HEAT TRANSPORT IN SALT FLATS: THE ROLE OF THE LAND-SURFACE ENERGY FLUXES IN DEEP SUBSURFACE TEMPERATURE DISTRIBUTIONS

The Salar de Atacama (SdA) located in Northern Chile is an endorheic basin with a hyper-arid climate. The basin has been closed from the end of the Miocene and has developed a thick evaporite succession and a major source of the worlds lithium-rich brine. The groundwater thermal regime of the basin has not been previously investigated. Characterization of heat flow in SdA may be useful to identify controls on groundwater temperature distributions in areas of discharge and identify sources of brine to the basin. Coupling temperature-depth profiles with heat transport models is an effective method for identifying the controlling mechanisms on heat in discharge areas. Specifically, the transition zone (TZ), which is an area of focused groundwater discharge resulting in lagoons and is a location for freshwater to become brackish/brine in SdA.

Multiple field campaigns from 2013-18 have collected 100’s of temperature-depth profiles in summer and winter seasons. The profiles range from 30 to 400 meters in depth. Higher elevations in the southern margin alluvial fans have temperature depth profiles with a range between 23-27 °C. Down-gradient about 2.5 km into the TZ, the range decreases to 12-17 °C in a mix of halite, gypsum, and carbonates. The temperature range in the TZ is below the mean air temperature of 16 °C. Farther north in the halite nucleus, the temperature range rises to 18-22 °C. Cooling of the subsurface occurs at depths greater than 100 meters below in the TZ. This thermal phenomenon is anomalous and difficult to explain with standard groundwater tracing theory.

One-dimensional and 2D models have been created to replicate different heat transport mechanisms in the TZ of SdA. Results demonstrate the loss of heat in the TZ cannot be accounted for solely with evapotranspiration; anomalous cooling can be explained by higher albedo in the TZ causing less heat to be absorbed from incoming solar radiation. In combination with high evaporation rates in the TZ and less heat from higher albedo, temperature-depth profiles and heat flow distributions are reasonably explained. Proper interpretation of temperature depth profiles along with heat flow distributions in the TZ places critical constraints on the influence of heat flow, surface energy budgets and the connectivity between the halite nucleus and TZ on groundwater flow patterns in the SdA.