The last 60 years has seen unprecedented groundwater extraction and overdraft as well as development ofnew technologies for water treatment that together drive the advance in intentional groundwater replenishment known as managed aquifer recharge (MAR). This paper is the first known attempt to quantify the volume ofMAR at global scale, and to illustrate the advancement of all the major types ofMAR and relate these to research and regulatory advancements. Faced with changing climate and rising intensity ofclimate extremes, MAR is an increasingly important water management strategy, alongside demand management, to maintain, enhance and secure stressed groundwater systems and to protect and improve water quality. During this time, scientific research—on hydraulic design offacilities, tracer studies, managing clogging, recovery efficiency and water quality changes in aquifers—has underpinned practical improvements in MAR and has had broader benefits in hydrogeology. Recharge wells have greatly accelerated recharge, particularly in urban areas and for mine water management. In recent years, research into governance, operating practices, reliability, economics, risk assessment and public acceptance ofMAR has been undertaken. Since the 1960s, implementation of MAR has accelerated at a rate of 5%/year, but is not keeping pace with increasing groundwater extraction. Currently, MAR has reached an estimated 10 km3/year, ~2.4% of groundwater extraction in countries reporting MAR (or ~1.0% of global groundwater extraction). MAR is likely to exceed 10% of global extraction, based on experience where MAR is more advanced, to sustain quantity, reliability and quality ofwater supplies.

A modelling study was carried out to provide a process-based quantitative interpretation of the biogeochemical changes that were observed during an ASR experiment in which reclaimed water was injected into a limestone aquifer at a field-site near Bolivar, South Australia. A site-specific conceptual model for the interacting hydrodynamic and biogeochemical processes that occur during reclaimed water ASR was developed and incorporated into an existing reactive multi-component transport model. The major reactive processes considered in the model were microbially mediated redox reactions, driven by the mineralisation of organic carbon, mineral precipitation/ dissolution and ion exchange. The study showed that the geochemical changes observed in the vicinity of the ASR well could only be adequately described by a model that explicitly considers microbial growth and decay processes, while an alternative, simpler model formulation based on the assumption of steady state biomass concentration failed to reproduce the observed hydrochemical changes. However, both, the simpler and the more complex model approach were able to reproduce the geochemical changes further away from the injection/extraction well. These changes were interpretated as a result of the combined effect of ion exchange, calcite dissolution and mineralisation of dissolved organic carbon.

Managed aquifer recharge is an increasingly popular technique to secure and enhance water supplies. Among a range of recharging techniques, single-well aquifer storage and recovery (ASR) is becoming a common option to either augment drinking water supplies or facilitate reuse of reclaimed water. For the present study a conceptual biogeochemical model for reclaimed water ASR was developed and incorporated into an existing reactive multicomponent transport model. The conceptual and numerical model for carbon cycling includes various forms of organic and inorganic carbon and several reactive processes that transfer carbon within and across different phases. The major geochemical processes considered in the model were microbially mediated redox reactions, driven by the mineralization of organic carbon, mineral dissolution/ precipitation, and ion exchange. The numerical model was tested and applied for the analysis of observed data collected during an ASR field experiment at Bolivar, South Australia. The model simulation of this experiment provides a consistent interpretation of the observed hydrochemical changes. The results suggest that during the storage phase, dynamic changes in bacterial mass have a significant influence on the local geochemistry in the vicinity of the injection/extraction well. Farther away from the injection/extraction well, breakthrough of cations is shown to be strongly affected by exchange reactions and, in the case of calcium, by calcite dissolution.