Optimization of groundwater remediation strategies in aquifers affected by slow desorption processes
Most of the major groundwater contamination in California, including that inundating the San Fernando, San Gabriel and San Bernadino Valleys, will be addressed using some variation of the pump-and-treat technology. The pump-and-treat strategy is often judged to be unsuccessful because of difficulties encountered in recovering the contaminants from relatively stagnant zones within stratified aquifer systems. These zones can exist at the particle scale, as intraparticle or intra-aggregate porosity, and at the larger scales, as low permeability layers or lenses interspersed in substantially more permeable layers. This work focuses first on achieving an efficient numerical solution to a system of groundwater flow and contaminant transport equations that sufficiently captures the dynamics of slow desorption in a two-dimensional porous medium. The upstreamweighted, multiple cell balance (UMCB) method is developed and verified here to provide such a solution. Next, this work focuses on coupling the simulation model with a management model to provide a design tool for pump-and-treat remediation of real aquifer systme. Zeroth, first and second moments are calculated for mobile and immobile aqueous concentration distributions, and tested as potential design objectives. In a departure from conventional approaches, spatial moment analysis is also applied to local differences between simulated mobile and immobile aqueous concentrations.
Results suggest that pump-and-treat systems in heterogeneous domains might best be designed as two phase operation. The first phase addresses the early time removal of mobile (i.e., readily accessible) phase contaminant, and suggests the conventional approach of placing extraction wells slightly downgradient of the plume centroid. The second attacks fractions of the contaminant plume that are either harbored within immobile porosity, or that have penetrated impermeable layers. The latter stage can be accomplished through maximizing the desorption driving force distribution, or be minimizing the spreading (variance) of this distribution. It is recommended that the techniques developed in this research be applied to one or more of California's on-going pump-and-treat systems.