234U/ 238U activity ratios of dissolved uranium in groundwaters from a jurassic limestone aquifer in England

Abstract

Uranium contents and 234U/ 238U activity ratios have been determined for groundwaters from the Lincolnshire Limestone artesian aquifer in eastern England. Changes in the quantitative and isotopic chemistry of the dissolved uranium are explained in terms of a mixing model involving the rapidly moving fissure water and much older water stored in the pore system of this oolitic limestone. The western part of the aquifer, closest to recharge, is dominated by oxidising groundwaters which then enter a reducing zone towards the east, where there is an abrupt decrease in Eh and the chlorinity of the groundwaters begins to increase. Uranium contents in the oxidising zone range from 0.7 to 3.4 μg kg −1 and 234U/ 238U activity ratio of this dissolved uranium is close to unity, the equilibrium value. The uranium content decreases abruptly when the grounwaaters enter the reducing zone, averaging 0.04 μg kg −1 east of the oxidation/reduction barrier. Simultaneously with the decrease in uranium content, there is an increase in 234U/ 238U activity ratio and this ratio increases to a maximum within 7 km of the oxidation/reduction barrier. This increase in activity ratio is attributed to enhanced 234U solution due to 234Th recoil from uraniferous fissure surfaces east of the oxidising zone. The activity ratio of dissolved uranium in the ancient pore waters could in principle reach high values due to 234Th recoil from the oolith surfaces. However, the activity ratio actually declines further east and this can only be explained as a consequence of mixing with pore waters in which the uranium activity ratio is closer to equilibrium. 234Th recoil from the oolith surfaces has probably been inhibited by sealing of the uranium-bearing surfaces in the process of oolith cementation.