The relationship between pore water velocity and longitudinal dispersivity of Cl −, Br −, and D 2O in soils

Abstract

Dispersivity was measured as a function of pore water velocity for saturated columns of sandy soil from breakthrough curves of pulse injections of bromide, chloride and deuterated water. Organic carbon contents of the soils ranged from 1 to 10%. Pore water velocities ranged from 10 to 4500 cm d −1 and mechanical dispersivities from 0.02 to 0.7 cm. The halides and D 2O showed identical elution behaviour in columns packed with sandy soils of low organic carbon content. At higher organic carbon and clay contents, charge exclusion increased the dispersion of D 2O in soil relative to that of the halides. Data were fitted to Hiby's equation (Hiby, 1962), describing the nonlinearity between the longitudinal hydrodynamic dispersivity and pore water velocity in columns packed with glass spheres. Below 2 cm 2d −1 the longitudinal hydrodynamic dispersion coefficient was dominated by molecular diffusion and dispersivity decreased with increasing pore water velocity. In the region above 2 cm 2d −1 mechanical dispersion was dominant and dispersivity increased with increasing pore water velocity. From extrapolation of the data obtained for this region it follows that effects of flow rate on dispersivity, though small, are still measurable for dispersion coefficients up to 1–10m 2d −1.