Isotopic database for precipitation and groundwater in the Sierra de Segura. Province of Jaén. Spain.

This dataset contains stable isotope observations of precipitation and groundwater collected in the Sierra de Segura karst aquifer system, southern Spain. The dataset was developed to characterize the spatial and temporal variability of the isotopic composition of precipitation and groundwater and to support the assessment of recharge processes, groundwater circulation, and hydrological connectivity in a Mediterranean multilayer carbonate aquifer system. The dataset comprises 884 water samples, including 403 precipitation samples and 481 groundwater samples. Precipitation was collected at eight rainfall collectors (RC1–RC8) distributed across the study area, whereas groundwater samples were collected from 20 springs, including 11 springs draining the Upper Cretaceous aquifer and 9 springs draining the Jurassic aquifer. Precipitation observations cover the period from December 2017 to March 2023, while groundwater observations extend from January 2020 to October 2023. For each sample, the dataset provides the sampling date, sampling site, sample type, and stable isotope composition of water expressed as δ¹⁸O and δ²H (‰ VSMOW). Derived isotope parameters include deuterium excess (d-excess) and local line-conditioned deuterium excess (Lc-ex). The Lc-ex parameter is calculated as the deviation of the measured δ²H value from the value predicted by the Local Meteoric Water Line (LMWL) for the study area, following the approach of Landwehr and Coplen (2006). The dataset is accompanied by detailed site metadata, including site identification, aquifer, geographic coordinates, elevation, geological and lithological information, hydrogeological unit, sampling-point characteristics, and monitoring period. A data dictionary is also provided to describe the variables and units included in the dataset. The dataset is intended to support research on stable isotope hydrology, precipitation–groundwater interactions, karst hydrogeology, groundwater recharge, aquifer connectivity, isotopic signal attenuation, and hydrological integration. It can also be used for comparison with isotope datasets from other Mediterranean mountain and karst systems, for the characterization of meteoric water lines, and for the development and evaluation of isotope-based hydrogeological conceptual models. The dataset provides the underlying observational data associated with research on the use of Lc-ex as an environmental tracer of isotopic signal transformation in the Sierra de Segura aquifer system. The data are provided in tabular format together with metadata and documentation to facilitate reuse and reproducibility.