Assessment of future trends on groundwater quality in a nitrate vulnerable zone (Esposende–Vila do Conde sector, NW Portugal): towards a combined conceptual and mass transport modelling

Nitrate contamination of groundwater is an important public health issue worldwide. For environmental and public health reasons, water should not contain more than 50 mg/L NO₃. An aquifer for which this limit is exceeded can be designated as a nitrate vulnerable zone (NVZ) and subject to action programs to minimize the NO₃ input. The study aims to assess future trends of groundwater quality and to predict the time required for groundwater to achieve the environmental goals in the Esposende–Vila do Conde NVZ (Portugal). Flow and transient nitrate transport modelling were performed using the FEFLOW software. The numerical model represents the saturated zone of phreatic aquifers, designed in a three-dimensional three-layer model. The calibration process was completed through the tool FEPEST. Sensitivity analysis was performed to investigate the model response to changes in hydraulic parameters and aquifer recharge. Two major simulations of mass transport were performed considering different options on nitrogen loads: (1) agricultural nitrogenous loads of diffuse origin; (2) nitrogen loads from agricultural and livestock sectors together. The results show that the minimization measures imposed in the NVZ are effective, shown by the groundwater nitrate concentration decreasing over time; however, concentrations above 50 mg/L will persist for the next two decades in both simulated scenarios. Combining the conceptual hydrogeological model, geovisualization techniques, and numerical flow and mass transport modelling has been shown as a comprehensive approach to understanding the measures needed for sustainable water resources management and particularly to predicting hydraulic heads and NO₃ dispersion in aquifers.