Groundwater flow in the Ischia volcanic island (Italy) and its implications for thermal water abstraction Hydrogeology Journal 28 (4): 1-23 This study improves knowledge on the hydrogeology of Ischia Island, an active volcano of southern Italy. Combining previous published and unpublished data with newly collected data, and through the results of simplified numerical models, the existing conceptual hydrogeological model of the volcanic island has been reviewed and the present impact of intensive withdrawals on the island’s groundwater resources has been examined. Two areas with different hydrogeological characteristics have been recognized: (1) the resurgent block of Mt. Epomeo, where an independent and uplifted basal groundwater circulation exists; (2) the external areas of Mt. Epomeo, where a continuous basal aquifer and local discontinuous perched aquifers have been distinguished. The marginal faults of the resurgent block of Mt. Epomeo represent the preferential pathways of ascending deep fluids. Innatural conditions, the island’s aquifersare recharged byrainfalland bydeepfluids frombeneath,dischargingtowards the sea and the springs. Since the start of activity at the spa facilities, involving intensive pumping from wells, a further and significant rechargeof the aquifershas come fromseawaterand fromincreased upwelling ofthe deepfluids. Althoughthisdoes notcompromisethesustainabilityofthegroundwater withdrawalsinquantitativeterms,thepumpingratesdeterminethequality ofthewater capturedbythewells.Thegreatvariabilityintemperatureandchemicalcompositionofgroundwater oftheislandis alsoinfluencedbylocalhydrogeologicalparametersandcharacteristicsofthewellsfromwhichthewatersamplesweretaken,as well as the phenomena of interaction among different end-members.