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124 Chapter Four

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Fig. 4.1 The δ H versus δ O relationship
for western and central European
palaeowaters (groundwaters having a
radiocarbon age of between approximately
15 and 30 ka). Old European groundwaters
2
are depleted in deuterium ( H) by about
12‰ compared with modern recharge
waters (as shown in the frequency
histogram of deuterium excess values, d,
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where d = δ H − 8δ O). The continental
gradient in deuterium is very similar to
modern meteoric water as indicated by the
essentially identical comparison with the
World Meteoric Water Line (WMWL)
and strongly suggests a constant
atmospheric circulation regime over
Europe during the past 35 ka years. After
Rozanski (1985).

ice from high latitudes while tropical samples show Since both condensation and isotope separation
very small depletions relative to ocean water. This are temperature dependent, the isotope composition
distribution is expected for an atmospheric Rayleigh of meteoric water displays a strong seasonal variation
process as vapour is removed from tropospheric at a given location. In interpreting groundwater iso-
air that is moving poleward. Linear correlations with topic compositions, individual recharge events are
coefficients only slightly different to equation 4.7 mixed in the region of water-table fluctuation such
are obtained from studies of local precipitation. For that isotopic variations over short timescales become
example, in the British Isles, a 20-year monthly obscured. Thus, it is possible to use a weighted mean
dataset from 1982 to 2001 for Wallingford, a station in isotopic composition to represent the isotopic signa-
central, southern England, gave the following regres- ture of the seasonal recharge. In Fig. 4.3, a 1-year
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sion: δ H = 7.00δ O + 0.98, with a slope a little less record of monthly precipitation amount and com-
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than the WMWL but consistent with those from other posite δ O values is shown for a rain gauge situated
long-term stations in north-west Europe (Darling & in north Norfolk, eastern England. The data show
Talbot 2003). the general trend of isotopically enriched precipita-
In addition to latitudinal and, similarly, altitudinal tion in the warmer summer months and isotopically
effects of temperature, the location of a site in rela- depleted precipitation in the colder winter months.
tion to the proximity of the evaporating water mass is The winter rainfall provides a representative volume-
also important. As water vapour moves inland across weighted mean isotopic composition for ground-
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continental areas, and as the process of condensation water recharge of −7.20‰ and −47.6‰ for δ O and
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and evaporation is repeated many times, rain or snow δ H, respectively.
becomes increasingly isotopically depleted. This con- Recognition of the effects of season, latitude, alti-
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tinental ‘rain-out’ effect of the heavy isotopes has tude and continentality is the basis for using δ O and
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been shown for Europe by Rozanski (1985), as illus- δ H isotope values as non-reactive, naturally occur-
trated in Fig. 4.1, and is also apparent in the stable iso- ring tracers to identify the climatic and palaeo-
tope composition of recent groundwaters measured geographic conditions of groundwater recharge.
for the British Isles (Fig. 4.2). For example, modern recharge waters in the Chalk