Entrapment and mobilization of residual halogenated organic liquids in saturated aquifer materia

This research demonstrates through theoretical analysis and experimental work that the residual saturation of dense non-aqueous phase liquids (DNAPLs) entrapped in saturated porous media depends on the relative magnitudes of the viscous, buoyant, and capillary forces acting on the liquids. The relative magnitudes of these forces are expressed in terms of two dimensionless groups: the Capillary Number and the Bond Number. The Capillary Number is defined as the ratio of viscous to capillary forces; the Bond Number is defined as the ratio of buoyant to capillary forces.;A nondimensional mathematical model is developed for estimating residual saturation of DNAPLs as a function of the Capillary and Bond Numbers. Analysis of the model suggests that residual saturations resulting from upward displacement of DNAPLs at low flow rates decrease with increasing Capillary Number and increase with increasing Bond Number. At high flow rates, residual saturation becomes independent of both the Capillary and Bond Numbers. This theoretical analysis is corroborated by the results of upward displacement experiments. DNAPLs were displaced by upward flow of water in order to maximize the role of buoyancy or density difference in the displacement process. Residual saturations observed at the lowest flow rates tested ranged from 30% to 55% of the pore volume, with the highest values corresponding to the most dense compounds. As the flow rate was increased, residual saturations decreased, asymptotically approaching a minimum residual saturation value of approximately 15% at the highest flow rates tested.;The model and experimental results suggest that a Capillary Number of at least 1 $times$ 10$sp{-5}$ in fine sands, corresponding to a hydraulic gradient of at least 4 m/m, is required to reduce DNAPL residual saturation to the minimum possible by upward displacement. Gradients of this magnitude may be infeasible in groundwater remediation. In horizontal displacement, however, the required hydraulic gradients are likely to be at least an order of magnitude smaller and, therefore, more easily attained than those required in upward displacement. Nevertheless, it is clear that some DNAPL will remain entrapped regardless of the displacement direction, even after remediation by groundwater pumping.