Page 139 - Hydrogeology Principles and Practice

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Environmental isotope hydrogeology

4.1 Introduction 4.2 Stable isotope chemistry and nomenclature

The stable and radioactive isotopes of the common Modern double inlet, double collector mass spec-
elements of oxygen, hydrogen, carbon, sulphur and trometers are capable of detecting small changes
nitrogen have a wide range of applications in hydro- in relative isotopic abundances with the results ex-
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geology. The stable isotopes of water ( O, O, H, pressed using the δ notation. In general, the δ nota-
2
H) can be used as tracers of the origin of groundwa- tion, normally expressed in parts per thousand (per
ter recharge and, together with noble (inert) gases mil or ‰) with respect to a known standard, is written
(Ne, Ar, Kr, Xe), used to provide information on as follows:
aquifer evolution. Sulphur and nitrogen isotopes
have applications in contaminant hydrogeology in R sample − R stan dard
=
×
δ 1000 eq. 4.1
the identification of pollution sources and their fate in R
stan dard
the groundwater environment. For further discus-
sion of sulphur and nitrogen isotopes refer to Bottrell where R and R are the isotopic ratios (for
sample standard
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2
et al. (2000), Heaton (1986) and Kendall (1998). The example O/ O and H/ H) of the sample and stand-
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radioactive isotopes of water, H (tritium), carbon ard, respectively. With this notation, an increasing
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( C) and chlorine ( Cl) are useful in providing esti- value of δ means an increasing proportion of the rare,
mates of aquifer residence times that can assist in heavy isotope. In this case, the sample is said to have a
managing groundwater resources. heavier, more positive or enriched isotope composi-
The following sections describe, with examples, tion compared with another, isotopically lighter sam-
the basis for the application of environmental iso- ple. For water, the accepted international standard
topes in groundwater investigations with emphasis is V (Vienna Standard Mean Ocean Water) with
SMOW
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given to groundwater source identification and age values of δ O and δ H equal to zero. Measurements
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2
dating using the stable and radioactive isotopes of δ O and δ H can usually be determined to an accur-
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of water, C and Cl. In discussing the origin of acy of better than ±0.2‰ and ±2‰, respectively.
groundwater recharge, a section is also included to The mechanisms of isotope separation that lead to
demonstrate the application of noble gas concen- enrichment and depletion of isotopic ratios between
tration data in reconstructing present and past phases or species can be divided into three types
groundwater recharge temperatures. The combined (Krauskopf & Bird 1995):
interpretation of environmental isotope and noble 1 mechanisms depending on physical properties, for
gas data can therefore enable reconstruction of example evaporation or precipitation;
palaeoenvironmental conditions and give insight into 2 exchange reactions resulting in isotopic equilib-
the history of aquifer evolution. rium between two or more substances;
3 separation depending on reaction rate.