ARSENIC ATTENUATION AND RELEASE DURING SUCCESSIVE AQUIFER STORAGE RECOVERY (ASR) CYCLE TESTS IN THE UPPER FLORIDAN AQUIFER

Aquifer storage recovery (ASR) involves successive recharge, storage, and recovery of treated surface water in subsurface permeable zones. In south Florida, ASR is used by utilities to augment drinking water supplies during dry periods, and also as a mechanism to store and recover large volumes of water for the Comprehensive Everglades Restoration Plan. Arsenic mobilization due to pyrite oxidation and subsequent redox reactions in the aquifer represent a challenge for extensive implementation of ASR in some permeable lithologies that include the Upper Floridan Aquifer. We use geochemical modeling methods to test the following hypotheses: 1) redox evolution from oxic to sulfate-reducing conditions is the overall condition that defines the pattern of arsenic mobilization, sorption, and release that is observed during ASR cycle tests; and 2) that an almost stoichiometric decline in recovered water arsenic concentrations will result during successive ASR cycles where oxic treated water is recharged (arsenic attenuation).

The Olga ASR system stores treated surface water for Fort Myers, FL. The system consists of one recharge/recovery well (1 million gallons per day) and two monitor wells located ca. 350 ft. away. Water is stored in permeable zones of the Suwannee Limestone (850-920 ft bls), which includes the upper part of the Floridan Aquifer System. During recharge, pyrite oxidation releases arsenic into this drinking water source, although arsenic concentrations at monitor wells typically remains less than 5 ppb throughout each cycle. During recovery (typically 30-75 percent of total volume recharged), arsenic concentration increases only at the ASR well, often exceeding the new 10 ppb drinking water maximum contaminant level. We infer that the process of arsenic transport occurs during recovery due to reductive dissolution of iron oxyhydroxides with subsequent release of sorbed arsenic, all occurring under sulfate-reducing conditions. Recent cycle tests that show long (70 percent) recovery periods do show declining maximum arsenic concentrations. This pattern suggests that the arsenic source is being depleted from aquifer material by successive oxic cycles.