Multi-isotopic approach for the assessment of groundwater-surface water interactions in a complex hydrogeological and stratigraphic context: the case study of the middle Farfa River Valley (Rieti, Central Italy)

The Farfa River is one of the most important tributaries on the left bank of the Tiber River, upstream Rome in Central Italy. Its water has a very high quality and a geochemical composition mainly associated with the calcium-bicarbonate facies, primarily due to the overflow of the Capore karst Spring, which is one of the main two spring supplying the city of Rome. In the final stretch toward the confluence with the Tiber River these geochemical characteristics completely change and according to the Regional Environmental Protection Agency's classification the river water quality status degrades from good to sufficient, suggesting that this may be due to potential pollution or contamination by human-related activities. This new study, using discharge and physico-chemical field measurements over four monitoring campaigns, together with the help of geochemical and multi-isotopic tracers (C, S, O, H) analyzed on water samples collected, reveals that this change is actually natural and related to the river's interaction with a group of springs, whose origin and recharge areas were largely unknown until now. The use of specific isotopes, such as ¹³C/¹²C and ³⁴S/³²S, has allowed to better understand the groundwater flowpaths, recharge areas and potential interactions with deep fluids upwelling along fault planes present in the study area.