Source of saline groundwater in basal Pennsylvanian sandstones, southwestern Illinois: Implications for fluid mixing and water-rock interactions

Saline groundwater is common in basal Pennsylvanian sandstones in southwestern Illinois. It is an environmental concern because well water is an important source of drinking water for local residents and farms. The origin of this saline groundwater is poorly understood. In this study, I combine hydrochemistry, isotope geochemistry, and petrography to evaluate the importance of fluid mixing and water-rock interactions in producing the saline groundwater.; The hydrochemistry of groundwater in basal Pennsylvanian sandstone changes from Ca-Mg-SO{dollar}sb4{dollar}-HCO{dollar}sb3{dollar} near the recharge zones to Na-Cl at depth. Na-Cl-Br relationships show that saline groundwater in deep zones is formed by fluid mixing between groundwater and basinal brine. Brine leaks into the aquifer from underlying bedrock and mixes with groundwater. In the Du Quoin area, saline groundwater is found in wells less than 10 m deep. Hydrochemically, those waters are indistinguishable from saline groundwater in basal Pennsylvanian sandstone at much greater depth. Mixed water from the deeper part of the aquifer must have migrated upward through structural faults and fractures along the Du Quoin Monocline.; The high sulfate concentration in groundwater near recharge zones is attributed to pyrite oxidation that occurs in the unsaturated zone and gypsum dissolution in the saturated and unsaturated zone. H{dollar}sp+{dollar} ions produced by pyrite oxidation dissolve calcite and dolomite to produce Ca{dollar}sp{lcub}2+{rcub},{dollar} Mg{dollar}sp{lcub}2+{rcub},{dollar} and HCO{dollar}sb3sp-{dollar} ions. As a result, the dominant ions in groundwater in the shallow zone (group A) are Ca{dollar}sp{lcub}2+{rcub},{dollar} Mg{dollar}sp{lcub}2+{rcub},{dollar} SO{dollar}sb4sp{lcub}2-{rcub},{dollar} and HCO{dollar}sb3sp-.{dollar} In the shallow zone, hydrogen ions also promote the dissolution of siderite and the transformation of illite/smectite to kaolinite. As groundwater migrates farther downdip, organic matter oxidation provide electrons to reduce iron-oxides and sulfate. In addition, mixing with brine increases the concentration of Ca{dollar}sp{lcub}2+{rcub}{dollar} and Mg{dollar}sp{lcub}2+{rcub}.{dollar} The combined effect of those processes promotes carbonate precipitation in the groundwater.; Variations in dissolved sulfate content and sulfur isotopic composition suggest that bacterial sulfate reduction to sulfide occurs in the aquifer. Sulfur isotopic ratios suggest that low-sulfate groundwater in deep zones can be formed by high-sulfate groundwater through a 30 to 45 percent reduction based on a 50 permil fractionation between sulfate and sulfide.; Carbon isotopic ratios of dissolved inorganic carbon of most of the groundwater suggest input from both organic carbon and carbonates. High {dollar}deltasp{lcub}13{rcub}{dollar}C of brine and coal seepage water indicates extensive methane production. Oxygen and carbon isotopic ratios of calcite and siderite cement in the sandstone indicate that those carbonate minerals are not in isotopic equilibrium with modern groundwater or brine. Oxygen and isotopic evidence suggest that of calcite and siderite formed at higher than in situ temperatures. Instead, they could be formed at a slightly higher temperature, which suggests that the diagenetic event occurred when basal Pennsylvanian sandstones were buried at a greater depth.