Use of Hydrogen Isotopes in Ground-water Studies

8/2/2019 Use of Hydrogen Isotopes in Ground-water Studies

http://slidepdf.com/reader/full/use-of-hydrogen-isotopes-in-ground-water-studies 1/29



Isotopes are variants of atoms of a particular chemical

element, which have differing numbers of neutrons. –

Wikipedia

Natural isotopes

Synthetic isotopes



1H is the most common hydrogen isotope with an

abundance of more than 99.98%.

Deuterium comprises 0.0026 – 0.0184% (by

population, not by mass). Water enriched in molecules that include deuterium

instead of normal hydrogen is called heavy water.

Deuterium is not radioactive, and does not represent a

significant toxicity hazard.



Heavy water is used as a neutron moderator and

coolant for nuclear reactors.

Deuterium is also a potential fuel for commercial

nuclear fusion.



Stable oxygen and hydrogen isotopes have been used in

ground water studies to investigate -

recharge,

mixing, ground water/surface water interaction,

advective-diffusive transport,

paleohydrogeologic interactions and to

estimate ground water ages.



As issues of source water protection of drinking water

supplies have come to the forefront, the methodology

to effectively manage semi-confined aquifers is still

unclear.

Commonly, the area around the wellhead is considered

the most risk sensitive area, but in semi-confined

settings the most sensitive areas may be located some

distance away from the wellhead.



A synthetic test case was developed to determine the

suitability of the technique for identifying localized

areas of recharge to a wellhead in aquifers where

evidence of modern water infiltration exists.

Results of the model based on the synthetic test case

indicate that the technique presented is capable of

identifying localized areas of recharge contributing to a

wellhead, in a semi-confined aquifer setting, with onlya limited amount of required data.



Confined aquifers are typically less susceptible toanthropogenic contamination than unconfined aquifers;

however, their vulnerability should not be ignored due

to the fact that confined aquifers are not always

perfectly isolated systems.

Aquitard windows, regions of focused recharge through

an aquitard, can provide a direct conduit for potential

contaminants from anthropogenic sources and elevatedrisk in otherwise confined hydrogeologic settings.

http://en.wikipedia.org/wiki/File:Aquifer_en.svg



http://en.wikipedia.org/wiki/File:Aquifer_en.svg



The model adequately represents experimentalobservations of isotope profiles during evaporationfrom saturated or unsaturated soils, under both non-isothermal and non-steady conditions.

Groundwater replenishment occurs by both indirect (orlocalized) recharge through streambeds, depressions,etc., and direct (or local) recharge through the surfacialmaterials.

It is this latter form of recharge which often leads to adifference in the isotopic signature between rainfall andthe unconfined groundwater.



To obtain hydrologic information from the isotopiccomposition of groundwater, the processes occurring inthe unsaturated zone which .lead to changes in isotopiccomposition must be taken into account.

EVAPORATION FROM SATURATED SOILS

For the existence of a steady-state concentration gradient nearthe surface, equality of the diffusive and convective fluxesimplies that:

D*dR/dz = E(R - Rre~)

where D* is the effective diffusivity of the isotope in the pore waterand E is the evaporation rate. Rre s is the isotope ratio of the waterentering the column from below



The solution of this differential equation gives:R - Rros + (R0 - Rres)exp( - z / z l )

where:

zl = D*/E and D* has been assumed constant, which willbe the case provided only that the pore space is

homogeneous with depth.

•

Isotope concentrations are usually expressed as relativedeviations from the concentration of a standard water, usually

Standard Mean Ocean Water (SMOW)_multiplied by 1000



The past two decades has seen considerable progress inthe use of oxygen-18 and deuterium for tracing watermovement in the unsaturated zone.

The stable isotopic species of water have been used to

investigate the processes of infiltration, evaporationand mixing, and to make quantitative estimates of groundwater recharge and evaporation rates.

A principal advantage of using stable isotopic tracers

to determine water movement is the limited variabilityof the effective diffusivities with varying water content,compared to the marked variation of the soil water .



Temperature effects on soil water profiles and fluxescan be significant, but appear to have little effect on

isotope profiles.

Further experimental clarification of the interaction of

the isotope profiles with temperature gradients is

required in order to obtain greater precision in

interpreting unsaturated zone profiles;

in particular, this is necessary for accurate estimation of evaporation rates.



Sukhija and Shah (1976) found that 3H-peak displacementmethod gave drainage estimates 20 - 40% higher than the3H mass balance method, at field sites in northern India.This suggests either:

(a) that 3H fallout has consistently been overestimated,

(b) that 3H is being lost from the soil profiles, or(c) that too much water is being counted in the peak-

displacement method.

The latter could be due to either(i)the presence of immobile water, or(ii) including water in the plant root zone, which may not

become drainage.



At recharge rates greater than approximately 20 mm yr-’ the results of the 3H mass-balance, peak-displacement

and chloride mass-balance studies, all appear to agree

within 30 - 50%. The results of peak-displacement

methods using artificial 3H tagging also compare well

with those of the chloride mass-balance method.



Of all the water on Earth, only 2.5 per cent is freshwater,the rest is salty. Of this freshwater, most is frozen in icecaps

present as soil moisture, or inaccessible in deep

underground aquifers, leaving less than 1 per cent

accessible for use. It is estimated that more than one third of the global food

production is based on irrigation, a significant portion of

which may rely of unsustainable groundwater sources.

Isotope hydrology is a nuclear technique that uses bothstable and radioactive environmental isotopes to trace the

movements of water in the hydrological cycle.



Isotopes can be used to investigate undergroundsources of water to determine their source, how theyare recharged, whether they are at risk of saltwaterintrusion or pollution, and whether they can be used in

a sustainable manner. During evaporation and condensation, the

concentration of oxygen and hydrogen isotopes in awater molecule undergoes small changes.

When water from the ocean evaporates, the heavierisotopes will condense first and fall as rain before thelighter ones.



At each stage of the hydrological cycle, there is a smallchange registered by a difference in the concentrationof oxygen and hydrogen isotopes in water that is asunique as a fingerprint.

The isotopes of pollutants, such as trace metals, orchemical compounds dissolved in water, also offerclues about its origin.

The picture that emerges allows hydrologists to map

groundwater sources and climatologists to betterassemble climate history, setting signposts for theimpact of future events as climate change occurs.



The International Atomic Energy Agency (IAEA)supports the use of isotope hydrology to improve

knowledge of water resources.

Each year, the IAEA allocates nearly US $3 million to

its water resource program.

The Agency has also invested about US $30 million in

150 projects in 60 countries to improve water

management using isotope hydrology and, in theprocess, has trained hundreds of young scientists.



The residence time of groundwater in an aquifer or thegroundwater age is an important parameter in any

palaeo-hydrologic and geo-hydraulic study.

Water-rock interactions occur during groundwater

recharge within days/ weeks and during flow in the

aquifer within years to even millions of years. Isotope

hydrological studies give at least an idea about

approximate ages of the various ground waters.



Dating by radioactive decay : The physical process of radioactive decay is the basis

of the age determination of groundwater.

Radioactive decay of a certain nuclide is completely

independent of any environmental parameter such aspressure, temperature, pH or chemical bonds, andonly depends upon a characteristic degree of instability, expressed into a half-life.

There are, however, physical processes andgeochemical reactions which secondarily change thespecific activity (= activity per L or per g).



3H is known as tritium and contains one proton and twoneutrons in its nucleus.

It is radioactive, decaying into helium-3 through beta-

decay with a half-life of 12.32 years.

Small amounts of tritium occur naturally because of theinteraction of cosmic rays with atmospheric gases.

Dating by 3H determines the residence time of shallow

groundwater and of spring water in fissured and fractured

rocks less than about 150 years.

The classical 3H method (Libby 1953) was based on the

environmental cosmogenic 3H activity in rain water.



Other applications of 3H is studying lake dynamics,and the estimation of groundwater recharge rates in

humid, arid and semi-arid regions.

In regions with low precipitation samples from dug

wells offer a unique possibility to estimate upper limits

of the groundwater recharge.



J. van den Akker, C. T. Simmons, J. L. HutsonThe use of stable isotopes,deuterium and oxygen - 18 to derive evaporation from flood irrigation on thebasis of pan evaporation techniques Journal of Irrigation and DrainageEngineering. Submitted July 5, 2010; accepted February 23, 2011; postedahead of print March 4, 2011. doi:10.1061/(ASCE)IR.1943-4774.0000361Stephanie S. Ivey, M.ASCE1; Randall W. Gentry, M.ASCE2; Dan Larsen3;and Jerry Anderson, F.ASCE4, Inverse Application of Age-Distribution

Modeling. Using Environmental Tracers 3H/3He( 1002 / JOURNAL OF HYDROLOGIC

ENGINEERING © ASCE / NOVEMBER 2008) Downloaded 13 Sep 2011 to210.212.97.131. Redistribution subject to ASCE license or copyright. Visithttp://www.ascelibrary.org

Maloszewski, P. and Zuber, A., 1993. Principles and practice of calibration and

validation ofmathematical models for the interpretation of environmental tracerin aquifers. Advances in Water Res., 16: 173-190. Libby, W.F., 1953. The potential usefulness of natural tritium. Proc. Nat. Acad.

Sci., 39: 245-247.



Beekman, H.E., Gieske, A. and Selaolo, E.T., 1996. GRES -groundwater recharge studies in Botswana (1987 - 1996).Botswana J. Earth Sci. III: l- 17.

Sukhija, B.S. and Shah, C.R., 1976. Conformity of groundwaterrecharge rate by tritium method and mathematical modelling. J.Hydrol., 30: 167-78.

Stute, M. and Deak, M., 1989. Environmental isotopic study(14C, 13C, “0, D, noble gases) on deep groundwater circulationsystems in Hungary with reference to paleoclimate. Radiocarbon,3 1 (3): 902-9 18.

Hubner, H., Kowski, P., Hermichen, W.-D., Richter, W. and

Schutze, H., 1979. Regional and temporal variations of deuterium in the precipitation and atmospheric moisture of Central Europe. In: Isotope Hydrology 1978, IAEA, Vienna, Vol.1: 289-305.



Zuber, A., 1986. Chapter 1. Mathematical models forthe interpretation of environmental radioisotopes in

groundwater systems. In: P. Fritz and J. Ch. Fontes

(Editors), Handbook of Environmental Isotope

Geochemistry Vol. 2, Elsevier, Amsterdam: l-59.

IAEA BULLETIN-VOL.19, NO.1

http://en.wikipedia.org/wiki/Isotopes_of_hydrogen

https://www.llnl.gov/str/Davisson.html http://www.ncbi.nlm.nih.gov/pubmed/11341004


https://www.llnl.gov/str/Davisson.html

http://www.ncbi.nlm.nih.gov/pubmed/11341004





























Use of Hydrogen Isotopes in Ground-water Studies

Documents

Transcript of Use of Hydrogen Isotopes in Ground-water Studies