Isotopic Insights into Redox Processes Driving Uranium Distribution in Eastern Karnataka Groundwater

Area of Science:

• Environmental Science
• Geochemistry
• Isotope Hydrology

Background:

• Uranium contamination in groundwater poses significant health risks when exceeding permissible limits.
• Groundwater samples from Eastern Karnataka revealed high uranium concentrations, with a majority exceeding regulatory thresholds set by the USEPA and AERB, India.
• The elevated uranium levels are predominantly geogenic, indicating natural geological sources.

Purpose of the Study:

• To investigate the redox processes governing uranium cycling in groundwater using stable isotopes.
• To determine the spatial distribution of uranium and its isotopic signatures in Eastern Karnataka groundwater.
• To identify distinct redox environments controlling uranium behavior in the subsurface.

Main Methods:

• Analysis of groundwater samples (n=46) for uranium concentration and isotopic ratios (δ238U and 234U/238U).
• Determination of geochemical and field parameters to assess redox conditions.
• Application of statistical analysis to integrate isotopic and geochemical data for redox environment characterization.

Main Results:

• Uranium concentrations ranged from 1.9 to 2744 μg/L, with 78% and 66% of sites exceeding USEPA and AERB limits, respectively.
• δ238U values varied from -0.95‰ to 0.3‰, indicating a generally oxidizing environment with localized reducing conditions.
• Groundwater 234U/238U activity ratios ranged from 1.2 to 5.4, suggesting α-recoil effects, preferential leaching, and dissolution of uranium-rich zones.

Conclusions:

• The study identified three distinct redox environments (oxidized, intermediate, U(VI) reducing) controlling uranium cycling in the region.
• Uranium isotope compositions, combined with redox indicators and statistical analysis, provide a robust method for understanding subsurface uranium behavior.
• This integrated approach is valuable for assessing uranium contamination in large, data-limited regions.