Quantifying water quality changes during managed aquifer recharge in a physically and chemically heterogeneous aquifer

[Truncated abstract] In many parts of the world, depleting water resources and their management are recognised as a fundamental problem. The impact of this problem is enhanced by seasonal as well as long-term imbalances between fresh water supply and demand. Managed aquifer recharge (MAR) is increasingly used to mitigate these imbalances. MAR operations often involve the injection of oxic waters into anoxic media, which will generally trigger a wide range of mineral dissolution/precipitation, ion exchange and complexation reactions that can alter the water quality. However, while the influence of physical heterogeneity on MAR processes is increasingly recognised, little attention has been devoted to the superposed impact of physical and geochemical heterogeneity on water quality. A comprehensive series of experiments, at both laboratory and field scale, were conducted in the context of a pilot aquifer storage and recovery (ASR) implementation in Perth, Western Australia, to develop a quantitative understanding of the coupled physical and hydrogeochemical processes that affect the quality of the recovered water. In the first part of this study a detailed aquifer characterisation was carried using high-resolution sediment sampling. The minerals that were likely to act as reductants for the oxygen introduced by the injection water and participate in the redox chemical reactions during a MAR operation were identified and quantified. These minerals included: pyrite, sedimentary organic matter (SOM), Fe(II)-carbonates and Fe(II)-silicates. The sediment characterisation was used in conjunction with incubation experiments to investigate correlations between reactive and physical parameters of the aquifer material. Subsequently, long-term batch and column experiments were performed to quantify the kinetics of the reactive processes that emerge under MAR conditions.