Isotope Hydrology
Managing Groundwater Budgets
Mike Kane Geo 361
What is Isotope
Hydrology?
Isotope Hydrology is a nuclear technique that uses
isotopic dating to measure and estimate the age,
origins, and the movement of water within the
hydrologic cycle.
The isotopes in water can be traced through water
sources to determine where the source is, how it is
recharged, and if it is at risk of contamination.
Why Use Isotope
Hydrology?
Of all the water on Earth about 2.5% of it is freshwater
sources.
Most of this water is locked in the icecaps, in soil
moisture, or in deep underground aquifers.
Water is key for the development of life. According to the
IAEA about 1/3 of the global food comes from irrigation,
which requires sustainable groundwater sources.
About 1.1 billion people today go without fresh water and
in 2025 2/3 of the world’s population may live in countries
with high shortages of water.
How Isotope Hydrology
is Used
During the processes of ET and
condensation the oxygen and hydrogen
isotopes in water molecules begin to
undergo changes.
When water is passed around in the
Hydrologic cycle it becomes naturally
stamped with “isotopic fingerprints”.
These finger prints vary depending on
the body of water and how the water
molecules travel through out the cycle.
The isotopes of the atoms are chemically
identical but physically different. For
example Oxygen becomes split into
Oxygen 16 and Oxygen 18 isotopes.
Certain isotopes have a heavier nuclear
weight which allows them to be
distinguished from other isotopes.
Oxygen 18> Oxygen 16.
How Isotope Hydrology
is Used Cont.
When ocean water evaporates, heavier isotopes will
condense and precipitate first. Since most evaporation
occurs over the ocean, the rain furthest from the coast
will be less enriched with heavier Isotopes.
As water changes stages in the hydrologic cycle the
composition of water isotopes will vary.
With the different isotopic signatures occurring, this
allows hydrologist to map groundwater sources and to
determine how they gain/lose water by following the trail
isotopes.
Application of Isotope
Hydrology
Lake Chala is a volcanic crater
that’s on the southeastern slope
of Mt. Kilimanjaro.
Lake Chala has an area of 4.2
km2, a volume of 3 X 108 m3, and
a depth of 100 m.
Lake Chala is not subject to
surface inflow or out flow.
During the 1960’s this lake
became of interest when local
farmers were looking for more
sources of water to expand their
irrigation processes.
Application Cont.
A problem arose because of nearby underground springs
that were already in use for irrigation purposes.
No one knew if the lake was the source of water for the
springs and by taking water from the lake could effect the
amount of water in the springs.
To determine if the two water sources were connected 10
water samples were taken from the lake on two occasions
nine months apart.
Over the course of three years samples were taken from
the local springs.
O18 (0/00)
σ
D (H2) (o/00)
σ
Lake Chala
1967
2.51
.07
7.3
2.7
Lake Chala
1968
2.43
.16
4.9
4.4
Homer Spring
-3.96
.05
-20.5
2.8
Kileo Spring
-4.72
.08
-23.2
1.3
Lenoya Spring
-4.11
.43
-17.3
1.9
Little Lumi
Spring
-4.20
.21
-18.3
.9
Magi ya Waleni
Spring
-3.97
.20
-17.2
2.1
Njoro Kubwa
-4.05
.43
-19.4
2
Njoro Ndogo
-4.04
.24
-18.6
2.1
Latema
Borehole
-4.53
.14
-22.1
3.1
Kitovo Spring
-4.56
.21
-21.9
2.6
Application Cont.
Proven by the data table, the composition of isotopes
O18 and D (H2) didn’t vary with time.
The values for the isotope composition of Lake Chala
and the nearby springs were greatly different.
This means that the lake water had no connection
with water from the springs.
Conclusion
Isotope hydrology is a useful tool in helping
countries and people develop water management
strategies.
It is very cost effective way to assess the location of
groundwater, where it is being recharged and help
decisions on where to extract the water.
The IAEA has put in over 30 million dollars in 150
projects to help 60 countries develop water
management strategies and techniques.