Decoding signature of water stable isotopes in Iran’s strategic aquifers

Ten Iranian strategic aquifers Groundwater is a critical component of Iran’s water supply, yet the environmental factors shaping the spatial variability of groundwater stable isotopes have not been comprehensively examined. Here, we analyzed 202 groundwater samples from 10 Iranian strategic aquifers—systems that collectively provide drinking water for over half of the country’s population—to identify spatial patterns in isotope composition and the environmental drivers behind them. Oxygen stable isotope (δ¹⁸O) and deuterium (δ²H) exhibited strong spatial relationships with latitude, elevation, and climatic gradients. Local meteoric water lines (LMWLs) in most aquifers displayed slopes lower than the global meteoric water line (GMWL), pointing to evaporative enrichment and variations in moisture sources. Spatial contrasts in deuterium-excess ( D -excess) and line-conditioned excess (LC-excess) further highlighted regional differences: northern aquifers exhabited elevated D -excess associated with humid winter precipitation, whereas central and eastern regions showed reduced values linked to arid conditions and intense evaporation. Approximately 65% of sampled wells had LC-excess values above –1.8‰, indicating notable contributions from return-flow and active recharge processes. Overall, these findings demonstrate how climate, topography, recharge dynamics, and evaporation jointly shape groundwater isotope signatures across Iran. The results offer a robust basis for future hydrological investigations and support national-scale strategies for sustainable groundwater management. • Decode signature of groundwater stable isotopes (GSIs) in Iran’s strategic aquifers. • We examined the spatial distributions and environmental controls on GSIs. • δ¹⁸O and δ²H exhibited strong correlations with latitude, topography, and climate. • Around 65% of the sampled wells showed LC-excess higher than the –1.8‰ threshold.