Assessing nitrate origin in a volcanic aquifer using a dual isotope approach

Identifying the origin of nitrate is important for the control and management of groundwater quality in aquifer systems. In the southern Apennines (Italy), the Mount Vulture volcanic aquifer is a large and valuable resource of potable and mineral water supply. Unfortunately, signs of anthropogenic impact, especially nitrogen contamination, have recently become evident. In this study, and for the first time, stable isotope ratios (15N and 18O) of NO3 were determined in groundwater to identify their origins and evaluate the presence of transformation processes. The Mount Vulture groundwaters are meteoric in origin, as demonstrated by measurements of D and 18O, and can be divided into two distinct areas based on their NO3 content. In the southeastern area, characterized by active agricultural land use, the high NO3 content and the 15N-NO3 isotopic values are due to anthropogenic contamination (inorganic fertilizer). In groundwaters from the western area, the NO3 contents below 4 mg/L and the 15N-NO3 values can be associated at organic soil N. Evidence for local denitrification may be assumed in a few groundwater samples of the western area showing relatively heavy 15N values and low concentrations of nitrate. Finally, the low measured 18O values indicate that nitrification occurred in both investigated areas.