A study of uranium and thorium series isotopes in Chalk groundwaters of Lincolnshire, U.K.

Abstract

A groundwater investigation is described in which uranium and thorium series radionuclides are applied to the study of the hydrogeologically well-documented Lincolnshire Chalk aquifer system of North Lincolnshire, England. The dominance of Eh in controlling the uranium content of groundwaters is discussed, and the 230 Th 234 U activity ratio used as an indicator of the speciation of uranium. In highly reduced waters the 230 Th 234 U activity ratio approaches unity, suggesting that the uranium in solution is behaving similarly to thorium in the +4 valence state. In the oxidised environment the higher uranium content groundwaters exhibit lower 234 U 238 U activity ratios. A model of mixing young, low 234 U 238 U activity ratio fissure waters with old, high 234 U 238 U activity ratio pore waters is discussed, but rejected. It is considered that preferential dissolution of 234U in the lower rate weathering regimes produces the observed relationship between uranium content and activity ratio. A method of in-situ ^Rn (random release factor) determination is suggested; however, the calculated values for ^Rn are disproportionately high for a fissure flow system. This, along with evidence of a radium depleted aquifer matrix indicates that radium is being mobilised and transported from the pore water system to the fissure system, and subsequently adsorbed by clay minerals lining the Chalk fissures.