Silicon-32 hydrology

Fairly extensive studies of cosmogenic 32Si activity in wet precipitations, surface and sub-surface waters and laboratory experiments with rain-water and rock/soil systems have established its usefulness for (1) dating groundwaters younger than 2000 years and (2) checking the validity of 14C ages between 2000 and 5000 years. It is shown that if a concordant age is found based on both 32Si and 14C, the deduced apparent age (irrespective of the model used) is in fact close to the 'true age' of the groundwater. However, if this is not so, it would be expected that a given water body will look younger on the basis of a shorter-lived radioisotope and that the 'true age' would be even greater than that obtained on the basis of the longer-lived isotope. Such a situation would result from mixing of old and young waters in the aquifer. Thus, the fact that the validity of the 32Si dating method has been demonstrated, an intercomparison of 32Si and 14C ages would be extremely valuable in the delineation of groundwater characteristics. (author)