Appraising the factors favouring uranium mobilization and associated health risk assessment in groundwaters of north-western India

Abstract

An attempt has been made in this study to evaluate the factors favoring the uranium mobilization into the groundwater of Northwest India using uranium isotope activity ratio (²³⁴U/²³⁸U), radon (²²²Rn) and environmental isotopes of water (²H, ¹⁸O and ³H). The values range from 23 - 597 µg/L for total uranium and 634-3210 Bq/m³ for radon and the corresponding annual effective dose is estimated to be 18.9-490 µSv/a and 6.2-31.5 μSv/a respectively. Uranium activity ratio (UAR) varies from 0.68 - 1.17 and maximum samples indicate secular equilibrium. Environmental isotopic data indicates that the source to groundwater is vertical percolation of rainwater in the case of shallow zone while regional flows from outcrop areas recharge the deep groundwater. A wide scatter is noticed in environmental ³H content (0.23-6.62 TU) indicating both fast and sluggish water flows. The UAR phase diagram suggests that leaching process controls the uranium mobilization into the groundwater. The correlations among UAR, uranium and U_excess further indicate oxidative nature of leaching process. Statistical treatment of the obtained data along with available geochemical and isotope evidences suggest that source of uranium is common but the driving processes are different for shallow and deep zone. Influences of root zone CO₂, oxic species from irrigation return flows and water level fluctuations are also evaluated. Low uranium, low UAR, low ³H and high ²²²Rn activity in deep zone suggest uranium being released from the roll front as well as transported from outcrop regions. This study highlights the application of uranium isotope ratio, radon and environmental isotopes in assessing vulnerability of alluvial aquifers towards uranium contamination.

Keywords: Activity ratio; Environmental isotopes; Groundwater contamination; Leaching; Punjab; Uranium isotopes.