Role of uranium series disequilibrium studies of fluids and solids in geological systems with potential for radioactive waste disposal (H-C)

Uranium series radionuclides provide one of the most sensitive natural isotopic signatures in groundwater studies and as such have been used successfully to identify water masses and to evaluate quantitatively the mixing of waters on a regional scale. An example of such an application is a recently completed study of a multilithological system in the Harwell region. The principal aim of the program was to establish the groundwater flow patterns using hydrogeological and geochemical methods in association with isotope contents and uranium series disequilibrium and thus provide an independent approach to the study of effective permeabilities of clay lithologies in a sedimentary sequence. The uranium isotopic signatures in groundwater samples collected from the region have been interpreted in terms of the regional groundwater circulation and mixing, and the most significant zones of groundwater movement through clay lithologies have been identified. A complementary approach is to measure isotopic signatures in various phases of the solid matrix. This latter approach is illustrated by a recent study of the distribution of the longer-lived radionuclides 238U, 234U, 230Th, 232Th and 238Th between the pore water, iron oxyhydroxides and clay mineral phases in a thick surface deposit at Elstow, England