Ground-water hydraulics and hydrochemical facies in Eocene aquifers of the East Texas Basin

Eocene stratigraphic units in the East Texas Basin are composed of a thick sedimentary sequence (approximately 2,000 ft [600 m]) of fresh-water aquifers and aquitards covering an area of approximately 15,000 mi2 (51,000 km2). Analysis of abundant data on hydraulic head, pressure, and water chemistry from water wells tapping the Wilcox, Carrizo, and Queen City aquifers reveals the regional ground-water circulation patterns, locations and mechanisms of major recharge and discharge areas, and evolution of ground-water chemistry. The Queen City aquifer is primarily a water-table (unconfined) system in which effects of topography create a series of local ground-water basins. The Wilcox-Carrizo aquifer system includes (1) an artesian (confined) section that is separated from the Queen City by the Reklaw Formation, a leaky aquitard, and (2) a water-table (unconfined) system where the Wilcox-Carrizo crops out along the west, north, and east margins of the basin. Structure and topography are major controls on ground-water circulation in the Wilcox-Carrizo. A structural ridge in the Wilcox-Carrizo corresponds roughly to a ground-water divide that separates a southward component of flow from one that is directed northeastward toward the Texas-Louisiana border. Topographically controlled vertical leakage between the Queen City and Wilcox-Carrizo aquifer systems affects circulation in the Wilcox-Carrizo. The vertical head differentials and the distribution of flowing wells indicate downward leakage over most of the basin and upward leakage only beneath the Trinity and Sabine Rivers. Similarly, pressure-versus-depth data from the Wilcox-Carrizo indicate a predominantly downward component of vertical flow that intensifies toward topographically higher areas and reverses direction beneath the Trinity River. As ground water flows from outcrop down the hydraulic gradient into the artesian part of the Wilcox-Carrizo aquifer, it is consistently altered chemically from an acidic oxidized calcium-magnesium-bicarbonate sulfate water to a basic reduced sodium bicarbonate water. This change in the water chemistry is predominantly controlled by two reactions: calcite dissolution and cation exchange with montmorillonitic clays. Water samples with anomalous chemical composition (compared with regional chemistry) indicate salt dome dissolution or anomalous hydrologic conditions, such as relatively high rates of recharge to the artesian part of the Wilcox-Carrizo through leaky aquitards.�