|Edwards Aquifer courtesy of Gregg Eckhardt, for educational use only
The Edwards Aquifer is an underground layer
of porous, honeycombed, water-bearing rock that is between 300-700 feet thick. It includes the Edwards and some associated limestones.
The San Antonio segment of the Aquifer extends in a 160 mile arch-shaped curve from Brackettville in the west to near Kyle
in the northeast, and is between five and 40 miles wide at the surface. At these two locations, groundwater divides
separate the San Antonio segment of the Aquifer from other Edwards limestones, so their waters do not mix. The Barton
Springs segment extends from Kyle to south Austin (see map above). The San Antonio segment is where most of the major
natural springs occur, where much of the use by humans takes place, and where the issues are most hotly-debated. A few
major Edwards water features like Barton Springs and San Felipe Springs occur on the other side of the
groundwater divides, to the north and west of the San Antonio portion.
The image below shows a typical cross-sectional view of the Aquifer:
The Aquifer is divided into three main zones: the contributing zone,
the recharge zone, and the artesian zone. The contributing zone occurs on the Edwards Plateau, also called the Texas
Hill Country. It is about 5,400 square miles, and elevations range between 1,000 and 2,300 feet above sea level.
The rugged, rolling topography is covered with thick woodlands of oak and cedar. Today, the Edwards Plateau bears little
resemblance to the prairies the pioneers to the area saw, but it is home to several endangered species and is itself the subject
of increasing environmental concerns. The contributing zone is also called the drainage area or the catchment area.
Here the land surface "catches" water from rainfall that averages about 30" per year, and water runs off into
streams or infiltrates into the water table aquifer of the plateau. Runoff from the land
surface and water table springs then both feed streams that flow over relatively impermeable
limestones until they reach the recharge zone.
The recharge zone is a 1,250 square mile area where highly faulted and fractured
Edwards limestones outcrop at the land surface, allowing large quantities of water to flow into the Aquifer. For this
reason, the Edwards is often called a fault-zone aquifer. About 75-80% of recharge occurs when streams and rivers cross the permeable formation and go underground. A small percentage
occurs when precipitation falls directly on the outcrop. A surface water reservoir
built partly on the recharge zone, Medina Lake, contributes large amounts of water to the Aquifer.
Also, some recent models suggest that significant amounts of recharge enters the Edwards from the Trinity Aquifer, perhaps as much as 10% of the annual total. Most of the annual average recharge of about 711,600 acre feet (for the period 1934-2006) occurs in the western
counties of Medina and Uvalde, where the Edwards outcrop is very wide at the surface. But rainfall is highly variable
and so recharge amounts also vary widely from year to year. In the recharge zone there
are no other rock formations overlying the Edwards - it is exposed at the surface. So the Aquifer here is "unconfined"
and has a water table that rises and falls in response to rainfall. However, the major portion of the Edwards, the artesian
zone, is confined between the Glen Rose limestone and the Del Rio clay, and it has no water table.
Seco Creek Recharge Project
|Aquifer being recharged
|It is hard to get
an idea of scale here, but the sinkhole on top is over 60 feet wide! The same sinkhole is shown below with a gigantic
amount of recharge water going into it. This is the Seco Creek Recharge Project, where water is purposefully collected
and diverted into the sinkhole to increase Aquifer recharge. Sinkholes can quickly receive large volumes of recharge
during rainstorms and transmit the recharge directly into the aquifer. (Photos contributed by Jeremiah Friddell.)|
Helotes Creek in recharge zone
One way to figure out if a stream
is on the recharge zone is to look for rocks that increase in elevation going downstream. In the photo at left, we are
in the bed of Helotes Creek, looking downstream, in old downtown Helotes. Normally, streambeds decrease in elevation
as they meander downstream and eventually confluence with another stream or reach sea level. But here, notice there
are large blocks of limestone that are several feet higher than the creekbed in the foreground. This is an indication
there was not enough water with sufficient velocity and erosive power to create a gradually graded streambed. The water
disappeared underground instead of eroding the limestone.
There is a thin strip of land south and
southeast of the recharge zone from San Antonio to Austin where limestones that overlie the Edwards are faulted and fractured
and have caves and sinkholes, so it is possible that surface water can still go into the Edwards limestone below. This
area is called the transition zone. There are plenty of good wells and numerous springs in the transition zone, so it
is actually also part of the artesian zone (see below), which is the area where we can pump good water out through wells or it comes to the surface
on its own through springs.
|The transition zone was established to
regulate petroleum storage tanks, so there are places where the boundaries follow particular streets or railroad lines.
Above is an image of an official US Geological Survey map on which the Transition Zone is delineated. The area is in
San Antonio, in northwest Bexar county. Notice how the bold dashed line defining the southern limit of the transition
zone follows Braun Road and Hwy 16.|
Contributing Zone Within the Transition Zone
There are some very small
areas south and southeast of the recharge zone where limestones other than the Edwards crop out at a higher elevation than
the Edwards, so water drains to stream courses that overlie the recharge zone. These areas are called the Contributing
Zone Within the Transition Zone.
is an image of an official US Geological Survey map on which a Contributing Zone Within the Transition Zone is delineated.
The area shown is just south of the intersection of I-10 and 1604 in San Antonio.
On the map at the top of this page, the area shown at left is the little green spot within the recharge
zone in northwest Bexar county.
Once recharge water works its way by gravity down into the artesian zone, there are other rock
formations lying over the Edwards, and water is trapped inside. The artesian zone of the Edwards is confined between
two relatively impermeable formations - the Glen Rose formation below and the Del Rio clay on top. The sheer weight
of new water entering the Aquifer in the recharge zone puts tremendous pressure on water that is already deeper down in the
formation. Flowing artesian wells and springs exist where hydraulic pressure is sufficient
to force water up through wells and faults to the surface. Major natural discharge occurs at San Marcos Springs and Comal Springs in the northeast. San Antonio Springs
and San Pedro Springs in San Antonio are dry most
of the time because large amounts of water are pumped from the ground by users in Bexar county, but they flow when Aquifer
levels are very high. Water moves generally from southwest to northeast through the Aquifer and
there are a number of barrier faults that make it difficult for waters in the various units of the Aquifer to mix together.
These faults, along with varying porosities and permeabilities of the limestone, control the movement of water in the Aquifer
fault. The J17 index well is
used to monitor the amount of pressure that water in the artesian zone is under. Changing pressure is reflected in rising
or falling well levels.
|Artesian well in San Antonio, circa 1895 |
Antonio began to rely on artesian wells for its water supply in 1891. These are two of San Antonio's first municipal
water supply wells. The photo shows the tremendous amount of pressure that Aquifer water was under at that time.
If we estimate the two men in the photo to be around 5 1/2 feet tall, then the column of water shooting up from the well is
around 25 feet high! The effect of releasing all this pressure through wells was that springflows began to decline immediately
and significantly. By 1896 there were approximately 40 wells in the San Antonio area. By around 1900 San Antonio
Springs had been reduced to just a trickle in most years.
This photograph appeared in R.T.
Hill and T.W. Vaughan's 1896 report on the geology and underground waters of the Edwards
Plateau. Hill and Vaughan were the first geologists to recognize that wells such as these had impacted springflows.
They were the first people to accurately describe the Edwards and how it works. Although they never used the word 'aquifer',
they referred to the Edwards as an artesian groundwater system, accurately described the catchment and transmission of water
in the Aquifer, and recognized its large extent from Brackettville to Austin.
|They even accurately predicted the existence of the
large contiguous artesian zone between San Antonio and Del Rio in which good water can be obtained anywhere. Before
their publication, the widely held belief was that waters supplying the artesian wells and spring rivers in south Texas came
from the distant Rocky Mountains. They recognized that was impossible, and they explained the true source is the rainfall
of the Edwards Plateau. |