Upper Trinity: how deep the wells are
Home water wells that draw from the Upper Trinity in the Texas Hill Country are typically 185 to 340 feet deep (median 245 ft, 467 wells drilled in the last 10 years). In the state's model it is limestone and marl. The county with the most wells in this layer is Gillespie (188). Estimated cost of a complete well at the median depth: $8,500 to $21,000.
Checked against the state's own aquifer codes: where our method names this layer, the state agrees in 59% of 347 wells. Read it as the likely layer, not a certain one.
Updated every day from Texas Water Development Board driller reports. Last update: November 11, 2026, using the state file of November 10, 2026.
Under Helotes, Bexar County
Dot: median. Bar: middle half (25th to 75th percentile). Counties with the most wells in this layer.
Digging in…
What this layer is
The Upper Trinity is the upper member of the Glen Rose Limestone: alternating beds of limestone and marl that make the stair-step hills of the Hill Country. It is the first Trinity layer a driller meets and usually the least productive, so many home wells pass through it and are completed deeper.
Units the model puts in this layer: upper Glen Rose Limestone.
Source of the layers: TWDB's Hill Country Trinity model, with 1 mile cells.
Key figures
- Typical depth (middle half)
- 185 to 340 feet
- Median depth
- 245 feet
- Deepest 10% are past
- 457 feet
- Water level (median)
- 118 feet
- 379 wells
- Yield (median)
- 13 gpm
- 345 wells, gallons per minute
- Wells used
- 467
- In the last 10 years
- All years on file
- 1,228
- median 245 ft, since 1982
- Confidence
- Standard (30 or more wells)
Counties where homes draw from this layer
Only counties with at least 10 wells in the layer. The share is of the county's wells that the model covers.
| County | Median depth (ft) | Wells | Share of the county's wells |
|---|---|---|---|
| Gillespie County | 200 | 188 | 14% |
| Kerr County | 280 | 153 | 17% |
| Kendall County | 186 | 32 | 3% |
| Blanco County | 441 | 28 | 4% |
| Comal County | 480 | 23 | 2% |
| Bexar County | 450 | 21 | 5% |
| Bandera County | 215 | 12 | 1% |
Places where it is the main layer
- Helotes 505 ft
Where it sits among the model's layers
From top to bottom, with the median depth of the home wells that draw from each:
| Layer (state's model) | Rock | Median depth (ft) | Wells |
|---|---|---|---|
| Edwards Group | limestone | 205 | 233 |
| Upper Trinity | limestone and marl | 245 | 467 |
| Middle Trinity | limestone and sand | 480 | 5,005 |
| Lower Trinity | sand and limestone | 510 | 1,875 |
Depth over time
Home well depth in this layer has not changed meaningfully in 20 years: the median was 245 feet in 2003 to 2007 and 240 feet in 2021 to 2025 (a difference under 10% or within the margin of error).
| Period | Median depth (ft) | Wells |
|---|---|---|
| 2001–2005 | 245 | 323 |
| 2006–2010 | 240 | 235 |
| 2011–2015 | 244 | 179 |
| 2016–2020 | 260 | 210 |
| 2021–2025 | 240 | 259 |
| 2026 | 200 | 20 |
Common questions
How deep is a well in the Upper Trinity?
Home water wells that draw from the Upper Trinity in the Texas Hill Country are typically 185 to 340 feet deep (median 245 ft, 467 wells drilled in the last 10 years).
What is the Upper Trinity?
The Upper Trinity is the upper member of the Glen Rose Limestone: alternating beds of limestone and marl that make the stair-step hills of the Hill Country. It is the first Trinity layer a driller meets and usually the least productive, so many home wells pass through it and are completed deeper.
How deep does the water stand in these wells?
When the wells were finished, the water stood a median 118 feet below the surface (379 wells).
What a well in this layer costs
At the median depth for this layer (245 feet), a complete home well would cost roughly $8,500 to $21,000: $25 to $60 per foot for drilling, casing and seal, plus $2,500 to $6,500 for the pump, pressure tank, controls and hookup at the well. Across the whole middle half of depths (185 to 340 feet) the range runs from $7,000 to $27,000. This is TerraFunded's estimate from published Texas prices, not a quote, and it leaves out trenching and the line to the house. Get prices from at least two licensed drillers.
How these numbers are made
We count new and replacement domestic (home) wells with a valid depth on their report. We use the last 10 years when there are at least 30 wells; otherwise all since 2001. The median is the middle well and the middle half runs from the 25th to the 75th percentile.
To tell which layer a well draws from, we take the ground elevation at its location, subtract the depth of the interval where water enters (or the bottom of the hole when the report does not give one) and see which layer of the state's model that point falls in.
For 88% of these wells the report says where the water enters and we use the middle of that interval; for the rest we use the bottom of the hole, which tends to read one layer too deep.
How well it works: In 2,665 wells in the Texas Hill Country where the State of Texas recorded the aquifer, our estimate of the formation the well draws from names the same layer of the state's model in about 76% of them, and that layer or the one next to it in at least 98%.
- The layers come from the Texas Water Development Board's Hill Country Trinity model, with 1 mile cells. It is a regional model, not a survey of any lot: between one model point and the next, the real depth of each layer can differ.
- "Near or below the base of the Trinity" means the well reaches the bottom of the model or passes it, often by tens of feet. Older rock that the model does not include may lie there.
- We never use the model's water levels. The water level on this page is the one each driller measured when the well was finished.
These are depths of real wells drilled nearby, not a promise about any property. Groundwater can change over short distances and with drought. A licensed water well driller can tell you what to expect on a specific tract.
Read the full methodology · Download the data (CSV, CC BY 4.0)
How to cite
TerraFunded (2026). Texas Well Atlas: Underground. From Texas Water Development Board driller reports and groundwater availability models (data as of November 10, 2026). https://terrafunded.com/wells/formations/upper-trinity. License CC BY 4.0. https://doi.org/10.5281/zenodo.24126380.
Sources
- Submitted Driller's Reports database, Texas Water Development Board (TWDB) (file of November 10, 2026)
- Groundwater Availability Model for the Hill Country portion of the Trinity Aquifer System, Texas. Jones, I.C., Anaya, R., and Wade, S.C., Texas Water Development Board, 2009, GAM v2.01 (MODFLOW-96). TWDB groundwater availability model.
- Ground elevation: Terrain Tiles (Mapzen), Registry of Open Data on AWS
- TWDB well report viewer (each well's official report)