Implications of Rate - Limited Mass Transfer for Aquifer Storage and Recovery Efficiency

I grant The Pennsylvania State University the nonexclusive right to use this work for the University's own purposes and to make single copies of the work available to the public on a not-for-profit basis if copies are not otherwise available. ABSTRACT Pressure to decrease reliance on surface-water storage has led to increased interest in aquifer storage and recovery (ASR) systems, where boreholes are used to inject potable water into aquifers and subsequently recover the stored water during times of peak demand, seasonal deficit, or extended drought. The viability of an ASR scheme is usually measured by recovery efficiency, which is defined as the ratio of recovered water that meets a predefined standard to total volume of injected fluid. Recovery efficiency can be enhanced or degraded by a number of physical and geochemical processes. At ASR sites, rate-limited mass-transfer (RLMT), which describes the exchange of solutes between mobile and immobile porosity domains, may control transport behavior that cannot be explained strictly by advection and dispersion processes. RLMT at ASR sites, in addition to dispersive mixing, can have a significant impact on the use and viability of the ASR scheme due to its impact on recovery efficiency. Despite the apparent utility of ASR as an emerging technology, few studies have utilized numerical models in conjunction with field observations of flow and transport, or considered the impact of RLMT processes on ASR systems. We present data from a pilot-scale ASR study in Charleston, SC and develop a three-dimensional (3D) finite-difference model to evaluate the impact of flow and transport processes, including different mechanisms of mass transfer, on ASR efficiency. We present evidence that RLMT processes cause a rebound in salinity during freshwater storage in a brackish aquifer. Single ASR-cycle model results demonstrate maximum efficiencies near 60%, and are degraded by large immobile pore volumes and high mass transfer rates between mobile and immobile domains. iv Multi-cycle results show low efficiencies due to RLMT degrading water quality during storage over 1-3 ASR cycles; however, efficiencies can evolve and improve markedly over ten cycles, exceeding efficiencies generated by advection-dispersion only models. For an idealized ASR model where RLMT is active, our simulations show a discrete range of diffusive length scales over which storage of freshwater in brackish aquifers may not be viable.