Managed Aquifer Recharge (MAR) is a promising technique for water management. It comprises a group of technologies that enhance the infiltration of various water sources into aquifers. The water stored underground can serve different uses, such as irrigation, industrial and drinking water supply, and the recovery or preservation of environmental assets. Globally, water resources are increasingly under pressure. MAR offers the potential to buffer water resource availability and meet demands in periods of water scarcity. Consequently, the uptake of this technique will likely soar in the next decades due to current pressure on water resources and the consequences of climate change, requiring highly trained practitioners that can provide the knowledge and know-how on MAR implementation. The present report is part of the Managed Aquifer Recharge Solution Training Network (MARSoluT ITN, 2019-2023), which aimed to train experts in MAR. The report deals with the objectives of work package four (WP4) of the MARSoluT project and aims at helping to improve water quality by optimising MAR design at active MAR sites in Spain. The report tackles four knowledge gaps on MAR in relation to water quality: (i) the need for a review of current regulations on MAR and how they use Maximum Allowable Concentrations (MACs) to prevent water pollution during MAR; (ii) a discussion on the current challenges for MAR water quality; (iii) the lack of a conceptual model that can help in the creation of regulation, rules, and guidelines on MAR; and (iv) the need for testing the efficacy of MACs at the national level with a focus on Spain. Although this report focuses on Spain, it does not restrict its development to this country and has a relatively broad scope, providing solutions for MAR worldwide. The methodologies in the current report are mostly based on literature review of scientific, technical and regulatory documents and the exploitation of published and unpublished water quality analyses and water quality standards involving MAR. For the first knowledge gap, a comprehensive review of existing regulations and guidelines for MAR was conducted, and an analysis of MACs in water quality standards from diverse sources globally was completed. To respond to the second knowledge gap, a discussion on modern water quality challenges for MAR is provided, including the context, history, future and the need for a risk-based approach. The Monitored and Intentional Recharge (MIR) conceptual model is a response to knowledge gap three, comprising a set of nine blocks that elaborates on the most critical aspects that must be considered to draft MAR regulations and guidelines. This section also entails a series of recommendations directed to future regulations on MAR, including developing a common terminology with legal implications, considering a permitting process, new legal development, the inclusion of budgetary aspects, regard to the technical background for authorisation, and the fact that water standards for MAR must be designed at the aquifer level. The fourth knowledge gap was addressed through an analysis of source water quality in MAR sites in Spain and a comparison between them and three European MAC-based standards (Italy, Spain, and the Netherlands) to evaluate the efficacy of MAC in dealing with water pollution at MAR sites. This section showed the high variability in terms of water quality at 11 MAR sites and the inadequacy of MAC-based standards to control pollution while allowing for MAR implementation at the national or European levels. This sort of standard could be useful aquifer-wide. This analysis also concluded that focus on quality should be given to the final quality of the water after MAR rather than the original water quality, and that the attenuation processes in the saturated and unsaturated zones should be regarded. Finally, biochar, a promising low-cost material to remove multiple pollutants during MAR, has been thoroughly studied. This report also emphasises that, nowadays, MAR is an option, but in the future will likely become a necessity, and that there is a need to continue investing in (1) applied research to improve insights into the development of freshwater cones in saline environments, (2) define measures to increase the efficiency of MAR systems and social acceptance, and (3), consider the beneficial impact of the unsaturated and saturated zones.