Review of dispersivities for transport modeling in soils

The one-dimensional convection-dispersion equation is often used to estimate the risk of nonpoint source groundwater contamination and the dispersivity in this equation is known to be a sensitive parameter for predicting the mass that leaches through the vadose zone to the groundwater. We derived a database of dispersivities from leaching studies in soils. Besides dispersivities, the database contains information about experimental parameters: transport distance, scale of the experiment, flow rate, boundary conditions, soil texture, pore water velocity, transport velocity, and measurement method. Dispersivities were found to increase with increasing transport distance and scale of the experiment. Considerably larger dispersivities were observed for saturated than for unsaturated flow conditions. No significant effect of soil texture on dispersivity was observed, but the interactive effects of soil texture, lateral scale of the experiment, and flow rate on dispersivity were significant. In coarse-textured soils, lateral water redistribution may take place across relatively larger distances, which explains the larger dependency of dispersivity on lateral scale of the experiment in coarse-than in fine-textured soils. The activation of large interaggregate pores may explain the increase in dispersivity with increasing flow rate in fine-textured soils, which was not observed in soils with a coarser texture. The distribution of dispersivities was positively skewed and better described with a lognormal than a normal distribution. Different experimental factors explained 25% of the total variability of log(e)-transformed dispersivities. The unexplained variance of the dispersivity was large and its coefficient of variation was 100%.