Geographic Journey: Karst and the Edward’s Aquifer

Geographic Journey: Karst and the Edward’s Aquifer


the following program is intended for
mature audiences water it’s one of our most precious
resources yeah a few of us know where our water comes from for most of us it
comes out of the tab for many of us it comes from a bottle when you get up in
the morning Dazed and Confused ready for a hot shower to kick start your day you
sell them give a damn where your water comes from as long as it’s hot where
does your water come from i’m scott walker i’m a geographer a professor and
a traveler and i’m taking you on a geographic journey the Edwards Aquifer this unique
groundwater system is one of the most prolific artesian aquifers in the world
it’s one of the greatest natural resources on earth serving diverse
agricultural industrial recreational and domestic water needs for almost 2
million users in recent decades demand for water has surpassed this ancient
aquifers capacity to provide and their increasing concerns about the welfare of
endangered species and regional economies that depend on its spring
flows some smart and forward-thinking people have been instrumental in setting
aside areas like this one to protect the aquifers recharge zone trust me was one
of the favorite sayings of one of these people Inga Van Nynatten who died of breast
cancer at only thirty years old Inga helped to create this green space in San
Marcos that’s the beginning of waters journey to your tab the chemical
reactions involving underground water are relatively simple although pure
waters of poor solvent almost all underground water is laced with enough
chemical impurities to make it a good solvent the compounds that make up a few
common minerals so basically underground water is a weak solution of carbonic
acid because it contains dissolved to carbon dioxide now dissolution is an
important weathering erosion process for all rocks but particularly it’s
effective on this type of rock here carbonate sedimentary rock or limestone
and you can see the three holes that are in this rock right here these holes are
not from somebody doing something to it these three holes are from a weak acid
because the rainwater falling through carbon dioxide in the atmosphere it
lands on this rock and it essentially dissolves the rock this is called karst
limestone this limestone is all over Central Texas you can see and just close
up here these holes in the rocks these are all surface rocks how jagged the
edges are these rocks are also underground and the
really pours rain water falls on this ground in this area and then parka
lights down into the aquifer this area is protected because we’re in the
Edwards Aquifer recharge zone this air is full of recharge features let’s take
a look at one that was found right here see this little cave right here looks
like a little rabbit hole or something like that what this is is a recharge
feature water goes comes percolated down through here and goes down into this
it’s coming with leaves down into the aquifer goes down into the underground
water system percolates through that cost goes into a larger area and these
things are all over the place out here now let’s go take a look at some larger
recharge features all right now what I want to do is take up tour the short
tour of this recharge feature we take the camera down in here and see what we
can find are you ready are you ready okay let’s take a look inside this
recharge feature right here take my good jacket off follow me down
with that Cameron okay here right now imagine I’m a drop of water I would just
be coming right down through here inside this recharge feature camera steady you
can look around let’s hope there’s nothing crazy in here this goes back
quite a ways back through here it goes way down into here let me see if I can
zoom in to that it’s just a big black hole it goes down through there goes way
back up into here I would say maybe 20 feet and then it just kind of gets too
small where where people can’t actually get into it and take a look at the rocks
up here you can see how they’re kind of porous you lot of cracks in the rocks
this is where the water comes down through here just imagine being under
the ground him which is where we are and the water comes down fills these spaces
and then runs on down this direction so I kind of touch what it’s like inside
it recharge feature let’s go look at some more not all the recharge features
on the Edwards Aquifer is as small as the ones we just looked at for example
this one here this fenced-off is called as l’s cave right in the middle of San
Marcos Texas and this area is fenced off because you can actually climb down into
this cave and reach the aquifer level it’s fenced off now it’s owned by the
Nature Conservancy to keep people from getting down into the aquifer and
potentially contaminating it see if we can get a shot of the zealous cave
interest where it’s blocked off now of course we’re locked out so I’m going to
do is I’m going to get the camera I’m going to hold it up high so we can take
a look down in there now here’s the entrance to zellas cave this is inside
the fence I can actually get to it but this is a close-up look at it the
Edwards Aquifer is just one of many aquifers in Texas it cuts a swath
through the center of the state and within this area lies San Antonio
America’s seventh largest city the city has a semi-arid climate and water from
the Edwards is the reason that 18th century Spanish missionaries were able
to establish footholds like the city of San Marcos in San Antonio for over two
centuries San Antonio and many other cities in the surrounding area we’re
able to grow and prosper without developing surface water because of the
clear running waters of the Edwards Aquifer kanya que esta this is what the
Tonkawa Indians called this place right here means where the river starts where
comal springs in new braunfels texas in landa park and the stream flow right
here is coming right straight out of the ground behind me at 407 cubic feet per
second right now now 1950s this got as low as 50 2 cubic feet per second and it
wasn’t enough water to supply fresh water for the fountain daughter
then what is now in endangered species to live and the actual the fish
population was decimated now since then fish have been brought in from San
Marcos Springs where they now thrive back here again now 1968 this water was
coming out of his ground at 600 of more cubic feet per second and this is all
coming straight out of the aquifer in general the movement of groundwater in
the aquifer is from areas of higher elevation in the south west toward major
discharge areas in the Northeast the flow pattern is controlled primarily by
locations of barrier faults that disrupt the continuity of permeable Edwards
karst limestone the presence of many faults and fractures makes the flow
patterns highly complex because of this complexity the waters we pump from the
ground and drink our mixture of waters of many different ages in some places
water moves only a few feet a day but in other places it may move a thousand feet
a day or more the average residence time for water in the aquifer is about 200
years much of the water in San Antonio and drink today probably went
underground around the time of the American Revolution although there are
many conduits and large caverns in the Edwards limestone you can’t picture the
underground reservoir as a vast pool most of the water is traveling in small
pore spaces within the carse limestone that are probably no larger than your
finger 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 like the cave I climbed into quickly become filled up
with water Edwards Aquifer is not a good storage aquifer where water can be
placed and expected to stay in for use tomorrow transmissivity is high enough
that as long as enough hydraulic pressure exists to force water up to the
level of Springs significant amounts of water will flow out these flows create
the San Marcos River and create the comal river that contributes to the
guadalupe river this entire process happens from water
flowing into recharge features to a moving underground and being discharged
by Springs before it’s ever tapped by your water utility know where your water
comes from and appreciate it the next time you take your shell Oh

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