Application of uranium-series disequilibrium to studies of groundwater mixing in the Harwell region, U.K.

Abstract

Regional groundwater systems in a mixed sedimentary sequence beneath the Harwell site have been studied as part of a national research programme of investigation into the feasibility of disposal of low- and intermediate-level radio-active wastes into argillaceous rocks. The principal aim of the programme is to establish the groundwater flow patterns using hydrogeological and geochemical methods. Basic groundwater chemical patterns in association with variations in isotope content and uranium-series disequilibrium can provide data on the permeabilities of clay lithologies in a sedimentary sequence, which can be independent of hydraulic models, and thus lend support to them. Groundwater samples derived from the high-permeability, formations in the Harwell region have been analysed for U and Th content and 234U/ 238U 230Th/ 234U and 230Th/ 232Th activity ratios. The U isotopic signatures have been interpreted in terms of the regional groundwater circulation and mixing patterns. The most significant zones of groundwater mixing determined from hydraulic and hydrochemical data, and supported by the U isotopic data presented here are situated just beneath the edge of the confined strata. Such zones coincide with the locations of hydraulic lows particularly in the Corallian formation towards which the regional groundwaters move. The work reported here concludes that U isotopic signatures can be used to identify water masses and to evaluate mixing of groundwaters in a sedimentary sequence on a regional scale, thus effectively complementing hydrogeological, hydrochemical and isotopic methods.