Throughout human history, the balance between water demand and supply, i.e., water availability, has varied both spatially and temporally. In this balance, the (perceived) quality of the water present with respect to the requirements for its intended use is of critical importance. Due to increasing weather variability caused by climate change, growing population and increasing urbanization which concentrates agricultural, industrial and drinking water demands, the world is increasingly challenged to provide a sustainable long term supply of fresh water (e.g., [1
]). This challenge is particularly pronounced in geographic areas where fresh water is naturally scarce, such as deserts (e.g., [2
]) and coastal areas (e.g., [5
]) that may further suffer from sea water intrusion by sea level rise, land subsidence and a decrease in seasonal discharge by major rivers (e.g., [7
]). As a consequence, problems such as seasonal water shortage, overexploitation of groundwater resources, saltwater intrusion, and disappearance of wetlands are already commonly occurring phenomena. Considering that economic growth, population increase, and climate change will further aggravate water availability, water crises were identified as a major global risk [10
As managed aquifer recharge (MAR) techniques involve the intentional subsurface recharge and storage of water into an aquifer for subsequent recovery or for environmental benefits, they have the potential to alleviate or even prevent water crises. Important advantages of these techniques are the support of higher water demands [11
], overcoming the temporal mismatch between water supply and demand [13
], water quality improvements, and the protection of water against evaporation losses, algae blooms, and atmospheric fallout of pollutants during subterranean storage [13
]. Although the application of MAR techniques has a history that goes back to pre-modern times (e.g., [7
]), these benefits together with today’s worldwide challenge for the sustainable management of water resources have caused MAR to be increasingly considered as a sustainable, water resource-efficient technique to meet both current and future water demands (e.g., [15
To fulfill the promise of MAR, however, to become globally significant in securing fresh water availability for the future [7
], MAR techniques need to be applied in much larger numbers. Moreover, they also require application at much larger scales, and for a wider range of conditions and purposes. This maturation of MAR systems requires technological and scientific advances in addition to policy and economic innovations [3
]. The water made available by MAR systems aims to be fit-for-purpose, meaning that depending on the intended use (e.g., counteracting salinization, irrigation or drinking water), different requirements may have to be met (e.g., [18
]). Therefore, consideration of water quality is central in developing the full potential for MAR application. This ranges from the improvement or deterioration of water quality during soil passage to operational issues (e.g., well clogging) or sustainability concerns (e.g., the re-use of treated WWTP effluent and stormwater). To date, the rise of anthropogenic contamination and environmental awareness, and analytical developments have led to ever-increasing attention for water quality aspects. With the application of MAR expanding into a wider range of conditions, from deserts to urban and coastal areas, and purposes, from large scale strategic storage of desalinated water to the reuse of waste water, the importance of these considerations are only expected to rise further.
Various water quality issues that contribute to this development, are addressed by the 18 papers in this Special Issue “Water Quality Considerations for Managed Aquifer Recharge Systems” and were presented in part at the 9th International Symposium on Managed Aquifer Recharge (ISMAR-9) in Mexico City, 20–24 June 2016.
MAR techniques are increasingly considered for alleviating wide-spread, current and future water scarcity problems, as they provide robust, effective, sustainable, and cost-efficient freshwater management solutions. MAR is therefore expected to be increasingly applied in a wider range of conditions and settings, from drinking water production and ecological support to fulfilling industrial and agricultural water demands, as well as urban water demands for the creation of water resilient green cities. As illustrated by the papers in this Special Issue, these developments will lead to new water quality challenges and increasingly complex scientific questions. These relate to new source water types, the application of MAR in previously considered less favorable conditions such as brackish/saline groundwater environments, and the application at larger, regional scales as well as the development of simulation tools to optimize the design and operation of MAR facilities. Addressing these aspects appropriately will contribute to a greater understanding, operational reliability and acceptance of MAR applications, and lead to a range of successfully engineered MAR systems that help increase their effectiveness to help secure water availability for the future.