Tracing nitrate fate in Malta’s hydrogeological system using an intensive vadose-groundwater monitoring network

Study region:: Maltese Islands Study focus:: This study integrates five years of vadose zone (VZ) and groundwater monitoring to identify the dominant non-point sources of nitrate pollution across various agricultural systems. A network of 16 VZ monitoring stations was installed beneath six representative crops (potatoes, fodder, vegetables, vineyards, greenhouses, and orchards) to quantify nitrate buildup and storage. Groundwater samples were collected from the three major aquifer systems of Malta. The analysis couples chemical monitoring with Modified DRASTIC vulnerability modelling to determine how VZ properties, land use, and geological structure influence nitrate delivery to groundwater. New hydrological insights for the region:: The results show that nitrate accumulation in the VZ varies strongly with crop type and geology. Potato cultivation produces the highest VZ nitrate storage and is the dominant source of nitrate loading to the MSLA, while intensive vegetable and greenhouse farming create local contamination hotspots above the Blue Clay and Coastal aquifers. Orchards and vineyards exhibit substantially lower nitrate buildup. Modified DRASTIC modelling highlights VZ thickness and nitrate storage as key controls on aquifer vulnerability. These findings clarify the coupled role of land use and subsurface structure in governing nitrate transport, providing a practical framework for identifying non-point nitrate pollution sources and enhancing groundwater management.