Evaluation of conceptual and numerical models for arsenic mobilisation during managed aquifer recharge

Ilka Wallis, CSIRO Land and Water, Floreat, WA, and Flinders University, Adelaide, South Australia.
Coauthors: Henning Prommer, Craig Simmons, Pieter Stuyfzand, Vincent Post, Thomas Pichler.

ABSTRACT

Managed aquifer recharge (MAR) is widely seen as a promising technique to meet growing water demands. An impediment to MAR applications, where oxygenated water is recharged into an anoxic aquifer, can be the mobilization of trace metals and metalloids including arsenic (As), which potentially can lead to elevated metal(loid) concentrations in the recovered water. Well-documented examples of As mobilization exist for MAR schemes operating in west-central and southwest Florida. Other studies have documented As mobilization during artificial recharge e.g. in the Netherlands, Denmark and Australia. 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 examples of arsenic mobilization, during 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. In the models that best reproduced field observations, the release of arsenic was modeled by oxidative dissolution of As-pyrite during the recharge phase of the MAR operations. Subsequent adsorption of arsenic to neo-precipitated amorphous iron oxides controlled aqueous concentrations. A return to reducing conditions during recovery phases of ASR operations induced reductive dissolution of iron oxides with renewed release of sorbed As leading to elevated aqueous concentrations in the recovered water.