Assessment of in situ solvent extraction interrupted pumping for remediation of subsurface coal tar contamination

In situ solvent extraction involving continuous injection-recovery of solvent-water solutions has been proposed for removal of coal tar and other nonaqueous phase liquids from the subsurface environment. A modified in situ solvent extraction technique involving an interrupted injection-recovery procedure was examined with a view toward improved recovery efficiency. Design of an interrupted pumping process for in situ solvent extraction was examined, and an initial comparison of the interrupted pumping process with the continuous pumping process was performed by simulation. A two-dimensional solute transport model was modified to incorporate the mass transfer processes involved during the shut-in and continuous flow periods. This model was thereafter used to simulate application of the interrupted and the continuous pumping processes at a hypothetical manufactured gas plant site. The predicted coal-tar recovery rates did not differ significantly for the interrupted and continuous pumping processes, but the interrupted pumping process was consistently more efficient in terms of solvent requirements and pumping duration needed. It appears that interrupted pumping offers somewhat faster mass removal rates and somewhat reduced pumping requirements to reach a particular remediation goal.