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EXTENDED ABSTRACT: Geochemical Modeling of Regional Hydrochemical
Evolution in the Wilcox Aquifer of Missouri and Arkansas
Estifanos Haile and Alan E. Fryar
Department of Earth and Environmental Sciences, 101 Slone Bldg., University of Kentucky, Lexington, Kentucky 40506
The Wilcox Group is part of the large Gulf Coast sedimentary sequence of Paleocene to Miocene age, deposited in a fluvio-deltaic environment (Hosman, 1996). To examine regional trends in groundwater quality, we sampled 22 municipal wells in the Wilcox Group in southeast Missouri and eastern Arkansas during 2006 and 2007 (Fig. 1) for field parameters (pH, Eh, and electrical conductivity), major and minor solutes, and selected isotopes (2H and 18O of H2O; 13C of dissolved inorganic C [DIC]; 34S of dissolved SO42-; and 36Cl). In the confined Wilcox aquifer, which occurs at depths greater than approximately 150 m (492 ft) (primarily in Arkansas), certain major and minor solutes show distinct trends along the regional hydraulic gradient, while concentrations of other solutes remain relatively constant. In particular, Na+ (Fig. 2) and Cl- tend to increase; Ca2+ (Fig. 2), Mg2+, K+, Fe, and SO42- tend to decrease; and HCO3 and Si are relatively constant. Dissolved organic C (DOC) was below detection limit for all sampled wells.
In this study, we have attempted to resolve mechanisms of hydrochemical evolution along the regional hydraulic gradient in the Wilcox aquifer using geochemical modeling tools. There are two basic approaches to geochemical modeling: (1) tracing the relation between initial and final water compositions with or without mixing (inverse modeling), and (2) deriving a final water composition from a given (initial) composition (forward modeling) (van der Kemp et al., 2000). In this work we have used inverse modeling by specifying a set of reactions that involve plausible solid mineral phases and evaluating mole transfers between wells that are thought to lie along flow lines (Parkhurst and Appelo, 1999). Cross-formational diffusion of saline water from confining units was posed as a bivariate mixing problem. Because the wells from the unconfined section did not reveal any relation with down-gradient wells, only samples taken from the confined section have been modeled.
Using speciation calculations, saturation-index calculations, and inverse modeling in PHREEQC (Parkhurst and Appelo, 1999), we identified a combination of processes that are likely to control groundwater chemistry. These are (1) dissolution of calcite, (2) oxidation of goethite and organic matter (as CH2O), (3) precipitation of siderite and pyrite, (4) release of Na+ in exchange for Ca2+ (Fig. 2), Mg2+, and K+ on clay mineral surfaces, and (5) diffusion of Cl- and SO42-from adjoining confining units containing connate water. By analogy with previous studies of regional aquifer systems in the Atlantic Coastal Plain (Chapelle and Knobel, 1983, 1985), we hypothesize that dissolution of shell calcite occurs in the overlying Claiborne Group. Organic matter exists as lignite in the Wilcox and / or Claiborne sediments; mole transfers of CH2O are larger for up-gradient pairs of wells than for down-gradient pairs. Goethite is inferred as a product of weathering of glauconite, which has been observed in the Wilcox sediments along with pyrite. Silicate weathering is assumed to be minor and is neglected in the models because, as mentioned above, dissolved Si tends not to vary with depth or distance (i.e., it remains in an equilibrium state).
σ13C of DIC ranged from -7.3 to -12.4 ‰ VPDB (per mille Vienna Pee Dee Belemnite), with no discernible trend along the flow path (Fig. 3). This may reflect the irregular distribution of lignite in the aquifer or bulk mixing between enriched σ13C from calcite and depleted σ13C from organic matter. Mixing with saline water from confining units appears to be insignificant volumetrically.
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