Trinity Aquifer: Not Relevant for Purposes of Joint Planning
GMA 13 Technical Memorandum 16-05, Draft 1
William R. Hutchison, Ph.D., P.E., P.G.
March 29, 2016
Introduction
The Texas Water Development Board, in its July 2013 document, Explanatory Report for
Submittal of Desired Future Conditions to the Texas Water Development Board, offers thefollowing guidance regarding documentation for aquifers that are to be classified not relevant for
purposes of joint planning:
Districts in a groundwater management area may, as part of the process for
adopting and submitting desired future conditions, propose classification of aportion or portions of a relevant aquifer as non-relevant (31 Texas Administrative
Code 356.31 (b)). This proposed classification of an aquifer may be made if thedistricts determine that aquifer characteristics, groundwater demands, and current
groundwater uses do not warrant adoption of a desired future condition.
The districts must submit to the TWDB the following documentation for the portionof the aquifer proposed to be classified as non-relevant:
1. A description, location, and/or map of the aquifer or portion of theaquifer;
2. A summary of aquifer characteristics, groundwater demands, andcurrent groundwater uses, including the total estimated recoverable
storage as provided by the TWDB, that support the conclusion thatdesired future conditions in adjacent or hydraulically connected
relevant aquifer(s) will not be affected; and3. An explanation of why the aquifer or portion of the aquifer is nonrelevant for joint planning purposes.
This technical memorandum provides the required documentation to classify the Trinity Aquifer
as not relevant for purposes of joint planning.
Aquifer Description and Location
As described in George and others (2011):
The Trinity Aquifer, a major aquifer, extends across much of the central andnortheastern part of the state. It is composed of several smaller aquifers contained
within the Trinity Group. Although referred to differently in different parts of thestate, they include the Antlers, Glen Rose, Paluxy, Twin Mountains, Travis Peak,Hensell, and Hosston aquifers. These aquifers consist of limestones, sands, clays,gravels, and conglomerates. Their combined freshwater saturated thicknessaverages about 600 feet in North Texas and about 1,900 feet in Central Texas. In1

Trinity Aquifer: Not Relevant for Purposes of Joint Planning
GMA 13 Technical Memorandum 16-05, Draft 1
William R. Hutchison, Ph.D., P.E., P.G.
March 29, 2016
general, groundwater is fresh but very hard in the outcrop of the aquifer. Total
dissolved solids increase from less than 1,000 milligrams per liter in the east andsoutheast to between 1,000 and 5,000 milligrams per liter, or slightly to moderately
saline, as the depth to the aquifer increases. Sulfate and chloride concentrations
also tend to increase with depth. The Trinity Aquifer discharges to a large numberof springs, with most discharging less than 10 cubic feet per second. The aquifer is
one of the most extensive and highly used groundwater resources in Texas.
Although its primary use is for municipalities, it is also used for irrigation,
livestock, and other domestic purposes. Some of the state’s largest water level
declines, ranging from 350 to more than 1,000 feet, have occurred in counties along
the IH-35 corridor from McLennan County to Grayson County. These declines areprimarily attributed to municipal pumping, but they have slowed over the past
decade as a result of increasing reliance on surface water. The regional waterplanning groups, in their 2006 Regional Water Plans, recommended numerous
water management strategies for the Trinity Aquifer, including developing newwells and well fields, pumping more water from existing wells, overdrafting,reallocating supplies, and using surface water and groundwater conjunctively.
Figure 1 (taken from Wade and Bradley, 2013) shows the limited extent of the Trinity Aquifer in
GMA 13. Note that it occurs only in a small portion of Uvalde, Medina, Atascosa, and Bexarcounties.

2Trinity Aquifer: Not Relevant for Purposes of Joint Planning
GMA 13 Technical Memorandum 16-05, Draft 1
William R. Hutchison, Ph.D., P.E., P.G.
March 29, 2016

Figure 1. Location of Trinity Aquifer in GMA 13Aquifer Characteristics
Mace and others (2000, pp 54-55) provided maps of the distribution of hydraulic conductivity of
the area. Within GMA 13, the hydraulic conductivity appears to be less than 1 ft/day.

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Trinity Aquifer: Not Relevant for Purposes of Joint Planning
GMA 13 Technical Memorandum 16-05, Draft 1
William R. Hutchison, Ph.D., P.E., P.G.
March 29, 2016
Groundwater Demands and Current Groundwater Uses
The Texas Water Development Board pumping database shows 2012 groundwater pumping for
the Trinity Aquifer as follows:

Atascosa: 0 AF/yr
Bexar: 17,854 AF/yr
Medina: 338 AF/yr
Uvalde: 195 AF/yr

Please note that these are totals for the entire county, not necessarily in the GMA 13 portion of theTrinity Aquifer.
Total Estimated Recoverable StorageWade and others (2013) documented the total estimated recoverable storage for the Trinity Aquifer
in GMA 13 as follows:

Total storage is given in the first column. The recoverable storage is assumed to be between 25
and 75 percent of the total storage.
Explanation of Non-RelevanceDue to its limited areal extent and generally low use, the Trinity Aquifer is classified as not relevant
for purposes of joint planning in Groundwater Management Area 13.
References
George, P.G., Mace, R.E., and Petrossian, R., 2011. Aquifers of Texas. Texas Water Development
Board Report 380, July 2011, 182p.

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Trinity Aquifer: Not Relevant for Purposes of Joint Planning
GMA 13 Technical Memorandum 16-05, Draft 1
William R. Hutchison, Ph.D., P.E., P.G.
March 29, 2016
Mace, R.E., Chowdhury, A.H., Anaya, R., and Way, S-C., 2000. Groundwater Availability of theTrinity Aquifer, Hill Country Area, Texas: Numerical Simulations through 2050. Texas WaterDevelopment Board Report 353, September 2000, 122p.
Wade, S., and Bradley, R., 2013. GAM Task 13-036: Total Estimated Recoverable Storage forAquifers in Groundwater Management Area 13. Texas Water Development Board, Groundwater
Resources Division, July 8, 2013, 30p.

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