Using stable isotopes to investigate geochemical processes and hydrogeodiversity in a complex aquifer system

Stable isotopes represent a powerful approach for investigating hydrogeological processes in groundwater. Using δ¹³C-DIC helps tracing carbon sources, quantify microbial activity (respiration), understand water-rock interactions and carbonate weathering, and determine DIC origin (organic matter mineralization, carbon dioxide dissolution, geogenic sources). Further, water-rock interactions can be effectively investigated using the ⁸⁷Sr/⁸⁶Sr isotopic ratio, a method based on the principle that the ⁸⁷Sr/⁸⁶Sr ratio in groundwater reflects the ⁸⁷Sr/⁸⁶Sr ratio of the hosting formation that was inherited at the time of rock formation, hence supporting the appropriate attribution of groundwater samples to a specific geological formation.

It is known that peat-rich fluvio-marsh alluviums promote anoxic conditions that enhance arsenic (As), iron (Fe), and manganese (Mn) release, while volcanic deposits and travertines also play a key role in controlling As in groundwater.

This study aims to elucidate how isotopes ratios can support groundwater quality patterns and biogeochemical processes interpretation in a medium size river basin with diverse lithological complexes, and eventually define hydrogeodiversity across the basin by combining geostratigraphy with hydrogeochemical information.

Between November 2024 and December 2025, more than 90 samples were collected in the Sacco River Valley. The study area has an extensive presence of alluvial and lacustrine deposits locally rich in peat (Pleistocene-Holocene), mainly alkali-potassic volcanic products intercalations from the Middle Latina Valley (Pofi and Ceccano eruptive centers) and the Albani Hill apparatus (Pleistocene), and travertine lenses (Pleistocene) connected to past hydrothermal systems. These lithological complexes host a large water table aquifer fed by local precipitation and by lateral flow from the carbonate mountains on the eastern side. A thick sequence of sandy-marly Flysch (Miocene) separates this groundwater body from a deeper groundwater system circulating in Meso-Cenozoic limestones. The sandy levels host themself a local circulation of groundwater.

Major and trace cations (ICP-OES/ICP-MS), anions (IC and titration for HCO₃), and dissolved organic carbon (TOC analyzer) were analyzed at CNR-IRSA laboratories. Stable isotopes (δ¹⁸O, δ²H, δ¹³C-DIC) were determined at ISO4 s.r.l. (Turin, Italy) and ⁸⁷Sr/⁸⁶Sr at CNR-IGAG, at Sapienza University of Rome laboratories.

The collected groundwaters have a calcium-bicarbonate facies with a slight tendency to alkaline-earth facies. Average pH is neutral, though both strongly alkaline (9.57) and acidic (5.52) values were recorded. Encountered reducing samples were largely from alluvial deposits (41%), while oxidizing conditions dominate in volcanic rocks (48%). Higher variability for As (0.2-55 μg/L), Fe (43.4-6138 μg/L), and Mn (1.3-437 μg/L) was observed in reducing conditions.

Less negative/slightly positive δ¹³C-DIC values (-0.62 – 0.67‰) in few samples suggested deep CO₂ interaction and distinguished rainfall-fed waters from deeper circulating systems. Further, they might indicate where organic pollution is currently active. ⁸⁷Sr/⁸⁶Sr results were found generally consistent with the actual stratigraphic sequences observed in the sampled wells or inferred from available cartographic information, confirming its usefulness in the attribution of the samples to the appropriate aquifer (or a mixing among several of them).

Finally, hydrogeodiversity across the basin provides a framework to interpret the spatial variability of groundwater quality and the associated biogeochemical processes and ecosystems, also finalized to management purposes.