Page 140 - Hydrogeology Principles and Practice

HYDC04 12/5/05 5:36 PM Page 123

Environmental isotope hydrogeology 123

An example of mechanism 1 is the evaporation of The relationship between δ and α is given by the
water which leads to the concentration of the light expression:
1
16
isotopes O and H in the vapour phase and the
+
heavy isotopes in the liquid phase. This is because α = R A = 1000 δ A
−
water molecules containing the light isotopes move AB R 1000 + δ eq. 4.4
B B
more rapidly and thus have a higher vapour pressure.
Most samples of freshwater have negative values of For example, in the condensation of water vapour
18
16
δ O (ranging down to −60‰) in that the light O (phase v) to liquid (phase l), equation 4.4 becomes:
isotope is concentrated in the vapour evaporating
from the sea surface. Oxygen in air has a high positive 1000 + δ O
18
l
isotopic signature of +23.5‰. α = 1000 + δ O l eq. 4.5
v
18
As an example of mechanism 2, if CO containing v
2
18
16
only O is mixed with water containing only O, 18 18
and if δ O =−5‰ and δ O =−14‰, then the frac-
exchange will occur according to the following l v
tionation factor can be calculated:
reaction, until equilibrium is reached among the four
species:
(1000 +− ))
(
5
l
1
α = = .0092 eq. 4.6
v
14
(
16
18
16
18
1
1 /2C O + H O j /2C O + H O eq. 4.2 (1000 +− ))
2 2 2 2
Although the bond strengths in the two compounds 4.3 Stable isotopes of water
18
16
are different, at equilibrium, the ratio O/ O will be
nearly the same in the CO and H O.
2 2 The relative abundances of hydrogen and oxygen iso-
The variable separation of isotopes depending
topes found naturally in the water molecule are given
on reaction rates (mechanism 3 above) is particularly
in Table 3.1. Meteoric water shows a wide range of
associated with reactions catalysed by bacterial activ- 18 2
2− δ O and δ H values reflecting the extent of isotope
ity. For example, in the bacterial reduction of SO ,
4
2− − fractionation during successive cycles of evaporation
the production of sulphide (S , HS and H S) is faster
2 and condensation of water originally evaporated
32
for the light isotope, S, than for the heavy isotope,
34 from the sea. When condensation occurs to form pre-
S, such that the light isotope becomes concen-
cipitation, the isotopic concentration changes accord-
trated in the sulphide species and the heavy isotope
2− ing to a Rayleigh distillation process for which the
enriched in residual SO .
4 isotopic ratio, R, in a diminishing reservoir of reactant
Regardless of mechanism, the extent of isotope
is a function of its initial ratio, R , the remaining reser-
separation between two phases A and B can be rep- o
voir fraction, f, and the fractionation factor, α, such
resented by a fractionation factor, α, where: 2 1
that R = R f(α − 1). The H/ H fractionation is pro-
o
portional to, and about eight times as large as, the
R 18 16
α = A eq. 4.3 O/ O fractionation. Both fractionations change
−
AB
R B proportionally as temperature changes. Craig (1961)
showed that δ values for meteoric water samples of
where R is the ratio of concentrations of heavy to global distribution, for the most part, define a straight
A 2 18
18
16
light isotope in phase A ( O/ O in liquid water, for line on a cross-plot of δ H against δ O, represented
16
18
example) and R is the same ratio in phase B ( O/ O by the approximate equation, know as the World
B
in water vapour). If equilibrium is established be- Meteoric Water Line (WMWL):
tween liquid water and vapour at 25°C, the value of
2
18
α is about 1.0092. Similar fractionation factors, very δ H = 8δ O + 10 eq. 4.7
slightly greater or less than 1, are obtained for other
18
2
examples of isotope separation and it is for this reason In general, samples with δ O and δ H lighter than
that the descriptive δ notation is adopted. −22‰ and −160‰, respectively, represent snow and