Isotope Hydrology: 3H/3He dating Peter Schlosser, February 19, 2008

Syllabus Date Reading Lecture HW Prof Tu 1/22 Orientation, Principles of tracer applications MS,PS Th 1/24 Surface water hydrology 101 PS Tu 1/29 Groundwater hydrology 101 Groundwater flow HW1 MS Th 1/31 Groundwater transport Tu 2/5 Dyes, particles, ions, and other deliberate tracers HW2 Th 2/7 Stable isotopes Tu 2/12    HW3 Th 2/14   3H Tu 2/19 3H/3He

Radiocarbon dating (14C) Syllabus Date Reading Lecture HW  Prof Th 2/21   Carbon Isotopes (14C, 13C) MS Tu 2/26 Radiocarbon dating (14C)  HW4 Th 2/28  CFCs, SF6, SF5CF3 PS Tu 3/4 85Kr, 39Ar HW5 Th 3/6 Noble gases (incl Rn) Tu 3/11 Noble gas thermometry

Transient Tracers Tracers: trace substances of natural or anthropogenic origin (stable and radioactive isotopes; chemical compounds. Sometimes toxic or otherwise harmful (contaminants) Transient tracers: ‘Dyes’ with known delivery rates to the environment (e.g., 3H, 3He, CFCs, 129I, SF6, 85Kr) Radioactive clocks (e.g., 14C, 39Ar, 3H/3He) Special sources (e.g., 18O, Ba, nutrients, Ra isotopes, Rn isotopes) Deliberately released tracers (e.g., SF6, 3He)

3He Radioactive daughter of 3H 1939: Luis Alvarez discovered an isotope of mass 3 and distinguished it from tritium. He noted that the isotope was the rare isotope of natural helium. Mass: 3.01603 amu (HD/H3+: 3.0219/3.0235 amu) Abundance in the atmosphere: 5.24 ppm * 1.384 *10-6 (3He/4He)atm = 1.384 * 10-6 Solubility of He in water: Ostwald coefficient: ca. 0.007 to 0.01 3He is less soluble than 4He: α ca. 0.983 for seawater at low temperatures Low abundance requires sophisticated analytical methods for precise measurement.

3H/3He method: motivation

Tritium/3He method

Tritium/3He method

Tritium/3He method

Tritium/3He method Determination of 3H/3He age requires measurement of tritium and helium isotopes at time t. No knowledge of tritium and 3He concentrations at time t0; i.e., at time of isolation of water parcel, is needed.

Tritiogenic 3He in TU R = [3He]/[4He]

Early 3H/3He literature Early theoretical work by Tolstikhin and Kamensky, Geochemical International, 6, 810 – 811, 1969

Basics of 3H/3He method

3H/3He profiles Adapted from Schlosser et al., 1988 (EPSL)

3H/3He profiles Adapted from Schlosser et al., 1989 (EPSL)

Separation of tritiogenic 3He From Schlosser et al., 1989 (EPSL)

Separation of tritiogenic 3He From Schlosser et al., 1989 (EPSL)

3H/3He profiles Adapted from Schlosser et al., 1989 (EPSL)

Future of 3H/3He method From Schlosser et al., 1989 (EPSL)

Future of 3H/3He method From Schlosser et al., 1989 (EPSL)

Future of 3H/3He method From Schlosser et al., 1989 (EPSL)

3He confinement From Schlosser et al., 1989 (EPSL)

3H/3He age: mixing The 3H/3He age is non – linear with respect to mixing. It favors the age of the water component with the higher tritium concentration. From Jenkins and Clarke, 1976 (DSR)

Summary Combined tritium/3He can extend the utility of tritium as absolute time marker They also can be used as a radioactive clock Terrigenic helium can complicate the separation of tritiogenic 3He from the total measured helium. In many cases this problem can be solved by additional measurement of Ne The 3H/3He age is an apparent age. It has to be interpreted with care to account for non – linearities introduced during mixing of two or more water masses 3H/3He can be used for another several decades to come before tritium will have decayed to a large degree and dispersion will have homogenized the 3He signal