RECHARGE AND REGIONAL-SCALE CHEMICAL EVOLUTION OF GROUNDWATER IN THE WILCOX AQUIFER, NORTHERN MISSISSIPPI EMBAYMENT, USA

Although recharge and hydrochemical evolution have been studied in various regional aquifer systems in the USA, such a study has not been conducted in the northern Gulf Coastal Plain. We focused on the confined lower Wilcox aquifer, which crops out along the margins of the Coastal Plain. We sampled 28 municipal wells on a 325-km north-south transect along the regional hydraulic gradient in Missouri and Arkansas. These included 20 Wilcox wells, 7 in the overlying Claiborne aquifer (to examine evidence of upward flow from the lower Wilcox), and 1 in the underlying McNairy-Nacatoch aquifer. Sampled well depths ranged from 116 to 518 m, increasing down-dip toward the south. Temperature and pH tend to increase with depth and distance from the recharge zone. The major-ion facies evolves from Ca-HCO₃ in the recharge zone to Na-HCO₃^- down-gradient. The lack of O₂ (27 wells ≤ 1.0 mg/L) and NO₃^- (undetectable in 23 wells), prevalence of Fe (18 wells > 0.3 mg/L), decreases in Mn and SO₄^2-, and increases in CH₄ down-gradient indicate that Wilcox groundwater becomes more anoxic with increasing residence time. Calcite dissolution, cation exchange, and redox reactions appear to be the primary controls on water quality.

Values of d²H and d¹⁸O are -42 to -28‰ and -6.8 to -5.1‰ (V-SMOW), respectively. These are slightly enriched with respect to the global meteoric water line (MWL) and fall along a regional MWL generated using precipitation samples from Paducah, Kentucky. The stable isotopes show two distinct, gradual enrichments along the flow path. There are two plausible explanations for these variations. First, the north-south increase in values is consistent with increasing proximity to the Gulf of Mexico (the moisture source area). Conversely, in a confined aquifer, increasing enrichment with distance from the recharge area is consistent with depletion of the isotopic composition of recharge over time. To resolve these explanations, we plan to estimate residence times and assess cross-formational leakage by modeling regional-scale flow with solute and heat transport.