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Ch.1. . Radioisotopes (Unstable isotopes) The isotopes do radioactive decay Mode of radioactive decay. From https://commons.wikimedia.org/wiki/Image:Radioactive_decay_modes.svg?uselang=ko. List of the radioactive ebvironmental isotopes  See Table 1-3 on p.17. T

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slide1
Ch.1.
  • Radioisotopes (Unstable isotopes)
    • The isotopes do radioactive decay
    • Mode of radioactive decay

From https://commons.wikimedia.org/wiki/Image:Radioactive_decay_modes.svg?uselang=ko

slide2

List of the radioactive ebvironmental isotopes  See Table 1-3 on p.17.

    • T
      • Formed by H-bomb testing (and minorly from cosmic ray bombardment on 14N)
      • Daughter: 3He
      • Measured in absolute amount (in TU). 1 TU=1 T per 1018H
      • Detected w/ LSC or gas counter (for the case of propane preparation) (see text)
slide3

14C

    • Formed by cosmic ray bombardment on 14N
    • Daughter: 14N
    • Measured against modern carbon (mC) whose 14C activity is 95% of that of 1950 NBS-oxalic acid standard
    • Due to the fractionation, should be normalized to d13C= -25‰
    • Converted into graphite and analyzed with AMS
slide4

36Cl

    • Formed by cosmic ray bombardment on 36Ar
    • Daughter: 36Ar
    • Often analyzed with AMS
slide5

Significance of environmental isotopes (in hydrogeology)?

    • Trace groundwater provenance
    • Estimate groundwater age
    • Interpretation of groundwater quality
    • Understanding geochemical evolution (reactions and reaction rates) and recharge processes
    • Recognizing geochemical & biogeochemical cycles of pollutants among soil-water-atmosphere
slide6

Isotopic fractionation

    • A process of resulting disproportion of isotopes due to their mass difference
    • Physicochemical fractionation vs. Diffusive fractionation: Compare the equations on p. 22 to 25.
    • Equilibrium vs. kinetic fractionation
    • See Fig. 1-5 on p.23.
slide7

The conditions of isotopic equilibria requires

    • Chemical equilibria: forward and backward reaction rates are same
    • Enough time for mixing between reactant and product reservoir
    • Enough time for complete mixing within each reservoir (homogeneity issues)
  • Temperature effect on fractionation
    • For an equilibrium fractionation, usually the fractionation is reciprocally proportional to T
    • For an kinetic fractionation, the fractionation is dependent upon the nature of the processes (e.g. biocatalysis vs. inorganic burning)
  • See Fig. 1-8 & Table 1-4 on p. 28, Fig. 1-7. on p.29, and Fig. 1-8 on p.30.
slide8

Isotope Fractionation, enrichment, and separation

    • Fractionation: a
    • Enrichment: e
    • Separation:D
    • See the equations on p.31 and Fig. 1-9 on p.32.
slide9

Assignments

    • Solve problems 1, 3, 5, 7, and 9 on p.33 to 34.