Age-Dating Groundwater
Environmental Poisons Turned Good
Can any good come from the atmospheric testing of nuclear bombs that happened
in the 1950s? How about from man-made chlorofluorocarbons (CFCs) that have depleted
our atmosphere's thin, protective UV-radiation barrier (commonly known as the
ozone layer)?
Maybe.
Contrary to common sense, man-made atmospheric gases produced since the 1930s
may actually be helping hydrologists age-date water from wells, springs, and
from deep inside the earth.
In 1928, Thomas Midgley, Jr. and Charles Kettering invented chlorofluorocarbons
(CFCs), also known as FreonTM. Since people in warmer climates wanted
ice in the summer, CFCs were a big hit in the refrigeration industry because
CFCs were a safer and more effective alternative to the widely-used and poisonous
ammonia. It was not until the 1970s that scientists discovered that CFCs were
destroying the ozone layer of the upper atmosphere. In 1987, members from sixty-eight
nations met and signed the Montreal Protocol which called for immediate reduction
of CFCs. However, much damage was already done. From about 1941 until the 1990s
copious amounts of CFCs were released into the atmosphere.
One
reason CFCs are good refrigerants is because of their molecular stability—CFCs
do not easily corrode the inner parts of a refrigerator. As it happens, molecular
stability is the characteristic that allows scientists to age-date water. Because
all gases dissolve in water, CFCs can be measured in water taken from wells
and springs. If the water contains no dissolved CFCs then we can safely assume
that the water trickled below ground, out of touch with the atmosphere, prior
to 1941. If the tested water contains CFCs then we can be confident that the
water has been in touch with the atmosphere since 1941. Scientists have also
found that recent precipitation can mix with very old water, which causes some
confusion with this age-dating method.
Notice on the satellite map at right that those sampling sites which did not
contain CFCs (red CFC markers) have water that is at least 60 years old. Samples
from sites marked with purple markers did contain CFCs and so are younger than
60 years—mixing of old and young water has occurred.
So what do nuclear bombs have to do with water age-date testing? In the 1950s
through the mid-1960s, humans got the grand idea to explode thermonuclear weapons
in the atmosphere, just to see what would happen. During that period of time,
radiation of all sorts was released into the atmosphere, including a radioactive
form of hydrogen, tritium (3H). Just like with CFCs, tritium dissolves in water
and trickles down into aquifers. Because of that brief period in our history,
hydrologists can test for tritium in wells, springs, and streams. On the map
at right, look for sampling sites that contained no tritium (red 3H
markers). These waters are older than the 1950s.
Scientists have many other ways to age date water, most of which involve testing
for dissolved gases. At the sampling sites that contained very old water, researchers
were also able to age-date water by seeking a naturally-occurring dissolved
gas—carbon-14, a radioactive form of carbon. Because of the constant bombardment
by radiation from the sun, carbon-14 is produced in the upper atmosphere. Over
time, carbon-14 slowly degrades back to regular carbon, but that process takes
thousands of years. So researchers can measure the quantity of carbon-14 in
water samples to tell how long it has been underground—the less carbon-14
that remains, the longer it has been underground. Remarkably, researchers have
discovered that two of the deepest wells, that reach down into the confined
aquifer, have water that is very, very old. The oldest water may have last been
in the light of day over 30,000 years ago.
Resource: Ground-Water
Flow Direction, Water Quality, Recharge Sources, and Age, Great Sand Dunes National
Monument, South-Central Colorado, 2000-2001, Michael G. Rupert and L. Neil
Plummer.