Arsenic control during aquifer storage and recovery cycle tests in the Floridan aquifer

More than 1.6 billion gallons of fresh water are lost to tide each day in Florida, while there are ecosystem and drinking water interests that could benefit from the retention of those flows. The Comprehensive Everglades Restoration Plan includes many components intended to improve flow and ecosystem functions of the Everglades, including the enhancement of drinking water supplies. One major category of interventions is increasing storage of wet season flows to supplement dry season water supplies. Aquifer storage and recovery (ASR), also known as aquifer recharge, is a large source of potential storage in Florida. However, many of the specific groundwater chemistry concerns have not yet been tested in the field.  Arsenic, a toxic metal, is one of the elements of greatest concern.
 
            A team of researchers at the US Army Corps of Engineers, Jacksonville District undertook a rigorous testing program to determine if surface water from the Kissimmee River could be stored in the Upper Floridan Aquifer, and meet arsenic and other water quality standards when withdrawn. The scientists hypothesized that though the introduction of surface water would mobilize arsenic and violate quality standards, this effect would only be temporary. This is because surface water contains much more dissolved oxygen than groundwater, and the interaction of oxygen and the pyrite minerals in the aquifer releases arsenic. As the oxygen disappears, the arsenic should return to its original state. In chemistry terms, this is an alteration in the reduction-oxidation state followed by a return to the original reduction-oxidation state.
 
            Three tests of increasing length (three months, eight months, and 17 months) were conducted to monitor the arsenic concentration of water recharged in the aquifer, its chemical behavior, and the quality of the water upon its returned to the surface. In addition, the amount of water successfully recovered compared to how much was sent into the aquifer is important for any potential drinking water storage. The testing was conducted through several wells situated around the recharge site, and scientists measured arsenic, oxygen, and a host of other water quality parameters. They found that upon introducing surface water into the aquifer, arsenic levels would spike as long as new water was being pumped, but as the recharge periods ended, arsenic levels quickly fell below the 10 μg/L regulatory maximum for drinking water. Essentially, the aquifer quickly returned to its native condition and the surface water had no lasting effect on the chemical composition.
           
            The implications of this study of recharge water-induced arsenic in the aquifer are positive. A major geochemical concern of ASR in South Florida should be moderated by the data showing that increased arsenic levels are not a permanent alteration of the groundwater geochemistry, but rather isolated to recharge periods. Water managers previously skeptical of the ability of ASR water to meet water quality standards for arsenic and other minerals should be more comfortable with the idea of subsurface storage as a serious option. The widespread effects of ASR on hydrogeologic conditions are still under investigation, but localized or smaller scale systems appear to have the scientific green light. With changes in climate expected to bring more variability to rainfall and to surface flows, storage options largely insulated from contamination, evapotranspiration, and other compromising conditions will be in greater demand.