Solving the ASR Arsenic Problem: Pilot-Tested Approach Shows Promising Results

Aquifer storage and recovery (ASR) has become a major component of water resource management initiatives worldwide. Recent challenges regarding implementation of ASR have discouraged utilities from developing this water management tool. Operational data for existing ASR systems and geochemical modeling have shown that injecting potable water with higher dissolved oxygen (DO) levels and oxidation-reduction potential (ORP) than native groundwater may cause naturally occurring minerals in the aquifer matrix, i.e. arsenopyrite, to be oxidized, releasing arsenic into the stored water.  CDM and the St. Johns River Water Management District, in cooperation with Sanford, Florida, have been investigating the use of ASR. Since 2003, the project has progressed through a desktop study, exploratory well program, regulatory permitting, and system design and construction. Before initiating cycle testing, CDM designed a pretreatment system to minimize the potential for arsenic leaching. The goal of pretreatment is to produce recharge water quality similar to the storage zone native groundwater through chlorine residual (Cl2) removal and DO reduction.The pretreatment system includes the Membrana Liqui-Cel^® degasification system, which reduces injected water DO levels. The degasification system requires removal of Cl2 to protect the membranes which also contributes to a decreased ORP, an added benefit. Implementation of the pretreatment system was initiated in two phases. Phase I began in April 2009, with Cycles Test 1 and 2, which instituted the dechlorination only with sodium bisulfite. Data collected during the injection periods show significant decreases in ORP in the potable water after dechlorination, Cycle testing will continue with both pretreatment systems in operation for Cycles 3 and 4. This presentation will present the cycle testing results and the effects of pretreatment on arsenic leaching. This project will enable the city to control future water resources through demand management, while maintaining water quality integrity by eliminating environmental challenges of open reservoirs.