Umbulan Spring is an Indonesian groundwater spring with a historically significant discharge up to 3,500 L/s which has significantly declined in recent years, has significantly declined from 5,262 L/s since 1987. In terms of hydrogeology, this declination of discharge indicates a depletion of water resources in the groundwater basin. This study aimed to identify the cause of the depletion in the Umbulan Spring discharge by analysing the aquifer system using an integrated methodology of hydrochemical and geological approaches. Results show that groundwater that discharges via Umbulan Spring is of the same groundwater facies as that of the surrounding artesian wells, having CaMgHCO3-type water with a bicarbonate anion facies and no dominate cation facies. The range of groundwater isotopes from -7.8 to -8.2 ‰ for 18O and -46.2 to -50.5 ‰ for deuterium correspond to the local Bromo Meteoric Water Line (BMWL). Groundwater isotope correlation with the BMWL and the site elevation shows that the recharge area is located on the northern slope of the Bromo-Tengger Volcanic Complex between 1,300 – 2,300 m above sea level. The groundwater source flows through a lapilli tuff aquifer which is covered by impermeable pyroclastic flow deposits. Due to its determination as part of the same aquifer system as the artesian wells, the depletion of the Umbulan Spring discharge is related to the increasing number of artesian wells around the spring.