PROTECTING WATER QUALITY FROM NATURALLY OCCURING CONTAMINANTS DURING MANAGED AQUIFER RECHARGE (Invited Presentation)

While managed aquifer recharge (MAR) can achieve numerous benefits for water supply, artificial recharge of exogenous recharge water can cause adverse impacts to local groundwater quality. Specifically, artificial recharge can perturb ambient geochemical and hydrological conditions of the receiving aquifer, potentially triggering the mobilization of toxic, naturally occurring contaminants from sediments to groundwater. Arsenic mobilization is increasingly reported at MAR sites globally and is a particular concern due to its ubiquity in soils and sediments and toxicity to human and ecosystem health at trace concentrations. Mitigating arsenic mobilization during MAR is complicated due to site specificity of geochemical and hydrogeologic conditions, chemical composition of source water, and variation of MAR techniques employed. We use case studies of arsenic mobilization via both infiltration and injection of advanced recycled wastewater in Orange County, California, United States to elucidate geochemical controls on arsenic mobilization during MAR. We demonstrate ho findings from laboratory experiments, field sampling, and reactive transport can be used to inform strategies for modifying recharge water composition to prevent unfavorable geochemical conditions. Pre-treatment strategies include controlling shifts in pH, redox conditions, and concentrations of competing ions in the receiving aquifer.

Synthesizing case studies of arsenic mobilization from Orange County and other MAR sites globally, we develop general guidance on site selection and implementation of aquifer storage and recovery projects in Texas, United States. Guidance focuses on developing a site-specific understanding in order to improving the geochemical compatibility of recharge water with receiving aquifers, thereby limiting the mobilization of naturally occurring contaminants during MAR. Consideration of geochemical compatibility issues throughout the design and implementation of MAR projects is critical for protecting water quality, particularly for projects intended for drinking water purposes.