Features

Isotopes in climatological studiesEnvironmental isotopes are helping us understand the world's climate

by Kazimierz Rozanski and Roberto Gonfiantini

X he fundamental motivation for the recent explosionof interest in climate studies is the growing scientificconcern that rapidly expanding human impact on theglobal ecosystem may significantly alter the world'sclimate in the near future. The major source for this con-cern is the observed change in the earth's atmosphere,probably the most vulnerable component of the entireecosphere.

Observation data clearly show that the concentrationin air of some trace constituents such as carbon dioxide,methane, carbon monoxide, ozone, chlorofluoro-hydro-carbons (CFCs), nitrogen and sulphur oxides, is chang-ing as a result of anthropogenic emissions. Thesechanges may have harmful, far-reaching consequencesin the near future via direct effects on the biosphere —including human beings—and, indirectly, via the altera-tion of the life-supporting conditions. Global warming asa result of the rapidly increasing concentration of someatmospheric components — known as the "greenhouseeffect" — may serve as a prominent example of suchconsequences.

As a matter of fact, the global average surface airtemperature has been rising for a century, probablybecause the earth is going through one of its minor cli-matic oscillations, which have been frequent in the past.But in the near future the rate of warming of the earth'satmosphere—which has been approximately half adegree Celsius in the last 100 years—is expected toincrease drastically, with consequences which have beenannounced as dramatic, such as the reduction of thepolar ice caps with rise of the sea level and flooding oflow coastal lands, changes in precipitation distributionpatterns and river regimes, and expansion of arid lands.

The reason for the predicted increase of the globalmean warming rate of the lower layers of the atmo-sphere lies in the additional energy trapped by someatmosphere trace gases intercepting the earth's infrared

Mr Gonfiantini is Head of the IAEA's Isotope Hydrology Section,Division of Physical Sciences, and Mr Rozanski is a staff member ofthe Section.

radiation, which otherwise would escape into space.Carbon dioxide and methane are the most importantgreenhouse gases, the concentration of which in the airhas been increasing since the middle of last century,mQiniw K»nr tint nnhf Hii£» tr\ the* nmwrinn /»ftncnrnn_

tion of fossil fuels. (See Table 1.)The predictions on the onset and extent of the green-

house effect are, however, admittedly imprecise due tothe complexity of environmental interactions, and thestill incomplete knowledge of global meteorological andclimatological mechanisms. For instance, we are still farfrom achieving a thorough understanding of the pro-cesses regulating the composition of the atmosphere andthe feedback mechanisms that operate between the majorcompartments (atmosphere, hydrosphere, biosphere,geosphere) of the global ecosystem, and determine itsclimatic response to internal and external forcing. Inrecent years, it has become apparent that a deeperinsight into the dynamics of this extremely complex,non-linear system can be gained only by an integratedapproach which combines two general directions ofresearch aimed at:

• Understanding of the present climate. This in-cludes evaluation of trends in observation data; refine-ment and improvement of the atmospheric generalcirculation models (GCMs); deeper understanding of therole of oceanic circulation on climate; identification andevaluation of important feed-back mechanisms; evalua-tion of sources and sinks of greenhouse gases;

• Understanding of the past climate. This includesbetter characterization of past climatic fluctuations, witha better definition of causes, timing and amplitude;detailed reconstruction of the global ecosystem be-haviour during major glacial/interglacial transitions;elaboration of models directed to meaningful projectionsof past climatic fluctuations into the future.

Environmental isotopes, the distribution of which innatural compounds is governed by environmental condi-tions, are one of the most powerful tools to investigatethe climatic variations and the response of the environ-ment to changing climate. This article briefly shows howthese isotopes have contributed to the progress of

IAEA BULLETIN, 4/1990

Features

Table

Gas

C0 2

CH4

N20CFC-11CFC-12

Source:ppmv =ppbv =

1. Trends in concentrations of

Concentrations

Pre-1850

275 ppmv0.7 ppmv

0.285 ppmv00

Adapted from V. Ramanathan, Science,part per million (in volume),part per billion (in volume).

major atmospheric

1985

345 ppmv1.7 ppmv

3.304 ppmv0.22 ppbv0.38 ppbv

240, (1989) pp. 293-299.

trace gases

Observed trendsfor 1975 85

(%)

4.611.03.5

103101

Mid-21 stcentury

400-600 ppmv2.1-4 ppmv

0.35-0.45 ppmv0.7-3 ppbv

2.0-4.8 ppbv

modern views in climatology and palaeoclimatology.The role played by the IAEA in the collection and elabo-ration of isotopic data of relevance for investigationsrelated to climate and its history will also be shown.

How do isotopes work?

Perhaps the most characteristic application ofenvironmental isotopes in geochemistry is their use as adating tool. A great variety of methods based on decayof radioactive isotopes and/or buildup of daughter iso-topes has been developed, which allows age determina-tion of groundwater, ice, rock, sediment, etc.

In recent years a renaissance of these methods hasbeen observed, owing to the development of acceleratormass spectrometry—a novel analytical technique allow-

ing the detection of low concentrations of environmentalradioisotopes in very small samples.

In climatology, many isotope dating methods in prin-ciple can be applied for establishing the proper timescale of past climatic events; the choice depends uponthe age range and nature of the system being inves-tigated. Both naturally produced (cosmogenic and insitu) and anthropogenic radioisotopes are used. By far,the most popular dating method was — and probably stillis — the one based on carbon-14, the radioactive isotopeof carbon, which, with a half-life of 5730 years, allowsthe determination of ages ranging from a few hundred toapproximately 40 000 years. Many other radioisotopescurrently are used in palaeoclimatology that have half-lives ranging from much shorter to much longer valuesthan that of carbon-14. (See Table 2.)

Table 2. Isotopes used in climatological

Isotope

Stable isotopes:Deuterium (Hydrogen-2)Oxygen-18Carbon-13

Anthropogenic radioisotopes:Tritium (Hydrogen-3)Krypton-85Caesium-137Carbon-14

Natural radioisotopes:Radon-222, Lead-210, Argon-39Carbon-14, Thorium-230, Protactinium-231Uranium-234, Uranium-238, Potassium-40

research

Type of materialstudied

— marine sediments— lacustrine sediments— ice cores— cave deposits— groundwater— precipitation

— ocean water— oceanic carbonates— lacustrine sediments— groundwater— precipitation— atmosphere

— marine sediments— lacustrine sediments— cave deposits— ocean water— groundwater— atmosphere

Type of climaticinformation derived

temperature of the ocean,ocean circulation, volume ofice sheets, air temperature,relative humidity, dynamicsof water cycle on regionaland global scale

dynamics of the ocean,dynamics of the atmosphere,sedimentation rates

dating of deep-sea sediments,dating of lacustrine sediments,dating of cave deposits,dating of ocean water,dating of groundwater,dynamics of the atmosphere

10 IAEA BULLETIN, 4/1990

Features

The concentration of stable isotopes in naturalcompounds varies due to the slightly different, mass-dependent behaviour of isotopes in natural physico-chemical processes. As the magnitude of these isotopiceffects, or fractionations, is a function of the parameterscharacterizing any given process, among whichtemperature is the most important, information on theenvironmental conditions at the time of formation can bederived from the stable isotope composition of water,ice, and minerals.

The stable isotope variations most frequently appliedin climatological investigations are those of the heavyisotopes of hydrogen and oxygen, i.e. hydrogen-2 (ordeuterium) and oxygen-18, which allow the effectivetracing of climate-induced changes in the hydrologicalcycle. Also carbon-13, the heavy isotope of carbon,appears to be a powerful climatic indicator.

In summary, we can say that in geochemical climatol-ogy, as in other fields of geochemistry, environmentalfadioisotopes are used as time indicators, and stable iso-topes as temperature indicators. The brief overviewwhich follows presents the major contribution ofenvironmental isotope techniques to the investigation ofthe earth's climatic history.

The ocean

Because of its relative homogeneity, the ocean is anideal environment to record major, long-term climaticvariations. In fact, due to its large mass and the resultingthermal, chemical, and isotopic inertia, short-termclimatic fluctuations would have a limited, often negli-gible impact on it.

Marine sediments were the first important source ofisotope data used in palaeoclimatology. The oxygen-18/oxygen-16 ratio of the carbonate shells of marineprotozoans called foraminifera, which are present inmarine sediment cores, is a function of the oxygen-18/oxygen-16 ratio of sea water and of its temperature.These two factors act in the same direction during theglacial/interglacial climatic fluctuations, enhancing theisotopic variations in the calcium carbonate formed. Ithas been found that the oxygen-18/oxygen-16 variationsof foraminifera shells are in reasonable agreementwith the expected variations of insolation induced bythe cyclic changes of earth's orbital parameters, andthe resulting climatic fluctuations predicted by theMilankovitch theory. By the parallel dating of sedimentcore sections using radiocarbon, the uranium series dis-equilibrium or the potassium-argon method, it has beenpossible to establish a detailed chronology and to eva-luate ocean temperature variations during most of theQuaternary. (See Figure IB.)

Also, the application of environmental isotope tech-niques to modern physical oceanography can beregarded as an indirect contribution to climatology. Infact, in spite of the considerable homogeneity of thepresent ocean, small differences in isotopic and chemicalcomposition do occur. These differences, together with

the tritium and carbon-14 variations, can be used to tracedifferent water masses, establish their circulation pat-terns at depth, and investigate the mixing between theshallow and deep ocean. The assessment of modernocean circulation is essential to understand the impact ofthe ocean on the global climate and meteorology.

The high-latitude continental environment:The ice caps

Large ice caps existing at high latitudes (Greenland,Antarctica) proved to be excellent archives of Pleisto-cene climate. Actually, a large part of our knowledge onQuaternary climatic variations is derived from the iso-topic composition of the polar ice cover.

Ice caps, formed by snow precipitation over large,almost flat regions with negative mean annual tempera-tures, preserve the vertical stratification, although thethickness of annual layers decreases with depth pri-marily due to the compaction process in which the snowis transformed firstly into firn or granular ice and theninto compact ice.

The oxygen-18/oxygen-16 and hydrogens/hydro-gen-1 ratios of precipitation are strongly correlated withthe temperature of formation: lower formation tempera-ture leads to isotopically depleted precipitation. Atmiddle and high latitudes, where the seasonal variationsof temperature are large, this implies parallel seasonalvariations of the isotopic composition of precipitation,with the heavy isotope concentration being higher insummer than in winter. These seasonal isotopic varia-tions are preserved in the ice cap down to a considerabledepth (about 1 to 1.5 km), despite the diffusion whichtends to smooth off the differences. The counting of theannual isotopic cycles along a vertical profile constitutesa method of ice dating and evaluating the accumulationrate.

The correlation between temperature and isotopiccomposition allows also the identification of icedeposited during the glacial periods, which is expectedto be considerably depleted in heavy isotopes withrespect to modern precipitation. In fact, this has beenobserved in all deep ice cap drillings carried out to date.Variations of oxygen-18 and deuterium content pre-served in ice made possible detailed reconstruction ofthe high-latitude palaeotemperatures during the lastglacial/interglacial cycle. (See Figure 1C.) Further-more, the chemical analyses of air bubbles trapped inice, coupled with the isotopic composition of ice, led tothe reconstruction of the history of the carbon dioxideand methane concentration in air during the last 160 000years. The profile of carbon dioxide concentrationderived from the Vostok ice core (Antarctica) (Fi-gure 1C) mimics the temperature curve derived from thedeuterium profile, pointing to a strong linkage betweencarbon dioxide and climate. It is believed that theobserved carbon dioxide variations are linked withclimate-induced oceanic circulation changes rather than

IAEA BULLETIN, 4/1990 11

Features

Figure 1. Environmental isotopes as palaeoclimatic indicators.

-6

GRAPHA

300 400 500Thousand years BP

600 700

oJ-.I -

2.5

0.0

-2.5

-5.0

-7.5

10.0

Td \Q ^

CO \

• i l l A'

460-

480

- 500

i

- \ }— ^^

i

20

k

w1 ~• i i

60

Thousand

rA/ \r V IW'1 , 1 ,

100 140years BP

280

260 •>

240 g.

220 ^

200 o

180

GRAPHC

BP = Before Present,defined as 1950.

These graphs show selected examples of climate-induced variations of oxygen-18 and deuterium concentration, preserved in environ-mental archives, namely oceanic sediments and polar ice sheets. The solid lines in graph A show ranges of the oxygen isotopic composi-tion of well-preserved marine macrofossils. This isotope record can be converted, under certain assumptions, to the temperature record.It is believed that beyond approximately 50 million years ago, the observed isotope variations are mainly caused by dramatic changesin ocean circulation, leading to substantial isotope differences between the shallow and deep ocean.

Graph B shows the composite curve of oxygen-18 variations in foraminifera shells extracted from five deep-sea cores. Isotope effectsaccompanying evaporation and condensation of ocean water lead to preferential storage of oxygen-16 in advancing ice sheets duringthe onset of the glacial period, and to corresponding enrichment of the remaining ocean. The retreat of ice sheets returns this isotopicallydepleted water back to the ocean. On the other hand, the oxygen-18 isotope fractionation between carbonate of foraminifera shells andsea water is temperature-dependent — shells formed at higher ocean temperatures tend to be enriched in oxygen-18. Calculations showthat approximately 70% of the total amplitude of the oxygen-18 variations in this graph can be explained by changes in the volume of icesheets. The remaining part is due to temperature variations.

Graph C shows the changes of carbon dioxide concentration in the atmosphere preserved in air bubbles (upper curve), and the surfaceair temperature derived from the deuterium isotope profile (lower curve), plotted against age in the Vostok ice core. The carbon dioxideprofile mimics the temperature curve, pointing to a strong linkage between variations of carbon dioxide content in the atmosphere andthe climate.

Notes: Concentrations of deuterium and oxygen-18 are expressed in d values, defined as per mille deviations from the standard (which is PDBfor oxygen-18 in carbonates and V-SMOW for oxygen-18 and deuterium in water). . . .

Sources: Graph A — adapted from L.B. Railsback, Geochimica et Cosmochim Acta, 54 (1990) pp. 1601-1609. Graph B — Imbrie et al.,Milankovitch and Climate, Part I, Eds A.L. Berger et al., D. Riedel Publishers (1984) pp. 269-305. Graph C — Barnola et al., Nature, 329, (1987)pp. 408-414.

12 IAEA BULLETIN, 4/1990

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directly with the greenhouse effect. A more preciseinterpretation would in particular require the determina-tion of the variation of carbon-13 content in the carbondioxide extracted from the air bubbles. The carbon dio-xide and isotope profiles retrieved from ice cores drilledin Greenland ice sheets reveal striking similarities to theAntarctic profiles, suggesting general synchroneity ofmajor climatic transitions in the Northern and SouthernHemispheres.

The middle- and low-latitude continentalenvironment

The studies briefly discussed above are restricted tomarine and polar environments. These are almost idealfor isotope investigations, the first one because of itshomogeneity, allowing the effects of relatively smallamplitude to be detected, and the second because theclimatic variations are more dramatic.

There is, however, a considerable interest in detect-ing climatic changes also in middle- and low-latitudecontinental areas. This interest is stimulated by the factthat relatively large differences both in magnitude andtiming of the climatic response to internal and externalforcing are expected for different geographical regions.It is clear that learning what happened in the past to theclimate in different regions of the world would beextremely instructive for predicting the consequences offuture climatic changes.

Lacustrine sediments are one of the most valuablecontinental materials for palaeoclimatic reconstructions.This material often contains carbonate whose oxygenisotope composition is controlled by that of the lakewater. For open lakes with fast water turnover, thewater isotopic composition corresponds to that ofprecipitation over the lake basin, which in turn istemperature-dependent. Thus, periods of cold and mildclimate are reflected by minima and maxima in theoxygen-18 content of the deposited carbonate.

More complex is the case of nearly closed lakes,where the isotopic composition of water is largely deter-mined by evaporation and exchange with atmosphericmoisture. Here the changes in water balance are respon-sible for the oxygen isotope variations recorded by thecarbonate deposited, with dry periods marked by highoxygen-18 content due to increased evaporation, whichpreferentially removes isotopically light water from thelake.

Isotope studies carried out to date resulted in adetailed reconstruction of climatic and hydrologic condi-tions prevailing during the last glacial-interglacial transi-tion and the early Holocene in Central Europe, NorthAfrica, and North America. Substantial differenceswere found among these regions with respect to bothtiming and structure of this transition. For instance,several humid-arid oscillations have been recorded inlacustrine deposits in the Sahara and the Sahel. In theSahara, the last important humid episodes occurredbetween 14 500 and 11 000 and between 9300 and

IAEA BULLETIN, 4/1990

7500 years ago. Isotopic evidence exists that probablymost of the groundwater stored in the Saharan aquiferswas recharged at this time.

Groundwater is also a source of palaeoclimatic infor-mation. The isotopic composition of groundwater gener-ally corresponds to the mean isotopic composition ofprecipitation in the recharge region. Consequently, theisotopic variations of precipitation connected with cli-matic changes are reflected by the groundwater's iso-topic composition. Also, atmospheric noble gasesdissolved in groundwater (neon, argon, krypton, xenon)may, under favourable conditions, provide valuablepalaeoclimatic information. In porous aquifers, concen-trations of noble gases in groundwater are directlyrelated to the mean annual temperature of the rechargearea and, consequently, can be used for the reconstruc-tion of temperature changes in the past.

There are, however, a number of difficulties in usinggroundwater as climate indicator. Groundwater canseldom be considered a closed system. Diffusion anddispersion in aquifers, mixing between groundwaterstored in different aquifers through leakages or discon-tinuities in the separating formations, and interactionswith the rock matrix, tend to mask or smooth off the cli-matic differences and make difficult the dating ofgroundwater by carbon-14. Nevertheless, a distinct cli-matic signal — a shift in the deuterium and oxygen-18contents accompanying the Late Glacial-Holocene tran-sition — has been identified in a number of confinedaquifers of the Northern Hemisphere.

Also the calcium carbonate deposited in karstic cavesin a variety of forms — stalactites, stalagmites,flowstones, all designated with the common name ofspeleothems — is of potential relevance in isotopicpalaeoclimatology. The speleothems are slowly precipi-tated from percolating groundwater over long periods oftime, in conditions close to thermodynamic equilibrium,in an environment where the seasonal variations of tem-perature, atmospheric humidity, and isotopic composi-tion of water and dissolved carbon dioxide are smoothedoff. Thus, only the long-term climatic variations shouldbe recorded by the isotopic composition of calcium car-bonate and of water inclusions trapped in speleothems,the dating of which is usually carried out by the uraniumseries disequilibrium method.

Another material suited for palaeoclimatic investiga-tions is the plant organic matter, the oxygen and hydro-gen isotopic composition of which is connected —through a rather complex chain of processes and chemi-cal reactions — to that of soil water and precipitation.This material, however, should be used with cautionbecause it may also reflect the variations of conditionsin the restricted environment where the plant is growing.

The atmosphere and global circulation models

Systematic determinations of the isotopic composi-tion of meteoric waters (precipitation, atmospheric

13

Features

vapour) help in understanding the links between precipi-tation and the main factors governing meteorologicalconditions and climate. In this field, a crucial role hasbeen played by the global network of stations jointlyoperated by the IAEA and World MeteorologicalOrganization (WMO) since the beginning of the 1960s,where monthly precipitation samples are collected fortritium, deuterium, and oxygen-18 content determina-tions. The majority of samples are analysed in theIsotope Hydrology Laboratory at IAEA headquartersin Vienna, and the remaining samples in several co-operating laboratories. Incidentally, in recent years thenumber of stations for which isotope composition ana-lyses of precipitation are performed has increased,because national networks have been set up in severalMember States.

This unique isotope database, accumulated during 30years of observations and still growing, provides thebasis for the understanding of isotope variations inprecipitation, both in terms of local geographic andmeteorological features, and global atmospheric circula-tion. The stable isotope variations are a consequence ofthe isotope effects accompanying each step of the water

cycle. As already mentioned, temperature is the mostinfluencing parameter. (See Figure 2. )

Stable isotope patterns in precipitation are particu-larly useful in the elaboration of global circulationmodels. These models simulate the behaviour of thewhole atmospheric system, including its interaction withthe ocean, and can be used to predict the response of thesystem to different types of internal and external forc-ing, such as the growing concentration of atmosphericgreenhouse gases and changes in insolation.

The predictive power of the models depends on theproper calibration of the parameters, and isotopes are ofhelp for this purpose. Transient tracers of anthropogenicorigin such as the bomb-produced tritium in precipita-tion, radiocarbon in atmospheric carbon dioxide andmethane, and krypton-85 released into the atmosphereby nuclear fuel reprocessing plants, are used to under-stand the dynamics of the ocean-atmosphere system, i.e.the general features of atmospheric and oceanic circula-tion and their interaction. Tritium and stable isotopes areapplied to investigate the water vapour transportbetween hemispheres and on the continents, to evaluatethe fraction recycled by evapotranspiration, and to study

Figure 2. Long-term fluctuations of isotopic composition of precipitation and surface airtemperature.

1°

2.0

1.0

0.0

1"of00 03

-2.0

3.0

-Oxygen-18

Surface air temperature

1960 1965 1970 1975 1980 1985

The graph shows the changes in surface air temperature and oxygen-18 isotope composition of monthly precipitation for eight Europeanstations of the IAEA/WM0 monitoring network over the period 1960-87. The composite curves represent 12-month running means,eliminating seasonal fluctuations. The reference level was arbitrarily set as the 1960-87 mean value for the Vienna station.

14 IAEA BULLETIN, 4/1990

Features

the interaction between troposphere and stratosphere.Radiocarbon and carbon-13 help to evaluate the contri-bution of various sources (biogenic versus fossil) to theincrease of carbon dioxide and methane in theatmosphere and to investigate their residence times,removal by sinks, and interchange with the ocean.

International research

The enormous quantity of isotopic data of varioustypes accumulated so far, has shown that environmentalisotopes are a powerful tool to investigate climate. Inparticular for the past climate, they may provide infor-mation which otherwise would not be accessible.Climatological investigations making use of isotopetechniques are being conducted in many laboratories inthe world, and in many cases the ultimate aim is thethorough understanding and the detailed prediction ofthe announced climatic changes, which should alreadybecome evident during the next century.

To date, the Agency's major activity related tometeorology and climate has been constituted by theIAEA/WMO network for isotopes in precipitation,which was originally initiated for hydrological applica-tions of environmental isotopes. In addition, theAgency, through its Isotope Hydrology Section, hasbeen active in the field of palaeohydrology, because thisis directly connected with the appraisal of waterresources in arid countries, which often consist of

palaeo waters, i.e. ground water recharged under past,more favourable climatic conditions.*

Also an Advisory Group Meeting on the applicationof isotope techniques to palaeoclimatic investigationsrelated to water resources was convened 10 years ago,and the proceedings were published by the IAEA underthe title Palaeoclimates and Palaeowaters: A Collectionof Environmental Isotope Studies. (STI/PUB/621)

In the near future, the IAEA's Division of Physicaland Chemical Sciences is planning to organize twoco-ordinated research programmes (CRPs) related toclimate studies. The first, devoted to isotope variationsof carbon dioxide and other trace gases in the atmo-sphere, has the objective of gaining deeper understand-ing on the role of greenhouse and other trace gases inthe global ecosystem. It involves investigating theirdynamics and improving the current knowledge on theirrates of release and removal by natural and anthropo-genic processes. This CRP is expected to start in 1991,after a preparatory consultants' meeting in December1990. .

The second CRP, scheduled to start in 1992, willfocus on palaeoclimatic reconstructions for continentalareas using isotope indicators. Also in this case, apreparatory consultants' meeting will be convenedtowards the end of 1991.

* See "Investigating the water resources of the desert: how isotopescan help", IAEA Bulletin, Vol. 23, No.l (1981).

IAEA BULLETIN, 4/1990 15

Features

Nuclear techniques for investigatingmigration of pollutants in groundwaterA report on some practical applications for preventing pollutionof water resources

by V.T. Dubinchuk, A. Plata-Bedmar, and K. Froehlich

X rotecting groundwater resources from pollution hasbeen a high-priority topic in recent years.

Usually purer and cleaner than surface water,groundwater is naturally protected by an excellent filter-ing system. The system includes soil, clays, and rockparticles that remove some soluble species, suspendedparticles, bacteria, and, to a large extent, viruses.

If such a filtering system is overloaded or bypassed,however, the aquifer itself may become polluted. Thesame type of pollutants that affect surface water —including domestic and industrial waste, seepage fromseptic tanks, mine drainage, sanitary landfills, andagricultural chemicals — may have a greater impact andmore prolonged effect on groundwater. Pollution inaquifers is retained for hundreds to tens of thousands ofyears, thus jeopardizing water supplies for futuregenerations.

Aquifers in some areas are less exposed to contami-. nation than in others because geological and hydrochem-ical conditions limit the risk. The sensitivity ofgroundwater quality to anthropogenic activities causingpollution is termed "aquifer vulnerability to pollution."A variety of processes, and soil and aquifer properties,affect aquifer vulnerability. The determination of theparameters characterizing these processes is essential formodelling and predicting the migration of pollutants ingroundwater systems. Major objectives are to preventpollution and degradation of groundwater resources, or,if contamination has already occurred, to identify its ori-gin so that remedies can be proposed.

At the IAEA, emphasis has been placed on practicalapplications of nuclear and isotope techniques that aredirectly or indirectly related to the solution of ground-

water pollution problems. This article reports on howsome of these techniques have been used in practice.

Migration of pollutants

An unsaturated (aeration) zone is a thin "skin" ofgeological bodies through which pollutants start theirway from the earth's surface to shallow groundwateraquifers. Special instruments are required to study howfast the groundwater and pollutants move through thesoil. A number of investigations have been, or are being,done using such instruments and isotope tracers withinsome national and international programmes, includingthose sponsored by the IAEA.

In the Crimean region, for example, investigationswere performed in 1985-86 to study salinization ofgroundwater. In the Chernobyl area, investigations aredirected at the migration of radionuclides released as aresult of the nuclear power plant accident in 1986.*

In both cases, a number of sites for complex isotopetracer investigations were constructed. At these sites,water containing tritium was injected into the ground tostudy its redistribution over space and time in the soil.This was done in order to measure the velocity of thegroundwater's movement in the unsaturated zone, and toevaluate the residence time (or age) of the groundwaterand its recharge rate to shallow aquifers. Simultaneouslyat the same sites, pollutants labelled with isotopes wereinjected and their movements were observed.

In the Crimea, results obtained near Djankoy showthat salt is transported upwards. This is due to highevapotranspiration and the low depth of the water

Messrs Dubinchuk, Plata-Bedmar, and Froehlich are staff members inthe IAEA Division of Physical and Chemical Sciences.

* Both of these studies were performed at the All Union ScientificResearch Institute of Hydrogeology and Engineering Geology inMoscow by the researchers V.T. Dubinchuk, Yu. A. Tsapenko,A.V. Borodin, and A.V. Gladkov.

16 IAEA BULLETIN, 4/1990