Page 145 - Hydrogeology Principles and Practice

HYDC04 12/5/05 5:36 PM Page 128

128 Chapter Four

Fig. 4.5 (a) Hydrogeological cross-section of bedrock aquifers of
Estonia showing the Voronka and Gdov aquifers of the Cambrian-
18
Vendian system. (b) Distribution of δ O values of groundwater
in Estonian aquifers grouped according to age as indicated by
14 C concentrations. Samples PK705 and PK897 are affected by
mixing with infiltrated modern water. The groundwater in the
Cambrian-Vendian system has a strongly depleted stable isotope
composition and low radiocarbon content and represents
periglacial or possibly subglacial recharge of meltwater through
tunnel valleys from the Fennoscandian ice sheet during the late
Weichselian (Devensian) ice age. After Vaikmäe et al. (2001).

quoted in years Before Present (pre-1950) using the inorganic carbon encountered along the ground-
notation a (for example, 1 ka indicates 1000 years water flowpath. The two principal sources are:
Before Present). 1 ‘biogenic’ carbon with a source in the CO of the
2
Allowance must be made in the interpretation of atmosphere and also respired by the decay of organic
the apparent age provided by measurement of the carbon in the soil zone, which is usually assumed to
14
C activity of a sample for reaction with sources of contain 100 pmc;