Interdisciplinary Research for the Delimitation of Catchment Areas of Large Deep Karstic Aquifers: Origin of the Thermal Springs of Alhama de Aragón and Jaraba (Spain)

The integration of different sources of geological and hydrogeological information and the application of interdisciplinary methods have informed the origin of the thermal springs of Alhama de Aragón and Jaraba, as well as other associated semi thermal springs (1200 L/s of combined flow, 711 L/s at over 30 °C), which is the main objective of this article. These springs come mainly from the autogenous recharge that occurs in the Cretaceous calcareous outcrops that border the Almazán Basin to the north, both in the Ebro Basin (Jalón Valley) and in the Duero Basin. The aquifer, shaped by upper Cretaceous limestones under the Palaeogene and Neogene rocks of the Almazán Basin, has extensive depths of more than 4000 m in the NE sector. This hydrostratigraphic unit has been affected by a generalized pre-Paleogene karstification that provides the main porosity to the aquifer. The underground flow moves in a NW–SE direction, crossing the Duero–Ebro divide, favoured by the topographic difference in elevation between the two basins. The regional flow is coherent with the progressive increase in temperature, infiltrating recharge water age (about 20–25 years in the semi-thermal springs, and more than 60 years in the Alhama and Jaraba springs), mineralization, and flow of the springs through which the system discharges. This issue is key to being able to design any sustainable conservation strategy in terms of quantity and quality of resources within the recharge area of the most important thermal springs in Spain. The Jaraba and Alhama de Aragón hot springs share the same or similar temperature, chemical composition, and geological contact of the spring. Their tritium isotopic composition and its evolution over time are practically the same. Their isotopic composition in D and 18O is also very similar. Both springs share the same recharge zone of similar altitude and constitute the end of flow tubes of similar length and flow rate.