The intensive exploitation of groundwater resources in arid and semi-arid regions poses serious challenges to the quantitative sustainability of aquifers. The Mashhad–Chenaran aquifer, one of the most important alluvial aquifers in north-eastern Iran and the main source of drinking water for the city of Mashhad, has experienced increasing stress in recent years. This study evaluated the quantitative sustainability of the aquifer during the 2015–2016 to 2021–2022 hydrological periods, based on an integrated analysis of well discharge, aquifer saturated thickness, specific yield (Sy), specific capacity (Q/s) and specific drawdown (S/Q). The results indicate a notable decline in well discharge, particularly in the southeastern and central parts of the aquifer. Concurrently, the saturated thickness of the aquifer decreased. Specific yield declined from approximately 0.95 to 0.25, corresponding to a reduction of around 74% in aquifer storage capacity. Additionally, the specific capacity decreased from approximately 0.63 to 0.43 MCM·yr⁻¹·m⁻¹, representing a reduction of around 32%. Meanwhile, specific drawdown increased from approximately 1.56 to 2.30 m, indicating a 47% increase in water-level decline per unit discharge, as well as a reduction in the hydraulic efficiency of groundwater exploitation. Areas of the aquifer were assessed for sustainability during the 2021–2022 hydrological period, and it was found that approximately 35% of the aquifer area was classified as unstable, 42% as semi-stable, and only 23% as stable. Overall, the findings demonstrate an intensification of quantitative instability in the Mashhad–Chenaran aquifer, emphasising the need to revise groundwater abstraction practices, control pumping rates and implement continuous monitoring to ensure the aquifer is exploited sustainably.
