Uranium series disequilibrium measurements at Mol, Belgium

The contract just completed has funded two parallel uranium series disequilibrium studies and the aims of and the progress to completion of these studies are given in this report. The larger study was concerned with the measurement of uranium series disequilibrium in ground waters derived from sand layers above and below the Boom Clay formation in North East Belgium. The disequilibrium data are analysed in terms of uranium, thorium and radium isotopic geochemistries and in terms of water types and their mixing in the regional groundwater system. It is concluded that most sampled waters are mixtures of younger and older waters. No true old water end-members have been sampled. Simple considerations of the uranium isotopic data indicate that the longest residence times of the sampled waters are not much in excess of 1 to 10 x 103y. Detailed mixing patterns could not be established from this limited data set particularly in the absence of more detailed modelling in conjunction with groundwater hydraulic pressure and flow direction data. (author)