Isotope hydrology (Water Management)

1. Isotope Hydrology
By
Zaidoon T. Abdulrazzaq
(Water Management)

2. Introduction
Isotope hydrology is a proven tool in understanding hydrological
processes such are recharge rate and recharge mechanism, surface
water groundwater interaction, time scale of processes, origin of
pollution etc. The tool is even more powerful when used in
understanding of hydrology of arid and semi arid regions. Isotope
hydrology could provide the much needed knowledge for water
resources management.
Isotope techniques are effective tools for fulfilling critical
hydrological information needs(Fig 1), such as:
 The origin of water.
 The determination of its age, velocity, and direction of flow.
 The interrelations between surface waters and groundwaters.
 The possible interconnections between different aquifers.
 Aquifer characteristics such as porosity, transmissivity, and
dispersivity.

3. The cost of such investigations is often relatively small in
comparison with the cost of classical hydrological techniques, and in
addition isotopes provide information that sometimes could not be
obtained by other techniques.
Applications of isotopes in hydrology are based on the general
concept of “tracing,” in which either intentionally introduced isotopes or
naturally occurring (environmental) isotopes are employed.
Environmental isotopes (either radioactive or stable) have the distinct
advantage over injected (artificial) tracers that they facilitate the study
of various hydrogeological processes on a much larger temporal and
spatial scale through their natural distribution in a hydrological system.
Thus, environmental isotope methodologies are unique in regional
studies of water resources to obtain time and space-integrated
characteristics of groundwater systems. The use of artificial tracers
generally is effective for site-specific, local applications.

4. Fig.1: Natural Water Cycle

5. Environmental isotopes, both stable and radioactive (Fig.2),
occur in the atmosphere and the hydrosphere in varying
concentrations. So far, the most frequently used environmental
isotopes include those of the water molecule, hydrogen (2H or D, and
3H) and oxygen (18O), as well as carbon (13C and 14C) occurring in
water as constituents of dissolved inorganic and organic carbon
compounds.
Thus, the hydrogen isotopes may be written:
1H—common hydrogen, 1 proton
2H—deuterium (also written D), heavy stable hydrogen, 1 proton + 1
neutron
3H—tritium (also written T), radioactive hydrogen, 1 proton + 2
neutrons.
Environmental Isotopes

6. Oxygen Isotope s are
16O—common oxygen, 8 protons + 8 neutrons
17O—heavy (very rare) oxygen, 8 protons + 9 neutrons
18O—heavy oxygen, 8 protons 10 neutrons
Carbon isotopes are
12C—common carbon, 6 protons + 6 neutrons
13C—heavy stable carbon, 6 protons + 7 neutrons
14C—radiocarbon, 6 protons + 8 neutrons
Most common and of interest to hydrochemists are 1H2
16O
(common), 1HD 16O (rare), and 1H2 18O (rare). The water molecules
may be divided into light molecules (1H2 16O) and heavy water
molecules (1HD 16O and 1H2 18O).

7. 2H, 13C and 18O are stable isotopes of the respective elements
whereas 3H and 14C are radioactive isotopes.
Fig. 2: Environmental isotopes (stable and radioactive)

8. Units of Isotopic Composition of Water
The isotopic composition of water is expressed in comparison to
the isotopic composition of ocean water. For this purpose an inter
nationally agreed upon sample of ocean water has been selected,
called Standard Mean Ocean Water (SMOW).
The isotopic composition of water, determined by mass spectrometry,
is expressed in per mil ‰ deviations from the SMOW standard. These
deviations are written D for the deuterium, and 18O for 18O:

9. Stable Isotopes
The stable isotopes are usually measured using an isotope ratio
mass spectrometer (Fig 3), in terms of the isotope ratios of the less
abundant to more abundant isotope, for example, 2H/1H and 18O/16O (1H
and 16O being the number of atoms of the most abundant isotopes of the
respective elements). The radioactive isotopes are measured either by
the counting of their radioactive decays (low-level counting, for example,
by liquid scintillation counter) or the number of atoms (using accelerator
mass spectrometry) in a given sample.
Where R is isotope concentration ratio (2H/1H, 13C/12C, 15N/14N, 18O/16O,
34S/32S) of a sample or a standard. The isotopic standard used for
hydrogen and oxygen isotopes is the Vienna Standard Mean Ocean
Water (VSMOW) with isotopic ratios of 155.76 ± 0.05·10-6 for 2H/1H and
2005.20 ± 0.45·10-6 for 18O/16O.

10. Fig. 3:Isotope Ratio Mass Spectrometry

11. Radioactive Isotopes
Among the environmental radioisotopes, tritium and carbon-14 have found
the widest application in groundwater studies. Radioactive isotopes (also called
radioisotopes) occurring in groundwater originate from natural and/or artificial
nuclear processes (see Table 1).

12. Table 1: Cosmogenic and anthropogenic radioisotopes used in hydrology

13. Cosmogenic radioisotopes are produced in nuclear reactions between the
nucleonic component of cosmic radiation and the atmosphere.
Anthropogenic radioisotopes are produced in nuclear bomb tests and
nuclear reactors. The concentrations of all these radioisotopes in
groundwater are very low and usually measured by counting their decay rate
A in a given sample. The number of atoms N in a sample can be derived from
A by the relationship: A = λ⋅ N
where λ, the decay constant, is related to half-life T1/2 by the equation
λ = ln 2 / T1/2. For long-lived radioisotopes such as 36Cl and 129I, the
decay rate becomes immeasurably small. In these cases the number
of atoms has to be measured directly, which is possible by the
Accelerator Mass Spectrometry (AMS) technique. The AMS
technique is superior to the conventional decay counting for 14C, since
AMS requires a very small sample size (up to 1000 times less than
conventional requirement) for analysis.

14. Water Sources Age Determinatio
Age determination is used as a guide for the classification of the
susceptibility of ground water to near surface contamination. Environmental
isotopes and tracers are used to determine the age of ground water. The
radioactive isotope Carbon14 is used to date ground water that is older than
1,000 years (Fig 4). Chlorofluorocarbons or Freon and tritium as a product of
above ground nuclear weapons testing in the 1960s techniques are used to date
ground water that is less than 50 years old (Table 2). The analysis of dissolved
gas samples can estimate the temperature of ground water at the time of
recharge. This is used in the age determination techniques.
Methodology Average time span
[years]
Radiocarbon C14 dating 1,000 – 30,000
Tritium 1T3 dating 0 - 50
Tritium to helium3, 1T3 / 2He3 isotopic ratio 0 - 30
Chlorofluorocarbons dating, CFC-11, CFC- 12, CFC-113 0 - 50
Table 2: Time span for the environmental isotopes and tracers used to assess the age of water sources.

15. Fig. 4: 14C Groundwater Dating

16. Isotopes in Groundwater
Tracing groundwater by means of environmental
isotopes (Table 3,4 and 5), offers unique and supplementary
information on the origin and movement of groundwater and its
dissolved constituents, as well as allows a quantitative
evaluation of mixing and other physical processes such as
evaporation and isotopic exchange in geothermal systems.
Often secondary water-rock interactions can be studied,
of which the occurrence is decisive for whether or not isotopes
act as conservative or non-conservative tracers. Under
suitable geochemical and hydrochemical conditions dating of
the groundwater is possible (e.g. by the 14C method).

19. Groundwater contamination and pollution is a major public health
issue throughout the world. It is increasing due to agricultural practices
as well as domestic and industrial wastewater releases, while rapid
urbanization contributes pollutants in complex ways. Wastewater is
artificially recharging aquifer systems. Shallow aquifers are inadvertently
recharged due to infiltration of wastewater as a consequence of
agricultural practices as well as the use of hydraulic fracturing (fracking)
for hydrocarbons extraction. The phosphate fertilizer added to
agricultural crops adds a level of radium and uranium isotopes to the
treated soil. The levels are significant to necessitate health physics
protection measures to the workers handling them. The ashes left from
burning coal in electrical energy production contain uranium and radium
isotopes that could lead to subsurface water contamination if used in
road construction or as fertilizer. The provision of safe drinking water
from deep tube wells is an important strategy being considered for the
mitigation of arsenic contamination in many locations.
Groundwater Contamination

22. Fig. 5: showing the LMWL, GMWL, and isotopic composition modifications
accompanying hydrological and geochemical exchanges in the hydrologic
compartment