, defined as the ratio of variance due to preferential flow vis-a-vis variance caused by hydrodynamic dispersion and two region effect, has been proposed to ascertain or compare the risk of preferential flow in soils. Though the transport parameters, temporal moments and PFSP were significantly correlated for all types of columns, these were not notably related with any soil property.