Insights into Andean slope hydrology: reservoir characteristics of the thermal Pica spring system, Pampa del Tamarugal, northern Chile

The thermal Pica springs, at ∼1,400 m above sea level (asl) in the Pampa del Tamarugal (Chile), represent a low-saline spring system at the eastern margin of the hyper-arid Atacama Desert, where groundwater resources are scarce. This study investigates the hydrogeological and geothermal characteristics of their feed reservoir, fostered by the interpretation of a 20-km east–west-heading reflection-seismic line in the transition zone from the Andean Precordillera to the Pampa del Tamarugal. Additional hydrochemical, isotope and hydrologic time-series data support the integrated analysis. One of the main factors that enabled the development of the spring-related vertical fracture system at Pica, is a disruption zone in the Mesozoic Basement caused by intrusive formations. This destabilized the younger Oligocene units under the given tectonic stress conditions; thus, the respective groundwater reservoir is made up of fractured Oligocene units of low to moderate permeability. Groundwater recharge takes place in the Precordillera at ∼3,800 m asl. From there groundwater flow covers a height difference of ∼3,000 m with a maximum circulation depth of ∼800–950 m, where the waters obtain their geothermal imprint. The maximal expected reservoir temperature, as confirmed by geothermometers, is ∼55 °C. Corrected mean residence times of spring water and groundwater plot at 1,200–4,300 years BP and yield average interstitial velocities of 6.5–22 m/year. At the same time, the hydraulic head signal, as induced by recharge events in the Precordillera, is transmitted within 20–24 months over a distance of ∼32 km towards the Andean foothills at Pica and Puquio Nunez.