THE USE OF UPWARD HYDRAULIC GRADIENTS TO ARREST DOWNWARD DNAPL MIGRATION IN ROCK FRACTURES

Upward water flow can arrest the downward migration of dense, nonaqueo us 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 di fference 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 exp eriments involving the migration of tetrachloroethylene (PCE) through two samples of fractured limestone demonstrate the ability of upward w ater flow to arrest downward DNAPL migration under both wetting and dr ainage conditions. It is suggested that upward gradients can be genera ted beneath contaminated regions of the subsurface to provide a ''hydr aulic bottom.'' A hydraulic bottom at a site would protect against pot ential downward mobilization of DNAPL in response to the application o f aggressive remediation technologies such as surfactant flooding, alc ohol flooding, and steam flooding. Upward gradients applied during dri lling may also protect against downward mobilization of DNAPL in the f ormation while drilling through source zones.