Hydrogeologic and Geochemical Distinctions in Salar Freshwater Brine Systems

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the transition zone water zones are distinct and separated from the brine in the halite nucleus. 23 Geochemical modeling indicates that the inflow and transition waters are saturated with respect 24 to calcite whereas lagoons, transition zone margin, halite nucleus margin and nucleus waters are 25 saturated with respect to calcite, gypsum, and halite, and the transition zone brines at depth 26 display a broader range of saturation states as compared to the nucleus brines. Long-term 27 remote-sensing of surface water body extents suggest that extreme precipitation events are the 28 primary driver of surface area changes (by a factor of 2.7 after storm). A major finding from this 29 work is that the subsurface brines in the transition zone and the halite nucleus are geochemically 30 and hydraulically disconnected from the groundwater discharge features (lagoons) over modern 31 time scales which has far reaching implications for understanding the link between brine and 32 freshwater. 33 Plain Language Summary 34 Salar systems are of intense global focus because of associated water and resource use issues. 35 The freshwater brine systems that are unique to these areas support both resource development 36 and community needs. Until now, rigorous, data-driven analyses of the hydrologic 37 characteristics of these systems have not been provided. This is the first integrated analysis of 38 freshwater-brine systems that characterize salars. The work includes an analysis of 1) 1 Introduction 50 The marginal environments of salar systems are unique ecological and hydrogeological regions 51 of great importance in arid to hyper-arid climates (Rosen, 1994 Figure 1. (carbonate, gypsum, and halite), major lagoon systems, wetlands and the open pools identified. The detailed hydrogeochemical transect is shown as a white line along the upgradient inflow zone through and across the transition zone, to the transition zone and nucleus margins and into the halite nucleus (inflow, TZ = transitions zone shallow and deep, lagoons, TZ margin, nucleus edge and nucleus, also refer to Movie S1 for a virtual field trip across these zones).

Geology and Hydrogeology of Transition Zones 171
The transitional zones of salars are known to be composed of a combination of alternating 172 sequences of evaporite deposits (ie. carbonate, gypsum, and halite), minor clastic material (clay, 173 silt, sand, and gravel), and in many cases volcanic ash and ignimbrite deposits. In the SdA basin 174 these geologic units make up the Vilama Formation, the stratigraphy of which is detailed in Lin transition to focused inflow. The overall morphology and the extent of flooding surfaces of the 183 lagoons may vary, but similar processes described in this paper apply to other lagoon systems. 184 The SdA marginal transition zone highlights the variability in lagoon morphology as well as the 185 regions between the lagoons and the nucleus margin. For example, the north Chaxa and Barros 186 Negros lagoon systems on the northeast transition zone appear to be fed by a large diffuse 187 groundwater region that becomes channelized into a small stream that feeds three lagoons which 188 are connected by small channels. Undoubtedly this lagoon system also receives inflow from the 189 eastern alluvial fans as small marshes and springs are observed to the east of these lagoons. In  196 Important to understanding the functioning of the transition zone and lagoon systems is the fact 197 that they are underlain by complex heterogeneous subsurface geology that is inherent in the 198 evaporite deposits (Warren, 2006) and interbedded ignimbrites, ashes, and clastic material which 199 together form the aquifer system. There are two main types of carbonate in the marginal zones 200 of the SdA which lie beneath the lagoons along with gypsum and minor halite. One type of 201 carbonate is that which is spatially most common near the edges of the basin where groundwater 202 has discharged in the past and/or in the modern, these carbonates are typically interbedded with 203 alluvial fan deposits and tend to have a vuggy or porous texture characteristic of tufa (Figure 3). 204 The other type of carbonate is the carbonate mud that forms in the lagoons associated with or 205 without microbialite and/or stromatolite deposits. These carbonate deposits can also be observed 206 forming in the modern environment (Sancho-Tomas et al., 2018) and are preserved in multiple 207 sediment cores extracted from the transition zone, these tend to be finely laminated and may 208 have gypsum crystals intergrown or in separate laminae/layers. In the subsurface these deposits 209 occur as lense shaped bodies which are consistent with the general morphology of the modern 210 lagoons and deposits forming today ( Figure 3 and Movie S1). 211 The gypsum deposits observed in core are characterized by either aggregates of prismatic 212 crystals usually with an upward growth pattern or lenticular aggregates which may represent 213 bottom-grown beds where gypsum crystals nucleate in evaporating brine. These textures are 214 found in the upper 10s of meters but at depths below that the texture is finer as gypsum mud or 215 compacted gypsum crystals. We define these textures generally at the meter scale as fine, 216 medium or course and conceptually illustrate this in Figure 3. Small open bodies of water not 217 larger than 10 m in diameter and 1-2 m deep that occur in the transition zone margin are at 218 saturation with respect to gypsum and are characterized by euhedral pyramidal aggregates of 219 gypsum crystals or rosette aggregates that form mounds on the sides and bottom of the pools 220 (these can also be observed in Movie S1 in the region salarward of the lagoons). Gypsum 221 crystals have also been observed to be forming at the surface of these features, these crystals 222 presumably accumulate in layers in these pools over time. Gypsum also occurs as secondary 223 crystals infilling voids as large euhedral crystals or as smaller crystals along fracture surfaces in 224 the ignimbrite. In the deepest (400 m) cores described from the nucleus there are both 225 recrystallized gypsum and halite beds that are highly compacted. Minor anhydrite has been 226 identified in sediment cores as elongate nodules or as thin beds.  Other geochemical data used in this paper originated from an internal industry report. These data 289 generally represent quarterly sampling over a period of up to a decade and are used primarily in 290 establishing seasonal variability and for modeling saturation indices for each hydrogeochemical 291 zone. All data used in this paper can be accessed at (https://doi.org/10.7275/qr40-z439). Modeling 292 results are contained in the supporting document for this paper in Table S1. Insitu measurements 293 of temperature, pH, and SC are reported as well as major and trace element concentrations and 294 anion concentrations. Methods of analysis for major elements are by ICP-OES, trace elements 295 by ICP-MS and anion concentrations by IC, bicarbonate was measured by titration in the 296 laboratory. SGS and ALS commercial geochemical laboratories were utilized for these analyses.  Finer scale characteristics such the heterogeneity in the transition zone geology, primary and secondary porosity and permeability features in the transition zone carbonate, gypsum and halite and halite nucleus are detailed in the circular insets. Important to note are the flow path arrows that depict diffuse groundwater movement in the shallow parts of the transition zone that ultimately end in the lagoons. Wider blue arrows indicate the relative amounts of infiltration (downward) and evaporation (upward). Our virtual field trip across the surface of these water zones is in Movie S1. the halite nucleus which are depicted in 3D view in Figure 3. Generally, the groundwaters in and 340 around the lagoons show significant spatial variability but have much less seasonality than the 341 lagoon waters themselves because they are sustained from inflow waters derived from the MNT 342 aquifer to the south and not as responsive to evaporation as the open water bodies. Along the flow path the general trend indicates that the inflow and transition zone waters are 490 undersaturated with respect to halite and gypsum (log Q/K < 0) but are at saturation with respect 491 to calcite (log Q/K > 0) and other carbonate, sulfate and silicate minerals (Table S1). The lagoon 492 waters which are represented here by waters from Laguna Brava are undersaturated with respect 493 to halite but are at saturation with respect to calcite and to a lesser extent gypsum. The TZ 494 margin, nucleus margin, and the nucleus waters are all saturated with respect to halite, gypsum 495 and calcite indicating that it is in these regions that concentrations/activities of the required ions 496 are elevated enough to cause the precipitation of all mineral phases. It is also apparent that the 497 lagoon, transition zone, and nucleus edge waters have the most range in SI values which is 498 expected because these waters are more susceptible to precipitation events and evaporation given that they are exposed at the surface or contain components of water that are exposed at the 500 surface. 501 502 Note that in the field there are areas within the transition zone that display vadose zone processes 503 are at work including formation of secondary mineral precipitates such as efflorescent salts and 504 chlorides that are precipitating within cracks and other openings in the primary salt crusts. We 505 attribute these to evaporation processes and the continual delivery of solutes above the water 506 table to form these secondary minerals.  paths into the TZ shallow zone that discharge at varying rates and locations throughout this area. 576 Some of the water forms springs that discharge at rates greater than the rate at which evaporation 577 can remove the water into the atmosphere. Water does not pool everywhere on the surface in 578 this zone for two reasons: 1) discharge appears to be smaller than the soil evaporation and 2) 579 once water is present at the surface evaporation rates increase substantially resulting in a non- The results of the geochemical equilibrium modeling presented in Figure 6 indicate that there are 641 definitive zones of predictable mineral precipitation in the inflow-transition-nucleus system but 642 that there is considerable variability particularly in surface water bodies directly exposed to the

Introduction
The supporting information contained in this section includes results from geochemical equilibrium modeling of the different water types analyzed in this paper for each of the hydrogeologic zones defined. These modeling results were used to produce figure 6 in the main text and all of the raw data are included at https://doi.org/10.7275/qr40-z439. Geochemist Workbench was used to produce these modeled results of saturation indices with the ionic strength approximation used for each sample based on the ionic strength of the solution (greater than or less than 0.1). Saturation indices values below 1.0 indicate undersaturation and values at or above 1.0 indicate saturation based on thermodynamic data and temperature of each sample. Inflow waters with lower ionic strength were modeled with the Debye-Huckel approximation and brackish waters and brines were modeled with the Harvie-Moller-Weare approximation.
Celestite (BaSO4) is predicted to be undersaturated in all water zones. Anhydrite is also predicted as undersaturated in all water zones which is consistent with our observations that this mineral although present is very sparse in outcrop and diamond drill cores recovered from the transition zone and the nucleus. Antarcticite (CaCl*6H2O) is included in the modeling results and is undersaturated in all waters analyzed. Table S1 where elemental concentration data were not available. Table S1. Modeled saturation indices for all water types and samples used in the analysis.

Saturation indices are blank in
A virtual field trip movie is included to aid in the reader's experience and understanding of the environment and defined water zones in this study. The major water zones and some of the important surface water bodies are identified with text in the movie, these correspond with the zones and features referred to in the text. Enjoy.
Movie S1. Drone video of the newly identified hydrogeologic and geochemical zones of the Salar de Atacama.