Validity and slopes of the linear equation of state for natural brines in salt lake systems
Validity and slopes of the linear equation of state for natural brines in salt lake systems
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DOI：
10.1016/j.jhydrol.2015.01.054
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发表时间：
2015-04
影响因子：
6.4
通讯作者：
C. Kohfahl;V. Post;E. Hamann;H. Prommer;C. Simmons
中科院分区：
文献类型：
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作者：
C. Kohfahl;V. Post;E. Hamann;H. Prommer;C. Simmons
许多与密度相关的地下水流模拟依赖于将流体密度与总溶解溶质含量 (TDS) 联系起来的线性状态方程。这种方法忽略了混合效应的非线性体积以及任何化学反应的影响。这些影响可以通过使用地球化学代码来考虑，这些代码实施基于单个溶质种类的浓度计算流体密度的算法。虽然原则上可以使用此类算法代替地下水模型中的线性状态方程，但由于计算开销较大，因此优选使用更简化的状态方程。这要求必须确定线性假设以及线性斜率的适当值。在这里，将已发表的 7 种化学成分不同的盐湖卤水的密度测量值与 PHREEQC-3 计算的密度进行了比较，证实了 PHREEQC 算法对盐湖卤水以及实验室实验中的海水卤水和人工卤水的适用性。此外，PHREEQC-3 的计算用于评估蒸发浓缩过程中矿物质沉淀反应的影响。结果表明，密度-TDS 关系在较宽的浓度范围内基本呈线性，斜率范围在 0.64 至 0.75 之间，该范围的上限适用于 Na-CO3-Cl 盐水，下限适用于 Na-Cl 盐水。高溶解度蒸发矿物（例如石盐和天然碱）的矿物沉淀限制了 TDS，并且可能导致基于高浓度线性状态方程的耦合流动模拟出现相当大的误差。斜率的错误表述可能会导致盐湖边缘卤水鼻的计算长度或瑞利数（表明密度分层是否稳定）的计算误差高达 20%。
Many density-dependent groundwater flow simulations rely on a linear equation of state that relates the fluid density to the total dissolved solute content (TDS). This approach ignores non-linear volume of mixing effects, as well as the impact of any chemical reactions. These effects can be considered by using geochemical codes that implement algorithms that calculate the density of a fluid based on the concentration of individual solute species. While in principle such algorithms could be used in-lieu of a linear equation of state in a groundwater model, the computational overhead is such that the use of a more simplified equation of state is preferred. This requires that the assumption of linearity as well as the appropriate value of the linear slope have to be determined. Here, published density measurements of 7 chemically-distinct salt lake brines are compared with densities calculated by PHREEQC-3, confirming the applicability of PHREEQC’s algorithm to salt lake brines, as well as to seawater brines and artificial brines from laboratory experiments. Further, calculations with PHREEQC-3 are used to assess the impact of mineral precipitation reactions during evaporative concentration. Results show that the density–TDS relationship is essentially linear over a wide concentration range, and that slopes range between 0.64 and 0.75, with the upper end of the range applying to Na–CO3–Cl brines and the lower end to Na–Cl brines. Mineral precipitation of highly-soluble evaporate minerals such as halite and trona limit TDS, and may lead to considerable errors in coupled flow simulations based on a linear equation of state at high concentrations. Misrepresentation of the slope may lead to an error of up to 20% in the calculated length of the brine nose bordering a salt lake, or of the Rayleigh number, which indicates if a density stratification is stable or not.