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136 Chapter Four

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Fig. 4.10 Sequential profiles of (a) tritium and (b) NO in the unsaturated zone of the Chalk aquifer beneath arable land in west Norfolk,
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Eastern England. After Parker et al. (1991).

of the Chalk, temporal variations in rainfall and zone and is isolated from the atmosphere below
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antecedent moisture content, and seasonal variations the water table, dissolved He concentrations from
in solute input (Parker et al. 1991). the decay of tritium will increase as the groundwater
becomes older. Although groundwaters contain helium
from several sources other than tritium decay, includ-
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4.4.5 H/ He dating ing atmospheric and radiogenic He and He, deter-
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mination of both tritium and tritiogenic He ( He*)
The limitations of using tritium alone to estimate can be used as a dating tool (Solomon et al. 1993). The
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groundwater residence times that arise from its short tritium-helium ( H/ He) age is defined as:
radioactive half-life and the decay of the bomb peak
can be addressed with the additional measurement of = 1 ⎛ 3 He* ⎞
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3 t 3 H/ He log e ⎜ 3 + 1 ⎟ eq. 4.17
He. As tritium in groundwater enters the unsaturated λ ⎝ H ⎠
T