Physics of Aquatic Systems II

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1 Physics of quatic Systems II (Isotope Hydrology) 1. Introduction Isotopes Werner eschbach-hertig Institute of Environmental Physics University of Heidelberg Contents of P II 1. Introduction to Isotope Hydrology 2. Stable Isotopes 3. Tritium and 3 He 4. Dating of young groundwater and modeling 5. Dating of old groundwater and paleoclimate 6. nalytical methods For details, updates and lecture notes see studium/lehre/quaphys/p2.html Contents of Session 1: Introduction What is Isotope Hydrology? Literature on Isotope Hydrology Introduction to Isotopes Radioactivity Overview of Isotopes/Tracers in Hydrology What is Isotope Hydrology? pplication of environmental isotopes/tracers to study (parts of) the hydrological cycle Scientific discipline that evolved since ~1950 by application of nuclear techniques to hydrology Mix between physics (analytical techniques) and earth/environmental sciences (problems) Strong role of IE (Int. tomic Energy gency) GIP: Global etwork of Isotopes in Precipitation Regular Isotope Hydrology meetings since 1963 Isotope Hydrology Lab: Standard materials Problems studied in Isotope Hydrology Origin of water masses and formation conditions Identification and separation of water components Recharge areas, flow paths, mixing Origin of contaminants Paleoclimate, paleotemperatures Residence time of water in the system Flow velocities, dispersion Fluxes, recharge rates, exchange rates Transport and degradation of contaminants What is a tracer? Substance, which: is present only in trace s ( Spurenstoff ) marks a trace in a natural system (marker) Tracers in aquatic systems Ideally mark the water or dissolved substances therein, and move passively with water or solutes Isotopes are often nearly ideal tracers, e.g.: isotopes of H and O mark the water molecule isotopes of C,, S etc. mark dissolved substances Conservative solutes (e.g. noble gases) are good tracers

2 Environmental Isotopes and Tracers Isotopes or other tracers that are widely distributed in the environment, especially the hydrosphere. They can be both of natural or anthropogenic origin. Transient tracers: nthropogenic, variable input e.g. Tritium, CFCs, SF 6, 85 Kr Geochemical tracers: atural origin, constant input e.g. stable isotopes, C, noble gases Contrast: rtificial tracers are released deliberately, in local tracer experiments (dyes, chemicals, also isotopes) Literature on Isotope Hydrology Mook, W.G. (ed.), 2001: UESCO/IE Series on Environmental Isotopes in the Hydrological Cycle - Principles and pplications. vailable online at Clark, I. D, Fritz, P., 1997: Environmental Isotopes in Hydrogeology. Lewis Publishers, Boca Raton. IUP 1868 Cook, P. G., Herczeg,. L. (eds.), 2000: Environmental Tracers in Subsurface Hydrology. Kluwer cademic Press, Boston IUP 1869 Kendall, C., McDonnell, J. J., 1998: Isotope Tracers in Catchment Hydrology. Elsevier, msterdam Mazor, E., 1997: Chemical and Isotopic Groundwater Hydrology. Dekker, ew York. IUP 1885 Moser, H., Rauert, W., 1980: Isotopenmethoden in der Hydrogeologie. Lehrbuch der Hydrogeologie, Band 8. Bornträger, Berlin. IUP 1883 UESCO/IE Series on Environmental Isotopes in the Hydrological Cycle Comprehensive overview of Isotope Hydrology in 6 Volumes Each sub-chapter downloadable as pdf-file Volume I: Introduction: Theory, Methods, Review Volume II: tmospheric Water Volume III: Surface Water Volume IV: Groundwater: Saturated and Unsaturated Zone Volume V: Man's Impact on Groundwater Systems Volume VI: Modelling Topics of this session: Vol. I, chapters 2, 5, 6 Isotopes uclei of atoms consist of protons and neutrons (= nucleons) Z = number of protons (atomic number) = number of neutrons = Z + = number of nucleons (mass number) Z determines to which element a nucleus belongs Z = number of electrons in a neutral atom number of electrons determines chemical properties Isotopes are atoms of the same element (same Z), with a different number of neutrons (varying and ) Iso topos (gr.) = same place (in the periodic system) otation Full: (e.g. C ) Short: X (e.g. C) ZX 6 8 Chart of the uclides Chart of the uclides Isotopes (const. Z) Isobars (const. ) Isotones (const. )

3 Stability of uclides Stability requires balance between Electromagnetic force: Repulsion between protons Strong force: ttraction between nucleons Some rules of thumb: Light elements: uclides with Z = (e.g. 12 C,, 16 O) or a slight neutron excess (e.g. 13 C, 15, 18 O) are stable. Heavy elements: Only nuclides with a strong neutron excess are stable (e.g. 208 Pb, Z = 82, = 126) Z and/or = 2, 8, 20, 28, 50, 82, 126 (magic numbers): high stability and consequently large natural abundance (e.g. 4 He, 16 O, 40 Ca, 208 Pb: doubly magic isotopes) Uneven Z: less stable isotopes (uneven-uneven: very rare) atural abundance of stable isotopes bundance mainly determined by processes during nucleosynthesis (big bang, supernovae) Symmetric (Z = ) and (doubly) magic nuclides tend to be most abundant (e.g. 4 He, 12 C,, 16 O, 20 e, 40 Ca) Element bundance of stable isotopes (%) Hydrogen 1 H H Helium 4 He He Carbon 12 C C 1.11 itrogen Oxygen 16 O O O eon 20 e e e 0.27 Radioactivity: modes of decay Radioactivity: modes of decay β - decay: n p + e + νe ZX Z+ 1Y -1 α decay: X Y + α α = He 4 4 Z Z β + decay: + p n + e + νe ZX Z 1Y + 1 e - capture: p + e n + ν ZX Z 1Y + 1 fission: X 2 daughter nuclides + some n γ decay: X X * Z Z + γ (no change of nuclide) Different modes of decay in the chart of nuclides Law of Radioactive Decay Radioactive decay is a purely statistical process, with constant decay probability. The number of decays in a given time interval is proportional to the total number of nuclides: d = λ dt with λ [T -1 ] = decay constant = decay probability per unit time Integration with initial condition (0) = 0 yields Equation of exponential decay. λt ( t) e = 0

4 Time information from radioisotopes Radioactive Decay Radioactive isotopes yield time information (age), if (t) and 0 are known (λ s are well-known constants): 1 ( t) t = ln λ 0 0 half-life: T 1/2 = ln2/λ mean life: τ = 1/λ (t) can be measured, the problem is to know 0! In practice, often not (number of atoms, ) is measured, but the number of decays per unit time: d ctivity: = λ dt Units for : Becquerel (Bq) = s -1 (decays per sec) or: Curie (Ci): 1 Ci = Bq (activity of 1 g Ra) 0 /2 0 /e 0 /4 T 1/2 τ 2T 1/2 t atural abundance of radioactive isotopes Radioactive isotopes can only occur in nature if they are Very long lived (half-life comparable to age of Earth) Constantly produced by nuclear processes Radioisotopes can be classified according to their origin: Primordial (remnants from formation of solar system) Cosmogenic (produced by interactions with cosmic rays) Subsurface (produced by nuclear reactions in the Earth) nthropogenic (produced by technical processes) Most radioisotopes have very low isotopic abundances, except very long-lived primordial isotopes Some heavy elements occur in nature, although they have no stable isotopes: produced in U/Th-decay series Primordial Radioisotopes (selection) Isotope bundance (%) Half Life (yr) Decay mode 40 K x 10 9 β -, EC 50 V x EC, β - 87 Rb x β - 4 d x α 8 Sm x α 176 Lu x β Hf x α 232 Th x α, SF 235 U x 10 8 α, SF, e 238 U x 10 9 α, SF Decay series of 232 Th (Thorium series, = 4n) Decay series of 238 U (Uranium series, = 4n + 2) half-life > 1 year 208 Pb half-life < 1 year 206 Pb half-life > 1 year stable half-life < 1 year stable

5 Decay series of 235 U (ctinium series, = 4n + 3) Production of Radioisotopes Radioisotopes can be produced by nuclear reactions induced by irradiation of nuclides. Sources of irradiation are tmosphere, surface: Cosmic rays Subsurface: U/Th-decay series and fission: α s and n s nthroposphere: uclear bombs, reactors, accelerators, etc. Examples: 207 Pb half-life > 1 year half-life < 1 year High energy: Spallation O + p Be + 2n + 5p 10 O( p,2n5p) Be stable Low energy: thermal n-capture + n C + p ( n,p) C Cosmogenic and subsurface production Main target elements are tmosphere:, O, r Rocks: Li, O, a, Mg, l, Si, Cl, K, Ca, Fe Some possible reactions: Cosmic Rays (CR) Cosmic rays (CR) are an important source of radioisotopes primary CR: 87 % p, 12 % α, 1 % heavier nuclei secondary CR: mainly neutrons tmosphere Subsurface 40 ( n,p) C 3 12 ( H) C n, 26 r( p,sp) l ( ) Li n,α r( p,3n2p) Cl 40 Ca ( n,2n3p ) 36 Cl r( n,2n) r 39 K ( n,p ) 39 r H 36 Cl( n, γ) Cl 37 Ca( n,α) r Cosmogenic production rates Production rates by cosmic radiation are very low Cosmogenic isotopes are very rare, e.g.: C/ 12 C ~ 10-11, 36 Cl/ 35 Cl ~ 10-15, 3 H/ 1 H ~ 10-18, global natural 3 H inventory: 3.6 kg Radioisotope Prod. Rate (atoms cm -2 s -1 ) 3 H Be Be C Cl nthropogenic production Major sources of anthropogenic radionuclides: Tests of nuclear weapons in the atmosphere (~1950s - 60s) ccidents of nuclear power plants ormal releases of nuclear power and reprocessing plants Waste from other applications (medical, science, etc.) nthropogenic nuclides of importance for isotope hydrology: Isotope Half-life (yr) Origin 3 H bomb tests C 5730 bomb tests 36 Cl 308'000 bomb tests 85 Kr 10.7 fuel reprocessing

6 Tritium Input: Bomb Peak Environmental Isotopes in quatic Systems Isotope Phase Half Life Use Deuterium ( 2 H, D) H 2 O (HDO) stable Marker, formation Tritium [TU] Tritium in precipitation verage over stations in northern Switzerland Tritium ( 3 H, T) H 2 O (HTO) yr Dating (input) Helium-3 ( 3 He) dissolved He stable Dating ( 3 H/ 3 He) Helium-4 ( 4 He) dissolved He stable Dating (accumulation) Carbon-13 ( 13 C) DIC, DOC, stable Marker Carbon- ( C) DIC, DOC, 5730 yr Dating (decay) itrogen-15 ( 15 ) diss. 2, O 3,.. stable Marker Oxygen-18 ( 18 O) H 2 O (H 18 2 O) stable Marker, formation Sulfur-34 ( 34 S) SO 4, stable Marker Chlorine-36 ( 36 Cl) Cl - 308'000 yr Dating (decay) Chlorine-37 ( 37 Cl) CHCs, stable Marker rgon-39 ( 39 r) dissolved r 269 yr Dating (decay) rgon-40 ( 40 r) dissolved r stable Dating (accumulation) Year Krypton-81 ( 81 Kr) dissolved Kr 210'000 yr Dating (decay) Krypton-85 ( 85 Kr) dissolved Kr 10.7 yr Dating (input) Radon-222 ( 222 Rn) dissolved Rn 3.8 d Dating (accumulation) U-series ( 238 U, 234 U,..) dissolved UO 2 variable Dating (disequilibrium) Environmental Tracers in quatic Systems radioactive decay Principles of Dating Methods accumulation C initial? 3 H, C 4 He Gases Tracer Phase Half Life Use He, e, r, Kr, Xe dissolved stable Formation (temp.) 39 r, 81 Kr slope = accumulation rate? itrogen ( 2 ) dissolved stable Formation (temp.) 40 r CFCs (11, 12, 113) dissolved stable (oxic) Dating (input) SF 6 dissolved stable Dating (input) Others Tracer Phase Half Life Use Temperature bulk stable Marker Conductivity (TDS) bulk stable Marker Ions (Cl -, Li +, ) dissolved stable Marker time mother - daughter pair 3 He time input variation 3 H CFC Kr 3 H time year Time Ranges of Dating Methods Summary 222 Rn SF 6 CFCs 85 Kr 3 H 3 3 H- He 4 He age [yr] 39 r 40 r 36 Cl 81 Kr C Isotope Hydrology: Use of environmental isotopes or tracers to study origin and flow of water Isotopes: toms of an element with different m () Light elements have 1 main stable isotope, few minor Radioisotopes are primordial or constantly produced Cosmogenic production is low: very rare isotopes Radioactive decay provides time information ( 0?) Dating also by using accumulation or input variation Large family of environmental isotopes and tracers Time scales (dating range) from days to Myr