The use of stable pore water isotopes in the study of nitrate leaching

In recent years, with the development of intensive farming and livestock breeding activities, nitrate contamination of aquifers has occurred. The importance of the unsaturated zone in the study of nitrate percolation is well recognized. The vadose zone represents a fundamental part of the water cycle, capable of storing water, providing water for vegetation, transporting solutes and degrading contaminants before they reach the aquifer.
This research involves two areas with different geological, hydrogeological and pedological properties located in Sardinia. The plain of Arborea has been designated as Nitrate Vulnerable Zone (NVZ) since 2005, but despite the reduction of nitrogen input no significant improvement in water quality has been achieved. In the Southern Campidano plain, an area with an agricultural tradition, significant nitrate concentrations were observed in groundwater.
The main objective of this research is to estimate the groundwater recharge rate using vadose-zone water stable isotope profiles.
Firstly, to understand the dynamics of water percolation through the vadose zone, soil samples were collected at different depths to analyzed for physical properties, including soil-water content, and grain size to estimate soil hydraulic properties. In addition, suction cups were installed at different depths at each site to extract pore water from the soil for chemical analysis.
In hydrological systems, the use of stable isotopes (¹⁸O and ²H) of pore water as environmental tracers is considered the most useful tool for establishing water flow and contaminant transport. In this study stable pore water isotope profiles combined with water content profiles were used to obtain insight into the transit time of water percolating through the vadose zone. At each of the two study sites, vertical soil sampling was made along the vadose zone and the soil samples collected were analyzed for stable water isotope ratios (¹⁸O and ²H) and volumetric water content. Through the seasonal signatures of stable isotopes in leachate water it is possible to quantify possible groundwater recharge, this approach, called the peak-shift method assumes advection-dominated transport.
Analysis of nitrate concentrations in soil water below the root zone using suction cups combined with groundwater recharge rates will allow an estimate of site-specific nitrate leaching. Through this approach, it is possible to evaluate potential and conservative propagation of nitrogen at specific depths to be compared with the concentrations measured to gain information on nitrate transformation processes in the soil.