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
limestone units, so their waters do not readily mix during wet and normal hydrologic conditions. 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 sides of the
groundwater divides, to the north and west of San Antonio portion. We are learning more all the time about the extent of the San Antonio segment and the connectivities between segments and other aquifers. There is new evidence that the portion of the Edwards in Kinney county is actually a separate pool, and also new research suggests the Barton Springs may not be so separate after all. In addition, new studies have shed light on the connections between the Edwards and the Trinity Aquifer, which is the major source of water for the Hill Country.
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
Cibolo Creek forms the border between Bexar and Comal counties and often
contributes all of its flow to Edwards recharge. Francis T. Bryan (1849) provided one of the earliest descriptions of what happens to streams that cross the recharge
After passing the Cibolo, four miles from Misenbergs, the road
becomes very good, being smooth and level. The Cibolo, where the
road crosses it, is a dry ravine. About two miles above there is
plenty of pure water.
It is not
uncommon for the Cibolo to flow 30 feet wide and a foot deep and, in the
space of a quarter mile, disappear completely into the Edwards
tumbles off the Edwards Plateau and reaches the recharge zone near downtown Helotes. The creek is almost always dry, but may
flow for months at a time during rainy spells such as occurred in 1992 and 2007.
Notice the large plates of fractured limestone in the creek bottom,
which allow recharge water to go underground. A few hundred
feet below this spot, the creek is almost entirely dry. Technically, this spot is in the Contributing Zone, not the Recharge Zone. It is Trinity limestone, and the Edwards limestone starts a few hundred feet downstream. But this rock is in contact and hydraulic communication with the Edwards, and current research is providing new information about the importance of inter-formational flow between the Trinity and the Edwards. On a map, the 'Recharge Zone' starts where the Edwards begins to outcrop at the surface - in reality, it's more complicated than that.
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
(see section on Faults
& Caves for fault map and photos). About 75-80% of recharge occurs when streams and rivers cross
the permeable formation and go underground. This is called allogenic recharge. Most of the remaining percentage of recharge occurs when precipitation falls directly
on the outcrop. This is called autogenic recharge. 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 706,000 acre feet (for the period
1934-2012) 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 (see Charts).
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 (see graphic below).
|Sinkholes can quickly receive large volumes of recharge during rainstorms and transmit the recharge directly into the Aquifer. The Vandina Farms sinkhole in western Medina county is located on private property just a few hundred feet from Seco Creek, which occasionally experiences tremendous floods. The Recharge Zone is very narrow here, so there is not much opportunity for these flood waters to enter the Edwards in the creekbed itself. In 1982, a dam was constructed on Seco Creek and a diversion channel was excavated to the sinkhole, so that floodwaters can recharge the Aquifer. (two photos above contributed by Jeremiah Friddell.)
A view of the Vandina Farms sinkhole in January of 2013. The diversion channel that directs flow to the sinkhole is seen at upper left.
Since 1982, annual recharge at the sinkhole has varied from zero to 12,915 acre-feet. The sinkhole can recharge up to 1,770 gallons per second. (Hammond, 1993).
|This little cave in a creekbed near UTSA is about six feet wide at the surface. Note the weathered remnants of a stalactite that formed when this was still a cave deep underground. When the climate was wetter, this was probably a spring; today water only goes in.
Most recharge occurs in streambeds, entering the Aquifer through sinkholes or fault planes, but about 25% occurs wherever the Edwards limestone outcrops at the surface. It is usually very difficult to detect exactly where such recharge occurs. In the photo at left, a roadcut has allowed us to visualize how water gets underground. There is a fracture at the surface that you would never see if you were wandering around the pasture. It leads to a solution cavity about 5' below ground, where rock has been dissolved away leaving smooth cave-like surfaces. The same fracture connects the solution cavity to a small cave opening below that is large enough for a child to squeeze into. There are many thousands of such recharge features, and most of them are not easily seen.
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.
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.
At left 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 (see Flowpath Map),
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 (see barrier
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.
San Antonio began to rely on
artesian wells for its water supply several years after the first large wells were drilled in 1891. These are two of San
Antonio's first municipal water supply wells, drilled at Market Street for George Brackenridge, who owned the water system and had a contract to supply the city. 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 & 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
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.
Mr. Hill was the geologist who recommended that George Brackenridge drill in this location.
When it was drilled in the early 1990s in southwest Bexar
county, this was the largest private water
well in the world. There
is still tremendous artesian pressure at this location. When this
well came in, it blew out rocks the size of basketballs 20 feet in the
air! Drilled by Ronnie Pucek and
his supporters to supply water to a controversial catfish farm, the well
also brought many of the region's water issues to the fore in the 90's. For more see the page on the Living
Waters Artesian Springs catfish farm.
Want to find out what zone of the Aquifer you are in? Check out
the TCEQ's Edwards
Aquifer Map Viewer
A water-table aquifer is one in which the water is under atmospheric pressure. Water will not rise above the level of the "table", and the table rises and falls in response to rainfall and recharge. Only a small portion of the Edwards is a water-table aquifer. The water-table portion of the Edwards is the recharge zone, where the Edwards limestone is exposed at the land surface. Here, because there are no confining rock layers on top of the Edwards, the water is under atmospheric pressure. Water will not rise in a well above the level of the water table. There may be an upper zone of unsaturated rock.
Most of the Edwards is an artesian aquifer, in which water is under pressure. In the confined or artesian zone, layers of impermeable rock overlie the Edwards limestone, trapping water inside with no easy way out. Just as diving to the deep end of a pool causes you to notice the pressure in your ears, the sheer weight of new water entering the Aquifer in the recharge zone causes water deeper down to be under great pressure. If water can escape through a well or through a spring opening, it will do so, rising above the top of the limestone formation. If there is sufficient pressure, water will rise all the way to the land surface and gush out in a huge volume. In this zone, all of the limestone is saturated at all times, and there is no rising and falling water table, only rising and falling pressure. New water entering in the recharge zone instantaneously exerts pressure on the entire system, so rainfall can cause rapid rises in well levels far distant from the rainfall itself. A good monitoring well is one that is very responsive to rainfall and pumpage, in a location where there is never enough pressure such that water will rise all the way up and flow out on the land surface (so there is always a well level to measure). The J-17 is one such well, and it is used to monitor Edwards pressure in the San Antonio pool. The well is on a major flowpath and quickly reflects rising and falling pressure. J-17 levels and springflows rates are used to trigger drought restrictions and pumpage cutbacks during dry times.
The fresh water / saline water interface, usually
known as the "bad water line" is the convergence of two flow systems
within the Aquifer. It is actually a zone and not a line. The
freshwater area is generally updip, closer to the land surface; while the saline
water area is farther downdip, deeper underground. In the freshwater
portion of the Aquifer, the limestone is highly permeable and transmission rates
are high, so water moves through it relatively quickly. By contrast,
deeper down in the formation, the saline water portion of the Aquifer has low
transmission rates and much higher residence times. When water is in
contact with limestone, it continually dissolves mineral solids from the
surrounding rock matrix. Eventually, the concentration of total dissolved
solids (TDS) becomes greater than about 1000 ppm,
and the water is considered saline and not
drinkable (seawater is about 33,000 ppm). The "bad water line"
is a natural phenomenon that occurs along the southern and eastern edges of the
fresh water zone where water has been in contact with limestone for a long
time. Since the rock in the saline water zone is less permeable and does
not transmit water as easily, the movement of water is slower. As a
result, water stays in contact with limestone longer and becomes more
During dry times or when the Aquifer is drawn
down to low levels, it is possible that salt water would intrude along the
interface between the fresh and saline sections of the Aquifer, but it would
probably be flushed back out again when water levels rose. A study by Ewing and Wilbert,
1991 concluded that water quality deterioration, in all cases except actual
ground-water mining, would be temporary and limited largely due to the
significant difference in permeability between the fresh and saline sections of
the Aquifer and to the flushing action that would occur with renewed increase in
In 1996 the United States Geological Survey
established a program to monitor possible changes in water quality near the
fresh / saline water interface that might result when drought occurs.
Water quality monitors were installed at four wells near the interface for early
detection of saline water encroachment into the fresh water zone and to provide
information on seasonal variations in water quality. Data collected in
1996-1997 will serve as a baseline to be used for comparison with new data that
might be collected during a future drought (USGS, 1998).
Edwards wells that produce
hot, sulfurous water are common near the fresh / saline water interface,
usually known as the "bad water line". The water is warm
and smelly (hydrogen sulfide gas causes a rotten egg odor) yet it is
safe to drink and was considered very healthful in the past. Some
of these wells, such as at the Hot Wells Hotel,
were legendary world-class destinations. One of the first Edwards
wells was drilled in the early 1890's by Colonel C. M. Terrell of the
United States Army. Hill and
Stock drink the
water freely - for months at a time having no other - and it is said
to free them from ticks. In one case, when given for the
purpose, it entirely freed a horse from bots and other intestinal
worms. Many sick people have visited the well to use the
water, both for drinking and for bathing. Afflicted persons
who have tried it claim that by drinking and bathing in the water
they have been completely cured of many diseases. (Hill
& Vaughan, 1893, p. 296).
|This abandoned hot well was
located on Ansley Road in the Terrell Wells area of south Bexar county,
in the immediate vicinity of where Col. Terrell's well was
drilled. It had been flowing for as long as anybody in the
neighborhood could remember - over 70 years. It was finally
plugged by the San Antonio Water System in November 2000. The
abandoned well structure and the surrounding area were coated with a
thick scale of elemental sulfur.
In 1918, twelve San Antonio civic organizations cooperated to produce a promotional brochure extolling the many virtues of the growing city, including its pure artesian water supply. It was entitled "Greater San Antonio, The City of Destiny And Of Your Destiny" and its production was directed by E. A. Luck from the basement of the Gibbs building. The bottom of this page asked:
What is there left in San Antonio for a community organization to do, except to say to the rest of the world - COME?
In the 1890s, George Dullnig had a resort about six miles southeast of downtown that featured bathing in hot sulfurous water, but his bottled mineral waters were more famous and successful than the resort. He offered various waters from 11 wells drilled around 1886 on his ranch, and won the highest award at the 1906 St. Louis Exposition for the best display of mineral waters. Water that was 104 degrees emerged from 2,205 feet down. After Dullnig's death in 1908, Herbert Gregory took over the ranch house and operated the Dullnig Wells Hotel and Bathhouse until 1925. The structures were demolished in 1935 (Valenza, 2000).
In the late 19th century, trading cards were very popular and came in an endless variety of themes. The back usually contained some advertisement, such as for tobacco or fine shoes. This one has some cute little dogs on the front.