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12 Natural Isotopes of Elements Other Than H, C, O

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12 NATURAL ISOTOPES OF ELEMENTS OTHER THAN H, C, O In this chapter we are dealing with the less common applications of natural isotopes. Our discussions will be restricted to their origin and isotopic abundances and the main characteristics. Only brief indications are given about possible applications. More details are presented in the other volumes of this series. A few isotopes are mentioned only briefly, as they are of little relevance to water studies. Based on their half-life, the isotopes concerned can be subdivided: 1) stable isotopes of some elements (He, Li, B, N, S, Cl), of which the abundance variations point to certain geochemical and hydrogeological processes, and which can be applied as tracers in the hydrological systems, 2) radioactive isotopes with half-lives exceeding the age of the universe (232Th, 235U, 238U), 3) radioactive isotopes with shorter half-lives, mainly daughter nuclides of the previous catagory of isotopes, 4) radioactive isotopes with shorter half-lives that are of cosmogenic origin, i.e. that are being produced in the atmosphere by interactions of cosmic radiation particles with atmospheric molecules (7Be, 10Be, 26Al, 32Si, 36Cl, 36Ar, 39Ar, 81Kr, 85Kr, 129I) (Lal and Peters, 1967). The isotopes can also be distinguished by their chemical characteristics: 1) the isotopes of noble gases (He, Ar, Kr) play an important role, because of their solubility in water and because of their chemically inert and thus conservative character. Table 12.1 gives the solubility values in water (data from Benson and Krause, 1976); the table also contains the atmospheric concentrations (Andrews, 1992: error in his Eq.4, where Ti/(T1) should read (Ti/T)1); 2) another category consists of the isotopes of elements that are only slightly soluble and have very low concentrations in water under moderate conditions (Be, Al). These are primarily used for sediment studies and for dating ice sheets; 3) two elements (N, S) have isotopic abundances revealing information on bio(geo)chemical processes in soils and associated waters; 4) three nuclides are cosmogenically produced isotopes from elements that occur as solutes in aqueous systems (Si, Cl, I) and are used for water dating; 213 Chapter 12 5) the decay series of uranium and thorium contain several isotopes of which the relations between the parent/daughter activities are useful pre-eminently. In the tables at the introduction of each section some relevant physical information is given about abundance and decay to finally a stable daughter nuclide. This chapter mainly serves to review the physical and chemical characteristics of the isotopes. Only a brief indication is being given of the possibilities of application for studying the water cycle. A thorough discussion is to be found in the other volumes of this series. Table 12.1 Solubilities of noble gases in water at various temperatures, the atmospheric concentrations and the concentration of the noble gases dissolved in water in equilibrium with the atmosphere at 10oC (Bensen and Krause, 1976; Andrews, 1992). Solubility in mL(STP)/L H2O Atmosph. Diss.conc. 0oC 10oC 20oC 30oC concentr. mL/L (10oC) Helium 9.53 9.07 8.82 8.74 5.24P10 6 4.75P10 5 Neon 12.6 11.4 10.5 10.0 1.82P10 5 2.07P10 4 Argon 53.5 41.7 34.1 29.0 9.34P10 3 3.89P10 1 Krypton 111.2 81.2 62.8 50.9 1.14P10 6 9.26P10 5 Xenon 226 154 112.5 86.7 8.60P10 8 1.32P10 5 Radon 510 12.1 HELIUM Table 12.2 Natural isotopes of helium. 3 4 2 He 2 He Stability Stable Stable Natural abundance fraction 1.3P10 6 100 % Natural abundance in air 1) 6.8P10 12 5.24P10 6 Natural abundance in groundwater 2) 10 10 to 10 13 10 3 to 10 5 1) mL gas /mL of air 2) mL gas /mL of water 214 Isotopes of other elements than H, C and O 12.1.1 ORIGIN AND CHARACTERISTICS The radioactive isotope of hydrogen, formed in the atmosphere by a nuclear reaction of nitrogen atoms with neutrons in cosmic radiation (Sect.8.3), decays through decay to 3He: 3H J 3He + (12.1) The maximum energy of 3H is 18 keV and the half-life is 12.43 a (Fig.8.2). The daughter product 3He is stable and chemically inert. 3He is also formed in the earth's crust (primarily in crystal rock) by the reaction: 6Li + n J 3He + 4He or 6Li (n,) 3He (12.2) The underground neutrons originate from spontaneous fission of uranium or thorium isotopes or by (,n) reactions in which the particles are from decays within the uranium and thorium decay series. 4He originates primarily from the natural decay of 235U, 238U and 232Th in the earth (Sect.12.13). 12.1.2 EXPERIMENTAL AND TECHNICAL ASPECTS In groundwater concentrations of helium are observed in the range of 10 5 to 10 2 mL(STP) of He/mL of water. With 3He/4He ratios of 10 8 to 10 7 this means that the 3He concentrations can be as low as 10 13 mL/mL of water. To measure these amounts of gas, specialised mass spectrometers are needed. This is even more true for the measurement of extremely low 3H contents (down to 10 2 TU) through 3He accumulation (Sect.12.1.5.2). In general, water samples are collected of 5 to 500 mL -depending on the laboratory's equipment- and thoroughly sealed before being shipped to the laboratory. 12.1.3 SOURCES OF 3He The main complication is that the principle of 3H/3He dating is based on the assumption that all 3He in the water sample is tritiogenic (coming from decay of 3H). However, part of the 3He may be of terrigenic origin (from in situ production according to Eq.12.2), part from the original equilibration of the water with the atmospheric gases. The latter contribution can be estimated from the Ne concentration, which can not be of terrigenic origin. A possible terrigenic contribution is to be estimated from the concentration of 4He (also corrected for the atmospheric component (Schlosser et al., 1998). These processes are summarised by: 33 3 3 Hetot = Heatm + Heterr + Hetrit (12.3a) 44 4 Hetot = Heatm + Heterr (12.3b) Netot = Neatm (12.3c) where the atmospheric concentration ratio Heatm/Neatm = 0.288. 215 Chapter 12 12.1.4 NATURAL ABUNDANCE The 3He/4He ratio in atmospheric helium is about 1.3P10 6, with a helium concentration of about 5 ppmv (partial pressure of 5P10 6 atm.). The relative constancy of this concentration is due to the state of equilibrium between the loss of helium from the crust to the atmosphere and the loss from the atmosphere to outer space. Groundwater concentrations are observed of 10 3 to 10 5 mL(STP)/mL of water, far exceeding the equilibrium value due to relatively high underground helium production. The 3He/4He ratio in groundwater is still within a narrow range of 5P10 8 to 10 7. Apart from the infiltration of tritium containing water, 3H can be produced underground in the aquifer, i.e. through in situ production: 14N + n J 12C + 3H (12.4) The presence of neutrons is due to spontaneous fission of U and Th. Because of its short half- life, 3H levels will be low, generally below 1 TU. The accumulated 3He levels will then depend on the degree of isolation of the water, and are thus a measure of that effect. 12.1.5 APPLICATIONS 3 3 12.1.5.1 PRINCIPLE OF H/ He DATING The application of 3H to obtain underground residence times ("ages") of groundwater was shown to depend on our knowledge of the 3H input from atmospheric precipitation (Sect.8.4.2). Studying the relation between 3H and 3He offers the possibility to determine ages without knowing the tritium input function. If the original 3H content of the infiltrating 3 3 precipitation is H0, the content after a residence time t, Ht, is presented by: 33 t Ht = H0 e The amount of 3He formed during this time is equal to the amount of 3H lost by decay: 33 3 3 t Het = H0 Ht = Ht(e 1) (12.5) The 3H/3He age is then: t = (12.43/ln2) P ln[3He/3H + 1] (12.6) Most applications are related to shallow, young groundwater, using the bomb peak of 3H in the precipitation during the early 1960s. The occurrence of 3H and 3He at great depth may be due to fractures, cracks or leaking boreholes, or to in situ production. 3 3 12.1.5.2 MASS SPECTROMETRIC MEASUREMENT OF H THROUGH He Very low 3H abundances (theoretically down to 0.0005 TU) can be measured by thoroughly degassing and storing 50 to 100 mL of water sample for at least half a year in a tightly sealed 216 Isotopes of other elements than H, C and O container and subsequently recollecting the decay product of tritium, 3He. The amount of this inert gas can be quantitatively measured mass spectrometrically. The 3He produced during this storage time (t) is (Eq.12.5): 33 3 3 t Het = H0 Ht = H0(1 e ) so that the original 3H content of the sample was: 3 3 t H0 = He/(1 e ) (12.7) To give an impression: the amount of 3He formed during half a year storage of 100 mL of water with a 3H concentration of 0.5 TU is: 3 18 (ln2)/12.43) P 0.5 3 Het = 0.5P10 P 2 P (100/18) P (1 e ) P 22.41P10 = 3.4P10 15 mL 12.2 LITHIUM Table 12.3 Natural isotopes of lithium 6 7 3 Li 3 Li Stability Stable Stable Natural abundance fraction 7.5 % 92.5 % 12.2.1 NATURAL ABUNDANCE In seawater the Li+ concentration is about 0.18 mg/L.
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