The last 60 years has seen unprecedented groundwater extraction and overdraft as well as development of new 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 of MAR at global scale, and to illustrate the advancement of all the major types of MAR and relate these to research and regulatory advancements. Faced with changing climate and rising intensity of climate 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 of facilities, 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 of MAR 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 km 3 /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 of water supplies. Keywords Managed aquifer recharge. Artificial recharge. Review. Water banking. History of hydrogeology This article is one of a series developed by the International Association of Hydrogeologists (IAH) Commission on Managing Aquifer Recharge

Bank filtration, either natural or induced through the river bed by pumping from a system of connected lateral or vertical wells, provides a means of obtaining public water supplies. The success of such schemes is dependent on the microbial activity and chemical transformations that are commonly enhanced in the colmation layer within the river bed compared to those that take place in surface or ground waters. The actual biogeochemical interactions that sustain the quality of the pumped bank filtrate depend on numerous factors including aquifer mineralogy, shape of the aquifer, oxygen and nitrate concentrations in the surface water, types of organic matter in the surface and ground water environments, and land use in the local catchment area. This paper provides an introduction to a series of nine papers contained in this Special Issue that highlight these factors and finishes with a list of recommendations for co-ordinated research into attenuation of groundwater pollution by bank filtration. q

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Riverbank filtration (RBF) is a process in which pumping wells located along riverbanks induce a portion of the river water to flow toward the wells. During RBF, which has many similarities to slow‐sand filtration, river water contaminants are attenuated from a combination of processes such as filtration, microbial degradation, sorption to sediments and aquifer sand, and dilution with background groundwater. RBF systems have been operating in Europe since the 1870s. In the United States, there has been renewed interest among large‐ to medium‐sized utilities to use it as a mechanism of water production to reduce treatment costs and to meet regulations on pathogens, disinfection byproducts, and other contaminants under the Surface Water Treatment Rule. Although filtrate water quality from RBF systems can vary based on river conditions, it is possible that appropriately designed systems can serve as pretreatment for drinking water, and at the same time, the utility can receive log‐removal credits for pathogens and particles. In addition, RBF can be used as a pretreatment for membrane filtration. In the United States, a knowledge gap exists on the benefits and limitations of using RBF. This paper addresses those gaps, outlining the benefits and limitations of the process. It also illustrates that RBF can be a viable alternative to surface water at suitable sites.

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