Groundwater resources in arid and semi-arid regions are increasingly subjected to intensive exploitation, posing serious challenges to the quantitative sustainability of aquifers. The Mashhad–Chenaran aquifer, one of the most important alluvial aquifers in northeastern Iran and the main source of drinking water for Mashhad city, has experienced increasing stress in recent years. This study evaluates the quantitative sustainability of the Mashhad–Chenaran aquifer based on an integrated analysis of well discharge, aquifer saturated thickness, specific yield (Sy), specific capacity (Q/s), and specific drawdown (s/Q) during the 2015–2016 to 2021–2022 hydrological periods.The results indicate a notable decline in well discharge, particularly in the southeastern and central parts of the aquifer. Concurrently, aquifer saturated thickness decreased, and the specific yield declined from approximately 0.95 to 0.25, corresponding to an approximately 74% reduction in the aquifer storage capacity. In addition, specific capacity decreased from about 0.63 to 0.43 MCM·yr⁻¹·m⁻¹, representing an approximately 32% reduction, while specific drawdown increased from approximately 1.56 to 2.30 m, indicating an increase of about 47% in water-level decline per unit discharge and a reduction in the hydraulic efficiency of groundwater exploitation. Areal sustainability assessment for the 2021–2022 hydrological period shows that approximately 35% of the aquifer area is classified as unstable, 42% as semi-stable, and only 23% as stable. Overall, the findings demonstrate a pronounced intensification of quantitative instability in the Mashhad–Chenaran aquifer and emphasize the need for revising groundwater abstraction practices, controlling pumping rates, and implementing continuous monitoring to ensure sustainable groundwater use.
