Reactive transport modeling of a multiple partially penetrating well (MPPW) system in a heterogeneous, brackish aquifer: Implications for coastal aquifer storage and recovery (ASR)

Abstract

The reactive transport impacts of a multiple partially-penetrating well (MPPW-)ASR system in a brackish, heterogeneous coastal aquifer are complex due to geochemical heterogeneities, cation-exchange, mineral dissolution, and redox processes, as well as complex fluid flow induced by density-differences between the injection water and ambient water. To understand the impact of relevant processes during MPPW-ASR, a reactive transport model was setup based on geochemical observations to evaluate the long-term impacts on the injected freshwater and the surrounding brackish water, which is diluted partially by the freshwater. It was found that the major impacts on the injection water during the initial cycles of MPPW-ASR were cation-exchange, Fe-Mn-carbonate dissolution, pyrite oxidation, and MnO2 reduction by Fe2+, whereas in later cycles the water composition was impacted by pyrite oxidation, trace metal sorption, oxidation of Fe2+ and Mn2+, and the reduction of MnO2 by Fe2+. The surrounding brackish water is diluted by the injected freshwater and forms a water type which is depleted in Na+ and enriched in HCO3- in the initial cycles but becomes enriched in Na+ and Cl- in later cycles. This water type is then transported away from the ASR-well, as well as towards the shallower intervals of the aquifer, maintaining its composition. The results of this project indicate that redox processes still affect the composition of the injected freshwater and the surrounding brackish water around the freshwater bubble is an undesirable water-type that could potentially harm the quality of the injection water.