Coupled Effects of Density and Dual-Domain Mass Transfer on Aquifer Storage and Recovery

Aquifer storage and recovery (ASR) involves the injection of freshwater into an aquifer for temporary water storage and later use. Previous studies have shown that density-driven convection may render part of the injectant unrecoverable when ASR is implemented in a saline aquifer. Where heterogeneity creates a significant proportion of immobile groundwater, dual-domain (immobile and mobile porosities) effects also lead to reductions in the efficiency of ASR schemes. Presuming constant total porosity (sum of the mobile and immobile porosities), the combined effects of dual-domain properties and water density on ASR are investigated for the first time in the present study, using numerical modeling of otherwise similar conceptual models to previous ASR analyses. Results show that the interactions between the density effect and dual-domain mass transfer lead to complex controlling patterns of the density contrast, the ratio of immobile-mobile porosities, the parameterization of immobile-mobile solute exchange, and the number of cycles. The influence of these factors on mixing processes and the recovery efficiency (proportion of freshwater injectant recovered by extraction) during ASR are shown. Dual-domain effects, where they occur in aquifers, must be included explicitly in models to reproduce recovery efficiencies, especially during early cycles, unless the immobile porosity is very small or the conductance between mobile and immobile regions is at extremes. Otherwise, a single-domain model can be utilized. The findings of this research illustrate the influence of both density effects and dual-domain processes on ASR and the need to account for both in assessing ASR feasibility in complex hydrogeological settings.