BOREHOLE TEMPERATURE PROFILES CONSTRAIN GROUNDWATER FLOW, EVAPORATION RATES, AND DISCHARGE TO THE SALAR DE ATACAMA, CHILE

The Salar de Atacama (SdA) in northern Chile provides a unique opportunity to study regional hydrogeologic processes in a hyperarid environment. The endorheic basin acts as a local and regional discharge zone for groundwater recharge off the Altiplano-Puna Plateau. The area is characterized by active modern (Late Miocene-Pliocene to present) volcanism and strato-volcanoes including Socompa (~6600 m) and Pular (~6200 m). The basin has been receiving groundwater discharge since ~ 7 Ma resulting in the accumulation of massive evaporite deposits in the halite nucleus (>1500 km³). Modern discharge of water along a 90km long flowpath evolves from fresh water to brine over an 8 km long transition zone. This study uses groundwater temperature profiles to constrain rates and hydrogeochemical processes impacting groundwater discharge in the southern transition zone of SdA.

Temperature profiles are analyzed for 13 wells along a 31 km hydrogeologic transect. The wells range in depth from 30m to 100 m with land surface elevations of 2450m at the most upgradient well and 2300m for wells in the heart of the halite nucleus. Wells were profiled seasonally so that we may observe seasonal temperature variations in the shallow subsurface. All wells are drilled into the southern transition zone aquifer and constrain the properties of the aquifer as fluids transition from the freshwater residing in the up-gradient alluvial aquifer to the brine found in the halite body to the north. We use a three dimensional adaptation coupled model of ground water flow and heat transport in a fully saturated aquifer system to interpret the temperature profiles.

A general cooling trend along the discharge zone has been observed in the vicinity of ephemeral lagoons. Temperature profiles generated from boreholes spanning the hydrologic transition zone reveal a drop of approximately 10^oC within the geothermal, discharge zone. Local volcanism, noble gas signatures, and upward flow of warm water seen in up gradient boreholes suggest a shallow magma body impacts available heat within the aquifer. We hypothesize that the enthalpy of vaporization of the brackish water found in the aquifer is one mechanism responsible for heat loss across the transect.