Improvement of the thermal spring protection area through numerical modelling and interdisciplinary studies

Abstract. 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 others associated semi thermal springs (1,200 l/s of combined flow, 711 l/s at over 30 °C). 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 Upper Cretaceous limestones and dolomites constitute the main aquifer of the Alhama and Jaraba thermal system. It extends continuously under the slightly permeable Tertiary of the Almazán Basin in the form of a NW-SE "synclinorium". Its bottom has extensive depths of more than 3,000 and 4,000 m in the NE sector, which constitute the focus of heat considering normal geothermal gradients. From the results of the modelling of the flow of this thermal system, it can be concluded that the origin of these springs comes 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 more distant Duero Basin. The underground flow follows a NW-SE direction flowing across the Duero-Ebro divide, favored by the topographic difference in elevation between the two basins. The modelled regional flow is coherent with the progressive increase in temperature, water age, mineralization, and flow of the springs through which the system discharges.