FIELD SCALE MASS ARRIVAL OF SORPTIVE SOLUTE INTO THE GROUNDWATER

The effect of spatial variability in sorption parameters is investigated for field scale solute transport in the unsaturated zone. The considered sorption reaction is kinetically controlled sorption-desorption coupled with a parallel linear equilibrium isotherm. The source of solute is assumed to be distributed over a heterogeneous field where the horizontal extent of the source area is significantly larger than the depth to the groundwater table. The transport model is expressed in terms of the mass flux and in dimensionless form, where the effect of equilibrium sorption is incorporated in the dimensionless parameters of the kinetic sorption model. The influence of different correlation properties between the sorption parameters and the hydraulic conductivity at saturation, K(s), on the expected field scale mass flux of solute is illustrated. The results indicate that spatially variable sorption rate coefficients with negative or no correlation to K(s) may result in an earlier mass arrival and a considerably altered form of the breakthrough curve compared to the case of constant sorption parameters. In addition, a comparison between the flux-averaged and the resident concentration models indicates that spatial variability in the sorption rate coefficients has similar effects on the temporal variation of the field-scale resident concentration as on the field scale mass flux of solute.