Abstract
Groundwater samples were collected at a site in N Norfolk, UK, comprising a borehole penetrating Upper Chalk and piezometers open within the overlyinf glacial deposits and intervening, weathered Putty Chalk. The samples were analyzed for δ 18O (water), δ 34S and δ 18O (SO 4) and δ 13 C (HCO 3) as well as major ions. Variations in solute concentration and isotopic composition with depth were found to be caused by limited groundwater circulation at the site and incomplete flushing of old groundwater. The isotopic data prove the existence of a mineralized palaeowater at depth, which has undergone SO 4 reduction and is being slowly modified by mixing with recent groundwater circulating at a shallow depth in a more transmissive Chalk horizon. One measured δ 18O (H 2O) value of −8.25‰ vsmow represents the lightest value reported for the Norfolk Chalk aquifer and is evidence for a component of Late Pleistocene recharge trapped at depth. The wide range of observed isotope ratios at the site, e.g. δ 34S and δ 18O SO 4 between 7.8 to 27.8‰ cdt and 5.2 to 20.0‰ ovsmow respectively, demonstrates the importance of vertical stratification of solutes and the implications this has for interpreting spatial hydrochemical surveys of groundwater.
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