Mechanistic Determination of Arsenic Remobilization at ASR Sites and Corresponding Adaptation Techniques

Climate change and socioeconomic developments have resulted in an ever increasing imbalance between the water supply and demand in many parts of the world, including the southwest U.S. To secure sustainable water resources, aquifer storage and recovery (ASR) techniques are investigated as a part of the climate change adaptation tools in the U.S. EPA Water Resources Adaptation Program (WRAP). This investigation is centered on arsenic dissolution and re-mobilization from arsenic-bearing pyrites during the ASR process. Various sizes of ASR applications are reported throughout the U.S. with the majority being located in the Southwest, Florida, and the Midwest. ASR wells are subject to the Underground Injection Control (UIC) Program's Class V regulations under the Safe Drinking Water Act. In addition to concerns of macronutrient and biological contaminant impacts, there are reports on the mobilization of metals from the aquifer matrix of these sites. Arsenic is one of the commonly observed metal contaminants in groundwater, potentially resulting from oxidation and dissolution of arsenic-bearing pyrite. A series of experimental studies are underway to quantify the mechanisms of arsenic dissolution from arsenopyrite host and the subsequent arsenic transport in an ASR application. In controlled bench-scale experiments, the arsenic-bearing pyrite dissolution rate is determined with the use of in-situ atomic force microscopy (AFM). AFM provides quantitative measurements of arsenopyrite morphology changes during the dissolution process. Separately, two ASR simulation soil columns measured at 6.1-m in height and 45.7-cm in diameter are used to simulate the physiochemical changes in aquifer materials and to quantify the arsenic dissolution-mobilization-transportation process. The two columns of identical experimental set-ups allow comparative studies of arsenic fate and transport in natural groundwater flow and in ASR operation using reclaimed wastewater after chlorination. In this paper, we will describe the preliminary experimental results and discuss the implications for ASR operations and the knowledge gaps for further studies.