CHEMICAL EVOLUTION AND RESIDENCE TIME OF GROUNDWATER IN THE WILCOX AQUIFER OF THE NORTHERN GULF COASTAL PLAIN

Author

Estifanos Haile, University of KentuckyFollow

Date Available

2-22-2012

Year of Publication

2011

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Earth and Environmental Sciences (Geology)

First Advisor

Dr. Alan E. Fryar

Abstract

This study aims to integrate groundwater geochemistry and mathematical modeling to determine the dominant geochemical processes and groundwater residence time within the Wilcox aquifer in the northern Gulf Coastal Plain. Groundwater samples were collected and analyzed for major ion chemistry, stable isotopes (¹⁸O, ²H, and ¹³C), and radioisotope ³⁶Cl content. Geochemical modeling enabled the identification of major sources and sinks of solutes in the aquifer. A two-dimensional, finite-difference, numerical model was used to determine the deep groundwater flow rate and transport of ³⁶Cl in the aquifer. Major ion chemistry shows a chromatographic pattern along the flow path in which a gradual increase of Na⁺ and decrease of Ca²⁺ and Mg²⁺ is evident. The most plausible inverse models in the downgradient section of the aquifer indicate that oxidation of organic matter (OM), which may be associated with discontinuous lenses of lignite, and consequent release of CO₂ sustain the reduction of Fe(III) (oxyhydr)oxides and sulfate and the dissolution of carbonate minerals (calcite and, in some instances, siderite). These processes, in turn, result in pyrite precipitation and exchange of Ca²⁺ for Na⁺ on clay-mineral surfaces. Models constrained with ¹³C are consistent with mole transfers between pairs of wells in close proximity, but not for the entire flow path. The observed range of δ¹³C of dissolved inorganic carbon (-7.3‰ to -12.4‰) is interpreted as a result of both oxidation of OM and dissolution of carbonates. Calculated values of ³⁶Cl/Cl show an abrupt discontinuity between the upgradient and downgradient sections that was also observed in δ¹⁸O and δ²H data. The gradual enrichment of ¹⁸O and ²H along the flow path could be the result of diffusion. The distinct differences in δ¹⁸O and δ²H between the upgradient and downgradient Wilcox aquifer suggest that the latter preserves a paleoclimatic signal.

Recommended Citation

Haile, Estifanos, "CHEMICAL EVOLUTION AND RESIDENCE TIME OF GROUNDWATER IN THE WILCOX AQUIFER OF THE NORTHERN GULF COASTAL PLAIN" (2011). Theses and Dissertations--Earth and Environmental Sciences. 2.
https://uknowledge.uky.edu/ees_etds/2