Role of molecular diffusion in contaminant migration and recovery in an alluvial aquifer system

Highly-resolved simulations and flow and transport in an alluvial system at the Lawrence Livermore National Laboratory (LLNL) site explore the role of diffusion in the migration and recovery of a conservative solute. Heteroge neity is resolved to the hydrofacies scale with a discretization of 10.0, 5 .0 and 0.5 m in the strike, dip and vertical directions of the alluvial-fan system. Transport simulations rely on recently developed random-walk techn iques that accurately account for local dispersion processes at interfaces between materials with contrasting hydraulic and transport properties. Solu te migration and recovery by pump and treat are shown to be highly sensitiv e to magnitude of effective diffusion coefficient. Further, transport appea rs significantly more sensitive to the diffusion coefficient than to local- scale dispersion processes represented by a dispersivity coefficient. Predi cted hold back of solute mass near source locations during ambient migratio n and pump-and-treat remediation is consistent with observations at LLNL, a nd reminiscent of observations at the MADE site of Columbus Air Force Base, Mississippi. Results confirm the important role of diffusion in low-conduc tivity materials and, consequently, its impact on efficacy of pump-and-trea t and other remedial technologies. In a typical alluvial system on a decada l time scale this process is, in part, fundamentally nonreversible because the average thickness of low-K hydrofacies is considerably greater than the mean-square length of penetration of the solute plume.