Geoscience Reference
In-Depth Information
The colder the temperature is at time of con-
densation the more enriched with deuterium and
18
O the water sample will be. In climates with
distinct seasons the amount of deuterium and
18
O in rainfall samples will vary according to
seasons. By taking a series of water samples from
rainfall and groundwater a comparison can be
made between the time series. If the groundwater
shows considerable variation in deuterium and/or
18
O concentrations then it is relatively recent
rainfall. If there is very little variation then it is
assumed that the groundwater is a mixture of
rainfall from both summers and winters in the
past and is therefore older. This is demonstrated
in Figure 4.8. The technique of looking at oxygen
and hydrogen isotopes is particularly common as
a way of determining whether water in a stream is
new (i.e. recently derived from rainfall) or old (has
been resident in groundwater for sometime).
By measuring the concentrations of contami-
nants like tritium or the ratio of isotopes of oxygen
an estimate of the age of groundwater can be
made. The different way that water moves through
the unsaturated and saturated zones of catchments
means that groundwater and streamflows contain
water with different residence times. The water
in a sample does not have a discrete age, but has a
distribution of ages. This distribution is described
by a conceptual flow or mixing model, which
reflects the average conditions in the catchment
(Stewart
et al
., 2007). Maloszewski and Zuber
(1982) provide an extensive review of these
mixing models; in short they account for the size
Rainfall
Young GW
Older GW
Time
Figure 4.8
Changing ratios of isotopes of oxygen and
hydrogen with time in a seasonal climate. Rainfall is
heavily influenced by temperature and shows large
variation between seasons. The older the groundwater
the more dampened down the time series.
of groundwater reservoir, the concentration of con-
taminant and compute the likely time of residence
within a well mixed groundwater reservoir.
Using the techniques outlined here, the average
age of groundwater or streamwater can be derived.
Studies of groundwater age frequently use a
combination of the different techniques to derive
an average residence time of water in a catchment,
or the groundwater age. It is important to realise
that it is an average residence time, not absolute.
The water contained in the groundwater reservoir
will be a mixture of water that has infiltrated
rapidly and some that moved very slowly through
the unsaturated zone.
unconfined aquifer near to the surface. If this is not
the case then the stream may be contributing water
to the ground through infiltration. Figure 4.9 shows
two different circumstances of interaction between
the groundwater and stream. In Figure 4.9(a) the
groundwater is contributing water to the streamflow
as the water table is high. In Figure 4.9(b) the water
table is low and the stream is contributing water to
the groundwater. This is commonly the case where
the main river source may be mountains a
considerable distance away and the river flows over
an alluvial plain with the regional groundwater
table considerably deeper than stream level. The
interaction between groundwater and streamflow is
discussed further in Chapter 5, especially with
respect to stormflows.
