Aquitard contaminant storage and flux resulting from dense nonaqueous phase liquid source zone dissolution and remediation

A one-dimensional diffusion model was used to investigate the effects of dense nonaqueous phase liquid (DNAPL) source zone dissolution and remediation on the storage and release of contaminants from aquitards. Source zone dissolution was represented by a power law source depletion model, which served as a time variable boundary condition to the diffusion equation used to describe mass transport in the aquitard. Two key variables were used to assess source zone dissolution behavior on aquitard mass storage and release: the power law exponential term G, which reflects the influence of the source zone architecture, and a new variable defined herein as the source to aquitard mass transfer coefficient beta, which reflects the influences of both the source characteristics and the aquitard media properties. As G increased or as beta increased because of more rapid source dissolution, the aquitard concentrations, depth of penetration, and long-term back-diffusion flux decreased. However, when beta increased because of increased sorption, concentrations and back diffusion increased but penetration decreased. The duration of aquitard mass loading was found to be significantly less than the duration of mass release. Moreover, the mass per unit area stored in the aquitard was 3 or more orders of magnitude less than the initial DNAPL source zone mass per unit area, and the back diffusion flux from the aquitard was typically 4 or more orders of magnitude less than the initial source zone flux. Additionally, the effects of partial source zone remediation were investigated, and the results suggest that source remediation can have a favorable effect on long-term back-diffusion risk.