|
Here are answers to
some of the questions I get asked most frequently regarding the Edwards
Aquifer and issues surrounding it. Let me know if you have
a question you would like to see answered here!
- Unless we start mining the resource by
using more than goes in on a long term basis, we will
always be able to get plenty of good water for critical
uses like eating and bathing. We have never seen the
Aquifer less than 90-95% full, so there is lots of water
down there we can use if we have to in an extreme
drought. However, we sometimes DO run out of water in the
top 5-10% of the Edwards formation, and when that happens
the springs stop flowing. Lots of people, along with
endangered plants and animals, depend on water from the springs. To keep them flowing we have to keep the
Aquifer almost full.
Maybe it's more correct to call it a money shortage. If money were no object, we could do very expensive things
like desalinate
ocean water and have an unlimited supply. What we are
running short on is the cheap, seemingly limitless
aquifer water that we have been used to using without
restriction. All of the alternative water sources seem
very expensive by comparison.
Yes.
Because of the complexity of the
Aquifer system, it is difficult to narrow down the range
of how much water we think it contains. One
thing that is clear is there's a big difference between
how much water the Aquifer contains and how much water
could be extracted. Some researchers have estimated the Aquifer may contain as much as 175 million acre feet. However, that figure includes water locked up in pore
spaces that are not connected to any other pores, so that
water can't move anywhere and is therefore not available. A more reasonable estimate of 25-55 million acre-feet is
based on effective porosity, which is a measure of the
percentage of pore spaces within the rock that are
connected to other pore spaces (see
Maclay, 1981 and
Ogden, 1986). Pores must be connected for water
to move through the rock and to the surface through
springs and wells. Additionally, one should not picture
the Aquifer as a vast underground pool. There are
undoubtedly many large caverns, but most of the water is
in small pore spaces that are probably no larger than
your finger.
-
If there's
25-55 million acre feet of water available, and if we use
only 450,000 acre feet a year, then it sounds like
there's enough water to last 200 years! However, the aquifer
contains a lot of water that we can't
really get to in legal or practical terms. The
problem is the springs go dry when the aquifer is still
95% full. So as long as we are going to maintain at least
minimal natural springflows for the sake of endangered
species, recreational economies, downstream ecosystems,
and downstream economies, then the large amount of water
below the level of the springs is essentially unavailable
to us. Even if we did decide to let all the springs go
dry and pump out as much water as we need, it is very
expensive to pump large amounts of water up from great
depths and it would soon get prohibitively expensive.
-
-
-
Water quality is the big upcoming issue that will need to be dealt with over the next several decades.
Users of the Edwards Aquifer are lucky
because no major water quality or pollution problems have been experienced
YET. In San Antonio there is little heavy industry and not much
potential for serious degradation. Although there is a Superfund site in Leon Valley and there have been other isolated instances of pollution, we have not yet experienced any widespread problems. Currently, Edwards water does not require treatment before distribution, other than disinfection by chlorination.
-
To keep the water clean for
future generations, however, vigilance in protecting the recharge and
contributing zones
will be necessary. Once water in the Aquifer becomes contaminated,
it will be very difficult and perhaps impossible to clean.
The only alternative might be to treat water by conventional means
after it is pumped to the surface, an expensive proposition that most
cities other than San Antonio have already had to invest in. In most
cities, water is drawn from a river or reservoir, treated in a central
location, and sent to customers over a wide area through very large
distribution mains. In San Antonio, there are many Edwards wells all
over the city where water is pumped out, stored in tanks, and distributed
mainly to the local area. There are no large conveyance systems and there
would be no way to move large amounts of water from a central treatment
facility if it became necessary. Instead, there would probably have
to be a large number of smaller treatment facilities located close to the wells, which would be more
expensive to build and operate. If it became necessary, the cost for
treating Edwards water would be a very big number indeed. Whatever
that number is, that is the value of the treatment the Aquifer is
currently providing for free.
-
In 1995, San Antonio enacted the first rules and guidelines for
development over the recharge zone. But only 2% of the zone is
within San Antonio's jurisdiction, so more widespread requirements for
ecologically sound development are needed. It is also important that
people living on the recharge zone be aware of how the Aquifer can become
polluted and refrain from dumping used motor oil, cleaning agents, and
other common household chemicals. In addition, it is important to
protect the quality of water that runs off the Edwards Plateau and ends up
as recharge. People living on the Plateau are not Edwards Aquifer
users, so restricting development and implementing rules for the sake of
protecting other people's water supply is a thorny political issue. Texas is a state that is very strong on private property rights, and many will simply not accept the notion that land use and development should be regulated to protect common resources like air and water.
In addition to cultural obstacles, the roots of our inaction on Aquifer protection also derive from the mindset of water managers and business leaders of a generation ago. In 1982, local officials strongly opposed the Sole Source Aquifer Protection Act, a federal bill that would have provided 50% federal funding for development of aquifer management protection plans and purchase of recharge zones of sole source aquifers. The bill was sponsored primarily by New York state representatives for the purpose of allowing the residents of Long Island to purchase the recharge zone of their aquifer, but provisions would have allowed residents of any designated sole source aquifer area to request financial assistance from the US EPA (the Edwards was the first aquifer to be designated as Sole Source). The directors of the Edwards Underground Water District said the bill would be contradictory to the Reagan Administration's policies of returning more responsibility for local matters to the state and local governments (Federal funding for New York aquifer opposed here, San Antonio Light, May 7, 1982). EUWD manager Tom Fox said Edwards Aquifer management "has been adequately addressed by the state and local governments" (Directors of EUWD opposing Aquifer Protection Act of 1982, Northeast Herald-News, May 20, 1982). The North San Antonio Chamber of Commerce also opposed the proposed law, saying it would result in only "a further expansion of the federal budget with little or no significant improvement to the environment" (Edwards Aquifer controls opposed, San Antonio Light, July 30, 1982).
-
Proponents of managing the aquifer as a
reservoir suggest there are many ways we could use the Aquifer to a
greater extent in dry times, and then let it fill back up when the rains
come. This is certainly the approach that water managers
use to get the maximum benefit from a surface water
reservoir. Such an approach could involve recirculation and/or
augmentation systems to keep the springs flowing. A number of
studies are underway regarding Aquifer optimization, and many are focused
on gaining a better understanding of the flowpaths and discrete units or
"pools" within the Aquifer. -
- It might be possible, for example, to draw more water
from particular units while leaving springflows largely unaffected.
Or it might be possible to bring water from behind the "Knippa
Gap", which is a narrow opening within an extensive, complex system
of barrier faults and a major controller of flow within the Aquifer.
Huge amounts of water cannot pass quickly through the Gap, so water piles
up in storage units behind it, causing water levels in wells to the west
to display much less variability than wells to the east. Water that
recharges in western Medina and Uvalde counties has to flow through the
gap to reach the main freshwater zones in Medina and Bexar counties.
-
- Many questions are unanswered. For example, there is
no evidence to suggest that recharge will be increased
during rainy times simply because we have drawn the level
down low in dry times. The recharge conduits are fixed in
size and may not be able to recharge the same volume of
water drawn out.
-
Though many questions and issues exist, there is
indeed some potential to build a few Type II recharge dams in addition to the
ones that already exist. They
would not be a magic-bullet type of solution and could only supply a small
percentage of overall projected water needs. The draft plan of the
South Central Texas Regional Water Planning
Group estimates an average of 21,000 acre-feet per year could be added to
the Aquifer. In short term droughts additional
recharge could help get us over the hump, but recharge dams
can't help in a long term drought because there will be
no water to recharge. The Edwards is not a good storage
aquifer where water stays put for use tomorrow. As long
as enough hydraulic pressure exists to force water up of
the level of springs, significant amounts of water will
flow out. In a 3-5 year drought, all the water that was
recharged during wet times will have left the Aquifer. And there
are some thorny issues involved, such as the fact that Type II dams are
constructed on the recharge zone which is the most environmentally
sensitive area. Also, most people don't want their land to be
sacrificed for such projects, but somebody's would have to be. Further, it is unclear who would pay and who
would benefit. For more on recharge dams, such as what the
difference is between Type I and Type II dams, see the section on
Edwards
Alternatives.
The US Fish & Wildlife Service
includes the possibly of salt water intrusion into the
fresh water zone among the reasons that some species have
been listed as endangered, but I have never talked to a
hydrologist who felt that saline water intrusion was
anything to be even remotely concerned about. During dry
times or when the Aquifer is drawn down to low levels,
very small volumes of salt water might intrude along the
interface between the fresh and saline sections of the
aquifer, but the saline water would be flushed back out
again quickly 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 recharge. The reason the water is
saline to begin with is because the rock in that area is
much less permeable, so water moves slowly and stays in
contact with the rock for a long time. Because the saline
water is tied up in the rock, it will be difficult for it
to move from that area. Also, the saline zone lies
downhill from the fresh water zone, and it seems very
unlikely that salt water would suddenly start to run
uphill toward the fresh water zone. Additionally, salt
water is more dense than fresh water, so it tends to sink
below fresh water and the two do not readily mix.
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
saline.
Because the movement of water in the Aquifer is highly complex, the waters we pump from the
ground and drink are a mixture of waters of many
different ages. In some places water moves only a few
feet a day, but in other places water may move 1000 feet
a day (see
Maclay, 1981
and
Ogden, 1986) . The average residence time for
water in the aquifer is around 200 years.
Average annual recharge for the period 1934-2005 is
about 719,000 acre feet. However, averages do not mean that
much in this region...recharge is highly variable from
year to year
(see recharge chart).
The Edwards provides treatment that would otherwise cost us billions
of dollars. Whether or not it is a filter depends on which
definition you like.
According to Webster's dictionary, a filter is
"a porous article or mass that serves as a medium for separating from a
liquid or gas passed through it matter held in suspension or dissolved
impurities or coloring matter." In
the field of water treatment, a filter is usually composed of paper or
sand, where pore spaces are small enough so that water will pass through
but small suspended particles will not. The Aquifer is not what people in the water resource field
traditionally think of as a filter. However,
there are many pore spaces within the Aquifer that are so small that
large suspended particles cannot pass through.In this sense, and according to a strict definition, the Aquifer is
indeed a filter and does provide some filtration. -
- Regardless of what one thinks is a filter, there's one thing for sure:
the Aquifer transforms dirty brown polluted recharge
water into crystal clear spring water and well water that we safely drink
without any additional treatment. The same processes used by a
conventional wastewater treatment plant occur naturally in the Edwards. For more on water
quality, filtration, and treatment that occurs within the Aquifer, see the
section on
water quality
None. Well, it receives chlorination, and that's a treatment, but Edwards water does not require the kind of conventional treatment that costs most other cities millions every year. All surface water sources require treatment, as do many other groundwater sources, but Edwards water is drinkable straight from the ground. It wasn't drinkable when it went into the ground, so Aquifer protection is all about protecting the ability of the Edwards to naturally filter and cleanse stormwater runoff, so that we don't have to pay for it.
The limestones of the Edwards appear
to have sufficient bearing capacity so that subsidence
will not be a major problem. On the other hand, no one
really knows if underground caverns would collapse or if
flow patterns would change should the level be drawn
lower than ever before.
Right now it seems highly unlikely
that any more large surface water reservoirs will be
built. Environmental concerns would be many, and
landowners would fight tooth and nail against sacrificing
their beautiful river valleys to slake San Antonio's
thirst. Reservoirs eventually silt in and have to be
replaced, so in terms of a management scheme that will
last hundreds and thousands of years, reservoirs have limited value.
We probably won't be able to build ourselves a
solution with concrete and pipes, at least not a solution that uses
Edwards water. Many of the world's
best engineers and planners have been working on this
problem for a long time and if there were technical
solutions, they probably would have found them by now. Technology
can help a little, but the
solution lies in developing effective management
institutions and changing cultural attitudes toward the
value and use of water. There is certainly much potential for
using traditional technologies like pipes and treatment facilities to
bring in new water from elsewhere, but that's different than using
technology to augment or manage Edwards water. The basic issue is we
are already using all the water the Aquifer can make available and there
is only limited potential for making it yield any additional water on a
sustainable basis.
Regarding traditional structural
projects like dams and pipes, one helpful
thing we can do will be to build a system for moving
recycled water to people who can use it. Farmers and
miners in the area use drinkable water from the Edwards
to grow crops and produce rock products, and these are
ideal uses for recycled water. In the long run it may be
necessary to build some small reservoirs for storing
recycled water. For more on this topic, see the sections
on
water
recycling and
using recycled water.
This one deserves its own section! See Brush
Management in the Edwards Alternatives section. -
No. Generally, confined aquifers do
not have water tables. The Edwards limestone is confined
between two relatively impermeable formations and is
always saturated; the only place where a water table
exists is near the recharge zone where there are no
overlying layers (see graphic in
Intro Section).
The "Aquifer level" reported
on the news has nothing to do with how far down it is
until you hit water in the main body of the Aquifer. It
is a measure of how much pressure is being exerted on
water in the formation at the location of the test well. When recharge enters the
Aquifer,
its weight exerts
pressure on water already inside. This pressure forces
water up through openings such as springs and wells. The
"Aquifer level" indicates the top of the water
surface in the test well, which is hundreds of
feet above the actual Edwards limestone. A good
"indicator well" is one that never becomes
artesian...water is never forced so high that it flows
out without pumping. See the section on the
J-17 index well for a graphic that illustrates what the Aquifer level means.
The Comal River population of the
fountain darter was completely eliminated by the 1950's
drought when the river was reduced to isolated pools of
water. The species was reintroduced using individuals
from another population in San Marcos, where the Springs
never dried up completely.
Water enters the Aquifer easily in the
recharge zone, but the subsurface drainage is generally
inadequate to hold all the water that falls in large rain
events. Recharge conduits and sinkholes quickly become
filled and the remaining water has to flow over the
surface. Flash floods are the result.
This seems like a question that would have a
straightforward answer, but the answer is not simple and it involves a
mystery!
The credit for naming the Aquifer goes to the first people
who accurately described the Edwards
and how it works, even though they never used the word
"aquifer". They were two geologists, R.T. Hill
and T. W. Vaughan, who wrote an 1898 report entitled:
The Geology of the Edwards Plateau and
Rio Grande Plain adjacent to Austin and San Antonio, Texas, with
references to the occurrence of underground waters. US Geological Survey
18th Annual Report, pt. 2-B, p. 103-321.
In this report, the authors gave the limestone that makes up the
Aquifer the name "Edwards". It had previously been known
as Caprina limestone. That name was abandoned because it is a
paleontologic term and geologists prefer that rock formations have
geographic names. The geographic area in the vicinity of the
groundwater that Hill & Vaughan were describing was the Edwards
Plateau, so they substituted the geographic name Edwards for
Caprina. The Edwards Plateau was named after Edwards county, which
was organized and named in 1883. And Edwards county was named for
Hayden Edwards, who lived in Nacogdoches in east Texas. So far,
historians have not been able to explain why Edwards county was named for
someone who lived so far away, but a few are trying to unravel the puzzle.
Although Hill & Vaughan never actually called it an
'aquifer', they
referred to the Edwards as an artesian groundwater system, accurately
described the catchment and transmission of water in the system,
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 a good water can be obtained
anywhere. Before their publication, the widely held belief was that
waters supplying the artesian wells and spring rivers came from the
distant Rocky Mountains. They recognized that was impossible, and
explained the true source is the rainfall of the Edwards
Plateau.
In 1849, some of the troops that had fought with William Jennings Worth in Mexico camped around springs in San Antonio during a cholera epidemic. Worth and 600 others died, and for a while the campsite of the troops was known as "Worth's Spring". There are least three different stories regarding the location of the campsite and springs, and it is not completely clear if the site was at San Pedro or San Antonio Springs.
In her well-researched history of San Pedro Springs Park, Cornelia A. Crook reported that after Worth's death, San Pedro Springs became known for a time as Worth Spring (Crook, 1967).
In an unpublished 1975 manuscript on the history of the Incarnate Word grounds, Betty Dunn reported that by 1852 the large spring on the property (presumably the Blue Hole at San Antonio Springs) was variously known as "Worth's Spring", the "North Springs", and "San Antonio Spring". All three names appear in an Abstract of Title prepared for the Incarnate Word Property in 1936 (Dunn, 1975).
In a 1973 newspaper article on the re-awakening of long dormant springs after heavy spring rains, Incarnate Word Information Director Dick McCracken told a reporter that Worth's Spring was located underneath Clement Hall, one of the dorms, and that water was being pumped out at a rate of 4,000 gallons per hour (Springs run again on grounds of IWC,
San Antonio Express-News, July 20, 1973).
In her 1989 report on the archaeology and early history of the area, Karen Stothert reported that many people believe Worth's Spring is the large spring located north of the east end of Olmos Dam (Stothert, 1989).
-
An archaeological survey by Anne Fox, in which she looked for 19th century military encampments in the Olmos Basin, did not turn up any evidence. Since Worth's time, the entire Basin has been substantially modified by farming and bulldozers, and superficial remains of a temporary camp could have easily been erased (Fox, 1979).
-
So the location of Worth's Spring remains a mystery!
I got interested in the Edwards Aquifer shortly after I came to San Antonio
in 1972. I started collecting reports and photographs and reading all
I could about it, and I always assumed I would write a book. During
those years when I was collecting a lot of material, I was also sort of a computer geek. When the web was invented in the
early 90's, that was about the same time that all the Edwards issues were
really coming to a head. I decided I wanted to learn this new web
programming language and started fooling with html tags using a text
editor. Meanwhile, since the Edwards was becoming such a hot issue, I
knew there was going to be a real need for untechnical, unbiased information, so that
the public could make informed decisions and choices. A that time, I
was doing consulting work for many of the regional water agencies, and I knew none of
them had the resources or wherewithal to put anything online. And even
if they did, I knew it would have your typical institutional bias, and that
sort of information doesn't always necessarily serve the public very well. So
I decided I would put my burgeoning webmaster skills together with all my
Edwards resource material and hack together a web page. The first
version went online in early 1995. Part of keeping the material
unbiased and unslanted has meant not accepting sponsorships or advertising.
I don't get anything from webmastering these pages except the satisfaction
of knowing they are widely regarded by educators, students, and interested
citizens as a reliable, unbiased resource. About 30,000 people visit
this site each month, each one viewing an average of about 12 pages. |
