Climate change and the expansion of urban areas worldwide is a major threat for sustainable and safe drinking water supplies. Managed Aquifer Recharge (MAR) is generally viewed as a powerful tool for replenishing depleted aquifers and for restoring ecological services of rivers with limited consumption of energy and chemicals[1]. Natural MAR systems based on water filtration during soil passage have proven to retain suspended particles and colloids, including microorganisms, and to favor biodegradation of contaminants, resulting in significant water quality improvement. However, periodic detection of pathogens in groundwater (GW), some with severe human health impacts[2], has led to strict quality requirements that effectively impede the use of low-quality water for MAR. This is paradoxical because potable water treatment during the XIX century simply consisted of sand filtering to remove pathogens and caused a life expectancy increase of some 20 years[3]. This paradox is well reflected on the on-going debate about quality requirements for GW recharge. Health protection authorities recommend strict controls on the water used for MAR, while several major cities experience that recharge using wastewater can be safe. As a result, the JRC[4] failed to reach a consensus on MAR water quality recommendations. The situation is unsatisfactory. Prudence demands regulations, while fear hinders actual implementation of MAR. New effective treatment strategies based on MAR can be the solution. The objective of MARadentro is to address the risk issue and provide recommendations on how to accomplish efficient MAR to guarantee human health, ecosystems protection and public acceptance on water reuse while increasing the fresh water resources.

Population increase is causing a severe shortage in water reserves, leading to aquifers overexploitation. The problem is especially severe in arid or semi-arid areas, where climate change is causing a reduction in rainfall. Lowering aquifer heads causes degradation of ecosystem functioning. For example, the loss of base flow in rivers implies the loss of hyporheic flow exchange, and thus biodiversity and its contaminants degradation services, causing rivers to become canals. Moreover, overexploitation impacts society and human well-fare as it directly affects food and fiber production, as well as prosperous development of urban areas. Reversing this negative trend requires the incorporation of low economic/energetic/water footprint technologies that allow the reuse/re-naturalization of water to increase the resilience and the adaptive capacity facing climate change.

In this context, MAR is an excellent option not only because it increases available water resources, thus increasing water use efficiency, but also because by recovering water levels it contributes to maintain wetlands and hyporheic flow in rivers. MAR may also be used to prevent seawater intrusion, thus helping in the protection of coastal ecosystems.

Despite its advantages, the implementation of MAR is still under debate. It was generally believed that pathogens did not move through porous media and that natural degradation processes removed most contaminants. However, outbreaks have shown that pathogens may reach pumping wells. Waterborne pathogens are diverse in size, infection dose, and survival and transport capacities in soils. Therefore, the behavior of different pathogens needs to be examined to reduce health risks in any water reclamation process5. However, a better understanding of factors influencing transport and fate of microorganisms is urgently needed to assess the risk of microbial contamination of aquifers, and to develop control strategies and treatment approaches. 

Anthropogenic pollutants have increased dramatically in all water resources during the last five decades and forced many countries to impose strict water quality requirements on the source water for MAR. In practice, these requirements are so strict that even rainfall does not qualify due to high levels of suspended solids and low pH. The debate caused the JRC4 to abstain from producing recommendations for MAR, as they would have made illegal numerous systems that have worked properly in several European countries for more than 50 years.

Moreover, growing pressures on water resources appear continuously, which makes it difficult for water engineers to address them in the framework of an integrated approach to manage the water cycle and to support circular economy. An attractive alternative is to use MAR as a tertiary treatment to ensure pollutants and pathogens removal by adding a reactive layer. By increasing the reactive surface, allowing diverse microbial communities to develop and creating a range of redox states, reactive layers have proven efficient for contaminants degradation. However, experience is limited regarding the optimal design for efficient functionality of these layers.

In addition to the technical aspects not yet resolved on MAR, the great challenge is the “human factor” on water reuse, which must be considered to achieve a positive response from the public leading to the smooth implementation of MAR.

Entidad Financiadora: MICINN

Referencia: PCI2019-1034254

Total financiado: 100.000€

Duracion: 17/05/2019-16/05/2022

Investigator principal: Maarten Saaltink.