Arsenic Mobilization Occurring at Aquifer Storage and Recovery Sites in the Southwest Florida Water Mangement District

Over the last ten years, the mobilization of naturally occurring arsenic in the Floridan aquifer during Aquifer Storage and Recovery operations has become the major issue affecting the success of this technology in Florida. This issue became especially critical when the Environmental Protection Agency (EPA) lowered the drinking water standard (DWS) for arsenic in 2005, from 50 ug/l to 10 ug/l. Most sites could operate within the old arsenic DWS but the new standard is proving to be elusive. There are approximately 25 ASR sites in the District and all that have tested for arsenic, approximately 10 sites, have found it at concentrations above the new DWS. Removal of arsenic occurs prior to distribution to the public and thus no arsenic above the DWS has reached the public. The issue is the presence of mobilized arsenic in the aquifer above the DWS. Extensive monitoring well networks at two facilities have demonstrated that the extent of arsenic is typically limited to less than 200 feet around each well and the extent shrinks and the concentration attenuates with successive cycles. Research has identified the probable source of arsenic and the geochemical process resulting in mobilization. Options to minimize arsenic mobilization through pre-treatment of the injection water are being explored. The primary focus of the pre-treatment effort is to remove dissolved oxygen from the injection water to lower the oxidation-reduction potential. Early results from the degasification pilot test cycle tests will be presented and discussed. Finding a solution to the arsenic mobilization issue is critical for the continued implementation of highly beneficial ASR projects in Florida. Data collected to date suggests that a zone of conditioning of several hundred feet would be adequate to allow for safe operation of ASR facilities in Florida.