Integrating isotope hydrology and hydrogeochemistry to assess recharge processes and vulnerability of drinking water springs: the case of Capannori (Tuscany, central Italy)

Ongoing climate change and increasing anthropogenic pressures are intensifying the challenges associated with the sustainable management of groundwater resources, particularly in Mediterranean regions characterized by pronounced climatic seasonality and recognized as climate change hot spots. Groundwater springs often represent a critical resource for local communities, serving both as a source of high-quality drinking water and as a key component of socio-ecological systems. Understanding the functioning, vulnerability, and resilience of the groundwater systems feeding these springs is therefore essential for effective and sustainable water management.

This study presents the first results of an ongoing investigation in the Municipality of Capannori (Lucca, Tuscany, central Italy), where publicly accessible springs are widely used for domestic water supply and have become the focus of increasing conservation efforts by local authorities. The collection of water from local springs represents a shared practice in this region that fosters a strong sense of community and reinforces the concept of water as a common good. Ensuring the sustainable management of these resources, therefore, requires a comprehensive understanding of groundwater systems, achievable through multidisciplinary approaches integrating isotopic fingerprinting with geochemical and hydrogeological tools.

A two-year monitoring programme started in April 2024 and involves 19 groundwater springs distributed across three hydrogeological sectors. Groundwater samples were collected for chemical and isotopic analysis (δ¹⁸O and δ²H), while temperature, pH, electrical conductivity and redox potential were measured in the field. Tritium and δ³⁴S-δ¹⁸O-SO₄ were determined on selected samples, and compositional data analysis (CoDA) was applied to the chemical dataset. In addition, a rain sampler was installed in February 2024 to collect monthly precipitation samples for isotopic analysis.

The results highlight pronounced chemical and isotopic heterogeneity among springs across the three hydrogeological sectors, reflecting the complexity of the groundwater systems involved. This heterogeneity points to distinct flow paths, water-rock interaction processes, and recharge dynamics, and suggests potentially different sensitivities of individual springs to hydroclimatic variability and anthropogenic pressures. From a management perspective, these differences imply that springs commonly perceived as part of a single resource may in fact exhibit contrasting levels of vulnerability under ongoing and future environmental change.

Stable water isotope data, together with deuterium excess, provide robust constraints on the recharge elevations of the aquifers feeding the springs, allowing the identification of recharge areas and the possible extent of recharge catchments. At the same time, part of the observed isotopic variability may reflect a climatic signal related to recharge seasonality rather than elevation alone. However, seasonal isotopic shifts were negligible across all springs, indicating well-mixed recharge systems and relatively slow groundwater circulation that dampens the pronounced isotopic variability of precipitation. Consistently, tritium values show no significant differences among springs (2.5-2.9 TU), indicating young groundwater with residence times not exceeding 5-10 years.

Overall, this study demonstrates how integrated isotopic and geochemical approaches can provide process-based insights that are directly relevant for groundwater protection and management under changing hydroclimatic conditions.