Below is the Bexar Audubon Society newsletter
article about Dr. George Veni's talk on karst aquifer geology at the
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"The best management practice is to stay off the karst."
That was the bottom-line message from Dr. George Veni,
one of the
world's top experts on karst aquifers -- like the
Edwards Aquifer on which
San Antonio depends -- when he spoke
at the Jan. 13 meeting of the Bexar
Audubon Society.
There may be a "least bad choice" but the only good choice is
no construction at all. Karst aquifers are just too
vulnerable, Veni explained. That has been proven by
an endless string of bad experiences on karst
aquifers all over the world, he said.
"Whenever there has been significant development over karst there has
been significant contamination of the aquifer," he stressed.
Despite a new meeting site in
The program, entitled "The Edwards Aquifer: A hydrogeological
and
biological primer," was supplemented by a Sunday field trip to see several
springs and other aquifer features.
Karst aquifers are the most vulnerable kind
because they have the largest holes in them and thus do not filter water, Veni explained. Any contaminant carried into a karst aquifer will come out of it in spring or well water.
Because the Edwards Aquifer holds so much water, he said, it has managed
-- so far -- to dilute contaminants enough to remain below the
level that would endanger people.
However, if pollution ever gets too heavy,
construct costly treatment plants to protect its people.
Authorities try to protect the aquifer by plugging recharge features
such as sinkholes, caves, etc., when development creates pollution hazards.
Not only does this reduce recharge
quantities but, Veni emphasized, it does
NOT prevent pollution.
"I did an experiment, spraying 6,000 gallons of water on a field
above
a cave," he said. "The field had no visible recharge features
at all, but
by the time I entered the cave ten minutes later, the water was raining
into it from the ceiling."
Microscopic pores exist in all parts of the karst,
he explained, and every part of the aquifer is vulnerable to pollution, not
just the places with visible recharge features.
Despite those microscopic seeps, it is the large water conduits that
define a karst aquifer and allow it to show both
rapid recharge and extremely rapid discharges of water.
In sand aquifers, water is packed into tiny spaces between sand grains.
In fracture aquifers, the water is found in narrow splits in the rock.
Karst aquifers offer much larger holes because
they are made of rock
types that can be dissolved by weak carbonic acid created when water absorbs
carbon dioxide from the air and soil. Over ages, this chemical action makes
openings called conduits, ranging from 5-10 millimeters across up to
man-accessible caves.
Karst has pores and fractures, too, and at any
one time, the conduits
in the Edwards Aquifer only hold about 6 percent of the total water supply.
But over time, 95 percent of the
water will move through the conduits.
Conduits are wide enough to allow turbulent flow, and that allows the
water to carry contaminants without any filtration effect. Furthermore, the
complexity of water flow in karst's myriad channels makes it hard to predict where
contaminants may go.
Conduits have another role. They are habitat for numerous animal
species, many of which have evolved in isolation within specific karst conduits and thus are rare.
This has been the root of
political controversy locally, as aquifer-management policies have been
influenced by species survival needs
as well as human water-management goals.
Ironically, Veni said,
if humans would just manage the aquifer
intelligently to ensure sustained water yields for human use, species
survival would pretty much take care of itself.
Only under quite rare drought conditions would any special measures
need to be taken to protect animals. But humans and animals alike are
jeopardized by the reckless, every-man-for-himself traditional approach to
groundwater.
sidebar
Some karst
tidbits:
--
constitution mandates that karst get the highest level of environmental
protection!
-- About 20 to 25 percent of the
-- While surface flow in the Edwards
region is from north to south, water
that has entered the aquifer actually flows east or east by north.
-- Cave species are often special
because they must adapt to a sunless
world where photosynthesis does not happen and thus food is rare and hard
to find even when it does exist.
-- Many cave species are relics of
widespread ancient groups of animals. One
-- There are four kinds of creatures
found in caves: accidentals (animals
that don't reside in caves and have no cave adaptations but may fall in or
take short-term refuge in one); trogloxenes (animals
that spend part of
their time in caves, often daylight hours, but must emerge to feed, e.g.,
bats); troglophiles (species that have some
adaptations and can spend their
whole lives underground, but don't have to); and troglobites/stygobites
(terrestrial
and aquatic species respectively that are obligate cave-dwellers, have extreme
adaptations such as absence of eyes and pigment, and would die on the surface:
all federally listed endangered cave species are in these groups).
-- Cave species have value to
humans, e.g., sources of medicine, indicators
of ecosystem health, and bats as insect-eaters and pollinators.
--
government in 2000.