June 16, 2016

DRAFT
Explanatory Report for Proposed Desired Future Conditions of
the Trinity Aquifer in Groundwater Management Area 10

Table of Contents
Section ...................................................................................................................................... Page
APPENDICES .............................................................................................................................. iv
FIGURES ........................................................................................................................................v
TABLES ........................................................................................................................................ vi
ABBREVIATIONS ..................................................................................................................... vii
1.

Description of Groundwater Management Area 10 .......................................................1

2.

Aquifer Description ...........................................................................................................2

3.

Desired Future Conditions ................................................................................................3

4.

Policy Justification .............................................................................................................4

5.

Technical Justification .......................................................................................................4

6.

Consideration of Designated Factors ...............................................................................5
6.1
Aquifer Uses or Conditions .................................................................................. 5
6.1.1
Description of Factors for the Trinity Aquifer in GMA 10 ............ 5
6.1.2
DFC Considerations............................................................................ 7
6.2
Water-Supply Needs ............................................................................................. 7
6.2.1
Description of Factors for the Trinity Aquifer in GMA 10 ............ 7
6.2.2
DFC Considerations............................................................................ 7
6.3
Water-Management Strategies ............................................................................ 8
6.3.1
Description of Factors for the Trinity Aquifer in GMA 10 ............ 8
6.3.2
DFC Considerations............................................................................ 9
6.4
Hydrological Conditions....................................................................................... 9
6.4.1
Description of Factors for the Trinity Aquifer in GMA 10 ............ 9
6.4.1.1
Total Estimated Recoverable Storage ............................................... 9
6.4.1.2
Average Annual Recharge ............................................................... 10
6.4.1.3
Inflows ................................................................................................ 10
6.4.1.4
Discharge ........................................................................................... 11
6.4.1.5
Other Environmental Impacts Including Springflow and
Groundwater/Surface Water Interaction ....................................... 11
6.4.2
DFC Considerations.......................................................................... 12

7.

Subsidence Impacts ..........................................................................................................128.

Socioeconomic Impacts Reasonably Expected to Occur ..............................................13
8.1
Description of Factors for the Trinity Aquifer in GMA 10 ............................ 13
ii

9.

8.2
DFC Considerations............................................................................................ 13
Private Property Impacts ................................................................................................13
9.1
Description of Factors for the Trinity Aquifer in GMA 10 ............................ 139.2
DFC Considerations............................................................................................ 14

10.

Feasibility of Achieving the DFCs ..................................................................................14

11.

Discussion of Other DFCs Considered ...........................................................................14

12.

Discussion of Other Recommendations .........................................................................15
12.1 Advisory Committees.......................................................................................... 1512.2 Public Comments ................................................................................................ 15

13.

Any Other Information Relevant to the Specific DFCs ...............................................15

14.

Provide a Balance Between the Highest Practicable Level of Groundwater
Production and the Conservation, Preservation, Protection, Recharging, andPrevention of Waste of Groundwater and Control of Subsidence in the Management
Area .................................................................................................................................. 15

15.

References .........................................................................................................................16iii

List of Appendices
Appendix A—Proposed Desired Future Condition Resolution
Appendix B—Water Management Strategies in the 2012 State Water Plan for Caldwell,
Hays and Travis Counties
Appendix C—Socioeconomic Impacts Analyses for Regions K, L, and M
Appendix D—Evaluation of Pumping at the Electro Purification Well Field in Hays
Countyiv

FIGURES
Figure ....................................................................................................................................... Page
1

Map of the administrative boundaries of GMA10 designated for joint-planning purposes
and the GCDs in the GMA (From Texas Water Development Board website) ..................22

Map showing the extend of the Trinity Aquifer in the GMA 10 (From Texas Water
Development Board website) ...............................................................................................3v

TABLES
Table ....................................................................................................................................... Page
1

Total groundwater pumpage values by county from the Trinity Aquifer in acre-ft/yr. Notethat pumping estimates may include areas of the Trinity Aquifer outside of GMA 10.......5

2

Total groundwater pumpage values in Uvalde County according the UCUWCD (2011) inacre-ft/yr ...............................................................................................................................63

Total groundwater pumpage values for Middle Trinity Aquifer and Lower Trinity
Aquifers according to BSEACD (acre-ft/yr) .......................................................................6

4

Total groundwater pumpage values for Middle Trinity Aquifer and Lower Trinity
Aquifer according to BSEACD (acre-ft/yr) .........................................................................7

5

Proposed Trinity Aquifer development in Regions L and K from 2010 to 2060 ................8

6

Total estimate of recoverable storage by county for the Trinity Aquifer within the GMA
10 limits (Values in acre-ft; Reference: Jones et al., 2013) .................................................97

Recharge value report for the Trinity Aquifer provided by the Medina County
Groundwater Conservation District (acre-ft/yr) and TWDB Aquifer Assessment 10-06 .10

8Lateral inflow to the Trinity Aquifer in GMA 10 (all values in acre-ft/yr) ......................11

9

Estimated value of cross-formational flow from the Trinity Aquifer to the Edwards
Aquifer (acre-ft/yr).............................................................................................................12

10

Estimated value of cross-formational flow from the Trinity Aquifer to the Edwards
Aquifer (acre-ft) .................................................................................................................12vi

Abbreviations
DFC

Desired Future Conditions

GCD

Groundwater Conservation District

GMA

Groundwater Management Area

RWPG

Regional Water Planning GroupMAG

Modeled Available Groundwater

TWDB

Texas Water Development Board

vii

1.

Description of Groundwater Management Area 10

Groundwater Conservation Districts (GCDs, or districts) were created, typically by legislative
action, to provide for the conservation, preservation, protection, recharging, and prevention ofwaste of the groundwater, and of groundwater reservoirs or their subdivisions, and to control
subsidence caused by withdrawal of water from those groundwater reservoirs or theirsubdivisions. The individual GCDs overlying each of the major aquifers or, for some aquifers,
their geographic subdivisions were aggregated by the Texas Water Development Board (TWDB)
acting under legislative mandate to form Groundwater Management Areas (GMAs). Each GMA
is charged with facilitating joint planning efforts for all aquifers wholly or partially within its
GMA boundaries that are considered relevant to joint regional planning.
GMA 10 was delineated based primarily on the extents of the San Antonio and Barton Springs
segments of the Fresh Edwards (Balcones Fault Zone) Aquifer, but it also includes theunderlying down-dip Trinity Aquifer. Other aquifers in GMA 10 include the Leona Gravel, Buda
Limestone, Austin Chalk, and the Saline Edwards (Balcones Fault Zone) aquifers. The planningarea of GMA 10 includes all or parts of Bexar, Caldwell, Comal, Guadalupe, Hays, Kinney,
Medina, Travis, and Uvalde counties (Figure 1). GCDs in Groundwater Management Area 10
include Barton Springs/Edwards Aquifer Conservation District, Comal Trinity GCD, Edwards
Aquifer Authority, Kinney County GCD, Medina County GCD, Plum Creek Conservation
District, and Uvalde County Underground Water Conservation District (UWCD) (Figure 1).
As mandated in Texas Water Code § 36.108, districts in a GMA are required to submit Desired
Future Conditions (DFCs) of the groundwater resources in their GMA to the executive
administrator of the TWDB, unless that aquifer is deemed to be non-relevant for the purposes ofjoint planning. According to Texas Water Code § 36.108 (d-3), the district representatives shallproduce a Desired Future Conditions Explanatory Report for the management area and submit to
the TWDB a copy of the Explanatory Report.
GMA 10 has designated the Trinity Aquifer as a relevant aquifer for purposes of joint planning.
This document is the preliminary Explanatory Report for this aquifer.

1Figure 1. Map of the administrative boundaries of GMA 10 designated for joint-planningpurposes and the GCDs in the GMA (From Texas Water Development Board website)
2.

Aquifer Description

The Trinity Aquifer consists of Cretaceous-age formations of varying viability as water sources.
The upper Trinity Aquifer (comprising the upper Glen Rose Limestone) has low yields and poor
water quality due to its evaporite beds. The middle Trinity Aquifer (comprising the lower Glen
Rose Limestone, the Hensel Sand, and Cow Creek Limestone) is the most widely used portion of
the aquifer. The lower Trinity Aquifer (comprising the Hosston Sand and Sligo Limestone) is as
widely used due to its depth and water quality (SCTRWPG, 2010). The Trinity Aquifer outcrops
very little within GMA 10 and exists as a confined aquifer underlying the Edwards (Balcones
Fault Zone) Aquifer. It is currently used as a minor source of groundwater in Uvalde, Medina,
Bexar, Comal, Guadalupe, Hays, and Travis counties, but is increasingly becoming a major
source due to rapid development and increased water demands.

2Figure 2. Map showing the extent of the Trinity Aquifer in GMA 10 (From Texas Water
Development Board website)
3.

Desired Future Conditions

The desired future conditions (DFC) adopted on 8/23/2010 for the Trinity Aquifer are as follows:
Average regional well drawdown not exceeding 25 feet during average recharge conditions(including exempt and non-exempt use); within Hays-Trinity Groundwater Conservation
District: no drawdown; within Uvalde County: 20 feet; not relevant in Trinity-Glen Rose GCD.
(TWDB, 2015)
GMA 10 has proposed to maintain the same DFCs in the second round as in the first round forthis aquifer, with the exception of Hays-Trinity GCD, which is no longer in GMA 10. Thissecond round of proposed DFCs was approved at the GMA 10 meeting on March 14, 2016 to beavailable for consideration during the 90-day public comment period and a public hearing heldby each GCD. After the comment period and public hearings, the proposed DFCs were adopted
at the GMA 10 meeting on XXXX, XX, XXXX. Resolution No. 2016-XX is attached inAppendix A.

3

4.

Policy Justification

The DFCs in the Trinity Aquifer within GMA 10 were adopted after considering the following
factors specified in Texas Water Code §36.108 (d):1.

Aquifer uses or conditions within the management area, including conditions that differ
substantially from one geographic area to another;a.
for each aquifer, subdivision of an aquifer, or geologic strata; andb.
for each geographic area overlying an aquifer

2.

The water supply needs and water management strategies included in the state water
plan;

3.

Hydrological conditions, including for each aquifer in the management area the total
estimated recoverable storage as provided by the executive administrator, and the averageannual recharge, inflows, and discharge;

4.

Other environmental impacts, including impacts on spring flow and other interactions
between groundwater and surface water;

5.

The impact on subsidence;

6.

Socioeconomic impacts reasonably expected to occur;

7.

8.

The impact on the interests and rights in private property, including ownership and therights of management area landowners and their lessees and assigns in groundwater asrecognized under Section 36.002;
The feasibility of achieving the DFC; and9.

Any other information relevant to the specific DFCs.

These factors and their relevance to establishing the DFCs are discussed in detail incorresponding sections and subsections of this Explanatory Report.
5.

Technical Justification

The TWDB developed a method described in GTA Aquifer Assessment 10-06 (Thorkildsen andBackhouse, 2010) that uses an analytical solution to estimate modeled available groundwater forvarious drawdown scenarios.
The GCDs in GMA 10 regard the Trinity Aquifer as an alternative water supply that poses little
threat to the overlying Edwards Aquifer—and in fact can lessen demands placed upon it. The
proposed DFC is an expression of average drawdown of the potentiometric surface. Table 1 is an
estimate of modeled available groundwater using the analytical approach used by TWDB. As
described in Thorkildsen and Backhouse (2010), the modeled available groundwater (MAG) is
estimated by multiplying the average drawdown by the storage coefficient and the area and then4

adding in estimated lateral inflow. As other inflows and outflows are considered to be negligible(described later in this report), this approach treats the aquifer as a closed system.
Table 1. Estimation of Modeled Available Groundwater (MAG)
Estimated Annual Modeled Available
CountyGroundwater(acre-ft/yr)
Bexar
19,998Caldwell0Comal
29,284
Guadalupe
0Hays3,557*
Medina5,369
Travis641
Uvalde639
Total
59,488**The Hays County total has been reduced by 258 acre-ft/yr to account for the Hays-Trinity GCD, which was
included in Thorkildsen and Backhouse (2010), but is no longer in GMA 10.

6.

Consideration of Designated Factors

In accordance with Texas Water Code § 36.108 (d-3), the district representatives shall produce a
Desired Future Condition Explanatory Report. The report must include documentation of hownine factors identified in Texas Water Code §36.108(d) were considered and how the proposed
DFC impacts each factor. The following sections of the Explanatory Report summarize theinformation that the GCDs used in their deliberations and discussions.
6.1

Aquifer Uses or Conditions

6.1.1 Description of Factors for the Trinity Aquifer in GMA 10
The Trinity Aquifer does not serve as the primary source of water for counties in GMA 10.
However, given restrictions on groundwater withdrawals from the Edwards Aquifer, withdrawals
from the Trinity Aquifer have been growing. The aquifer is stressed due to increasing numbers ofwells to supply rapidly developing areas of central Texas. In addition, wells that were poorly
cased through evaporite beds in the Upper Trinity formation have diminished the water quality inparts of the Middle Trinity Aquifer (SCTRWPG, 2010). Another concern is potential movementof the “bad water line” (where total dissolved solids concentrations exceed 1,000 milligrams perliter) due to increased groundwater withdrawal. Water quality becomes progressively poorer inthe downdip sections of the Trinity Aquifer, with the “bad water line” stretching east-west
through southern Uvalde and Medina counties, and then southeast-northwest through central
Bexar, and along the southeastern edge of Comal and Hays counties (SCTRWPG, 2010).
The TWDB provides historical groundwater pumpage values by county and aquifer. Table 2
provides the amount of groundwater in acre-feet supplied by the Trinity Aquifer for the period
2000-2013. The Trinity Aquifer does not provide the majority of groundwater in any county,5although the Trinity Aquifer share has increased from 2000 to 2012 in Comal, Hays, and Travis
counties. The TWDB does not report any pumping from the Trinity Aquifer in Caldwell or
Kinney counties.
Table 2. Total groundwater pumpage values by county from the Trinity Aquifer in acre-ft/yr.
Note that pumping estimates may include areas of the Trinity Aquifer outside of GMA 10.
Values from https://www.twdb.texas.gov/waterplanning/waterusesurvey/historical-pumpage.asp
CountyBexar
Comal Guadalupe Hays
MedinaTravisUvalde
2000
7,974
2,895
0
2,236
42
1,868
49
2001
8,761
2,422
0
2,441
33
1,969
46
2002
9,425
2,229
0
2,212
35
1,944
45
2003
8,681
2,169
0
2,115
36
1,944
43
2004
9,301
5,642
0
2,024
35
1,754
40
2005
11,579
5,404
0
2,249
186
1,929
61
2006
11,353
6,916
4
3,497
248
3,591
96
2007
8,698
6,896
4
3,818
242
2,838
91
2008
10,020
4,270
4
3,670
220
3,461
170
2009
11,675
4,166
6
4,262
248
4,594
163
2010
15,475
2,456
9
4,985
356
8,801
246
2011
18,530
4,678
6
6,110
479
10,364257
2012
17,854
7,119
8
5,286
338
7,636
195
2013
14,763
4,180
7
5,061
332
8,808
180
District-level water use numbers compiled by two GCDs in the GMA 10 area are also available,
but only for recent years. Uvalde County UWCD values are sourced from their annual water use
report database and provided in Table 3. These numbers are higher than the county-wide values
provided by the TWDB, particularly in 2009 and 2010.
Table 3. Total groundwater pumpage values in Uvalde County according the UCUWCD (2011)in acre-ft/yr. Values from UCUWCD (2011).
Aquifer
2007
2008
2009
2010
Trinity
228
267
1,667
908
The Barton Springs Edwards Aquifer Conservation District (BSEACD) values are based onmeter readings from district wells and are provided in Table 4. The numbers are smaller than the
county-wide numbers given by TWDB because the BSEACD only covers a portion of Travis
County.

6

Table 4. Total groundwater pumpage values for Middle Trinity Aquifer and Lower Trinity
Aquifer according to BSEACD (acre-ft/yr). Values from BSEACD (2013).
County Aquifer
2007
2008
2009
2010
2011
Hays
Middle Trinity
0
0
0
0
27
Lower Trinity-----Travis
Middle Trinity
0.4
0.3
0.4
0.3
5
Lower Trinity
11
28
18
20
17
6.1.2 DFC Considerations
The Trinity Aquifer in GMA 10 is not the primary water source for much of the area. However,pressure on the freshwater Edwards (Balcones Fault Zone) Aquifer has led to the need for viable
alternative supplies. The proposed DFC allows for a modeled available groundwater that issignificantly above the current use of the aquifer and allows room for development of the aquifer
as an alternative supply while protecting existing groundwater supplies.
6.2

Water-Supply Needs

6.2.1 Description of Factors for the Trinity Aquifer in GMA 10
For estimating projected water-supply needs (i.e., water demand vs. supply), the districts useddata extracted from the 2017 State Water Plan and provided by the TWDB. The TWDB provides
water-supply needs estimates by decade as well as by county. A summary of the projected watersupply needs is provided in Table 3 by decade in acre-ft/yr. Also shown in Table 3 are demands,
existing supplies, and water-supply strategies. Note that these are county totals, not just the
portions of each county in GMA 10.
The projections in Table 5 show that for the 2017 State Water Plan planning period (2020-2070),
there is a progressively increasing water-supply deficit, increasing from 135,000 acre-ft in 2020
up to 497,000 acre-ft in 2060. As in prior plans, some of the water-demand deficits in the area inthe out-years (the later years in the planning period) include numerous contractual shortages.
These contractual shortages will be addressed on an ad-hoc basis, through the renewal andexpansion of contracts with wholesale water suppliers and the contractual reallocation of existing
supplies in order to address the projected water demands for these and other area water-usergroups. But even so, it is projected that there will be unmet needs under drought-of-record
conditions and in the out-years.
6.2.2 DFC Considerations
Population growth throughout GMA 10 is creating demand for additional water supplies from all
sources. The DFC allows for drawdown of the Trinity Aquifer to allow for its use in the future
as water supply of growing importance to the region.

7

Table 5. 2017 State Water Plan information for counties in GMA 10 containing the Trinity
Aquifer. All values are in acre-ft/yr. Note that these are county totals and are not limited to the
portion of each county in GMA 10.
CountyBexar

CaldwellComal

Guadalupe

Hays

MedinaTravisUvaldeTotal

6.3

Aquifer
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing Supplies
Needs
Strategy Supplies
Demands
Existing
Supplies
Needs
Strategy
Supplies

2020
367,664
348,478
61,498111,676
7,939
10,563201
2,953
42,66041,807
5,348
20,102
36,48750,6791,486
9,021
38,01755,922580
14,07368,171
39,51432,5102,142
290,697423,296
3,199
148,005
75,59547,88830,7472,642
927,2302030
404,641
350,452
87,009139,674
8,992
10,606701
2,869
50,55543,5508,434
27,74342,64253,7494,320
14,14348,14056,144
4,148
28,57966,67339,78330,5272,601
346,067
421,001
19,203
193,633
73,69447,48028,756
3,109
1,041,404

2040
438,621352,909110,801
172,615
10,06910,6271,368
2,938
58,562
45,235
14,81233,28548,28754,937
7,660
16,304
61,37656,441
12,63540,651
65,14740,05628,5803,208
398,642
419,022
27,658228,20371,70547,55926,657
3,791
1,152,409

2050
473,953
353,419
139,602
211,590
11,191
10,6402,223
3,540
66,45946,693
21,30438,881
54,22954,805
12,37524,352
74,24957,07022,756
51,23863,68840,267
26,707
3,745
436,992
411,952
41,766
275,798
69,99347,664
24,8154,559
1,250,754

2060
509,657
354,103
169,573
259,448
12,362
10,648
3,154
4,291
74,986
48,391
28,19844,98961,97754,70817,412
28,173
93,141
58,244
38,59469,74162,36440,51324,9384,306
470,440
401,880
85,617306,286
68,45147,742
23,135
5,168
1,353,378

2070
543,989354,936
199,085304,681
13,557
10,6604,080
5,305
83,56250,20035,02251,406
68,632
54,69622,35637,388115,037
59,67957,222
88,522
61,252
40,76823,445
4,918
509,035
392,060
134,438
338,800
67,17947,742
21,744
5,797
1,462,243

1,018,147
135,569

1,022,765
183,098

1,026,786
230,171

1,022,510
291,548

1,016,229
390,6211,010,741
497,392

310,614412,351

500,995

613,703

722,402

836,817

Water-Management Strategies

6.3.1 Description of Factors for the Trinity Aquifer in GMA 10
Both Regional Water Planning Groups K and L plan to further develop the Trinity Aquifer aspart of their water management strategies to cover future water needs. Table 6 provides the
proposed Trinity Aquifer withdrawals developed by Regional Water Planning Groups K and L
for the 2012 State Water Plan. Additionally, Table 6 above shows the total of water management
strategies developed as part of the 2017 State Water Plan.
8

Table 6. Proposed Trinity Aquifer development in Regions L and K from 2010 to 2060. Values
from SCTRWPG (2010) and LCRWPG (2010)
County

Bexar

Hays

Hays

Water Utility
Group
Regional Water
Planning Group

Bexar Metropolitan
Water District*County Line Water
Supply Company

Manufacturing

L

L

K

Water Management
Strategy

Development of Local
Groundwater Supplies(Trinity Aquifer)

Development of Local
Groundwater Supplies(Trinity Aquifer)

New well field for
Trinity Aquifer

Source Name

Trinity Aquifer

Trinity Aquifer

Trinity Aquifer

2010
2020
2030
2040
2050
2060

2,016
2,016
2,016
2,016
2,016
2,016

--1,129
1,452
1,613
1,936
2,420

--75
200
301
400

*Bexar Metropolitan Water District was acquired by San Antonio Water System in 2012

6.3.2 DFC Considerations
The proposed DFCs allow for development of the Trinity Aquifer in GMA 10 as contemplated in
the water management strategies in the 2012 State Water Plan. The estimated MAG of 59,488
acre-ft/yr is greater than estimated current use and water-management strategies targeting theaquifer.
6.4.

Hydrological Conditions

6.4.1 Description of Factors for the Trinity Aquifer in GMA 10
6.4.1.1 Total Estimated Recoverable Storage
Texas statute requires that the total estimated recoverable storage of relevant aquifers be
determined (Texas Water Code § 36.108) by the TWDB. Texas Administrative Code Rule§356.10 (Texas Administrative Code, 2011) defines the total estimated recoverable storage as theestimated amount of groundwater within an aquifer that accounts for hypothetical recovery
scenarios that range between 25 percent and 75 percent of the porosity-adjusted aquifer volume.
Total estimated recoverable storage values may include a mixture of water-quality types,
including fresh, brackish, and saline groundwater, because the available data and the existing
9Groundwater Availability Models do not permit the differentiation between different waterquality types. The total estimated recoverable storage values do not take into account the effectsof land surface subsidence, degradation of water quality, or any changes to surfacewater/groundwater interaction that may occur due to pumping.
Table 7 provides the total estimated recoverable storage values for the Trinity Aquifer in GMA
10. The percentage values for the 25 percent of total storage and 75 percent total storage shown
here were rounded within one percent of the total.
Table 7. Total estimate of recoverable storage by county for the Trinity Aquifer within the GMA
10 jurisdiction (Values in acre-ft)(Jones et al., 2013)
CountyTotal Storage
25 percent of Total
75 percent of Total
Storage
StorageBexar
5,500,000
1,375,000
4,125,000
Caldwell
24,0006,000
18,000Comal
2,300,000
575,000
1,725,000
Guadalupe
43,00010,75032,250Hays2,400,000
600,000
1,800,000
Medina11,000,000
2,750,000
8,250,000
Travis690,000
172,500
517,500
Uvalde1,100,000
275,000
825,000
Total
23,057,000
5,764,250
17,292,750
6.4.1.2 Average Annual Recharge
The Trinity Aquifer is confined throughout most of the extent of GMA 10, therefore it does notreceive direct recharge in this area. Rather the aquifer is recharged in the Trinity Aquifer outcrop
area located in GMA 9 where the aquifer is not confined. The GMA 10 area is located south andeast of GMA 9. Recharge estimates from previous studies varied from 1.5 to 11 percent of theannual rainfall falling on Trinity Aquifer outcrop areas. Recharge also occurs from losing
streams crossing the aquifer outcrop (Jones et al., 2009). Table 8 includes recharge values
calculated for the Medina County Groundwater Conservation District. Note that this district
includes some Trinity Aquifer outcrop area that falls outside the GMA 10 boundary and thisrecharge occurs in that area, rather than within the GMA 10 extent. As shown in TWDB Aquifer
Assessment 10-06 (Thorkildsen and Backhouse, 2010), there are small outcrop areas within
GMA 10. In this assessment, TWDB estimates recharge to the aquifer to be approximately 4
percent of precipitation.
6.4.1.3 Inflows
Lateral Inflow Table 9 provides the estimated annual volume of flow into the Trinity Aquifer in
GMA 10 from the Hill Country portion of the Trinity Aquifer across the Balcones Fault Zone
(from Thorkildsen and Backhouse, 2010).

10

6.4.1.4 Discharge
Cross-formational flow: BSEACD (2013) suggests that there might be some vertical leakage
from the Edwards Aquifer into the Trinity Aquifer, but this input is likely limited to the top 100
feet of the Upper Trinity Aquifer, as the bottom portion of the Upper Trinity Aquifer acts as anaquitard and prevents leakage from reaching the Middle Trinity Aquifer. In general, crossformational flow is out of, not into, the Trinity Aquifer in GMA 10. Jones et al. (2011) estimatedthat cross-formational discharge from the Hill Country portion of the Trinity Aquifer to the
Barton Springs and San Antonio segments of the Edwards Aquifer were 660 acre-ft/yr per mileof aquifer boundary in Uvalde and Medina counties; 2,400 in Bexar and Comal counties; and350 in Hays and Travis counties. Table 10 provides estimated cross-formational flow from the
Trinity Aquifer to the Edwards Aquifer within the Edwards Aquifer Authority (EAA).
Table 8. Recharge values for the Trinity Aquifer provided by the Medina County Groundwater
Conservation District (acre-ft) and TWDB Aquifer Assessment 10-06
Estimated annual amount of recharge
Area
SourceAquiferfrom precipitation to the district
MCGCD GAM Run 09-31
Trinity Aquifer
6,918
UvaldeTWDB AquiferCo.
Trinity Aquifer
36
Assessment 10-06
UWCD
Comal
TWDB Aquifer
Trinity Aquifer
206
CountyAssessment 10-06
Hays
TWDB Aquifer
Trinity Aquifer
107
CountyAssessment 10-06
Natural Discharge: Since the Trinity Aquifer is confined in the GMA 10 study area, no directdischarge from the aquifer is expected. Discharge occurs in the outcrop areas, north andnorthwest of GMA 10, where springs flow from the Trinity Aquifer and streams are net gaining
from Trinity Aquifer discharge (Jones et al., 2009). No major springs issue from the Trinity
Aquifer itself within GMA 10. BSEACD (2013) does mention that some Upper Trinity Aquifer
water may flow laterally or vertically into the Edwards Aquifer and thus, indirectly, feed
Edwards Aquifer springs, such as Barton Springs. However, Middle Trinity Aquifer does notappear to discharge in the Balcones Fault Zone.
6.4.1.5 Other Environmental Impacts Including Springflow and Groundwater/Surface
Water Interaction
As described in previous sections relating to inflows and discharges, the Trinity Aquifer in GMA
10 is confined and largely separated from surficial processes and the overlying Edwards Aquifer
except the upper portion of the Upper Trinity Aquifer. While the current conceptualization of theaquifer includes flow from the Hill Country portion of the Trinity Aquifer (GMA 9) into the
Trinity Aquifer in GMA 10, it is possible that large-scale development in GMA 10 could impact
up-dip areas outside the GMA. There is not currently a groundwater availability model toevaluate the extent to which these impacts could occur.

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Table 9. Lateral inflow to the Trinity Aquifer in GMA 10 (all values in acre-ft)
AquiferCountyLateral Inflow from Hill Country Trinity
Upper Trinity
Bexar
8,530
Upper Trinity
Caldwell0Upper Trinity
Comal
15,346Upper Trinity
Guadalupe0Upper Trinity
Hays
2,512
Upper Trinity
Medina1,576
Upper Trinity
Travis267
Upper Trinity
Uvalde176
Middle Trinity
Bexar
11,560
Middle Trinity
Caldwell0
Middle Trinity
Comal
13,678Middle Trinity
Guadalupe0
Middle Trinity
Hays
913
Middle Trinity
Medina3,751
Middle Trinity
Travis374
Middle Trinity
Uvalde417
Total
59,100
Table 10. Estimated value of cross-formational flow from the Trinity Aquifer to the Edwards
Aquifer (acre-ft)
Estimated net annual volume of flow
District
SourceAquifer
between each aquifer in the district
from Trinity Aquifer to
EAA
GAM Run 08-67 Edwards and associated
13,622
limestones
6.4.2 DFC Considerations
Analysis of the hydrological conditions of the Trinity Aquifer in GMA 10 indicates that theaquifer can continue to serve as an alternative water supply to the freshwater Edwards (Balcones
Fault Zone) Aquifer. However, since it has not seen large development historically in many areasof GMA 10, there is limited information on how the aquifer will respond to significant pumping.
The proposed DFC allows for considerable drawdown and a significantly larger modeled
available groundwater than is the current amount of groundwater use.
7.

Subsidence Impacts

Subsidence has historically not been an issue with the Trinity Aquifer in GMA 10. The aquifer
matrix in the northern subdivision is well-indurated and the amount of pumping does not createcompaction of the host rock and/or subsidence of the land surface. Hence, the proposed DFCs
are not affected by and do not affect land-surface subsidence or compaction of the aquifer.

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8.

Socioeconomic Impacts Reasonably Expected to Occur

8.1

Description of Factors for the Trinity Aquifer in GMA 10

Administrative rules require that regional water planning groups evaluate the impacts of not
meeting water needs as part of the regional water planning process. The executive administrator
shall provide available technical assistance to the regional water planning groups, upon request,
on water supply and demand analysis, including methods to evaluate the social and economic
impacts of not meeting needs [§357.7 (4)]. Staff of the TWDB’s Water Resources Planning
Division designed and conducted a report in support of the South Central Texas Regional Water
Planning Group (Region L) and also the Lower Colorado Regional Water Planning Group(Region K). The report “Socioeconomic Impacts of Projected Water Shortages for the South
Central Texas Regional Water Planning Area (Region L)” was prepared by the TWDB in supportof the 2011 South Central Texas Regional Water Plan and is illustrative of these types ofanalyses.
The report on socioeconomic impacts summarizes the results of the TWDB analysis anddiscusses the methodology used to generate the results for Region L. The socioeconomic impact
report for Water Planning Group L is included in Appendix C as an example. These reports aresupportive of a cost-benefit assessment of the water management strategies and thesocioeconomic impact of not promulgating those strategies.
8.2

DFC Considerations

The proposed DFC allows for development of the Trinity Aquifer above what is called for in thewater-management strategies in the 2012 State Water Plan. For this reason, the proposed DFC
will not have a socioeconomic impact associated with an unmet water need.
9.

Private Property Impacts

9.1

Description of Factors for the Trinity Aquifer in GMA 10

The interests and rights in private property, including ownership and the rights of GMA10
landowners and their lessees and assigns in groundwater, are recognized under Texas Water
Code Section 36.002. The legislature affirmed that a landowner owns the groundwater below thesurface of the landowner's land as real property. Joint planning must take into account the
impacts on those rights in the process of establishing DFCs, including the property rights of both
existing and future groundwater users. Nothing should be construed as granting the authority todeprive or divest a landowner, including a landowner's lessees, heirs, or assigns, of thegroundwater ownership and rights described by this section. At the same time, the law holds thatno landowner is guaranteed a certain amount of such groundwater below the surface of his/herland.
Texas Water Code Section 36.002 does not: (1) prohibit a district from limiting or prohibiting thedrilling of a well by a landowner for failure or inability to comply with minimum well spacing or
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tract size requirements adopted by the district; (2) affect the ability of a district to regulate
groundwater production as authorized under Section 36.113, 36.116, or 36.122 or otherwise
under this chapter or a special law governing a district; or (3) require that a rule adopted by a
district allocate to each landowner a proportionate share of available groundwater for production
from the aquifer based on the number of acres owned by the landowner.
9.2

DFC Considerations

The DFC is designed to allow for additional development of the Trinity Aquifer as an alternative
water supply in a manner that does not harm other property owners. The DFC does not prevent
use of the groundwater by landowners either now or in the future, although ultimately total useof the groundwater in the aquifer is restricted by the aquifer condition, and that may affect theamount of water that any one landowner could use, either at particular times or all of the time.
10.

Feasibility of Achieving the DFCs

The feasibility of achieving a DFC directly relates to the ability of the GCDs to manage theTrinity Aquifer to achieve the DFC, including promulgating and enforcing rules and other board
actions that support the DFC. The feasibility of achieving this goal is limited by (1) the finite
nature of the resource and how it responds to drought; and (2) the pressures placed on thisresource by the high level of economic and population growth within the area served by thisresource.
Texas state law provides Groundwater Conservation Districts with the responsibility andauthority to conserve, preserve, and protect these resources and to ensure the recharge andprevention of waste of groundwater and control of subsidence in the management area. State law
also provides that GMAs assist in that endeavor by joint regional planning that balances aquifer
protection and highest practicable production of groundwater. The feasibility of achieving thesegoals could be altered if state law is revised or interpreted differently than is currently the case.
The caveats above notwithstanding, there are no current hydrological or regulatory conditions
that call into question the feasibility of achieving the DFC.
11.

Discussion of Other DFCs ConsideredNo other expression of DFC of the Trinity Aquifer in GMA 10 was considered. GMA 10
evaluated alternative amounts of drawdown for the DFC expression, including larger amounts ofdrawdown. The proposed DFC specifies an amount of drawdown that is not unreasonably large
or small, and that should be readily achieved on the basis of currently known information about
the aquifer.

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12.

Discussion of Other Recommendations

12.1

Advisory Committees

An Advisory Committee for GMA10 has not been established.
12.2

Public Comments

Each GCD must hold a public meeting within 90 days after the GMA approves its DFCs. During
this meeting, the GCD needs to document stakeholder input. This input is to be submitted by areport from the GCD to the GMA within 90 days after the GMA approves its DFC.
GCDs in GMA 10 have not yet approved its second round of DFCs. The GCDs have not yet held
public meetings to gather public comment on the DFCs. No public comments have yet beenoffered regarding the Trinity Aquifer.
This draft chapter of the Explanatory Report may be used as supporting documents to inform thepublic before such hearings and meetings are held.
13.

Any Other Information Relevant to the Specific DFCs

During the process of DFC development the GCDs in GMA 10 reviewed and evaluated thepotential impacts of a planned development of the Cow Creek formation of the Middle Trinity
Aquifer in central Hays County. The evaluation focused on 1) the potential for drawdown
impacts within the Cow Creek to propagate to other portions of the Trinity and Edwards aquifers,
and 2) the viability of production over the 50-year planning period at a wide range of pumping
rates. This evaluation is documented in Appendix D.
14.
Provide a Balance Between the Highest Practicable Level of Groundwater
Production and the Conservation, Preservation, Protection, Recharging, and Prevention ofWaste of Groundwater and Control of Subsidence in the Management Area
The “DFC Considerations” discussed in previous sections (especially 6.x.2, 8.2, 9.2, 10, and 11)
provide the context in which the balancing factor is being addressed. But the TWDB has notdeveloped guidance on how to approach this factor. It is up to the GCDs to determine how toapproach it for each relevant aquifer, whether in a qualitative, quantitative, or combinationmanner. In addition, the GCDs need to include stakeholder input so that this factor can be more
confidently addressed. GCD management plans will also be used to complete this requirement.
Each GCD must hold a public meeting within 90 days after the GMA approves its DFCs. During
this meeting, the GCD will document stakeholder input regarding whether the DFCs provide abalance between the highest practicable level of groundwater production and the conservation,
preservation, protection, recharging, and prevention of waste of groundwater and control ofsubsidence in the management area. This input is to be submitted by a report from the GCD to
the GMA within 90 days after the GMA approves its DFCs. The information in the aggregated15

reports from the GCDs in GMA 10 will then be incorporated into the final Explanatory Reportsubmitted to the TWDB for promulgation and use in calculating the MAG.
15.

References

Barton Springs Edwards Aquifer Conservation District (BSEACD). 2013. District Management
Plan. Approved by Texas Water Development Board, January 7, 2013. Website: http://www.
bseacd.org/uploads/Financials/MP_FINAL_TWDB-approved_1_7_2013_Body.pdf
Jones, I.C., J. Shi, and R. Bradley. 2013. GAM Task 13-033: Total Estimated Recoverable
Storage for Aquifers in Groundwater Management Area 10. August, 2013.
Jones, I. C., R. Anaya, and S.Wade. 2009. Groundwater Availability Model for the Hill Country
Portion of the Trinity Aquifer System, Texas. Prepared for the Texas Water Development Board.
Website: http://www.twdb.texas.gov/groundwater/models/gam/trnt_h/TRNT_H_2009_Update_
Model_Report.pdf
Jones, I.C. 2011. Interaction between the Hill Country Portion of the Trinity and Edwards
Aquifers: Model Results included in Interconnection of the Trinity (Glen Rose) and Edwards
Aquifers along the Balcones Fault Zone and Related Topics. Karst Conservation Initiative
February 17, 2011 Meeting Proceedings. Austin, TX. Website: http://www.bseacd.org/
uploads/AquiferScience/Proceedings_Edwards_Trinity_final.pdf
Lower Colorado Regional Water Planning Group (LCRWPG). 2010. 2011 Region K Water Plan:
Volume I. Prepared for the Texas Water Development Board, July 2010.
Mace, R., A. Chowdhury, R. Anaya, and S. Way. 2000, Groundwater Availability of the Trinity
Aquifer, Hill Country Area, Texas: Numerical Simulations through 2050: Texas Water
Development Board, Report 353, 117 p.
South Central Texas Regional Water Planning Group (SCTRWPG). 2010. South Central Texas
Regional Water Planning Area 2011 Regional Water Plan Volume I: Executive Summary andRegional Water Plan. September 2010. http://www.twdb.texas.gov/waterplanning/rwp/plans/
2011/L/Region_L_2011_RWPV1.pdf
Texas Water Development Board (TWDB). 2015. Summary of Desired Future Conditions for
GMA 10, website: http://www.twdb.texas.gov/groundwater/management_areas/dfc_mag/
GMA_10_DFC.pdf
Uvalde County Underground Water Conservation District (UCUWCD). 2011. Management Plan
2011-2021. Adopted July 26, 2011. Website: http://www.twdb.texas.gov/groundwater/
docs/GCD/ucuwcd/ucuwcd_mgmt_plan2011.pdf

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