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Environmental isotope hydrogeology 137

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Fig. 4.11 Hydrogeological section of the Linsental aquifer, north-east Switzerland, indicating the H/ He ages (in days) of samples from
boreholes and two pumping stations, Sennschür (PS1) and Obere Au (PS2), for various sampling dates. The ages of samples collected from
shallow and deep (point d6) sections of the aquifer in February and July 1996 are also shown. In February, older groundwater dominated in
the shallow aquifer during a time of decreased river water infiltration. In July, heavy rain and flood events were common and locally
infiltrated river water with a younger age dominated in the shallow aquifer. After Beyerle et al. (1999).

where λ is the tritium decay constant. Analytical Quaternary gravels reworked by fluvial erosion. The
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techniques for tritium and He determinations are lower aquifer boundary is formed by a low perme-
such that under ideal conditions, ages between 0 and ability molasse bedrock. Hydraulically, the Linsental
about 30 years can be determined with typical analyt- aquifer is predominantly influenced by groundwater
ical uncertainties of <10% (Solomon et al. 1993). abstraction from two pumping stations, Sennschür
Further confidence in deriving an apparent age is and Obere Au (PS1 and PS2, respectively in Fig. 4.11),
obtained when multiple techniques are applied to the and infiltration from the River Töss.
same sample, for example combining measurements Groundwater and river water samples were col-
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of H/ He and chlorofluorocarbons (CFCs) (Section lected between 1995 and 1997 and analysed for their
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5.8.3). Applications for H/ He dating are found in tritium and helium contents and the resulting
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the accurate dating of groundwater residence times 3 H/ He ages interpreted along a hydrogeological sec-
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of a few tens of years (Plummer et al. 2001), in identi- tion. As shown in Fig. 4.11, the H/ He groundwater
fying groundwater flowpaths (Beyerle et al. 1999) ages from the deep part of the aquifer are similar
and in tracing sources and rates of contaminant (about 600 days), whereas water ages of the shallow
movement (Shapiro et al. 1999). aquifer depend on the time of sampling. As river
As an example, Beyerle et al. (1999) applied infiltration is reduced at times of low river stage, only
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H/ He dating at a bank filtration site at Linsental in older groundwater remains and the H/ He ages
north-east Switzerland. The Linsental aquifer is in the close to the river become comparable to the age
lower reach of the River Töss valley and has a max- of groundwater in the deeper aquifer. The ages
imum thickness of 25 m and an average width of of groundwater abstracted from the two pumping
200 m. The aquifer is composed of heterogeneous stations are about 1 year old, much higher than