Comparative assessment of groundwater recharge rates and transit time through the unsaturated zone for groundwater protection

The intensification of human activities, particularly in agriculture, has resulted in the over-exploitation and depletion of groundwater resources leading to widespread contamination. This study aims to evaluate and compare different methodologies for estimating groundwater recharge rates and transit times through the unsaturated zone, for enhancing the understanding of water percolation dynamics in the vadose zone. The study area is in the Arborea plain in Sardinia (Italy), an area designated as a nitrate vulnerable zone since 2005, where groundwater is severely affected by diffuse nitrate contamination. Three distinct approaches were employed to estimate groundwater recharge: (i) the peak-shift method based on isotopic profiles of soil pore water within the vadose zone, (ii) the water balance concept incorporating the results from the application to the one-dimensional flow and transport model FLOWS to simulate water balance in the unsaturated zone, and (iii) the Soil Water Balance code, which employs a modified version of the Thornthwaite-Mather soil-water balance approach at a daily resolution, integrating spatially distributed soil, meteorological, and land cover data. The recharge estimates obtained from these methodologies show agreement with each other and with previous studies, demonstrating the consistency of these approaches as simplified and effective tools for quantifying net water infiltration. The assessment of percolation times within the unsaturated zone is crucial for evaluating aquifer vulnerability and optimizing agricultural management practices. Accurate quantification of the spatial and temporal distribution of groundwater recharge contributes to improving land and water resource management, enabling the implementation of targeted mitigation measures to minimize groundwater pollution and enhance long-term protection strategies in similar hydrogeological settings.