Importance of Hydrogeochemical Bench Studies for Assessment of Potential Water-Quality Issues Related to Managed Underground Storage

Managed underground storage (MUS) activities affect physical and chemical characteristics of the hydrogeologic system. Changes in redox conditions, for example, may yield water-rock interactions that degrade native groundwater and recovered-water quality. The changes are highly dependant on the MUS project scale, operating conditions, physiochemical and microbiological aspects of all waters involved (recharge, native groundwater and mixtures thereof) and hydrogeologic properties of the aquifer, to name a few.

Aquifer storage and recovery (ASR) is a specific type of MUS that often involves highly variable redox conditions in the aquifer system. To elucidate processes that may yield potential water-quality changes, lithogeochemical characterization of the aquifer media, hydrochemical characterization of native and recharge water and bench-scale studies are important. These studies have been ongoing in Florida focusing on several municipal ASR facilities and the Comprehensive Everglades Restoration Program to address an issue of critical concern: potential mobilization of arsenic from the Floridan aquifer system matrix during ASR activities.

Trace-mineral analyses have identified first-order sources of mobilized metals and metalloids during these activities. In the case of arsenic, which is often mobile during ASR cycle tests, microcrystalline pyrite is the principal source. Batch reactor studies under variable redox conditions help identify hydrogeochemical processes extending beyond the initial arsenic mobilization. For example, it is now widely believed that arsenic is released via pyrite oxidation during oxic recharge conditions and is rapidly sorbed by hydrous ferric oxyhydroxides (HFO), which become a second-order source of arsenic. Upon recovery of water during ASR and reversal of redox conditions, reductive desorption or dissolution of arsenic from the HFOs is hypothesized. Experimentation employing batch reactors appear to confirm the hypothesis in laboratory conditions and may further reflect relative mobilization of numerous constituents (molybdenum, uranium, etc) during ASR. Assessment of bench-scale studies as a predictive tool is pending field-scale ASR testing.