how do the radio isotopes used in ground water study

2. Radioisotopes in ground water
research
SUBMITTED BY
PREETHI.K.D
2014602009, I M.Sc. Soil Science

3. Introduction
• Groundwater is one of the smallest components of
the hydrosphere
• Most groundwater is of meteoric, i.e. atmospheric
origin
• Groundwater is also often withdrawn
for agricultural, municipal, and industrial use by
constructing and operating extraction wells. The
study of the distribution and movement of
groundwater is hydrogeology, also called
groundwater hydrology.

4. Composition of total global water

5. Reasons for research
• There is a lack in comprehensive assessment
of the quality and availability of groundwater
resource.
• Resource is often poorly understood and
poorly managed.

6. Effective tool for research
• Stable and radioactive isotope techniques are cost
effective tools in hydrological investigations and
assessments, and are critical in supporting effective
water management.

7. Isotopes
• Environmental isotopes – unique in regional
studies of water resources.
• Artificial isotopes – effective for site
specification and local application

8. Environmental isotopes
Stable isotopes 2H,
13C, 18O
Radioisotopes 3H,
14C
• SI – measured by isotope ratio mass
spectrophotometer
• RI – measured by counting of their
radioactive decays.

9. Radiocarbon
• Carbon-14, 14C, or radiocarbon, is a radioactive
isotope of carbon with a nucleus containing 6 protons and
8 neutrons.
• Carbon-14 was discovered by Martin Kamen and Sam Ruben.
• Half life – 5370 years
• Beta - 0.156476 MeV
• Radiocarbon dating is a radiometric dating method that uses
(14C) to determine the age of carbonaceous materials up to
about 60,000 years old.

10. SAMPLING PROCEDURES
There are two techniques used in radiocarbon dating:
(1) the radiometric technique for normal size samples, which
counts the beta radiation coming from a prepared material, and
(2) the AMS (Accelerator Mass Spectrometry) technique, which is
suited for very small amounts of samples.
Beta Analytic Inc. uses the AMS technique.

11. • For ground water dating, the AMS technique is most common
due to the reduced physical labor for the collection of the
sample in the field and, afterwards, the laboratory.
• With a sample sent for AMS dating, we need one liter of the
well water.
• A standard wide-mouth plastic bottle available in all chemical
supply firms is generally used.
• Please note that the persons sampling the well or working in
the laboratory should not be wearing luminous watches - this
can cause tritium contamination.

12. Radiocarbon
The basic radiocarbon age determination calculation is as
follows:
t = - 8035 ln (δC14final / δC14initial )
• t = the radiocarbon age of the sample
• 8035 = the decay constant of radiocarbon, i.e., the half-life
divided by ln 2. A half-life of 5730 years for carbon 14 is used,
as per international convention
• ln = the natural logarithm
• δC14final = the measured net radiocarbon content of the
sample
• δC14initial = the net radiocarbon content of the modern
standard

13. Tritium
• Tritium is produced naturally in the upper atmosphere by
interaction of nitrogen, and, to a lesser extent, oxygen with
cosmic rays.
• Tritium (3H), and other chemical and isotopic substances in
ground water, can be used to trace the flow of young water
(water recharged within the past 50 years) and to determine
the time elapsed since recharge.
• Information about the age of ground water can be used to
define recharge rates, refine hydrologic models of ground-
water systems, predict contamination potential, and estimate
the time needed to flush contaminants from ground-water
systems.

14. Cont..
• In water it is expressed in TU
• Half life – 12.43yr
• Concentration is generally low in natural water
• Electrolytic enrichment is often carried out
prior to decay counting using liquid
scintillation or proportional counters.
• In precipitation – 2 to 5 TU

15. Cont…
• It is difficult to evaluate age information from tritium data alone, age
commonly can be reliably determined from data on tritium (3H) and its
decay product, helium-3 (3He).
• The 3H/3He age is based on a calculation that determines the amount
of 3He derived from radioactive decay of 3H in the water.
• Several conditions are necessary to solve the calculation and interpret the
age:
(1) The sample must contain detectable 3H (greater than approximately 0.5
tritium unit, or TU, which is defined as one 3H atom in 1018 hydrogen
atoms) and
(2) if the sample contains terrigenic helium from the Earth’s crust and mantle
sources, the relative abundances of helium-3 and helium-4 isotopes in the
terrigenic helium must be known, and data on dissolved neon
concentrations in the sample are needed to help determine how much
helium-3 is derived from tritium decay.

16. Tritium dating
• In principal, the determination of the
tritium/3He age of groundwater is simple. If
both the tritium and 3He concentrations are
measured in TU, it can be calculated as

17. Tritium dating
• Tritium (H-3) dating of ground water is sometimes used as
ancillary data for the radiocarbon dating study. It is less
successful than radiocarbon dating for two reasons:
• (1) the half-life of tritium is merely 12 years (versus
approximately 5568 years for radiocarbon), meaning that only
young ground water would show measurable values, and
• (2) the contamination of the atmosphere with nuclear testing
fallout tritium was extensive, reaching thousands of times the
normal amount, resulting in a serious ambiguity.

18. Applications
Isotopes are commonly employed to investigate:
• sources and mechanisms of groundwater
recharge
• groundwater age and dynamics
• interconnections between aquifers
• interaction between surface water and
groundwater
• groundwater salinization and
• groundwater pollution.

19. Source and mechanism of groundwater
recharge
• To ensure the sustainable development and
management of groundwater resources.
• Isotopes - used to identify and evaluate
present day groundwater recharge

20. Groundwater recharge
• The isotopic composition of groundwater (oxygen-18
and deuterium) is determined by the isotopic
composition of recharge.
• If most of the recharge is derived from direct
infiltration of precipitation, the groundwater will
reflect the isotopic composition of that precipitation
• Differences in isotopic composition of groundwater
resulting from different recharge sources, there can
be differences due to how recently recharge
occurred.

21. Cont…
• It is possible to identify, quantify, modern recharge —
within 40 to 50 years — by measuring isotopes and
dissolved gases (e.g. tritium, tritium and helium-3,
chlorofluorocarbons (CFCs) and sulphur hexafluoride
(SF6)) in soil water in an unsaturated zone or in
groundwater from shallow.
• The tritium–helium-3 method used to estimate
groundwater recharge rates by determining the
residence time of different groundwater samples
collected at different depths.

22. Recharge rate determination by
tracer peak displacement
• The assumption of the piston-flow model is that the tracer
and all water in the soil move simultaneously.
• The tracer peak at the position z and the time t is the
integrated result of the downward (infiltration) and upwards
(evaporation) movement that occurred during the period t –
to (to — starting time).
• The amount of water stored in the soil section between z and
zo represents the actual recharge (or the actual evaporation
loss if z is above the initial position zo).

23. Groundwater age
• Residence time, also called groundwater age, is the length of
time water has been isolated from the atmosphere.
• unconfined aquifers – a vertical gradient of groundwater ages
(increasing age with depth).
• Gradient α inverse of the recharge rate (volume/time)
• confined aquifers – horizontal or lateral gradient (age
increasing with distance from area of recharge).
• Gradient α inverse of the flow velocity

24. Cont…
• Groundwater movement – few meters per yr
• Km/yr - hundreds or thousands of years old
In large aquifers with long flow paths :
• carbon-14 (5730 years ) - a suitable tool for
the dating of groundwater in an age range of
about 2000 to 40,000 years.
In deep confined aquifers : ages of tens and
even hundreds of thousands of years
•krypton-81, chlorine-36 and iodine-129

25. Interconnections between aquifers
• stable isotopes, can be used to investigate
such interconnections (isotopic composition
of groundwater in the aquifers being
measured is different.)
• Stable isotope data can be used to estimate
the flow of groundwater from adjacent
aquifers.

26. Interaction between surface water
and groundwater
• Groundwater often consists of a mixture of
recharge from surface water (lakes or rivers)
and local precipitation.
• A simple isotopic balance equation can then
be used to estimate the relative proportions
of surface water and precipitation in recharge

27. Groundwater salinization
In areas where salinization of groundwater is occurring, it
is necessary to identify the mechanism of salinization in
order to prevent or alleviate the cause.
Isotope techniques can be used to distinguish the
importance of the following processes which may lead to
the salinization of groundwater:
• leaching of salts by percolating water
• intrusion, present or past, of salt water bodies such as
sea water, brackish surface water or brines and
• concentration of dissolved salts through evaporation

28. Groundwater pollution
• Environmental isotopes can be used to trace
the pathways of pollutants in aquifers and
• predict spatial distribution and temporal
changes.
• Concentration and stable isotope composition
of hydrocarbons – a powerful tool for
pollution assessment and remediation.

29. Current use
• The isotope hydrology program at the International Atomic
Energy Agency works to aid developing states (including 84
projects in more than 50 countries) and to create a detailed
portrait of Earth's water resources.
• In Ethiopia, Libya, Chad, Egypt and Sudan, the International
Atomic Energy Agency used such techniques to help local
water policy deal with fossil water.
• An arsenic pollution crisis in Bangladesh that the World
Health Organization calls the "largest mass poisoning of a
population in history" has been investigated using this
technique.

30. Reference
• www.iaea.org
• www.iisc.ernet.in
• www.ijird.com
• Bohlke, J.K., and Denver, J.M., 1995, Combined use of groundwater dating,
chemical, and isotopic analyses in two agricultural watersheds, Atlantic
coastal plain, Maryland: Water Resources Research, v. 31, p. 2319–2339.

31. Thank you