Abstract Salt flat brines are a major source of minerals and especially lithium. Moreover, valuable wetlands with delicate ecologies are also commonly present at the margins of salt flats. Therefore, the efficient and sustainable exploitation of the brines they contain requires detailed knowledge about the hydrogeology of the system. A critical issue is the freshwater-brine mixing zone, which develops as a result of the mass balance between the recharged freshwater and the evaporating brine. The complex processes occurring in salt flats require a three-dimensional (3D) approach to assess the mixing zone geometry. In this study, a 3D map of the mixing zone in a salt flat is presented, using the Salar de Atacama as an example. This mapping procedure is proposed as the basis of computationally efficient three-dimensional numerical models, provided that the hydraulic heads of freshwater and mixed waters are corrected based on their density variations to convert them into brine heads. After this correction, the locations of lagoons and wetlands that are characteristic of the marginal zones of the salt flats coincide with the regional minimum water (brine) heads. The different morphologies of the mixing zone resulting from this 3D mapping have been interpreted using a two-dimensional (2D) flow and transport numerical model of an idealized cross-section of the mixing zone. The result of the model shows a slope of the mixing zone that is similar to that obtained by 3D mapping and lower than in previous models. To explain this geometry, the 2D model was used to evaluate the effects of heterogeneity in the mixing zone geometry. The higher the permeability of the upper aquifer is, the lower the slope and the shallower the mixing zone become. This occurs because most of the freshwater lateral recharge flows through the upper aquifer due to its much higher transmissivity, thus reducing the freshwater head. The presence of a few meters of highly permeable materials in the upper part of these hydrogeological systems, such as alluvial fans or karstified evaporites that are frequently associated with the salt flats, is enough to greatly modify the geometry of the saline interface.

中文翻译：

---

类似于阿塔卡马盐沼（智利）的盐滩淡水-盐水混合区的 3D 绘图、流体动力学和建模

摘要 盐滩卤水是矿物质的主要来源，尤其是锂。此外，盐滩边缘还普遍存在生态脆弱的宝贵湿地。因此，有效和可持续地开发它们所含的卤水需要有关系统水文地质学的详细知识。一个关键问题是淡水-盐水混合区，这是由于补给淡水和蒸发盐水之间的质量平衡而形成的。盐滩中发生的复杂过程需要采用三维 (3D) 方法来评估混合区的几何形状。在本研究中，以阿塔卡马盐沼为例，展示了盐滩混合区的 3D 地图。该映射程序被提议作为计算高效的三维数值模型的基础，前提是淡水和混合水的水头根据它们的密度变化进行校正，以将它们转换为盐水水头。修正后，作为盐滩边缘带特征的泻湖和湿地的位置与区域最小水（盐水）水头重合。已使用混合区理想横截面的二维 (2D) 流动和传输数值模型解释了由该 3D 映射产生的混合区的不同形态。该模型的结果显示混合区的斜率与通过 3D 映射获得的斜率相似，并且低于以前的模型。为了解释这种几何形状，使用 2D 模型来评估混合区几何形状中的异质性影响。上层含水层的渗透率越高，斜率越低，混合区变得越浅。发生这种情况是因为大部分淡水侧向补给由于其更高的透射率而流经上层含水层，从而降低了淡水水头。在这些水文地质系统的上部存在几米高渗透性材料，例如经常与盐滩相关的冲积扇或岩溶蒸发岩，足以极大地改变盐分界面的几何形状。