Geoscience Reference
In-Depth Information
Case study
METHOD - AGEING GROUNDWATER
An important piece of information for somebody
managing groundwater resources is the age of the
water contained in the aquifer. This information
will give an idea of how quickly any contaminated
water may move towards an extraction zone, or
how long ago the contamination occurred. Darcy's
Law gives an indication of the possible flow rates
within an aquifer but the measurement is at too
small a scale to be scaled up to estimate how long
it has taken for water to reach a certain position.
Frequently there is little idea of where the water
has actually come from, so even if you could esti-
mate water velocities you don't know the distance
travelled and therefore can't estimate age. In order
to overcome this problem groundwater scientists
use the chemistry of different substances dissolved
within the water to estimate its age.
Carbon dating is a common technique for the
dating of terrestrial deposits but is problematic
for young groundwater, since it is only accurate
when the sample is more than 1,000 years old.
Groundwater is frequently more than 1,000 years
old so it is possible to use the technique of carbon
dating, looking at the rate of decay of
14
C in
dissolved organic carbon. Another form of carbon
dating looks for
14
C resulting from the testing of
thermo-nuclear weaponry.
Another dating method, particularly for
younger groundwater, is to look for concentrations
of materials that we know have been added to the
atmosphere by humans. Fortunately for ground-
water dating, humans have been very good at
polluting the atmosphere with substances that are
then dissolved in precipitation. Figure 4.7 shows
the concentration of four gases that have been
added to the atmosphere in differing amounts.
The relative concentrations of these gases in water
samples give an estimate for the average age of
the groundwater. Tritium, a radioactive isotope
40
600
Tritium
CFC-12
CFC-11
SF6 x100
35
500
30
400
25
20
300
15
200
10
100
5
0
0
1940
1950
1960
1970
1980
1990
2000
Figure 4.7
Tritium concentrations in rainfall, CFC
and SF
6
concentrations in the atmosphere
1940-2002. Tritium units (TU) are 1 tritium atom
in 1,018 hydrogen atoms. CFC and SF
6
are in parts
per trillion by volume (pptv).
Source
: Figure redrawn from Stewart
et al
. (2007)
of hydrogen with 3 neutrons, was added to the
atmosphere in large quantities through explosion
of hydrogen bombs, particularly in the 1960s and
1970s. Tritium concentrations reached a peak in
1963 and have since declined to almost back-
ground concentrations. Tritium has a radioactive
half-life of 12.3 years. Chlorofluorocarbon (CFC)
compounds were used in aerosols and refrigeration
from the 1940s until their banning in the 1990s.
CFC-11 concentration has slowly declined since
about 1993, while CFC-12 concentration is still
increasing, but at a much slower rate than before
1990. Sulphur hexafluoride (SF
6
) is used for
cooling and insulation, particularly in electronics.
Another dating method is to look at the ratio
of two isotopes of oxygen and/or two isotopes of
hydrogen found in water molecules. When water
in the atmosphere condenses to form rain there is
a preferential concentration of heavy isotopes
of hydrogen and oxygen in the water molecules.
The heavy isotope of hydrogen is deuterium
(1 neutron) and the heavy isotope of oxygen is
18
O.
