Geochemical Modeling of the Madison Aquifer in Parts of Montana, Wyoming, and South Dakota
Geochemical Modeling of the Madison Aquifer in Parts of Montana, Wyoming, and South Dakota
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DOI：
10.1029/wr026i009p01981
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发表时间：
1990-09
影响因子：
5.4
通讯作者：
L. Plummer;J. Busby;R. W. Lee;B. Hanshaw
中科院分区：
文献类型：
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作者：
L. Plummer;J. Busby;R. W. Lee;B. Hanshaw
溶解碳酸盐、硫酸盐和硫化物的稳定同位素数据与水组成数据相结合，构建了怀俄明州、蒙大拿州和南达科他州部分麦迪逊含水层沿八条流动路径的地球化学反应模型。硫同位素数据被视为同位素稀释问题，而碳同位素数据被视为瑞利蒸馏。所有反应模型都重现了观测到的最终水体的化学和碳、硫同位素组成，并通过预测麦迪逊石灰岩中白云石和硬石膏的碳、硫同位素组成进行了部分验证。地球化学反应模型表明，麦迪逊含水层中地下水的主要反应是去白云化作用(方解石沉淀和硬石膏溶解驱动的白云石溶解)。硫酸盐还原、[Ca++Mg2+]Ina+阳离子交换和岩盐溶解在当地是重要的，特别是在蒙大拿州中部。地下水系统被视为对来自外部来源的二氧化碳气体是封闭的，如土壤带或交叉地层渗漏，但对二氧化碳是开放的，因为有机物的氧化加上在含水层内发生的硫酸盐还原和其他氧化还原过程。计算了麦迪逊硬石膏的矿物质量转移和模拟的硫同位素组成，绘制了整个研究区的地图。根据模拟的碳质量转移进行调整的地下水碳14年龄范围从现代到约23,000年B.P.，指示的流速为7~7ftlyr(2.1-26.5rnlyr)。根据达西定律利用14C平均流速计算出的水平导水率大多在基于数字模拟的5倍之内。计算的矿物质量转移和调整的14C地下水年龄可以确定含水层中的表观反应速率。有机质的表观氧化速率通常为0.12JLIDOl/Uyr。硫酸盐和次要的铁是主要的电子受体。在25℃时，34-S在硫酸盐和硫化氢之间的(动力学)生化分馏约为-44%o，温度变化为
Stable isotope data for dissolved carbonate, sulfate, and sulfide are combined with water compo sition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota. The sulfur isotope data are treated as an isotope dilution problem, whereas the carbon isotope data are treated as Rayleigh distillations. All reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the Madison Limestone. The geochemical reaction models indicate that the dominant groundwater reaction in the Madison aquifer is dedolomitization ~calcite precipita tion and dolomite dissolution driven by anhydrite dissolution). Sulfate reduction, [Ca + + Mg2+ ]INa+ cation exchange, and halite dissolution are locally important, particularly in central Montana. The groundwater system is treated as closed to C02 gas from external sources such as the soil zone or cross-formational leakage but open to C02 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer. The computed mineral mass transfers and modeled sulfur isotopic composition of Madison anhydrites are mapped throughout the study area. Carbon 14 groundwater ages, adjusted for the modeled carbon mass transfer, range from modem to about 23,000 years B.P. and indicate flow velocities of 7~7 ftlyr (2.1-26.5 rnlyr). Most horizontal hydraulic conductivities calculated from Darcy's Law using the average 14 C flow velocities are within a factor of 5 of those based on digital simulation. The calculated mineral mass transfer and adjusted 14 C groundwater ages permit determination of apparent rates of reaction in the aquifer. The apparent rate of organic matter oxidation is typically 0.12 JLIDOl/Uyr. Sulfate and, to a lesser extent, ferric iron are the predominant electron acceptors. The (kinetic) biochemical fractionation of 34 S between sulfate and hydrogen sulfide is approximately -44%o at 25•c, with a temperature variation of