THE RELATIONSHIP BETWEEN PYRITE STABILITY AND ARSENIC MOBILITY DURING AQUIFER STORAGE AND RECOVERY IN SOUTHWEST CENTRAL FLORIDA

Elevated levels of arsenic are common in water recovered from aquifer storage and recovery (ASR) systems in southwest-central Florida that store oxygenated surface water. Mineralogical investigations of the Suwannee Limestone of the Upper Floridan aquifer, the preferred storage zone for ASR systems, have shown that the highest concentrations of arsenic are associated with the occurrence of arsenic-rich pyrite in zones of high moldic porosity. This investigation employed geochemical modeling to examine the stability of pyrite in limestone during simulated injections of oxygenated surface water. Nineteen wells with open-hole intervals encompassing only the Suwannee Limestone with known mineralogy and water chemistry were included in the investigation. Injections were simulated for a subset of these wells that had representative water chemistry. The goal was to determine if aquifer redox conditions could be altered to the degree of pyrite instability. Increasing amounts of injection water were added to the storage zone water in a series of steps and the resulting reaction paths were plotted on pyrite stability diagrams. The unmixed storage zone water in the wells plotted within the pyrite stability field indicating that redox conditions were sufficiently reducing to allow for pyrite stability. Thus arsenic is immobilized in pyrite and its concentration in ground water should be low. This was corroborated by actual analysis of arsenic in water samples; none of the 19 wells sampled had concentrations above 0.036 µg/L. During simulation, however, as the ratio of injection water to storage zone water increased, redox conditions became less reducing and pyrite became unstable. The result would be the release of arsenic from the aquifer matrix into the storage zone water.