Background According to climate change predictions, Spain, as most other Mediterranean countries, faces increased water shortages, pollution and loss of water-dependent ecosystems in the near future. The Llobregat river delta aquifers - at the south-western corner of the Barcelona metropolitan area - are an important component of the water system supplying the city. Overexploited in the past, they face serious ecological problems. The river undergoes severe droughts in summer, riparian (river bank) vegetation has disappeared, and there is constant seawater intrusion in freshwater aquifers. Solutions to water stress problems are urgently needed, but they must be sustainable, economical and safe. Managed Aquifer Recharge (MAR) is a favoured option, because it stabilises storage capacity to buffer seasonal variations of water supply and demand, protects coastal aquifers from saltwater intrusion, mitigates the depletion of groundwater levels, and enhances natural treatment when low-quality water is recharged. Objectives The ENSAT project’s objective was to demonstrate the application of a well-established technology for aquifer remediation, but in an innovative way. This involved Soil Aquifer Treatment (SAT) during aquifer recharge episodes, using a reactive organic layer in the bottom of an infiltration pond that promotes micro-biological activity to break down pollutants. Specifically, the project aimed to improve the quality of groundwater at the Sant Vicenç dels Horts aquifer recharge site; develop a modelling tool for the prediction of the impact of an organic substrate in terms of hydraulics and geochemistry during infiltration; and adapt and transfer its tools and methodology to other aquifer recharge sites in Europe. Results The ENSAT project started with the selection of an organic substrate, through which water would pass during aquifer recharge. The results showed that the most suitable substrate was vegetable compost, ideally a mixture of forest and garden compost. Project partner CSIC-IDAEA then developed a numerical model and conducted laboratory studies to define the best materials to be used in the reactive layer, and designed the composition of the layer. For the demonstration studies, this was defined as 60 cm of reactive layer (50% sand, 50% vegetable compost, and 1-2% of clay) plus 5 cm of local ground and 0.1% of iron oxides. The laboratory tests provide a starting point for future applications of reactive layers. The demonstration site comprised two existing ponds. The first pond is for the decantation of the suspended material that arrives in the river water, after which the water is transferred to the infiltration pond where it recharges the aquifer. After being prepared, five piezometers were installed for monitoring: two downstream and three inside the infiltration pond. The reactive layer was installed as designed in laboratory tests, and the infiltration pond constructed so that the water entering it did not damage the ground materials. The project started its sampling campaign as soon as the infiltration began. The aim of the analyses was two-fold: firstly, an assessment of the organic substrate efficiency and its impact on groundwater quality, and, secondly, surveillance for emerging pollutants. In all, more than 4 000 analyses were carried out by two laboratories over a six-month infiltration period, which included the use of new analytical methods. The results showed that the organic layer had an effect on water quality and that, after one year of recharge, the water in the aquifer had the same quality parameters (showing the recharge and/or the added layer does not alter the aquifer). The analysis of the emerging pollutants showed good results, as the concentration of emerging pollutants in groundwater was relatively high during recharge before the use of the reactive layer and decreased considerably with its use. The different removal rates for each pollutant were quantified. The project, therefore, achieved its main objective of demonstrating that the addition of a reactive layer for the infiltration of water had a positive effect on the reduction of groundwater pollutants, by enhancing their biological degradation, and therefore improving water quality. After more than one year of continuous recharge, the reactive layer was still active. Therefore, this technology can be useful for medium and long-term applications. CETaqua developed a methodology to facilitate the transfer of this technology to other aquifers, in the form of a software application called hydROL. This application simulates the reactive transport in an aquifer and predicts the performance of a particular reactive layer in terms of hydraulics and geochemistry during infiltration. The software was tested and validated at very different recharge sites in Budapest (Hungary) and Aubergenville (France). hydROL provides managers of operative or future aquifer recharge sites with an easy-to-use desktop tool for forecasting the behaviour of the system when an organic layer is added. The project demonstrated that physical, chemical and biochemical processes associated with water movement within the subsoil represent a natural and alternative way to reduce the presence of these contaminants, with economic as well as environmental benefits. Energy consumption is significantly reduced compared with the treatment of water with reverse osmosis and ultra-filtration (SAT consumes only 10% of the energy of these methods); costs and environmental impacts are also reduced because there is no need to use reagents; and the initial investment is cheaper. The project helped implement the Water Framework Directive (2000/60/EC), the Groundwater Directive (2006/118/EC) and other EU policy. Further information on the project can be found in the project's layman report (see "Read more" section).