RESOLVING WATER AND SOLUTE BUDGET IMBALANCE IN A GROUNDWATER-DOMINATED ARID CATCHMENT USING HYDROFACIES CHARACTERIZATION AND INVERSE MODELLING

The Salar de Atacama basin in northern Chile hosts a brine and fresh water aquifer that is subject to substantial and increasing water resource competition chiefly due to ongoing extraction of its massive lithium brine deposit and hard rock mining to the south. Despite the importance of these resources, recent work has shown that observed groundwater recharge rates are inadequate on the order of 9-20 times to close hydrologic and solute budgets (specifically Na⁺ and Cl^-) within the topographic catchment under the modern hydrologic regime. Shallow groundwaters discharging to the basin through springs and diffuse groundwater, representing components of the larger groundwater system show substantial variability in apparent sources, flowpaths and mean residence times. This has led to fundamental questions about the mechanisms controlling solute fluxes and their distribution. To address these questions, we present a detailed and comprehensive characterization of groundwater hydrofacies in the basin.

Discretization of groundwater fluxes into constituent hydrofacies, segregating water “compartments” spatially across the basin and with depth allow for investigation into the degree of connectedness within the local and regional groundwater system and the mechanistic distinctions that exist among compartments. We utilize a large dataset of ~1,300 individual water samples across an area ~20,000 km² from high elevation springs and salt lakes to groundwater discharge at the margins of the basin. Results show a groundwater system that is markedly heterogeneous, inflow water compartments exhibit dominant facies ranging from Ca-Na-Cl and Na-Cl in the east to Ca-Na-HCO₃-Cl and Na-HCO₃-Cl in the north and south with waters of the adjacent high elevation Altiplano plateau exhibiting a range of Ca-Na-HCO₃-Cl, Ca-Cl, Ca-Na-Cl and Na-Cl facies. From this analysis coupled with rigorous inverse geochemical modelling we present a process-based model for understanding the nature of solute flux within the groundwater system. Geochemical evidence coupled with hydrophysical constraints developed from our previous work point to the conclusion that the “missing” water and solutes cannot be accounted for without substantial influence from transient head-decay of pre-modern storage and/or solute rich inter-basin underflow.