San Antonio Express-News (Sunday)
Lesser-known aquifer vital for area
Experts watch health of Trinity, which feeds Edwards
In downtown Wimberley, signs warn about swimming at your own risk in Cypress Creek — but no one's swimming as the water crawls slowly through the Hays County town.
A few miles away at Blue Hole Regional Park, the rope swings that typically launch swimmers into the crisp water below now hang still and quiet, and white lines on the tree roots show how far the water level has fallen. Upstream, nobody is splashing in the water at Jacob's Well. The springs that feed the iconic swimming hole have stopped flowing for one of the few times in recorded history, closing the well for the summer.
And over on the Guadalupe River, there's not been much river at all since July — just a trickle of water here and there.
All those streams and springs are fed by the Trinity Aquifer — and their low water levels raise concerns about the Trinity's health and underscore just how important the groundwater system is for much of Central Texas.
The Trinity Aquifer is a key water source for the San Antonio and Hill Country area, and its health plays a key role in the health of another important water source, the Edwards Aquifer, which provides water for more than 2 million people. Protecting the Trinity and understanding how it functions and interacts with the Edwards Aquifer is of growing importance as drought and higher demand combine to make water supply an ongoing problem in Texas, experts say.
That's why the low water flows on the surface are so concerning for people such as Larry Hull, president of the Comal Trinity Groundwater Conservation District. The lower stream flows and spring flows, the underground water supplies falling, wells drying up — it's all interconnected.
“I look at them as the same,” Hull said. “To me, that's indicative of the level of the aquifer going down, the lack of rain, the lack of springs.”
Although the neighboring Edwards Aquifer gets more attention in and around San An
tonio, the Trinity Aquifer’s location and composition make it a key component of the Texas groundwater ecosystem.
‘A real complex formation’
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 in 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.
Though often referred to as a single aquifer, the Trinity 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.
Like the Edwards, the Trinity is a karst aquifer, meaning it is recharged by fractures in the surface that let water flow in. But the Trinity has fewer karst features in its surface than the Edwards does, hydrologist Doug Wierman said — so there are fewer ways for water to enter the system.
The amount of water entering the aquifer also varies by location, according to Marcus Gary, principal geoscientist for the Edwards Aquifer Authority.
Charlie Flatten, general manager of the Hays Trinity Groundwater Conservation District, said that “the Trinity is what we would call a tight aquifer,” so it’s hard to get water out of it and back into it.
Only about 5% of rainfall actually enters the Trinity to recharge it, Flatten said. When the ground is dry, as it has been most of this year, it’s even harder for water to make its way into the system.
The geology of the aquifer also means wells around the region can have vastly differing water levels. Because of fault lines or types of rock formation in the ground, even neighboring wells can be in different situations.
“Each area’s so uniquely different,” Hull said. “It’s just a real complex formation.”
The speed of water movement within the aquifer also can vary greatly, said Micah Voulgaris, general manager of Cow Creek Groundwater Conservation District, which is based in Boerne and manages groundwater in Kendall County.
In some parts of the county, “we might see really rapid water movement,” Voulgaris said, while in others, the formation is so tight that there’s little horizontal movement. He said he’s seen wells influence each other from a mile away, but in other areas, there’s little interaction.
The Edwards Aquifer has more permeability, which allows water to flow throughout the system more easily, along long flow paths. That makes it easier to monitor the Edwards’ water level using fewer wells because it acts like a bathtub: Water entering in one part of the region results in higher levels across the system.
While the Edwards Aquifer Authority keeps data from dozens of wells, it relies on levels from the J-17 index well in Bexar County, along with springflows in New Braunfels and San Marcos, as the key metrics for drought restrictions. Because the Edwards is more permeable, Gary said, an index well such as J-17 is more reflective of the entire system.
“We can get rain in Medina or Bexar County and see a pretty quick impact at J-17 or Comal Springs, not just in a localized area,” Gary said.
That’s not the case for the Trinity. You can have wells a few miles apart, both in the Middle Trinity, he said, but their levels can vary dramatically depending on how connected they are to the karst features in that area. “It’s so highly variable in the Trinity that you need a lot more monitoring wells,” Gary said.
When water is pumped from part of the Edwards, the high permeability allows more water to flow in and replace it. In the Trinity, because the zones aren’t as connected, there’s less refill in some areas.
Despite their different structures, the two aquifers also interact with each other. That interaction happens primarily in two ways, Wierman said: Water
The Trinity has fewer surface fractures, allowing less water in to recharge it.
Trinity water levels are harder to determine because different monitoring wells tap into different layers of the aquifer.
Edwards Aquifer
Trinity Aquifer
Unlike the Edwards Aquifer, the Trinity is comprised of multiple
sections, each with various layers or strata.