The Basis of Civilization - Water Science? (Proceedings of the UNFSCO/IAlIS/1WHA symposium held in Rome. December 2003). 1 AI IS I'ubl. 286. 2004 49

A short history of isotopes in hydrology

R. LETOLLE & Ph. OLIVE Laboratoire de géologie appliquée, box 123, Université P. M. Curie, F-75252 Paris Cedex 05, France r e n e . l e t o l l e @ w a n a d o o . f r

Abstract This paper recalls the birth and development of stable and radio­active isotope techniques in hydrology from the discovery of isotopes in 1912 to the beginning of the 21st century. K e y w o r d s l 4 C ; 2 H ; J H ; l s O ; his tory; h y d r o l o g y ; h y d r o g e o l o g y ; i so topes ; t e c h n i q u e

INTRODUCTION

In 1813 the English chemist Prout put forward the theory that chemical elements should have an atomic mass which is a multiple of that of hydrogen, that is an integer number. Although this theory held for many elements discovered after that, problems arose for elements with an atomic mass not an integer, such as chlorine (mass = 35.5). Several scientists speculated that chemical elements could be associations of more elementary substances, which Crookes in 1886 tentatively called "meta-elements". Other scientists, such as the Frenchmen Schutzenberger and De Marignac (the discoverer of gadolinium), following Crookes's hypothesis, in 1886 advocated the attribution of the exact mass 16 to oxygen. The discovery of radioactivity in 1898 by Pierre and Marie Curie led them to the hypothesis that some elements could have various atomic weights.

Soddy suggested in a paper presented at the second meeting of the Chemical Section of the British Association meeting in Birmingham in 1913, 27 years after Crookes's proposal, that " there exist several substances with identical or practically identical properties but with different atomic weights " (Aston, 1922, p. 6). At the same time, using an apparatus called the "J. J. Thomson positive ray parabola detector", Aston, analysing a very pure neon gas, discovered that neon with a mass of 20 existed alongside another species of neon of mass 22.

After World War I, several machines called mass spectrographs were built, using the properties of mass and electric charges of ionized substances, by Aston (using ionization of gases) and by Dempster (using fhermo-ionization for solids), and these became mass spectrometers when electrometers were used in place of photographic plates. At the same time, there were a number of attempts to separate isotopes, through physical and chemical methods, such as diffusion etc., which were more or less successful. Aston was awarded the Nobel Prize in chemistry in 1922 for his work in this field.

Subsequently the hunt for isotopes across the entire Mendeleev Table began. It took some time to discover the rarer stable (or "weakly" radioactive) isotopes of many elements (the last one was tantalum-180, in 1958), and it was only in 1929 that Giauque (Nobel Prize winner in 1949 for his work on very low temperatures) and

50 R. Létolle & Ph. Olive

Johnson discovered the rare isotopes of oxygen, 1 8 0 (abundance -1500 ppm) and 1 7 0 (-500 ppm) (Giauque & Johnson, 1929a,b). In 1932, Urey (Nobel Prize winner in chemistry, 1934) discovered the isotope of hydrogen with mass 2, which was called "deuterium" (2H or D), and showed that it existed in natural water with an abundance of about 100 ppm, which was very difficult to ascertain with precision (Urey et al, 1932). At the same time, it was known that it was possible to separate isotopes of some elements, or at least to enrich them, through classical methods such as fractional distillation. It was therefore possible to predict that evaporation and condensation of water should affect the abundance of deuterium in natural waters. This was the case in 1934 when Gilfillan studied the deuterium content of sea water, but his data were very spurious.

In 1932, a special mass spectrometer was devised to study the isotopes of oxygen, and Bleakney & Whipple (1935) made a short survey of these isotopes. Radioactive isotopes in water began to be discovered in 1937, as will be seen later.

STABLE ISOTOPE HYDROLOGY

The interest in deuterium in water was considerable, especially when it was demon­strated it had special properties in nuclear physics. In many laboratories, in the United States as well as in Europe and Japan, much research was undertaken to extract "heavy water" from ordinary water, and to find "naturally enriched waters". Searching for variations in the abundance of deuterium in nature gave erratic results; the sensitivity of the mass spectrograph was not great enough, so researchers used gravity methods, measuring the speed of fall of a small quartz float in a thermostatic tube filled with water and submitted to precise but small variations in temperature. At first, as 1 8 0 had just been discovered, arbitrary corrections had to be made to calculate the deuterium content, as oxygen isotope variations, which were not possible to measure easily at that time, could also influence the density measurement values. Results were erratic, but in any case they proved the reality of the variations in the concentration of deuterium in natural waters. Dole (1936) then attributed the greatest part of these density variations to 1 8 0 . Some papers of historic interest are: Dole (1934); Vereschagin et al. (1934); Okabe & Titani (1935); Riesenfeld & Chang (1936); Guntz & Beltran (1937); Paravano & Pesce (1938); Pesce & Cervone (1940); Demidenko (1940); Kassatkina & Florensky (1941); Vernadsky et al. (1940); and Oana (1948).

More precise and reproducible apparatus was necessary to measure accurately the small natural variations of the abundance of isotopes, and technology gave birth to monstrous machines weighing several tons, but those were rarely, if ever, used for geochemical studies. But in 1940, Nier constructed a light mass spectrometer, which is the grandfather of the precision mass spectrometers of today. Then he built 100 of these machines to monitor the isotope content of argon—argon was employed to cool the apparatus used for the preparation of uranium-235 to build the first nuclear bombs. Special machines were also built in secrecy for measurements of heavy water used for neutron absorption in the first nuclear power plants.

After World War II, most specialists who had worked on isotopes went back to civilian research (Kirshenbaum, 1951)—at that time very few hydrologists knew about isotopes or even imagined that they would later provide a powerful tool for their

A short history of isotopes in hydrology 51

studies, except perhaps, the Soviet scientists Teis and Florensky (Teis & Florensky, 1939-1950). However the geochemical status of isotopes was really born with papers in 1947 by Urey and by Bigeleisen & Mayer, who gave to a series of young students clues to geochemical problems, namely to: Friedman, Epstein, Craig and others. The fundamental discoveries in stable isotope hydrology are due to the teams in California and Illinois and also to Thode's team in Canada, who participated in the separation of heavy water at the Chalk River plant.

Quantitative progress in the knowledge of the natural history of oxygen and hydrogen isotopes (and also of other elements, such as S, C and N, of concern to hydrology) was made through technological advances in the mass spectrometry of gases by Thode et al. (1944, 1945, 1949), Nier et al. (1947), Kistemaker (1948) and McKinney et al. (1950). As memory effects in the MS were produced in direct analyses of water, advances came from the use of CO2, equilibrated at 25°C with the water to analyse, as a carrier gas (Epstein & Mayeda, 1953); with continuous monitoring of two ion beams of mass 44 and mass 46 ( 1 2 C I 6 0 1 6 0 + and 1 2 C 1 6 0 1 8 0 + ) ; and with small corrections due to interference of the 1 3 C and l 7 0 components of the CO2 molecule and instrumental effects; similar corrections for D/H ratios; measure­ment of the ratio with a Wheatstone bridge, and rapid switching between a standard gas as reference and the gas to be measured to cancel out instrumental defects. These achievements led to the elimination of most of the faults of previous isotope measure­ments, while precision and accuracy became of the order of 0.1 per mil ( % o ) of the 1 8 0 / 1 6 0 ratio difference to that of the reference water.

It appeared that as the various isotopes of an element have the same chemistry, except for very small differences in their chemical kinetics, this could provide a very useful tool to follow the destiny of a mass of water through "isotope tracing" with a small quantity of "water" enriched in respect of one or several rare isotopes, just as in chemical tracing. However, enriched isotopes are very expensive and there is no question in routine work of trying to recover the tracer. Therefore, tracing with enriched isotopes has mainly been used on the laboratory scale and rarely in the field.

Nevertheless, difference in the behaviour of various isotopes in reactions, either physical or chemical, led to thinking that natural variations could be detected and used to characterize a mass of water through its history: its evaporation, precipitation, chemical exchange, etc. These ideas had the immediate consequence that following the "prehistoric period" of rare isotope measurement with low precision and reproduc­ibility obtained through the previous generation of mass spectrometers and densito­meters, a clear picture of stable isotope "natural history" could rapidly be attained. Especially important was the work of Epstein, Craig, Friedman, Thode and others, followed by that of scientists from Europe, Japan and the USSR. However, in some countries, where laboratories could not afford to buy or build the new type of gas isotope mass spectrometers, there was for a time the persistence of density measure­ments, with the use of water standards, but calibrated against MS-measured standards (for example in Chile and Romania).

Epstein & Mayeda (1953) and Friedman (1953), using the new MS techniques devised by Nier, put an end to the generalized gravity measurements which were still used in most laboratories, and began to put some order into the great amount of discordant data obtained to date (and indicated as variations of density of water relative to ocean water). The only sure facts at the beginning of this research were that

52 R. Létolle & Ph. Olive

(a) atmospheric water was poorer in heavy isotopes than liquid water on the continents, which was itself poorer than sea water; (b) ice was poorer than liquid water; and (c) evaporated water was enriched in heavy isotopes, although all this had been pred­icted since Aston (1922). The wide spectrum of isotopic values for continental waters (and volcanic waters) could not be clearly interpreted due to the impossibility of using the gravity method to distinguish clearly between the D and , 8 0 effects.

Craig introduced the use of a water standard, the SMOW (Standard Mean Ocean Water), since the enormous mass of deep ocean waters had shown only very small variations in their deuterium and 1 8 0 content. This reference was adopted and disseminated by the International Atomic Energy Agency in the 1970s, under the name of the Vienna-SMOW; at the same time, Craig proposed the presentation of data in a so-called "5 % o TO SMOW scale":

S = [ ( ^ s a m p l e / - R S M O W ) - 1] X 10']

where R is 1 8 0 / l 2 0 (or 2H/'H). The same improvements were provided for the D/H measurements, with the use of hydrogen as a carrier gas, prepared through reduction of water on metal uranium at high temperature. Hence, data for deuterium, using the same SMOW standard, are also given in the SMOW scale, with absolute abundances rigorously established by Baertschi (1976).

This notation, which is equivalent (for slight variations in isotope abundances, as is the case for natural waters) in balance calculations to a concentration, became standard for many pairs of isotopes of other elements, such as S, N, C, etc. while for water it quickly replaced the usual "ppm" notation; a balance is easily written as:

Qn 8 „ = Zq\ 8 n / Tqi

The notion of an "isotope fractionation effect" when water molecules go from one phase (or component) to another is given by:

£phasel — £phase2 ~~ [ ( ^ p h a s c l — ^phase2) — X 10 ] ~ 8phasel — 8phase2

which justifies the ô notation that was defined at the same time. In fact, ô values are not strictly equivalent to absolute abundances, and calculations using big ô values may introduce quite important systematic errors: a fact that is often forgotten by inattentive researchers.

After the 1950s, there were two directions of research. The first was to make a catalogue of 1 8 0 and deuterium data in waters from a variety of origins, from ocean water to water from inclusions in minerals; the second was to understand how natural mechanisms, such as evaporation, precipitation, phase exchange, percolation in microporosities, etc., acted to modify isotope compositions. It appeared that if the highest content of deuterium and ' 8 0 was found in brines, the lowest in polar ice, there was no global correlation between the two isotope contents, as predicted by thermo­dynamic theory, which links fractionation of isotopes to the ratio of their mass, that is 2 for hydrogen isotopes and 18/16 (= 1.125) for oxygen.

In the following years, rapid progress was made in acquiring knowledge of the "natural history" of continental waters by Epstein & Mayeda (1953), Friedman (1953), Dansgaard (1953, 1964), and especially by Craig (1961a), together with an under­standing of phenomena leading to the variations which were observed. Of special interest for meteoric waters was the importance of the fractionating effects in the kinetics of evaporation, precipitation, the formation of ice, exchange with silicate or

A short history of isotopes in hydrology 53

Fig. 1 Map of mean values of l 8 0 in precipitation in France (from Lécolle, 1990).

carbonate rocks, in as much as l 8 0 and deuterium water molecules do not behave exactly as ordinary OH2 =16.

The accumulation of data relevant to precipitation (Fig. 1) showed several effects at different scales of time and space, such as the "continental effect". This demonstrates a decreasing trend in the content of heavy isotopes with distance to the ocean, which is due to the fact that "heavy water" condenses a little faster than "light water", together with an "altitude" effect, linked to the temperature at the time of precipitation. This made it possible to trace water vapour fluxes in the atmosphere.

The behaviour of isotopes in water during evaporation was also carefully studied theoretically in the laboratory and in the field (Hydrological Processes, 2000). The different origins of the water in river basins could then be traced independently from chemical studies, and percolating waters could be followed. Conversely, the stable isotope content of aquifers could indicate more precisely the origin of the water. Craig (1961) was able to build his general and celebrated deuterium/ l 80 relation for

54 R. Létolle & Ph. Olive

5 1 80(%.)

Fig. 2 Deuterium- l 8 0 diagram of continental waters.

precipitation: ÔD = 8Ô 1 80 + 10 (Fig. 2), which has been refined when introducing secondary phenomena, especially evaporation.

Water may exchange isotopes of oxygen with minerals, especially calcite and silicates. This exchange is faster at higher temperatures and when isotopic equilibrium is attained; specific equations may give the temperature to which the water was exposed. This "isotope thermometer" is used on solid phases for magmatic, volcanic and sedimentological studies, being easier to sample than fluids. Conversely, as there is little hydrogen in most minerals, the deuterium content of exchanged water is much less modified. This gave the clue to the origin of thermal waters; as the celebrated study by Craig of the Yellowstone waters (1961) showed, the thermal water was of local precipitation origin. Many studies of hydrothermal areas followed (IAEA, 1967, 1974, 1981a,b), showing that the so-called juvenile water is always a very subsidiary component of thermal waters.

From the 1950s on, progress was made in all aspects of water science, including planetary studies which are not considered here (through remote controlled and miniat­urized mass spectrometers), and in océanographie and atmospheric sciences. A few highlights of the more important theoretical advances are described: - the mechanism of exchanges between liquid and water vapour, especially by

Merlivat & Jouzel (1979), and the reconstitution of the evaporation history of lakes (Fontes & Gonfiantini, 1970);

- the evolution of the water in precipitation during storms, either rain or hail-storms (Jouzel et al., 1975);

- the mechanism and water balance of evaporation in soil (especially important in arid countries), and the exchange between soil and aquifers;

- the origin of palaeowaters and volcanic waters. As will be seen later, the radio­active dating of waters proved a most useful tool to prove the age of their origin, in connection with stable isotope studies;

- palaeoclimatology through ice coring in glaciers and icebergs (Dansgaard et al., 1960; Dansgaard, 1964), in connection with the study of tapped gases (CO?, methane);

- the interpretation of river flow hydrograms, in combination with hydrological and chemical studies (Buttle, 1994);

A short history of isotopes in hydrology 55

- problems of the origin of wines, fruit juices, etc. (plants use and transpire water, and the water constituting them depends upon climate, hydrology, and their own generic properties. Fraud in wines and fruit beverages can be detected through their O and deuterium content; fraud in alcohol and other organic substances can also be detected through the position of the deuterium atoms in the molecule by Nuclear Magnetic Resonance (NMR) which, however, is useless for water studies).

A large number of local and regional studies have used stable isotopes (Yurtsever & Gat, 1981; Rozansky, 1987; Gat, 1996; Froelich, 2000; and many others)—appli­cations are now for the most part routine. An extensive survey of recent progress is to be found in a special issue of Hydrological Processes (2000).

Of course, many of the case studies were supplemented by chemical and radio­active isotope measurements (tritium and/or radiocarbon). Parallel stable isotope measurements showed that the water in deep aquifers in arid and semiarid countries was mostly very old, leading to the conclusion that they were recharged in ancient times when conditions were more humid (pluvial episodes). Their deuterium and l s O content had no relation to present climatic conditions, and their comparison with the present precipitation D/' sO diagram confirmed that more humid conditions had existed in the catchment areas. There was a very thorough argument to show that these fossil waters have to be interpreted and used with the utmost care.

After the "prehistoric" period, the study of isotope pairs was developed for dissolved species containing sulphur (Thode et al., 1961-1963), carbon (Craig, 1953), nitrogen (Kohl et al, 1971), especially in biogeochemical systems, then for rarer components of substances dissolved in water, such as Sr (sedimentary balances of watersheds) or Rare Earths; of dissolved gases ( 4 0Ar/ 3 6Ar, 3He/ 4He for dating and tracing, CH 4 for metabolic studies). Details of this literature can be found in the IAEA reports given in the references.

Since 1980, progress in technology has led to better mass spectrometers, and to the miniaturization of samples, but this has in turn led to new problems relating to sampling and processing.

Real progress was made in the 1980s with the advent of the accelerator mass spectrometer (AMS), a combination of a particle accelerator and a high resolution MS. This device allowed very low concentrations be measured of any isotope (down to a few atoms), either stable or radioactive. It opened an entirely new perspective in isotope natural tracing. However, the cost of analyses using such instruments makes the choosing of samples to study quite a problem.

It may be noted that "artificial tracing", that is adding tracers artificially enriched in one or two isotopes, is never used in hydrology due to the very high price of such materials. The same will hold for radioactive tracers, which are dealt with next, due to their negative impact on the environment.

RADIOACTIVE ISOTOPES IN HYDROLOGY

The scarcity of radioactive substances in the hydrosphere made their potential application to water problems more difficult than for stable isotopes. After the discovery of tritium by Alvarez in 1938 (for which he was awarded a Nobel Prize), a

56 R. Létolle & Ph. Olive

paper by Urry (1941) foresaw applications of radioisotopes to water studies. However lengthy experiments were needed to prepare and measure adequately very low concentrations of these isotopes of very low radioactive energy and the first papers on radiocarbon were published by W. F. Libby in 1947 (his book, Radioactive Dating, dates from 1952). He was awarded the Nobel Prize in 1960. But at the beginning, few research programmes dealt with carbon compounds dissolved in water. At present, nearly 200 laboratories in the world possess the necessary apparatus, but most of them work on archaeological material.

Due to the very weak energy of its P rays which were hidden in the background noise, adequate tritium and radiocarbon measurements in water needed more time (Grosse et al., 1951). Isotope enrichment apparatus was necessary to detect very low radioactivity rates, and enormous progress was made in the 1980-1990s with more efficient p ray detectors.

Tritium: 3 H

This isotope was suspected by Grosse in 1922. J H is a P emitter (Emax = 18 keV), with a half life (T\n) of 12.43 ± 0.05 years, and it disintegrates into stable 3He. The concentrations are expressed in "tritium units", TU:

1 T U = - ^ x l 0 " 1 8 =0.118 Bal" 1

Its natural origin in precipitation is due to the interaction of neutrons in cosmic rays with atmospheric nitrogen:

* N + 0 n » » > ^ C + 3 H

The mean baseline value in precipitation at a mean latitude in the northern hemisphere was then assessed as between 5 and 10 TU (this evaluation was made using old glacier ice and vintage wines).

Tritium of artificial origin was added to the environment from 10 March 1952, when the first thermonuclear bomb test by the USA took place at Eniwetok in the Marshall Islands in the Pacific Ocean, until the series of tests ended in 1980; there was an interruption between 1963 and 1966. The Soviet tests took place during this period in the Arctic Islands, then in the Semipalatinsk test area in Kazakhstan.

Figure 3 shows the evolution of the mean annual tritium concentration for Thonon-les-Bains (Université de Paris, France) from 1963. Most remarkable is the peak in 1963 when tritium concentrations reached -3000 TU, that is three orders of magnitude greater than the natural concentration. Following the break in tests in the atmosphere in 1980 and notwithstanding the atmospheric emissions of civilian nuclear power plants (Létolle & Olive, 1983) and accidents such as the Chernobyl explosion of 26 April 1986, atmospheric concentration in precipitation fell back at the beginning of the 21st century to the previous level of 10 TU. This was due to radioactive decay and dilution in the Earth atmosphere-ocean system.

Global atmospheric tracing of atmospheric water vapour led to new appr­oaches to the water cycle, not only in hydrology (Begemann & Libby, 1957), but also in meteorology (transfer of water vapour from the stratosphere to the troposphere)

A short history of isotopes in hydrology 57

1963 peak

Fig. 3 Variation of tritium content in precipitation at Thonon-les-Bains (France).

and in oceanography (turbulent exchanges between the atmosphere and the ocean down to the thermocline). The beginning of the 1960s was a very fruitful time for the newborn subject of isotope hydrology, a time when a number of important papers were published for example by Brown (1961) in Canada, Eriksson (1963) in Sweden, Mûnnich et al. (1967) in Germany, Vogel (1967), and Fontes et al. (1967).

The IAEA organized the first meeting on isotope hydrology in Vienna in 1966 with fewer than 100 participants. This meeting is now held every four years. Two important hydrological problem areas have been dealt with, namely aquifer recharge and dating.

The assessment of the recharge, that is the flux of water feeding the aquifer, depends upon the distribution of 3 H through the year (summer peak), and the distri­bution of precipitation through the year. Together with , 8 0 and 2 H data in rain and water from the unsaturated zone (or data from a lysimeter) the J H yearly flux can be estimated with a precision of ±30%. In the unsaturated zone, the 1963 peak became the classical marker to determine the speed of infiltration of water (Smith et al., 1970).

In the saturated zone, two models were used to describe the way water travels through a porous medium. In the "piston flow model", recharge moves through the aquifer at a constant speed in the case for a confined aquifer. The tritium content S, at the output, results from the content at the input, x years ago:

S = ExeXxx = Exe0Mxx in TU

where X is the disintegration constant of tritium:

, In 2 0.693 ., A. = = = 0.056 year

TU2 12.43 In the case of an unconfmed aquifer, it may be argued that recharge mixes

completely with the aquifer water: this is the well-mixed model (Hubert et al, 1970). The tritium content S„ of the aquifer at year N results from the mixing of oc% of

58 R. Létolle & Ph. Olive

precipitation water during year N, the 3 H content of which is E„, and of (1 - a )% of the aquifer water from year N- 1 the content of which is S„.\ :

S„=ax E„ + 0.944 x (1 - a) x Sm

a is the renewal rate of water and 0.944 (= e" 0 0 3 6 1) the decay of J H during one year. The "average transit time", x is linked to the renewal rate a by:

x = — (years) a

In steady state conditions where every year the aquifer containing a water volume V receives and loses the same flow Q, then:

Q 1 , - K

« = 77 = - ( y e a r ) V x

The 1963 tritium peak allowed good evaluations of the transit time of underground waters until the 1980s. Now tritium concentrations no longer allow such evaluations as the 3 H content at the aquifer outputs are about 10 to 15 TU, too close to the precip­itation input value (Fig. 3). Either, the J H content is higher than one TU and recharge is subsequent to the thermonuclear tests, so the transit time will be of the order of a few years, or even some centuries (3000 TU in 1963 became 380 TU in 2000; if T = 400 years, a = 0.0025 in the mixing model: therefore one measures only 0.0025 x 380 = 1 TU coming from the 1963 peak after 37 years). Or, the 3 H content is lower than about 1 TU and the water is "old": one may then use the l 4 C content of the total dissolved carbon in the underground water. The problem of mixing of "recent" waters with "old" waters is dealt with later. The combination of data on 3 H and JHe, its disintegration product, is a promising tool (Schlosser & Shapiro, 1998; Solomon & Cook, 2000), as with other tracers (Ekwuzel et al., 1994).

Tritium was also used tentatively for artificial tracing, but finally abandoned for security reasons.

Radiocarbon: 1 4 C

1 4C was discovered by Kammen and Ruben in 1937 and used as early as 1947 by Libby for dating archaeological and geological material (Anderson et al., 1947). Broecker & Walton gave a synthesis of ideas on 1 4 C in continental waters in 1959.

Dating underground water with radiocarbon was set up by the Miinnich team at Heidelberg in 1957, and gave access to waters the recharge of which is pre-Eniwetok time ( JH < 1 TU) in a wide time span from 102 to 104 years (Miinnich, 1957). All progress on the age and history of palaeowaters worldwide derives from this pioneer work (Fig. 4).

It is necessary to explain the complex pathway of carbon from atmosphere to underground water here, in order to avoid mistakes in the calculation of the age of palaeowater, which has been often the case:

1 4C is a beta emitter (Emax =155 keV) with a period T\a = 5730 ± 40 years: l 4 C » » 1 4 N + p"

A short history of isotopes in hydrology 59

Atmosphere atmospheric C*02

100 pCm - 8 % 0 i

n '*

Unsaturated ground

Soil [C*H 2 0] n

100 pCm -27%o

n

Respiration and Soil

Mineralization Unsaturated

ground il Soil C 0 2

90 pCm -23%o

Acidity ^ HC*0 3" + H C 0 3 ' - ^ O pCm 0 pCm ^

-17%» n - 1 % »

alkalinity C a C 0 3

Unsaturated ground il

Soil C 0 2

90 pCm -23%o U "

HC*0 3" + H C 0 3 ' - ^ O pCm 0 pCm ^

-17%» n - 1 % » 0 pCm 0%0

Saturated zone

2HC0 3 " V 45 to 65 pCm -12 to -8% o

1 4 C acquisition and carbon mineralization of underground waters in limestone soils. Activities in pCm and 1 3 C in %o vs PDB standard.

Fig. 4 Diagram of the isotopic evolution of carbon isotopes in water.

Concentrations are expressed in "percent of modern carbon" or pCm with 100 pCm = 13.56 ± 0.07 dpm g"1 of C = 0.226 Bq g"1.

The other carbon isotope, 1 3 C, is stable. Its abundance is given in the § (delta) notation, in %o (per mil) relative to an international standard, PDB, also distributed by IAEA. In natural carbon, when the l j C abundance is about 1%, the 1 4 C abundance is about ten billion times smaller than that of 1 3 C.

I 4 C is formed like tritium through nuclear reaction with atmospheric gases, and its disintegration gives stable 3He. In the atmosphere, the mean abundances of 1 4 C and 1 3 C are as follows:

A14(co2)= 100 ± lOpCm and 813(C02) =-8.0 ± 0.5 This atmospheric C 0 2 is incorporated in vegetal biomass during photosynthesis,

where:

C02(alm) + H 2 0 » » [CH 20] + 0 2

100 pCm 100 pCm -8 ± 0.5%o -27 ± 5%o

For plants with a C-3 ("normal" photosynthesis cycle), carboxylation in chloroplasts produces an impoverishment of about -20%o in organic matter. It is less for C-4 cycle plants characteristic of the tropical zone.

Temperate climate soil C 0 2 comes from root respiration and oxidation of dead organic matter, the radiocarbon content of which is lower than 100 pCm since it has suffered radioactive decay. In uncultivated soils, before 1952, the radiocarbon content was 90 pCm, corresponding to an apparent age, %, of about 900 years:

r , , , . 1 0 0 . 1 0 0 . . .

60 R. Létolle & Ph. Olive

The 5 L l C content is -23 ± 5%o, slightly enriched in C 0 2 by 4%o due to a more rapid diffusion of ' 2CÛ2 vs 1 3 C 0 2 from soil to atmosphere, which slightly enriches the CO2 remaining in the soil with heavy carbon.

Soil CO2 with 90 pCm [ 1 4C] and -23%o [ I j C ] , dissolves in rain water percolating through the soil. Water acidity results from this process:

C 0 2 ( s o i i ) + H 2 0 « » HCO3" + H +

90pCm 90 pCm - 2 3 % -17%o

Bicarbonate is in isotopic equilibrium with dissolved C0 2 , and slightly enriched in l 3 C and its 8 U C value is most commonly around —17%o. This is the case, for example, of underground waters in granitic or volcanic soils.

Isotopic fractionation for 1 4 C is twice that for i 3 C: 6% for 1 3 C for the C0 2 /HC0 3

fractionation, therefore 12%o and 1.2 pCm for 1 4 C. The activity of dissolved mineral carbon (DMC) stays unchanged at about 90 pCm.

In a limestone context this water is aggressive (pH from 5 to 6), dissolves calcite and, hence the pH shifts and stabilizes around 7 to 8:

C a C 0 3 ( s ) + H + < o > Ca 2 + + HC0 3" 0 pCm 0 pCm

0%o - l ° / o o

Bicarbonate is practically unfractionated relative to calcite and its l 4 C content reflects the zero content of "dead carbon" of calcite. The fraction, n), of active carbon with 90 pCm depends upon the proportion of dissolved atmospheric CO2 in the total dissolved mineral carbon (TDMC), i.e. the ratio [acidity/(acidity + alkalinity)]. This concept of alkalinity vs acidity, taken from Stumm & Morgan (1996, p. 168-179), gives the relation:

acidity _

acidity + alkilinity

1

2x(10 / f»~ p H +1)

where Ko is the dissociation constant of the C0 2 /H 2 C03 equilibrium. For pH < 8.5 and temperatures between 10 and 30°C:

<|> = 0.076 x (pH) 2 - 1.24 x pH + 5.57

The original activity of l 4 C, Ao of the TDMC is then deduced:

A0 = <j) x 90 (in pCm)

The variation domain of the initial values of Ac, and 80 in the recharge zone in a limestone context is:

at pH 6.5: <|> = 0.72, then A 0 = 0.72 x 90 = 65 pCm, and 8 0 = 0.72 x (-17) = -12.2%;

at pH 8.5: <\> = 0.5, then A0 = 45pCm and 80 = - 8 % o .

Only if the 8 l j C values exceed - 8 % o , must isotopic exchange between TDMC and the carbonate matrix (0 pCm and 8 = 0%o), which creates an enrichment in 1 3 C and a dilution of l 4 C, be considered. As the isotope fractionation for 1 4 C is twice that for l 3 C, then, for example, an enrichment of 10%o with a measured S i 3 C T D M C of -2%o against a

A short history of isotopes in hydrology 61

Montbouy (A9) Boubligny (A2)

pH 6.28 + 0.02 6.62 ± 0.02 <|> = 0.076 x (pH) 2 - 1.24 x pH + 5.57 0.78 ±0.04 0.69 ± 0.03 v4o=()>x90 [pCm] 70.2 + 7.0 62.] ±6 .2 Ai [pCm]) 21.6±0.4 7.0 + 0.3 r = 8 2 6 7 x l n ( 4 M ) [years] 9740+ 1070 18 040 ±1070 ô 0 =( | )x ( -17) [ % o ] -13.3 + 1.3 -17.7 ± 1.2 5, [ % o ] -15.0 ±0 .2 -13.4 ±0.2 Piezometric data [in m NGF] + 126 +95

value -12%o calculated from (]), creates an "ageing" in l 4 C of 2 x 10%o = 20%o = 2pCm, which might considerably modify the conclusions of the hydrological study.

Let us take as an example the case of two boreholes on the same southeast-northwest flow line of the captive Albian aquifer of the Paris basin (Raoult et al., 1998):

The distance between the two boreholes is 35 km. The mean 1 4 C velocity is:

35000±100 m „ „ . t . - i v C = = 4.2 ±1.1 m year

8300 ±2000 years Isotope exchange between TDMC and the matrix may be neglected as TDMC S l j C are lower than — 8 % o and at equilibrium with the soil CO2 at -19%o.

Darcy's law between the two boreholes gives:

T„, Ah 1 U - — X X

em A/ wc

where 11 is the mean velocity (m s"') in pores, T,„ the geometric mean of the aquifer transmissivity (1.4 x 10"3 m 2 s"1), e,„ the mean thickness (99 m), Ah/Al the potential gradient (31m for 35 000 m) and wc the kinematic porosity (0.15):

1.4xl0~3 31 1 » .1 » , -1 it = x x = 8.35x10 m s = 2.6 m year

99 35000 0.15 Taking into account the uncertainties of each method, agreement between their results is rather good. This is due to the hydrological continuity between the two boreholes and the homogeneity of the chemical composition.

Estimation of the age of a water requires precise knowledge of the aquifer characteristics and chemistry, and especially pH precise to ± 0.02 pH units measured in situ (to estimate Ao). Present computers (Visual MODFLOW) can manipulate complex three-dimensional programs connecting hydraulic and chemical data. As the input and output flows of big aquifers can seldom be measured or even observed, the only possibility is to fit the model to piezometric data, taking into account the few existing hydrodynamic parameters, such as transmissivity and storage coefficient. Flow velocities calculated from the model could be (or should be) compared to those obtained from 1 4 C measurements, as shown by the example above.

As for stable isotopes, the use of tritium or 1 4 C for artificial tracing, sometimes used in the 1960s, has been completely abandoned for security reasons.

62 R. Létolle & Ph. Olive

OTHER RADIOACTIVE ISOTOPES

There are numerous attempts to use other radioactive isotopes, such as dissolved 2 6A1, 3 9Ar, l 0Be, 3 6C1, 3 2Si, 1 2 9 I , l 3 7 Cs (Wijngaarden et al, 2002), etc.; all are very rare isotopes (much more so than tritium itself) coming from various nuclear processes. Years ago, when the first tentative measurement of y ) Ar required 30 tons of water and a long counting time to obtain good statistics, such methods were really experimental. Now, it is possible, due to the extraordinary sensitivity of the AMS, to correlate data obtained through water radiocarbon dating with others coming from radionuclides of a completely different geochemical behaviour (Bentley et al, 1986; Philips et al, 1986; Elmore & Phillips, 1987; Liu et al, 1995; and others). In practice, as for the stable isotopes of a variety of elements, it does not appear yet that all the tentative work with new radioactive isotopes are of real interest for hydrologists, except in the case of distinguishing between various sources of pollution.

Chlorine-36 will be taken as an example. Like 3 H and l 4 C, 3 6C1 is a beta emitter (Emax = 716 keV) with TU2 = 301 000 ± 4000 years:

3 ^ C » » > 3 ^ A r + |3"

AMS directly gives the concentration of '6C1 which may be expressed in two ways:

3 6 CI (a) 3bR=R36Cl = is the ratio of the 3 6C1 atom number to the total CI atom

CI number. This ratio does not change during evaporation, but it is lower if water dissolves evaporite CI" without ' 6C1.

3 6 CI (b) ",e'A =A^6Cl = (^A, not to be confused with argon-36, is the number of

kg atoms of 3 6C1 per kg or litre of water sample. This concentration increases with evaporation but does not change during evaporite dissolution.

For instance, the following is the result of the analysis of an Australian underground water:

mCl"= 3.44 x 10"3 mol kg"' water 36Rx 10 1 5 = 53 ± 7.4

hence: ZhA = 3 6C1 kg"1 = 36R x mCl x 6.022 x 10 2 3

3bA = (53 x 10"'5) x (3.44 x 10"3) x (6.022 x 10 2 3) = 1097 x 10 + s

3 6 ^lx 10"5= 1097 j 6 Cl is produced in the high atmosphere, by the action of cosmic rays on argon

(cosmogenic production): 4 0 Ar + p » » 3 5C1 + n + a

Then it reaches and percolates through the recharge zones of aquifers through rainfall. As cosmic rays penetrate the upper metre of soil, they produce more j 6 Cl, through

neutron activation and spallation (fragmentation of nuclei) of 3 D C1, j 9 K and 4 0 Ca (epigenic or surface production). Deeper down, the neutron flux coming from disinteg-

A short history of isotopes in hydrology 63

ration of natural radionuclides: U, Th and K, irradiates j 5 Cl to give hypogenic 3 6C1: 3 5 Cl + n » » 3 6 C l + y

After five to six periods this 3 6C1 reaches the radioactive secular equilibrium, when disintegration equals production.

Returning to the Australian sample: :,6A x 10"5= 1097

The origin of the sample is a borehole located 150 miles (~95 km) from the recharge zone where 36A0 = 1200 x 103 is the initial concentration in water which contains tritium. I6AAQ = 55 x 105 is the hypogenic concentration for secular equilibrium, a value depend­ing on the amount of U, Th and K in the aquifer matrix, and also on the Cl~ content and porosity.

The age of water % is calculated by the classical disintegration equation:

T]N . 36AMES-6A 301000 1 0 9 7 x 1 0 s - 5 5 x l 0 5

T = - ^ x l n — -, - = x ; - = -41 000 years In 2 -^AQ-^A 0.693 1200x10= - 55x10*

The case of mixtures of waters of various origin

Due to natural causes (for example two different aquifers connected through a fault, diffusion through a clay bed considered as water-tight, etc.) and/or man-made causes (faulty tubing leading to the pollution of a lower aquifer by an upper aquifer, excess pumping from a deep aquifer leading to the introduction of water from an upper aquifer), sampling in a multi-bed aquifer may represent the mixture of waters of different ages in proportions which are obviously unknown if only one radioisotope is used. However, combining two dating methods with isotopes of different periods, such as tritium (Ty2 ~ 12 years) and radiocarbon (T\a ~ 5700 years), may give the clue to the problem of aquifer mixing.

CONCLUSION

In a little less than one century, the discovery of isotopes and radioactivity has changed the world. However, it is only during the last half century that enough technical progress such as: (a) better electronic devices (for instance, the sensitivity of 3 H measurements has increased a 100-fold since 1980); (b) new measurement techniques (e.g. micro-methods for l 8 0 analysis, AMS for i 4 C and many other radioactive isotopes; and (c) automatic samplers and analysers; was made for isotope and radioactivity science to be used to understand phenomena and resolve problems in the field of hydrology.

Regarding the most important contributions of isotope hydrology to fundamental hydrology, one may list: understanding mechanisms of infiltration and evaporation, analysis of flood hydrographs, distinguishing recent groundwater (with J H and NO3) from old groundwater ( 1 4C < 50 pCm), stratification of water in aquifers, dating of deep fossil aquifers (Sahara, Arabia, Paris basin) considered to be strategic reservoirs, meteoric origin of thermal waters, space and time distribution of infiltration characteristics, problems of saline intrusion in coastal areas (CF vs 1 8 0 ) , not counting

64 R. Létolle & Ph. Olive

applications in oceanography, glaciology, meteorology, magmatology and fraud detection, and others.

Most of the foundations of isotope hydrology were laid in the period 1950 to 1980. Further progress will come through the use of the natural history of isotopes yet to be investigated in dissolved solids and gases, with more sophisticated apparatus. Another important step would be the development of portable analysers for stable isotopes and/or radioactivity, but that will be limited the statistics of counting (o = N05). And, an impossible dream for isotope hydrologists would be the development of paper indicators, like those for pH and eH measurements.

REFERENCES

General The reports of the International Atomic Energy Agency ( IAEA) give the best overview of the advances in isotope hydrology. The following were all published by IAEA, Vienna, Austria:

i 967 Radioactive dating and methods of low-level counting. STI /PUB/152.

1974 Isotope techniques in groundwater hydrology. STI /PUB/373 .

1976 Interpretation of environmental isotope and hydrochemical data in groundwater hydrology. STI/PUB/429.

1976 Nuclear techniques in geochemistry and geophysics. STI /PUB/425 .

1979 Behaviour of tritium in the environment. STI /PUB/498.

1979 Isotopes in lake studies. STI/PUB/51 I.

1981 Methods of low-level counting and spectrometry. S T I / P U B / 5 9 2 .

1981 Low-level tritium measurement. I A E A - T E C D O C - 2 4 6 .

1981 Stable isotope hydrology: deuterium and oxygen-18 in the water cycle. STI /DOC/10/210.

1983 Tracer methods in isotope hydrology. I A E A - T E C D O C - 2 9 1 .

1983 Guidebook on nuclear techniques in hydrology. STI/DOC/10/9112.

1987 Isotope techniques in water resources evaluation. STI /PUB/757.

1987 Studies on sulphur isotope variations in nature. STI /PUB/747 .

1989 Isotope techniques in the study of the /jydrology of fractured and fissured rocks. STI/PUB790.

1990 Use of artificial tracers in hydrology. I A E A - T E C D O C - 6 0 1 .

1991 Isotope techniques in water resources development. ST1/PUB/S75.

1992 Isotopes of noble gases as tracers in environmental studies. STI /PUB/859.

1995 Isotopes in water and environmental management, (also on line).

1996 Isotopes In wetter resources management STI /PUB/970, vols 1 and 2.

1999 Isotope techniques In water resources development and management. C D - R O M (also on line).

O t h e r genera l re ferences Clark, I. & Fritz, P. ( 1997) Environmental Isotopes in Hydrology. Lewis Publ., Boca Raton, Florida, USA. Fritz, P. & Fontes, J. C. (eds) (1986) Handbook of Environmental Isotope Geochemistry, vol. 2 The Terrestrial

Environment. Elsevier, Amsterdam, The Nether lands .

Gibson, J. J. & Prowse, T. D. (eds) (2000) Isobalance. Special issue of Hydrological Processes 14(8) , 1339-1536 .

Létolle, R., Mariotti , A. & Bariac, T. (1993) Les isotopes stables (2 vols). ADIT, Strasbourg, France (in French).

Melander, L. & Saunders, W. IT. (1980) Reaction Rates of Isotopic Molecules (2nd edn). John Wiley, Chichester, UK.

Rankama, K. (1951) Isotope Geology. Pergamon Press, Oxford, UK.

Rankama, K. (1963) Progress in Isotope Geology. Pergamon Press, Oxford, UK.

References in text

Anderson, E. S., Libby, W. F., Weinhouse , S., Kirshenbaum, A. D. & Grosse, A. V. (1947) Natural radiocarbon from cosmic radiation. Phys. Rev. 7 2 , 9 3 1 - 9 3 6 .

Aston, F. W. (1922) Les isotopes. French translation by S. Veil. Hermann, Paris, France.

Bertschi, P. (1976) Absolute l s O content of Standard Mean Ocean Water. Earth and Planet. Sci. Lett. 3 1 , 3 4 1 - 3 4 4 .

Begemann, F. & Libby. W. F. (1957) Continental water balance, groundwater inventory and storage t imes, surface water mixinti rates and world wide circulation patterns from cosmic ray and bomb tri t ium. Geochim. Cosmochlm. Acta 12. 2 7 7 - 2 9 6 .

Bentley, H. W, Philips, F. M., Davis, S. N., Airey, P. L., Calf, G. E., Elmore, D., Habermehl , M. A. & Torgerson, T. (1986) Chlorine-36 dating of very old ground water: I. The great Artesian Basin, Australia. Water Résout: Res. 2 2 , 1991-2002 .

A short history of isotopes in hydrology 65

Bigeleisen J. & Mayer, M. G. (1947) Calculation of equil ibrium constants for isotopic exchange reactions. J. Chem. Phvs.

15,261. Bleakney W. & Hippie, J. A. (1935) A study of oxygen isotopes. J. Phys. 4 7 , 800.

Broecker, W. S. & Walton, A. (1959) The geochemistry of l 4 C in fresh-water systems. Geochim. Cosmochim. Acta 16, 15.

Brown, R. M. (1961) Hydrology of tritium in the Ot tawa Valley. Geochim. Cosmochim. Ada 2 1 , 199-216 .

Buttle, J. M. (1994) Isotope hydrogram separat ions and rapid delivery of pre-event water from drainage basins. Proc. Phys. Geogr. 18(1) , 1 6 - 4 1 .

Craig, H. (1953) The geochemistry of the stable carbon isotopes. Geochim. Cosmochim. Acta 3 , 53 .

Craig, H. (1961a) Isotopic variations in meteoric waters. Science 133 , 1702.

Craig, H. (196 lb) Standard for reporting concentrations of deuterium and oxygen 18 in natural waters. Science 133 , 1833.

Craig, H. & Gordon, L. (1965) Stable Isotopes in Océanographie Studies and Paleotemperatures. Sp. Publ. 9, CNR, Pisa,

Italy.

Craig, H., Gordon, L. & Horibe, G. (1963) Isotope exchange effects in the evaporation of water. I Low temperature

experimental results. J. Geophys. Res. 6 8 , 5079.

Dansgaard, W. (1953) Stable isotopes in precipitation. Tellus 5 , 4 6 1 .

Dansgaard, W . (1964) Stable isotopes in precipitation. Tellus 16, 4 3 6 - 4 6 8 .

Dansgaard, W., Nief, G. & Roth, E. (1960) Isotopic distribution in a Greenland iceberg. Nature 185 , 232.

Demidenko, S. G. (1940) Isotope composi t ion of a tmospher ic precipitations. Acts in Physicochemislry, USSR 13 , 305

(in Russian).

Dole, M. (1934) The natural separation of the isotopes of hydrogen. J. Chem. Phys. 2 , 337; 2 , 548; and 5 - 6 , 999.

Dole, M. (1936) The relative atomic weight of oxygen in water and air. J. Chem. Phys. 4 , 268 ; and 4 , 778.

Ekwuzel B. et al. (1994) Dating of shal low groundwater comparison of the transient tracers HeA'H, chlorofluorocarbons

and S 7 K r . Water Resour. Res. 3 0 , 1693-1708 .

Elmore, D. & Philips, F. M. (1987) Accelerator mass spectrometry for measurement of long-lived radioisotopes. Science 2 3 6 , 5 4 3 - 5 5 0 .

Emeleus, IT., Furrow, J. W., King, A., Pearson, T. G., Pureed, R. H. & Briscoe, H. V. (1934) The isotope ratio in hydrogen: a genera! survey by precise density comparison upon waters from various sources . . / . Chem. Soc. 1 2 0 7 , 1948.

Epstein, S. & Mayeda, T. (1953) Variat ions of O 1 8 content of waters from natural sources. Geochim. Cosmochim. Ada 4 , 2 1 3 - 2 2 4 .

Eriksson, E. (1963) The possible use of tritium for est imating groundwater storage. Tellus 10, 472^178 .

Fontes, J. C. & Gonfiantini, R. (1967) Compor tement isotopique au cours de I 'evaporation de deux bassins sahariens.

Earth Planet. Sci. Lett. 3 , 258 .

Fontes, J . C , Létolle, R., Olive, Ph. & Blavoux, B. (1967) Oxygène-18 et tri t ium dans le bassin d 'Evian . In: Isotopes in

Hydrology, 4 0 1 ^ 1 1 5 . IAEA, Vienna, Austria.

Friedman, I. (1953) Deuterium content of natural waters and other substances. Geochim. Cosmochim. Acta 4 , 89.

Froehlich, K. (2000) Evaluat ing the water balance of inland seas using isotopic tracers: the Caspian Sea experience.

Hydro!. Processes 14 , 1371-1383 .

Gat, J. R. (1996) Oxygen and hydrogen isotopes in the hydrological cycle. Ann. Rev. Earth Planet. Sci. 2 4 , 2 2 5 - 2 6 2 .

Gilfillan, E. S. (1934) The isotopic composit ion of sea water. J. Am. Chem. Soc. 5 6 , 4 0 6 .

Giauque, W. F. & Johnston, II. L. (1929a) An isotope of oxygen of mass 17 in the Ear th ' s a tmosphere. Nature 1 2 3 , 831 .

Giauque, W. F. & Johnston, H. L. (1929b) An isotope of oxygen, mass 18. J. Am. Chem. Soc. 5 1 , 1436.

Gross , A. (1922) An unknown radioactivity. J. Am. Chem. Soc. 56 .

Grosse, A. V., Johnston, W. H., Wolfgang, R. L. & Libby, W. F. (1951) Trit ium in nature. Science 113 , I.

Guntz , A. A. & Beltran, E. (1937) L 'eau lourde et les végétaux. Ann. Inst. Agron. Maison Carrée 1 , 15.

Hubert, P., Marcé , A., Olive, Ph. & Siwertz, E. (1970) Etude par le tritium de la dynamique des eaux souterraines. C.R.

Acad. Sci., Paris 2 7 0 , 9 0 8 - 9 1 1 .

Jouzel, J., Merlivat, L. & Roth, E. (1975) Isotopic study of hail . J. Geophys. Res. 8 0 , 36, 5015.

Libby, P. (1952) Radiocarbon Dating. Univ. Chicago Press, Chicago, USA.

Liu, B., Phillips, F. M., Hoines, S., Campbel l , A. R. & Sharma, P. (1995). Water movement in desert soil traced by hydrogen and oxygen isotopes, chloride, and chlorine-36, southern Arizona. J. Hydrol. 168 , 9 1 - 1 1 0

Kassatkina, 1. A. & Florensky, K. P. (1941) Isotope composi t ion of the water of some seas and lakes. Dokl. Akacl. Nauk. 3 0 , 822. (in Russian)

Kirshenbaum, I. (1951) Physical properties and analysis of heavy water. Nat. Energy Series, Manhattan Project Tech. Section III, 4A.

Kistemaker , J. (1948) The mass spectrometer and some chemical applicat ions. Anal. Chimica Acta 2, 522.

Kohl, D. H., Shearer, G. B. & Commoner , B. (1971) Nitrogen contribution to nitrate in surface waters in a corn belt

watershed. Science 174 , 1331-1334 .

Lecolle, P. (1990) Répartit ion des isotopes stables de l 'oxygène sur le territoire français. Thesis , Univ. P. M. Curie, Paris, France.

Létolle, R. & Olive, Ph. (1983) Isotopes as pollution tracers. In: Guidebook on Nuclear Techniques in Hydrology, 4 1 1 -422 . Technical Report series 9 1 , IAEA, Vienna, Austria.

66 R. Létolle & Ph. Olive

McKinney, C. R., McCrea , J. M., Epstein, S., Allen, H. A. & Urey, H . C. (1950) Improvements in mass spectrometry for

the measurement of small differences in isotope abundance ratios. Rev. Sci. Instruments 2 1 , 724.

Merlivat, L. & Jouzel J. (1979) Global cl imatic interpretation of the deuter ium-oxygen relationship for precipitations.

J. Geophys. Res. 8 4 , 5 0 2 9 - 5 0 3 3 .

Munnich, K. 0 . (1957) Messungen des l 4 C Gehaltes von harten Grundwasser . Naturwissenschaflen 4 4 , 32 .

Miinnich, K. O., Roether, W. & Thilo, L. (1967) Dating of groundwater with tritium and l 4 C . In: Isotopes in Hydrology,

3 0 5 - 3 2 0 . IAEA, Vienna, Austr ia .

Nier, A. O. (1940) A mass spectrometer for routine isotope measurements . Rev. Sci. Instruments 1 1 , 212.

Nier, A. O, Ney, E. P. & Inghram, M. G.( 1947) A null method for the comparison of two currents in a mass spectrometer. Rev.

Sci. Instruments 18, 294.

Oana, S.(1948) Distribution of heavy water in natural waters . Chemical Researches, Japan 3 , 7 1 .

Okabe, K. & Titani, T. (1935) Isotopen Zusammense tzung des Wasssers aus Petroleum Quellcn. Bull. Chem. Soc., Japan 9, 466.

Paravano, N . & Pesce, B. (1938) Composiz ione isotopica delle acque naturali. Aid Sci., 10th Int. Congress Chem. (Roma) ,

vol. II, 4 0 1 , and, ibid. Composiz ione isotopica del l 'acqua piovano, 412 .

Pesce, B. & Cervone, C. (1940) Composiz ione isotopica del l 'humidi ta atmosferica. Gazz. Chim. Italica 11, 26.

Phillips, F. M., Bentley, H . W.. Davis, S. N. & Elmore, D (1986) Chlorine 36 dating of very old ground water: II. Milk

River Aquifer, Alberta, Canada. Water Résout: Res. 22, 2 0 0 3 - 2 0 1 6 .

Raoult, Y., Lauvcrjat, J., Boulègue, J., Olive, Ph. & Bariac, T. (1998) Etude hydrogéologique d 'un ligne d 'écoulement de

l 'aquifère de l 'Albien dans le bassin de Paris entre Gien-Auxerre et Paris. Bull. Soc. géol. France 169 , 453^157 .

Ricsenfeld, E. II. & Chang, T. L. (1936) Uber des Vertcilung des schweres Wasser isotopen auf der Erdc. Ber. Deutsch Chem.Ges. 6 9 , 1308.

Rozanski , K. (1987) Oxvgen-18 and deuterium in atmospheric part of hydrological cycle. 5c/. Bull. Univ. Mining &

Metallurgy, Krakow 1098, 7 - 1 0 1 .

Schlosser, P., Shapiro, S. D., Stute, M., Aeschbach-Herl ig , W., P lummer , N . & Busenberg, E. (1998) Trit iumfITe measurements in young groundwater . In: Proc. Isotope Techniques in the Study of Environmental Change, 165-189. IAEA, Vienna, Austria.

Smith, D. B. , W e a m , P. L., Richard, H . J. & Rowe, P. C. (1970) Water movement in the unsaturated zone of high and low permeabil i ty strata using natural tritium. In: Isotope Hydrologv 1970, 7 3 - 8 7 . IAEA, Vienna, Austria.

Solomon, D. K & Cook, P. G. (2000) J H and J H e . In: Environmental Tracers in Subsurface Hydrology (ed. by P. G. Cook & A. L. Flerzeg), 3 9 3 - 4 2 5 . K h m e r Academic Publishers, Dordrecht, The Nether lands .

S tumm, W. & Morgan, J. J. ( 1996) Aquatic Chemistry. John Wiley, Chichester, UK.

Teis, R. V. & Florensky, K. P. ( 1939-1950) Numerous papers in Doklady Ak. Nauk (in Russian).

Thode, H. G. (1963) Sulphur isotope geochemistry. In: Studies in Analytical Geochemistry, 25 . Royal Soc. Canada, Sp. Publ. 6.

Thode, II. G., Graham, R. L. & Siegler, M. (1945) A mass spectrometer and the measurement of isotope exchange factors. Can. J. Res. 2 3 B , 40 .

Thode, H. G., McNamara , .1. & Collins, C. B. (1949) Natural variations of the isotope content of sulphur and their

significance. Can. J. Res. 2 7 B , 361

Thode, H. G., Monster, J. & Dunford, H . B. (1961) Sulphur isotope geochemistry. Geochim. Cosmoehiin. Acta 2 5 , 159.

Thode, II. G. & Smith, S. R. (1944) The Natural Abundance of the Oxygen Isotopes. Nat. Res. Council Canada, Report M C 5 7 .

Titani, T. & Marada, S. (1935) The isotope fractionation due to evaporat iona distillation. Japan Chem. Soc. Bull. 10, 101.

Titani ,T. & Morita, N. (1938) Die Vergleichung der Dichte des Lei lungswasser von Osaka und von Cambr idge USA.

Japan Chem. Soc. Bull. 13, 409.

Urey, FI. C. (1947) The thermodynamic properties of isotopic substances. J. Chem. Soc. 5 3 , 567.

Urey, H., Brickwedde, F. G. & Murphy, G. M. (1932) An isotope of hydrogen of mass 2 and its concentration. Phys. Rev. 3 9 , 864, and 4 0 , 1.

Urry, W. D. (1941) The radio elements in water and the sediments of oceans. Phys. Rev. 5 9 , 4 7 9 .

Van Wijngaarden, M., Rigollet, C. & De Meijer, R. J. (2002) Assessment of historic Hood plain sedimentat ion rates along the River Meuse in the Netherlands using l 3 7 C s dating with P H A R O S . In: The Structure, Function and Management Implications of Fluvial Sedimentary Systems (ed. by F. J. Dyer, M. C. Thorns & J. M. Ollev), 3 8 9 - 3 9 8 . IAHS Publ. 276 . IAHS Press, Wallingford, UK

Vogel , J. C. (1967) Investigation of groundwater flow with radiocarbon. Isotopes in Hydrology, 355 -369 . S'fP 152. IAEA, Vienna, Austria.

Yurtsever, Y.& Gal, J. R. (1981) Stable isotope hydrology: deuterium and oxygen-18 in the water cycle. In: IAEA Tech. Rep. 210 (ed. by J. R. Gat & R. Gonfiantini) , 103. IAEA, Vienna, Austria.