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Paper No. 82-22
Presentation Time: 9:00 AM-5:30 PM
USING HEAT AS A GROUNDWATER TRACER TO CONSTRAIN GROUNDWATER DISCHARGE TO LAGOON COMPLEXES IN SALAR DE ATACAMA, NORTHERN CHILE
The Salar de Atacama (SdA) is a topographically-closed, hyper-arid basin located in northern Chile. Long regional flow paths extending from the adjacent Andean Plateau recharge the 17,000 km2 watershed of the SdA. Like other arid regions, SdA exhibits immense amounts of evaporation; this flux of water leaving the basin produces a thick evaporitic crust of carbonate-gypsum sediment across the basin floor. In this study, we explore the freshwater-brine interface (Transition Zone) along the margin of the salar, where we see the first signs of groundwater discharge into the basin in the form of lagoon complexes. Our goal is to quantify groundwater flux into this subsurface environment, which is dominated by evaporative carbonaceous sediments. We do this by using heat as a tracer in the shallow saturated zone and by solving the coupled groundwater-heat transport equations . Temperature-depth time series collected in January 2015 show significant variations in amplitude and phase shift as function of depth and location within the seepage zone. We employ a program VFLUX to produce estimates of upward flux and thermal diffusivities and found that the calculated thermal diffusivities were greater than previously estimated thermal diffusivities in literature. Our results also reveal that magnitudes of upward fluxes range from ~145 mm/day in open water locations to ~1 mm/day in locations with no surficial standing water. To investigate the effectiveness of using heat as a tracer to estimate evaporation in arid environments, we compare previous estimates of average evaporation rates across SdA (0.01 – 5 mm/day) to our upward flux measurements, which are generally greater. The irregularities in our results and previously measured fluxes suggest that using point measurements for temperature time series may not necessarily be reflective of the entire Transition Zone environment. We interpret that a fraction of the Transition Zone exhibits focused discharge points like the ones in our study, therefore it may be necessary to couple these local measurements by defining a thermal signature and using distributed temperature sensing or thermal mapping techniques with drone-based FLIR; therefore it may be possible to use heat as a tracer to constrain upward flux at SdA and other playa environments in the future.