The use of upward hydraulic gradients to arrest downward DNAPL migration in rock fractures

Upward water flow can arrest the downward migration of dense, nonaqueous phase liquids (DNAPLs) through rough-walled fractures provided that a sufficient hydraulic gradient exists. An exact analytical solution to predict the arresting gradient demonstrates that there is little difference between the gradient required to arrest DNAPL migration near the top of the fracture, and the gradient required to arrest migration once DNAPL has extended to the bottom of the fracture. Laboratory experiments involving the migration of tetrachloroethylene (PCE) through two samples of fractured limestone demonstrate the ability of upward water flow to arrest downward DNAPL migration under both wetting and drainage conditions. It is suggested that upward gradients can be generated beneath contaminated regions of the subsurface to provide a ''hydraulic bottom.'' A hydraulic bottom at a site would protect against potential downward mobilization of DNAPL in response to the application of aggressive remediation technologies such as surfactant flooding, alcohol flooding, and steam flooding. Upward gradients applied during drilling may also protect against downward mobilization of DNAPL in the formation while drilling through source zones.