Estimation of Transit Time Along the Unsaturated Zone in the Protection of Groundwater Resources

The growing world population coupled with the expansion of agriculture and animal husbandry has led to excessive use of organic and chemical fertilizers in soils, resulting in widespread nitrate contamination of groundwater. Despite the restrictive regulations imposed by the Nitrate Directive, no improvements in groundwater quality have been observed in many Nitrate Vulnerable Zones. In studies of vulnerability to aquifer contamination, the focus is more on the groundwater body and the unsaturated zone is often neglected. The unsaturated zone is the main factor controlling water movement and pollutant leaching, depending on the soil properties and infiltration rates. Natural attenuation processes in unsaturated zones can reduce the leaching of contaminants into groundwater. The infiltration rate determines the transit time in the vadose zone and thus the water-rock interaction time that controls the transformation, the entrapment/adsorption of the transported solutes. This research compares two regions of Sardinia with different geological, hydrogeological and pedological features: the Arborea plain (designated as a NVZ since 2005), and the southern Campidano plain, in which nitrate concentration in groundwater frequently exceeds the limit of 50 mg/L. The main purpose of this study is to estimate the rate of groundwater recharge using stable water isotope profiles in the vadose zone at the two sites by comparing physical properties of the soils and land use. At each of the two study sites, soil samples were collected every 10 cm along a vertical profile and were analyzed to determine their stable water isotope ratios (δ2H and δ18O) together with the water content, grain size distribution and physical-chemical parameters of the soils. The peak-shift method was applied based on the assumption that seasonal effects on the isotopic composition of precipitation are traced through the soil vertical isotopic profile that were used along with the piston dis\placement method to estimate recharge rate. The study of the transit time of water percolation along the unsaturated zone is therefore an influential factor in the assessment of the vulnerability of the aquifer and has implications in the optimization of agricultural practices and actions to protect the groundwater resource.