DIFFUSIVE AQUIFER/AQUITARD INTERACTION DURING AND AFTER AQUIFER-SIDE REMEDIATION AT DOVER AIR FORCE BASE, DELAWARE

Vertical profiles of tetrachloroethene (or perchloroethylene, PCE) were used to explore a diffusive interaction between aquifer and aquitard during and after the aquifer-side remediation at Dover Air Force Base (DAFB), DE. The aquitard under study has been contaminated from above as a result of long-term contact with halogenated organic contaminants that were dissolved in the groundwater of the overlying aquifer. PCE distributions in the aquitard underlying the unconfined aquifer were sampled from core tubes obtained at 4 different times over the course of a 35-month field investigation within "test-cells" that were isolated from the surrounding groundwater by means of grout-sealed steel sheet pile barriers. The aquifer-side remediation during the initial 12 months of this period generated such boundary conditions at the aquifer/aquitard interface that a back diffusion of contaminants from the aquitard into the aquifer was induced. Modeling predictions of concentration changes were made on the basis of the earliest coring results and an assumption of sorption-retarded diffusion in the aquitard and advective-dispersion in the aquifer, and using laboratory information about sorption and transport characteristics of the media. The Predictions indicated substantial change in PCE concentrations within the upper aquitard (near the aquifer/aquitard interface) and the development of a "back-diffusion" profile up into the aquifer. Modeling also predicted comparatively minor profile changes in the deeper aquitard, and especially in the deep layer where sorption was strongest. All of these predicted effects were quantitatively comparable to the observations in the coring results, thus providing an evidence of the diffusive interaction between the aquifer/aquitard at this site.