Activity ratios of 234U/238U and 226Ra/228Ra for transport mechanisms of elevated uranium in alluvial aquifers of groundwater in south-western (SW) Punjab, India

Abstract

The concentrations of total dissolved uranium (U), its isotopic composition (234U, 235U, 238U) and two long lived Ra isotopes (226Ra and 228Ra) in alluvial aquifers of groundwater were determined to investigate the groundwater flow pattern in the south-western (SW) Punjab, India. Particular attention was given to the spatial variability of activity ratios (ARs) of 234U/238U and 226Ra/228Ra to predict the possible sources and supply process of U into the water from the solid phase. The measured groundwater 234U/238U ARs were ∼1 or >1 in the shallow zone (depth < 30 m) with high U concentration and <1 in the deeper zone (depth > 30 m) with relatively low U concentration. The simultaneous elevated U concentration and 234U/238U ARs in waters were possibly due to differences in imprints of rock–water interactions under hydrologic conditions. However, 234U/238U ARs < 1 clearly indicate the lack of recharge from surface water to groundwater leading to 234U deficit in groundwater. This deficit might be also attributed to alpha recoil processes under strong dissolution. Overall, the decreasing pattern of 234U/238U ARs observed from SE to SW or NW ward clearly indicates a groundwater flow paths from SE to SW/NW. Similarly, 226Ra/238U ARs < 1 for all water samples reflect that the precursor 238U is fairly mobile relative to 226Ra. This might be due to unusually high amount of 238U in groundwaters and subsequently the different geochemistry of the two isotopes. On the other hand, 226Ra/228Ra ARs in groundwaters varied widely and observed about 50–300 times higher than 238U/232Th ARs in granitic rocks or soils. Such elevation in ARs might be attributed to different dissolution properties of their parents during water–rock interactions or lattice damage during decay or local enrichments of uranium in the aquifers.