Evaluation of Conceptual and Numerical Model for Arsenic Mobilisation During Managed Aquifer Recharge

Managed aquifer recharge (MAR) is widely seen as a promising technique to meet growing water demands. It involves the injection of, for example, treated or reclaimed water into permeable formations for later withdrawal. An impediment for all applications, where oxygenated water is recharged into anoxic aquifers, can be the mobilization of trace metals, including arsenic. While conceptual models for the fate of arsenic under such circumstances exist, they are generally not rigorously tested through translation into numerical modeling approaches and subsequent application to field data sets.
In this study, we use well-documented examples of arsenic mobilization, i.e., a deepwell injection experiment in the Netherlands and ASR operations in west-central and southwest Florida for model development and evaluation. In all considered cases arsenic mobilization is induced during injection of oxygenated water into anoxic aquifers. 

 Several conceptual models of arsenic mobilization were evaluated through field-scale reactive transport modeling. Initially observed chloride data were used to calibrate the groundwater flow and nonreactive transport behaviour in the MAR systems before subsequently the impact of reactive processes was quantified. The calibrated reactive transport models were then able to provide a detailed description of the spatial and temporal hydrochemical changes that occurred in the investigated MAR operations. Pyrite oxidation and the formation and dissolution of amorphous iron-oxides were shown to be the key chemical processes for water quality changes, which in turn controlled the observed fate of arsenic during the experiments.