Dynamics of DNAPL penetration into fractured porous media

Dense nonaqueous phase liquids (DNAPLs) that pool above stratigraphic conta cts as a result of capillary forces can migrate downward through fractures in the underlying fine-grained layer, Although the DNAPL in the fracture is excluded from the surrounding porous matrix by capillary forces, the DNAPL constituent will dissolve and migrate by diffusion into the matrix. As a f ront of moderately soluble DNAPL advances into a fracture, the nux of disso lved material into the matrix increases until it becomes comparable to the rate of now into the fracture, reducing the pressure in the DNAPL phase and slowing its advance. If the DNAPL front encounters a constriction with suf ficiently large entry pressure, its advance will halt temporarily, But as t he concentration gradients driving diffusion into the matrix decrease, ther e will be less DNAPL lost by diffusion and the DNAPL pressure at the constr iction will increase until it exceeds the entry pressure, causing the downw ard advance of the front to suddenly resume. Because the time scales of dif fusion are much slower than those of density flow this analysis suggests th at under certain circumstances a DNAPL can suddenly resume its downward adv ance after a long period of apparent immobility. In one plausible example, a dichloromethane front passes through 5 m of fractured clay in 16 days, is immobile for more than four years, and then suddenly moves again. Where th is phenomenon is possible, removal of DNAPL to protect underlying aquifers becomes more important as a remediation goal.