Hydrogeological Analysis, Trinity Edwards Aquifer
-
Upload
michael-van-asten -
Category
Documents
-
view
65 -
download
2
Transcript of Hydrogeological Analysis, Trinity Edwards Aquifer
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>.