Understanding the influence of Malta litho‐structural features on the dynamics of nitrate transport in the vadose zone

Coastal aquifers in the Mediterranean are increasingly stressed by climate change, intensive agriculture, and rising water demand. In Malta island, where groundwater is a key resource, nitrate contamination remains a persistent issue despite regulatory efforts and the designation of the islands as a Nitrate Vulnerable Zone. To understand the causes of this persistence, a network of vadose zone monitoring stations was installed to collect real-time data on soil (bedrock) water content. Groundwater quality data (nitrate concentrations, major ions, and stable isotopes) were obtained following monthly laboratory analysis of percolating groundwater samples. Detailed analysis at selected sites shows that the litho-structural features of the unsaturated zone strongly influence water and solute movement. Low-permeability layers slow vertical infiltration, promote lateral flow, and act as natural hydraulic barriers. Nitrate concentrations in shallow groundwater can reach very high levels, indicating substantial accumulation in the unsaturated zone. Isotopic data suggest surface evaporation and deep, slow recharge. Groundwater levels show delayed responses to rainfall and are more closely linked to long-term precipitation trends, confirming the aquifer's low reactivity. The chemical composition of groundwater shows increasing nitrate and chloride levels, supporting the hypothesis that legacy nitrogen stored in the vadose zone continues to leach over extended timescales. This study proposes a revised conceptual model of the mean sea level aquifer that integrates the role of litho-structural features in controlling recharge and pollutant transport. Findings underline the need to reassess groundwater vulnerability by considering the dynamics of the unsaturated zone and implement long-term monitoring to better manage nitrate contamination.