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 [email protected] Abstract This paper recalls the birth and development of stable and radio­ active isotope techniques in hydrology from the discovery of isotopes in 1912 to the beginning of the 21st century. Key words l4C; 2H; JH; lsO; history; hydrology; hydrogeology; isotopes; technique 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, 180 (abundance -1500 ppm) and 170 (-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 demon­ strated 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 180 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 180. 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 (12CI60160+ and 12C160180+); and with small corrections due to interference of the 13C and l70 components of the CO2 molecule and instrumental effects; similar corrections for D/H ratios; measure­ ment 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 measure­ ments, while precision and accuracy became of the order of 0.1 per mil (%o) of the 180/160 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 reproduc­ ibility obtained through the previous generation of mass spectrometers and densito­ meters, 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 measure­ ments, 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).