TRINITY AQUIFER EDUCATION
Learning About the Trinity Aquifer
The Trinity Aquifer is one of nine major aquifers defined by the state, stretching from the Texas Hill Country into Oklahoma, and it is the primary water source within the Trinity Glen Rose Groundwater Conservation District (TGR).​
Pecos Valley
Seymore
Gulf Coast
Carrizo-Wilcox Outcrop
Carrizo-Wilcox Subgroup
Hueco-Mesilla Bolsons
Ogallala
Edwards-Trinity Outcrop
Edwards Outcrop
Trinity Outcrop
Trinity Subcrop
Edwards Subcrop
Edwards-Trinity Subcrop
Trinity Aquifer in Northern Bexar County
Edwards Aquifer Region in Blue
Though often referred to as a single aquifer, the Trinity Aquifer is really a group of aquifers, experts say. It’s divided into three units — the Upper, Middle and Lower Trinity — and within those are more layers, or strata, made up of different formations, all of which allow water to move through in different ways and at different rates.
The Trinity system stretches across 61 counties from northeast of Dallas to southwest of San Antonio covering more than 10,000 square miles at the surface and more than 21,000 square miles below ground. It runs throughout much of the Hill Country, including all or portions of Kendall, Comal, Hays and Blanco counties, and is considered one of the “most extensive and highly used groundwater resources in Texas” by the Texas Water Development Board.
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The Trinity Aquifer is a low production aquifer which recharges slowly, with only 4–5% of rain recharging it. Despite its slow recharge rate, it stands as one of Texas’ most extensive and heavily used groundwater resources. Primarily utilized by municipalities, it also serves as a vital source for rural homes, irrigation, livestock, and other domestic uses.
Trinity Aquifer
Trinity Aquifer Subgroup (down dip)
Trinity Aquifer in Northern Bexar County
Trinity Aquifer water well depths vary from shallow, hand-dug wells to drilled wells ranging from 100 feet deep to over 1,200 feet deep based on TWDB records for Bexar County. Depths are highly variable and depend entirely on site-specific topography and geology, especially faulting. Well yields in the Trinity are typically dependent on the nature of fractures and dissolution features intersected by the well bore, which can be highly variable over short distances. Well yields are typically low (less than 20 gallons per minute (gpm)) but may occasionally be significantly higher, with yields of 400-600 gpm being reported in some wells.
Water quality within a specific aquifer can be defined or characterized in a general sense, but can vary significantly based on local geology, hydrology, and structure.
The Trinity Glen Rose District is located in the southern portion of the Trinity Aquifer where it underlies the Texas Hill Country. The District includes the northern third of Bexar County as well as small parts of Comal and Kendall counties and is part of the Edwards Plateau. Although it is defined by the state as a major aquifer, the water quality and productivity of the Trinity Aquifer throughout the region, including the District, varies greatly.
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The District is located entirely within the San Antonio River basin and is divided by three sub-basins: the Cibolo Creek, Medina River, and Upper San Antonio River sub-basins. The Cibolo Creek, Leon Creek, Salado Creek, and the Upper San Antonio River watersheds provide for surface drainage generally from the northwest to the southeast within the District. Cibolo Creek is a tributary of the San Antonio River and drains from northwest to southeast across the Trinity Group of Aquifers, where it is a major recharge feature of these aquifers. Cibolo Creek is also the county boundary between portions of northern Bexar County and adjacent counties to the north and east. Salado and Leon Creeks flow from northwest to southeast through the middle of the District, eventually joining the San Antonio and Medina Rivers to the south.
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As seen below, the Trinity Glen Rose Groundwater Conservation District is divided into five precincts.
Recharge of the Aquifer Through Rainfall - Recharge to the Trinity Group of Aquifers occurs via local rainfall on its outcrop and flows in Cibolo Creek and through the overlying units where the Trinity Aquifer is present in the subsurface. When Cibolo Creek has water in the stretches along northern Bexar County, the losses of water from the creek are substantial and serve as a significant recharge source to the Trinity Aquifer in the District. Although this rapid recharge can occur in other isolated areas of the Trinity Aquifer in the Hill Country, it is a dominant recharge mechanism in the District.
The substantial volume of recharge that periodically occurs during wet periods and flooding along Cibolo Creek creates relatively large variations in water levels in the Trinity Aquifer in the District. When water levels are high in the District, there is a natural hydraulic gradient that drives groundwater laterally to the southeast and into the Edwards Aquifer.
Dolomitic Limestone
Fossiliferous Limestone
Shale and Sandstone
Pumping of Water from the Trinity Aquifer - While the Edwards Aquifer receives large volumes of water from the Trinity Aquifer, water wells are a primary source of discharge from the Trinity in Bexar County. Historic pumping from the Trinity Aquifer within Bexar County has increased over time, from approximately 11,000 acre-feet in 2009 to greater than 23,000 acre-feet per year since 2016. Production from wells can vary significantly based on water levels in the Trinity.
Water demand will continue to increase in the Hill Country, and this may add more pressure to increase groundwater production from the Trinity Aquifer in the District. Production from the Trinity Aquifer is primarily used for municipal purposes, with some production for domestic, irrigation, livestock and mining. More than 80 percent of the current groundwater production from the Trinity in the District is used for public water supply.