Regional vs. Local Isotopic Gradient: Insights and Modeling from Mid-Mountain Areas in Central Italy

Mountainous zones are often characterized by complex orography and contacts between different aquifers that usually complicate the use of isotope hydrology techniques. The Apennine chain (Italy) and 10 mountain and mid-mountain areas belonging to it are the objective of this study. An original isotopic data treatment, able to identify the most probable recharge area for several springs/springs' groups/wells, has been developed. The method consists of a two-step approach: (1) the determination of the spring/wells computed isotope recharge elevation; (2) an advanced δ¹⁸O precipitation distribution model over the study area supported by statistical and GIS-based procedures implemented by two processes: first, the clipping of precipitation δ¹⁸O values (depicted from the δ¹⁸O–elevation relationships obtained for each study area) over a most probable recharge area for each analyzed spring or well and, second, the calculation of the overlapping distribution between the spring/well mean δ¹⁸O values ± σ and the precipitation δ¹⁸O content for each outcropping aquifer. A new regional δ¹⁸O gradient covering 150 km latitudinal length of central Italy has been defined. Seven LMWL and δ¹⁸O–elevation relationships able to represent the local precipitation isotopic composition have been obtained. The mean elevation of the springs and wells recharge areas have been assessed by a comparison between the obtained gradient with nine δ¹⁸O gradients available in the literature and those obtained at a local scale. The new isotopic modeling approach can stress whether the mere isotope modeling based on the stable isotope of oxygen agrees with the hydrogeological setting of the study areas.