32Si in limestone aquifers

This paper reports an investigation into the potential application of 32Si as a dating tool for groundwater in limestone aquifers. Refinements to previous techniques are described, which produced a detection limit of 0.1 mBq m−3, compared to a level of ∼5 mBq m−3 in rainwater. Water from wells tapping Estonian limestone aquifers showed significant and decreasing 32Si concentrations down to 50 m. At greater depths, concentrations were below detection limits. This indicates input of cosmogenic 32Si into the aquifer with initial concentration estimated to be 2.5±0.5 mBq m−3, but also shows that 32Si is sorbed during passage through the soil, as well as in the limestone aquifer. Results from karst springs in the Thuringian Basin and Poland, and an aquifer in Saxony-Anhalt, were similar. Comparison with tritium results indicates that non-radioactive (geochemical) processes reduce the measured 32Si concentrations in many cases by more than one order of magnitude below expectation. Consequently, despite a favourable half-life (140 yr) between tritium and 14C, 32Si cannot be applied successfully to date karst waters in the age range between these two isotopes.

The depth profile and 32Si vs. 14C relationship for the Estonian aquifer show some regularities which may, after further investigation of geochemical factors, allow the application of 32Si as an additional age indicator for young groundwater, in cases where tritium data do not provide sufficient information or accuracy.