1

WATER RESOURCE ANALYSIS

MICHAEL VAN ASTEN

2

Michael Van Asten

Dr. James Vandike

3

Hydrogeology

Hometown Resource Project

Canyon Lake

Introduction

The area of study is to the South of Canyon Lake in Texas. This area, which lies within

the Balcones fault zone of the Edward aquifer is one of the most hydrologically active regions in

Texas. The Edwards aquifer has been deemed as the sole source of water for San Antonio in

addition to providing water for much of Austin, irrigation, and other uses along the I-35 corridor.

The rocks that comprise the Edwards aquifer are partially dissolved limestone, which has been

heavily faulted from normal stress, resulting in steep fault planes. In addition to the dissolution

of the limestone units, there is significant secondary porosity along the faults, which gives the

aquifer an anisotropic and highly permeable nature. Due to this secondary porosity, the

groundwater flow follows the dip direction of said normal faults, which gives it a Northeastern

flow direction. Due to the high permeability of the Edwards aquifer, water levels change rapidly

with fluctuating rainfall and pumping.

Structure and Stratigraphy

The structure of the Balcones fault zone dictates many characteristics of the Edwards

aquifer. Regionally, there exists a series of horsts and grabens, which cause the aquifer to be

highly transmissive. As a result of these normal faults, there is a large amount of secondary

porosity along said fault planes. This secondary porosity along steeply dipping faults results in

anisotropic flow. In addition to the porosity along faults, there are also dissolution caverns and

channels oriented parallel to faults and joints. The combination of secondary porosity from

4

faulting and partially dissolved limestone units creates a highly permeable aquifer (Kuniansky).

Figure 1 shows an example of the dissolution present in some portions of the Edwards aquifer.

Additionally, figure 1a depicts an example of stress fractures along a streambed, which recharge

the aquifer and are prevalent in the area of study. The normal faults, as illustrated in figure 2,

penetrate from the Edwards group down into the Trinity group. These serve as a recharge area

for the Edwards group, which is confined by the Del Rio clay. The underlying Trinity aquifer has

lower quality water, between 1,000 and 5,000 milligrams per liter of dissolved solids (George).

However, there is little leakage from the Trinity aquifer into the overlying Edwards aquifer. The

lateral movement of water into the Edwards aquifer from the Trinity aquifer is about 2.7 cubic

feet/s/mi (Kuniansky). Since the Edwards aquifer is of higher quality, it serves as the major

ground-water supply for the region. Figure 3 shows the stratigraphic column for the Balcones

fault zone. As illustrated, the Edwards group is underlain by the less permeable, lower-quality

Trinity group, and is confined by Del Rio clay. The Edwards group is dominated by highly

dissolved limestone units, while the Trinity group is composed of carbonates, sands, and shales.

The overlying clay unit has relatively low porosity and permeability, thereby providing an

effective aquitard. The highly faulted, dissolution-riddled limestone units that make up the

Edwards aquifer are provided not only high permeability, but a zone for rapid recharge as well as

an effective confining unit, resulting in a fruitful aquifer.

5

Figure 1 and 1a respectively, (George).

Figure 2, (George).

6

Figure 3, (Kandinsky).

Hydrologic Characteristics

The hydrologic characteristics of the study area have many qualities that are directly

related to both the rock units and structure. The Edwards aquifer is comprised of permeable

Limestone units which have been heavily faulted. The dissolution of the carbonate units within

the Edwards aquifer has lead to the formation of extensive caverns. Additionally, horst and

graben style faulting provides the unit with additional secondary porosity as well as increased

permeability. As a result of this faulting, the direction of groundwater flow is in a Northeasterly

direction, as illustrated by figure 4. The effective thickness of the Edwards aquifer ranges from

7

350-850 feet, and the effective porosity varies from anywhere between 15-35 percent effective

porosity (Kuniansky). One effect of highly permeable carbonate units that have experienced

significant horst-and graben faulting is a relatively high hydraulic conductivity.

In the area of interest, South of Canyon Lake, figure 5 shows that there is a hydraulic

conductivity of 50-100 feet perday. Also, figure 5 captures how the hydraulic conductivity

varies by several orders of magnitude within a relatively small area. Figure 6 depcits the

transmissivity for the Edwards and Trinity aquifers within the area of interest. According to the

Figure, the Edwards group has a transmissivity of 10,000 to 100,000 feet squared per day while

the Trinity group, which underlies it, has a transmissivity of 2,500 to 5,000 feet squared per day.

Transmissivity in the area is largely controlled by dissolution pathways as well as faulting, which

explains why there is a range of several orders of magnitude within a relatively small range. The

large difference in transmissivity is a contributing factor for why there is little vertical leakage

recorded for the Edwards-Trinity contact. The specific yield of the Edwards aquifer within the

Balcones fault zone area was estimated to be from 0.02 to 0.03 for the unconfined zone and from

10^-4 to 10^-5. The Edwards aquifer storage coefficient in the Balcones Fault area was

approximated as 0.02, while the Trinity storage coefficient ranges from 0.001 to 0.0001

(Kuniansky). This difference of several orders of magnitude between the adjacent units reflects

their proportionally different porosity and permeability. Due to the relatively high permeability,

there are rarely steady-state conditions within the Balcones fault zone, and water-level

fluctuations of 50-100 ft are common (Kuniansky). As can be seen from the data and

illustrations, there is a wide range of hydraulic characteristics within a relatively small area. This

is the direct result of varying amounts of dissolution and faulting within the Edwards carbonate

8

group, which has dramatic effects on the aquifers hydraulic characteristic and is the cause of the

significant local fluctuations.

Figure 4, (George).

9

Figure 5, George

Figure 6, (Kuniansky)

10

The water quality from the Edwards aquifer is desirable. Although the water in the

Balcones fault zone is hard, it is generally fresh, containing less than 500 milligrams per liter of

total dissolved solids (George). This is largely due to the fact that there is only roughly 2.7 cubic

feet per second per mile of water flowing into the Edwards aquifer from the Trinity aquifer,

which has much more dissolved solids according to George, roughly 1,000-5,000 milligrams per

liter (Kuniansky). Similarly to the Edwards Trinity contact, there is little water movement across

the saline-freshwater transition boundary. The amount of water moving from the freshwater part

of the Edwards aquifer to the saline part was roughly 10 cubic feet per second. This amount

increased when water levels were low (Kuniansky). The Edwards aquifer has high quality water

and has little risk of natural contamination.

Recharge of the Edwards aquifer comes from percolation through the Balcones fault zone

as well as through upward leakage from the Trinity aquifer. The Trinity aquifer leaks 80

thousand acre feet per year into the Edwards aquifer in the Balcones fault zone. However,

upward leakage from the Trinity is relatively small in comparison to recharge at the Balcones

fault zone outcrops, discharge, and pumping (Kuniansky).

Surface water in the area is largely connected to the Guadalupe River. In the Balcones

fault zone, “Baseflow accounted for 25 to 90 percent of the total stream flow for December 1974

to March 1977, and ranged from 14 to 147 cubic feet per second in the Guadalupe River Basin”

(Kuniansky). The Guadalupe River is the primary input to Canyon Lake, a large lake to the

North of the study area.

11

Usage, Recharge, and Law

The Edwards aquifer is the sole source of water to San Antonio. Additionally, it supplies

much of municipal Austin as well as irrigation along the I-35 corridor. Recharge to the Edwards

aquifer is extremely variable, ranging from 200,000 acre feet per year to 2,000,000 acre feet per

year while ground water withdrawals during 1990 were about 639,000 acre feet per year.

Averages for recharge from 1978-89 was 800,000 acre feet per year (Kuniansky). Due to the

wide recharge range, there is risk of declining water levels in the aquifer. When irrigation use

peaked in the1970’s total pumping far outweighed recharge, which resulted in a decline of 200

feet within the Edwards aquifer. However, the annual average net change in storage is 30 acre

feet per year. Water usage from the aquifer goes about 50 percent to public supply and 50

percent for irrigation (Kuniansky).

Although the Edwards aquifer is a major water source for the region, current recharge and

usage rates balance each other. However, although the high permeability of the Edwards aquifer

in the Balcones fault zone allows for rapid recharge rates, it also means that the water levels

fluctuate drastically from changes in pumping rate. As a result, it is important that water usage is

heavily monitored throughout the region.

The Edwards aquifer is one of the United States’ most productive aquifers. It is the primary

water supply for San Antonio and has been designated as a sole source aquifer by the U.S

Environmental Protection agency (Geohydrologic). In 1993, Texas Legislature created the

Edwards Aquifer Agency to manage groundwater withdrawals from the Edwards Aquifer. The

EAA, using historic data for the Edwards Aquifer, set regulations and limiting withdrawals from

the Edwards Aquifer during periods of drought. Additionally, in 2007, the Texas Legislature

12

passed Senate Bill 3, which established a pumping cap of 572,000 acre-feet. Other water law

comes in the form of the Recovery Implementation Program (RIP) for the Edwards Aquifer

region, which identifies and evaluates methods to protect threatened and endangered species

associated with the Edwards Aquifer, as required by state and federal law (Edwards). The

withdrawal cap placed by Senate Bill 3 is less than usage data for previous years, so the laws in

theory have a significant effect on usage. Since both San Antonio and Austin are largely

dependent on the aquifer, water laws are crucial to long-term preservation of the cities.

Conclusion

The Edwards aquifer in the Balcones fault zone is a fruitful aquifer. Normal faulting

resulting in horst and graben structures has created secondary porosity, which, in combination

with extensive dissolution of carbonate units, makes for a highly permeable aquifer. However,

aforementioned faulting and dissolution varies to a high degree within a relatively small area, so

many hydraulic characteristics of the aquifer change within a short distance. The Balcones fault

zone provides not only a highly permeable aquifer, but also an area of rapid recharge where the

Edwards group outcrops via percolation through fault planes and joints as well as dissolution

channels. Although the Edwards aquifer is ideal in many ways, it is the sole source for San

Antonio and much of the surrounding population is dependent upon the groundwater supply. As

a result, withdrawal caps in addition to well permits have been issued by the Texas Legislature in

order to preserve the immensely beneficial aquifer.

13

Work Cited

Ardis, Ann, and Eve Kuniansky. "Hydrogeology and Ground-Water Flow in the Edwards-Trinity

Aquifer System, West-Central Texas." (n.d.): n. pag. USGS. Web.

<http://pubs.usgs.gov/pp/1421c/report.pdf>.

Ardis, Ann and Rene Barker. “Hydrogeologic Framework of the Edwards-Trinity Aquifer

System, West-Central Texas”(n.d.): n. pag. USGS. Web.

<http://pubs.usgs.gov/pp/1421b/report.pdf>.

"Edwards Aquifer - Pumping Rights Acquisition." SAWS:. N.p., n.d. Web. 02 Dec. 2014.

<http://www.saws.org/your_water/waterresources/projects/edwards.cfm>.

“Geohydrologic Framework of the Edwards and Trinity Aquifers, South-Central Texas” (n.d.): n.

pag. USGS. Web. <http://pubs.usgs.gov/fs/2006/3145/pdf/fs06-3145_508.pdf>.

George, Peter G., Robert E. Mace, and Rima Petrossian. "Aquifers of Texas." (n.d.): n. pag.

Texas Water Development Board. Web.

<https://www.twdb.state.tx.us/publications/reports/numbered_reports/doc/R380_Aquifers

ofTexas.pdf>.