Minimizing Arsenic Mobilization During Aquifer Storage and Recovery by Source Water Degasification

The release of arsenic from native pyrite has become a critical issue in implementing Aquifer Storage and Recovery (ASR) as a water supply technology. A Degasification (Degas) Test has been completed at the Bradenton Potable ASR facility to confirm if the removal of oxidizers, primarily Dissolved Oxygen (DO) and chloramines, from ASR source water can inhibit the mobilization of arsenic. The Degas system uses hydrophobic membranes under vacuum to achieve greater than 99.93% removal of DO from oxygen saturated source water. Dechlorimination, up to 90%, is achieved via addition of sodium bisulfite. A previous Cycle Test included the injection of 160 million gallons (MG) of DO saturated (8.51 mg/L) and chloriminated (3.8 mg/L) source water resulted in a peak arsenic concentration of 75 µg/L, with the peak concentration occurring at 71% recovery of the stored water. A subsequent full-scale Cycle Test implementing Degas included the recharge of 160 MG of low-DO (6 µg/L) and low-chloramine (0.45 mg/L) source water resulted in a 50-fold reduction in the peak arsenic concentration, with the peak arsenic concentration of 1.48 µg/L occurring at 79% recovery. The pretreatment technique resulted in a 94% reduction of the total arsenic released at the Bradenton ASR site. These results indicate that the removal of oxidizers (i.e., DO and chloramine) from ASR source water can reduce the level of arsenic released from the aquifer matrix into solution to below the 10 µg/L Maximum Contaminant Level (MCL).