Continental Effect

Several workers noticed that the average isotopic composition of precipitation tends to have more negative values further away from the ocean coast. This continental effect is well reflected in European groundwaters (Fig. 9.14). The explanation lies in the history of the precipitating air masses. As they travel inland, rain is gradually precipitated by condensation, accompanied by more efficient condensation of water molecules with heavier isotopes (opposite to evaporation). The residual moisture in the air masses thus becomes gradually lighter in its isotopic composition, and lighter rain is progressively formed. The continental effect is often masked by other effects, for example, temperature (seasonal) effect and, as discussed in the next section, altitude effect.

Fig. 9.14 Lines of equal dD values for Europe, based on over 300 samples of groundwater. A trend of lighter isotopic composition is seen as a function of the distance from the ocean, reflecting the continental effect in the precipitation. (After Sonntag et al., 1979.)
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