UNDERSTANDING AQUIFER STORAGE AND RECOVERY EFFICIENCY IN A CLASTIC-LIMESTONE AQUIFER, CHARLESTON, SOUTH CAROLINA

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 to subsequently recover the stored water during times of peak demand or extended drought. Previous studies have found that injected water is subject to chemical change through a variety of processes. 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. Few ASR studies, however, have utilized numerical models in conjunction with field observations of flow and transport, or considered the impact of heterogeneity on system efficiencies. In this work, we use data from a pilot-scale ASR project from Charleston, SC to develop a three-dimensional finite-difference model, calibrated to field data, to evaluate the impact of flow and transport processes, including different mechanisms of mass transfer, on efficiency. Our results show evidence for local heterogeneity and dual-domain porosity effects impacting flow and solute transport in the aquifer fracture zones. Single cycle results do not demonstrate efficiencies of or near 100% as reported in previous ASR simulations. Ongoing results of this study will have implications for decisions regarding pumping and recovered water quality in the ASR system, and will allow us to make suggestions for improving recovery efficiency and utility of the system. This study is working toward the development of a comprehensive understanding of factors controlling efficiency in ASR systems using simulated results, through the consideration of a range of geologic heterogeneity, injection and pumping cycles, and flow and transport parameters.