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138 Chapter Four

initially considered. An estimated mixing ratio of 50% excess air component are possible, normalized to
between the younger infiltrated river water and older dissolved Ne contents (Elliot et al. 1999). Some of this
deep groundwater explains the age of the water excess air may be lost by gas exchange across the
abstracted from the production boreholes. Hence, water table. The rates of gas exchange of the indi-
3
3
the H/ He ages have provided useful information vidual noble gases decrease with molecular weight
on the recharge dynamics and residence times of the such that fractionation of the residual excess air
bank filtration system (Beyerle et al. 1999). occurs with significant gas loss. Typically, noble gas
recharge temperatures are determined by an iterative
procedure. Initially, unfractionated air is subtracted
4.5 Noble gases successively from the measured concentrations,
and the remaining noble gas concentrations are
The temperatures at which recharge water is equilib- converted into temperatures on the basis of solu-
rated with air in the soil zone can be determined bility data and the atmospheric pressure at the eleva-
from the noble (inert) gas contents of groundwater. tion of the water table. This procedure is repeated
The concentrations of Ar, Kr, Ne and Xe dissolved until optimum agreement among the four calcu-
during equilibration of recharge water with air are lated noble gas temperatures is achieved (Stute et al.
controlled by their solubility relationship with tem- 1995).
14
perature, generally decreasing in concentration with When combined with stable isotope data and C
increasing temperature. In temperate latitudes, the ages, noble gas recharge temperatures can further
potential effects of solar insolation at the ground sur- elucidate the history of aquifer evolution and pro-
face are minor and the recharge temperature gener- vide a proxy indicator of palaeoenvironmental condi-
ally reflects the mean annual temperature of the soil tions. Noble gas data are presented by Dennis et al.
zone. Any increase in groundwater temperature as (1997) and Elliot et al. (1999) for the Chalk aquifer
the water moves from the soil zone to greater depths of the London Basin and provide evidence for Late
in the aquifer will not result in exsolution of noble Pleistocene (cold stage interstadial) groundwaters
gases, since the increase in hydrostatic pressure main- recharged at temperatures 5–7°C cooler than pre-
tains the groundwater undersaturated with respect to sent in confined zones north of the River Thames
the gases. It is therefore valid to derive recharge tem- (Table 4.3). Recharge conditions at this time were
peratures from noble gas contents (Andrews & Lee probably controlled by the occurrence of areas of
1979). The quantity of a dissolved noble gas, for unfrozen ground within the summer permafrost
example Ar, is given by: (talik zones), and so the noble gas recharge temper-
ature may be more representative of the mean sum-
−3
3
[Ar] = s P (cm STP cm H O) eq. 4.18 mer air temperature. Mean annual air temperatures
T Ar 2
spanning the frozen winter period were probably
where s is the solubility of Ar at 1 atmosphere pres- lower than the noble gas recharge temperatures.
T
sure and the temperature of recharge, T; and P is the As shown in Fig. 4.12, similar mean recharge tem-
Ar
partial pressure of Ar in the atmosphere. Similar rela- peratures during the last glacial period of about 5°C
tionships apply to the other noble gases. cooler than modern Holocene waters are reported
Noble gas concentrations of groundwater com- for the East Midlands Triassic sandstone aquifer in
monly exceed those calculated for thermodynamic the United Kingdom (Andrews & Lee 1979) and
solubility equilibrium with air in the unsaturated Devonian sandstones in the semi-arid Piaui Province
zone (eq. 4.18). This additional component, termed in north-east Brazil (Stute et al. 1995). Hence, terres-
‘excess air’, is most likely the result of fluctuations trial records of palaeogroundwaters provide convinc-
in the water-table trapping and partially or entirely ing evidence for surface temperature cooling at both
dissolving small bubbles under increased hydro- high and low latitudes during the last (Devensian or
static pressure or surface tension. Corrections for the Weichselian) ice age.