Mechanisms of Arsenic Desorption from Clay Mineral Surfaces during Managed Aquifer Recharge.

Managed Aquifer Recharge (MAR) is a viable method for both augmenting local groundwater supplies and storing water for later usage. However, the addition of highly purified recharge water can alter native geochemistry and result in the mobilization of trace contaminants including arsenic. To investigate the mechanisms of As desorption during MAR, we conduct a series of batch and column experiments using sediments collected near infiltration basins in Orange County, CA. We use recharge water modified with varying ionic compositions to evaluate the effect of the ionic strength of infiltrating water on As retention and to determine which recharge water chemistry minimizes As release to the surrounding groundwater. Our results reveal that the clay fraction of the recharge basin sediments is dominated by vermiculite, and the clay-water interactions exert control on As retention. Additionally, we find that increased concentrations of Ca and Mg have a significant impact on As retention by forming ion-bridging complexes to the clay mineral surfaces. This work provides a basis for amending recharge water with modifiers (e.g., lime and/or dolimitic lime) as Ca and/or Mg sources to minimize As desorption and prevent the degradation of recharge water quality.