Managed Aquifer Recharge (MAR) technique based on the Forested Infiltration Area (FIA) is being tested to mitigate groundwater nitrate contamination, due to intensive agricultural activity, in a sandy aquifer within the Nitrate Vulnerable Zone of Arborea (central-western Sardinia). The research is being carried out within the NATMed project (https://natmed-project.eu/) (2023-2026), funded under the EU PRIMA Programme.
The FIA system is implemented in a pilot site of around 0.4 ha and it is supplied by drainage water with an average nitrate concentration of 70 mg L-1, coming from a nearby dewatering pumping station. The infiltration process occurs through six recharge trenches, with a total length of 300 meters and one meter deep, placed between rows of Eucalyptus and Poplar trees. The recharge water is treated before infiltration by a Passive Treatment System (PTS) installed within the trenches. It consists of a mixture of Eucalyptus wood chips (50% of volume) and inert material, whose function is to promote the denitrifying bacteria action, reducing nitrate (NO3) to atmospheric nitrogen (N2). Eventually, when the PTS effectiveness is reduced and the planting is fully developed, denitrification will be sustained by symbiotic bacteria residing on the tree root systems.
In two years of hydrogeochemical monitoring, a noticeable reduction of nitrates up to 85% in the infiltrating water and a significant decrease in groundwater nitrate concentration have been observed. These results suggest that the FIA technique is an efficient Nature-Based Solution for decontaminating waters polluted by nitrates. However, a considerable amount of soluble phosphate is released by the PTS, leading to an increase in the concentration of this pollutant in groundwater. Laboratory and field tests on some materials able to remove phosphate before infiltration are ongoing, with promising results