ABSTRACT The groundwater flow system composed of the Edwards-Trinity (Plateau) Aquifer and the Hill Country portion of the Trinity Aquifer together occupy an area of ~100,000 km 2 of west-central Texas. In addition to the common groundwater flow system, these aquifers also share a common, contiguous hydrostratigraphy—the Trinity Group hydrostratigraphic unit. The aquifers provide the primary source of water for the Edwards Plateau and Texas Hill Country and also sustain numerous springs and streams in the region. The sensitivity of the aquifers to drought and well discharge has raised concerns over the availability of water from these aquifers. Groundwater discharge takes the form of (1) discharge to streams and springs; (2) evapotranspiration; (3) pumpage from wells; and (4) cross-formational flow across the Balcones fault zone boundary to the Edwards (Balcones Fault Zone) Aquifer and underlying parts of the Trinity Aquifer. Groundwater inflow to these aquifers occurs by diffuse and discrete infiltration through the aquifer outcrops. Due to regional variability of lithologic compositions, hydraulic conductivity and storativity vary both vertically and laterally throughout the aquifer, with hydraulic conductivity decreasing with depth and from north to south.

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.