ABSTRACT Aquifers possess distinct mineralogy, diagenetic history, and geochemical constraints, which control the pore water’s hydrochemistry. Whenever aquifers exchange groundwater, the incoming groundwater establishes a new chemical driving force, imposing new reactions related to rock-water interactions. This study developed a simple approach to assess the mass balance of the rock-water interaction of the groundwater exchanged between interconnected aquifers. To illustrate the suitability of this approach, it was applied to the Bauru Aquifer System located in Sao Paulo State in southeastern Brazil. The hydrochemical characterization identified three major stratified hydrochemical facies with typical chemical compositions as well as the authigenic mineralogy assemblages. The first and shallowest facies was composed of a variable groundwater composition along with highly diluted water from a recent recharge. The second facies was composed of unconfined bicarbonate calcium-rich water with a mineralogy mainly composed of K-feldspar, Mg-montmorillonite, and calcite. The third facies, which was confined and the deepest, was composed of bicarbonate sodium-rich water and mineralogically dominated by Na-, Mg-, and Ca-montmorillonite, calcite, and analcime. Although the replacement of Na by Ca suggests an extensive cation exchange in the deep-confined portion, sodium-rich dissolution and calcium-rich clay precipitation were identified as the operating mechanisms promoting Na enrichment and Ca consumption in the water of the closed system. This finding is supported by the authigenic mineral characterization, δ13C of the dissolved inorganic compounds, and numerical analysis. The approach developed for this study, which used numerical simulations to calculate the mass balance, was capable of consistently reproducing the water composition and mineralogy changes.

中文翻译：

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具有地球化学分层的含水层系统中水流的岩溶质反应质量平衡

摘要含水层具有独特的矿物学、成岩历史和地球化学约束，它们控制着孔隙水的水化学。每当含水层交换地下水时，流入的地下水就会产生一种新的化学驱动力，从而产生与岩水相互作用相关的新反应。这项研究开发了一种简单的方法来评估相互连接的含水层之间交换的地下水的岩水相互作用的质量平衡。为了说明这种方法的适用性，它被应用于位于巴西东南部圣保罗州的包鲁含水层系统。水化学表征确定了具有典型化学成分的三个主要分层水化学相以及自生矿物组合。第一个也是最浅的相由可变的地下水成分以及最近补给的高度稀释的水组成。第二相由无侧限碳酸氢钙富钙水组成，矿物学主要由钾长石、镁蒙脱石和方解石组成。第三相是封闭的、最深的，由富含碳酸氢钠的水组成，矿物以钠、镁、钙蒙脱石、方解石和方沸石为主。尽管用 Ca 替代 Na 表明在深部限制部分存在广泛的阳离子交换，但富钠溶解和富钙粘土沉淀被确定为促进封闭系统水中 Na 富集和 Ca 消耗的运行机制。这一发现得到了自生矿物特征的支持，溶解的无机化合物的δ13C，以及数值分析。本研究开发的方法使用数值模拟来计算质量平衡，能够始终如一地再现水成分和矿物学变化。