Tracing nitrate sources and transformations in the Lomellina phreatic aquifer (Italy) using dual isotope analyses

Nitrate contamination in groundwater is a widespread environmental issue that significantly affects water quality. However, nitrate sources and transformations remain poorly investigated in European rice-paddy contexts, such as the Lomellina area, within the main Italian rice-growing district. This study applies systematically dual isotope analyses (δ¹⁵N and δ¹⁸O) in nitrate to trace the origin and biogeochemical evolution of nitrogen compounds in the shallow phreatic aquifer of Lomellina, where groundwater depths range from <1 m to approximately 15 m. Between June 2024 and December 2025, thirteen monitoring campaigns were conducted at 25 monitoring points tapping the phreatic aquifer. Isotopic data were interpreted using a locally adapted Kendall plot and integrated with nitrate concentrations, land-use information, and records of fertilizer application. Results indicate that groundwater nitrate concentrations, reaching up to 164.49 mg/L, are predominantly associated with ammonium-based inorganic fertilizers, as supported by the isotopic signatures (δ¹⁵N from −5 to +15‰; δ¹⁸O from 0 to +5‰). Nevertheless, contributions from nitrate derived from soil organic matter and atmospheric deposition are evident. In several samples, nitrate concentrations ranging from 5 to 15 mg/L can be related to a mixing among these sources. Instead, some other samples display atypical isotopic compositions that hinder a clear attribution of nitrate origin. Overall, the findings improve the understanding of nitrogen dynamics in intensively cultivated rice systems and provide a robust framework for future monitoring strategies. The results also support the development of more sustainable fertilizer management practices, both in the study area and in comparable agricultural settings where distinguishing fertilizer-derived nitrate is essential. This study was carried out in the context of the PROMEDRICE project (https://promedrice.org/; PRIMA-Section2–2022) funded, for the Italian partners, by MUR (Italian Ministry of University and Research).