ABSTRACT The northern segment of the Edwards (Balcones Fault Zone) Aquifer is an important source of water for municipalities, industry, and landowners in central Texas. Rapid population growth in this part of Texas has increased interest in the north segment of the aquifer and heightened concerns about groundwater availability. The aquifer consists of Cretaceous limestone stratigraphic units that crop out along its western margin and dip toward the east. Groundwater primarily flows from the aquifer outcrop recharge zones toward discharge zones along perennial rivers and streams in the outcrop area and to a lesser extent toward deeper parts of the aquifer, eventually discharging by cross-formational flow to overlying stratigraphic units, such as the Del Rio Clay, Buda Limestone, and Austin Chalk. Groundwater isotope compositions in the aquifer indicate that groundwater flow is most active in the unconfined parts of the aquifer and that most recharge occurs during late fall and winter months, even though highest monthly precipitation occurs during the spring. Pumping from the northern segment of the Edwards (Balcones Fault Zone) Aquifer is ~6.8 × 10 7 L/d, having peaked at ~1.0 × 10 8 L/d in 2004, but still up from ~3.4 × 10 7 L/d in the 1980s. Most of this pumping (~90%) is for municipal uses. However, in the rural northern and heavily urbanized southern parts of the aquifer, domestic and manufacturing uses, respectively, account for a significant portion of total pumping.

A chronostratigraphic framework was developed for the subsurface Eagle Ford of South Texas in conjunction with a log-based regional study that was extended across the San Marcos Arch and into East Texas using biostratigraphic and geochemical data to constrain log correlations of 12 horizons from 1729 wells in South and East Texas. Seven regional depositional episodes were identified by the study. The clayrich Maness Shale was deposited during the Early Cenomanian in East Texas and northern South Texas where it correlates to the base of the Lower Eagle Ford. After a fall in sea-level, East Texas was dominated by the thick siliciclastics of the Woodbine Group, whereas in South Texas deposition of the organic-rich EGFD100 marls of the Lower Eagle Ford began during the subsequent Lewisville transgression. A shift in depositional style to the limestones and organic-rich shales of the Eagle Ford Group occurred in East Texas during the Middle-Late Cenomanian EGFD200 and EGFD300 episodes produced by the continued rise in sea-level. Erosion along the Sabine Uplift shifted the focus of deposition in East Texas southward to the Harris delta and deposited the “clay wedge” of northern South Texas during the EGFD400 episode. The introduction of an oxygenated bottom-water mass onto the Texas shelf produced the considerable decrease in TOC preservation that marks the Lower/Upper Eagle Ford contact. This event coincided with the onset of Oceanic Anoxic Event 2 (OAE2) and the Cenomanian-Turonian Boundary sea-level high, which starved much of the Texas shelf of sediment. The only significant source of sediment was from the south; within the study area, the EGFD500 interval is essentially absent north of the San Marcos Arch. Deposition recommenced on much of the Texas shelf during the Late Turonian EGFD600 episode with the Sub-Clarksville delta of East Texas and the carbonate-rich Langtry Member of South Texas and eastern West Texas. Bottom-waters became oxygenated at approximately 90 Ma, initiating the transition from the Eagle Ford Group to the Austin Chalk.