A computational model was developed to simulate aquifer remediation by pump and treat for a confined, perfectly stratified aquifer. A split-operator finite element numerical technique was utilized to incorporate flow field heterogeneity and nonequilibrium sorption into a two-dimensional, radially symmetric advectivedispersive transport model. Simulations reproduced the tailing behavior commonly observed at remedial sites. Predicted remediation times were observed to increase as the degree of hydraulic conductivity heterogeneity and/or sorption nonequilibrium increased. Dimensional analysis was used to facilitate the general application of simulation results to a range of aquifer/contaminant systems. The effects of sorption nonlinearity, sorption capacity heterogeneity, and intermittent pumping were also analyzed.
IntroductionAquifer remediation by pump and treat (pAT) is the prescribed strategy at approximately two thirds of the existing 1200 Superfund sites [Travis and Dory, 1989], as well as numerous other sites of groundwater contamination. To date, however, only limited success has been achieved in the attainment of health-based cleanup objectives. In light of documented lengthy cleanup times, considerable effort has been expended toward enhancing the current understanding of PAT, its limitations, and alternative remedial strategies.In the evaluation of remedial options, predictive capability is desired for the degree of mass removal attainable, the rate at which such removal is achievable, and the ultimate cost of rernediation. Laboratory simulation for a specific site is difficult or impossible because of the complexities and scales that characterize the underlying processes. Consequently, mathematical models play a key role in remedial process selection and design.Conceptual analyses have identified a number of factors that contribute to the lengthy cleanup times observed for PAT systems. In general, success in achieving remedial objectives is hindered by the presence of contaminant in regions that are not readily accessed by the advective flow field created by pumping. Examples of difficult to remove material include (1) dissolved phase contaminant located in regions of low hydraulic conductivity, (2) nonaqueous phase liquid (NAPL) contaminant trapped by capillary forces at residual saturation, and (3) sorbed contaminant for which mass transfer to the aqueous phase is rate-limited [Kee!y, 1989; Mackay and Cherry, 1989; Haley et al., 1991; Wilson, 1992]. Most existing models which are applied to PAT systems consider advective-dispersive transport with equilibrium sorption, and consequently do not account for contaminant present in the less accessible regions. It is not surprising therefore that most predictions of PAT effectiveness have not been realized in practice, if indeed cleanup targets are ever reached. Depending upon the relative distribution of subsurface contaminants, a number of alternative strategies have been proposed to improve the effectiveness of PAT. For aquifers in which hydraulic conduct...