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

Uranium (U) contamination in groundwater
becomes a critical
concern
when its concentration exceeds permissible limits. Groundwater samples
(n = 46) from Eastern Karnataka exhibit U concentrations
ranging from 1.9 to 2744 μg/L, with 78% and 66% of sites exceeding
the United States Environmental Protection Agency (USEPA) and Atomic
Energy Regulatory Board (AERB), India thresholds, respectively. These
high U levels are primarily geogenic. The δ238U and
U activity ratios (234U/238U) were determined
on the same set of samples to understand the redox processes governing
groundwater U cycling. The δ238U isoscape displays
a variation of 1.3‰ from −0.95‰ to 0.3‰,
providing evidence of a generally oxidizing groundwater environment,
with localized U-reducing conditions in the southern part of the study
area. The (234U/238U) of groundwater varies
from 1.2 to 5.4, indicating a dynamic interplay of α-recoil
effects, preferential leaching of 234U, and the dissolution
of high U zone(s) in granitic rocks. Statistical analysis of geochemical
and field parameters reveals three distinct redox environments: oxidized,
intermediate, and U(VI) reducing, across the study area. Integrating
U isotope compositions and additional redox indicators with statistical
analysis in this manner provides a robust approach to understand the
subsurface environment controlling U cycling in large, data-limited
regions.

地下水中铀（U）污染在其浓度超出容许限值时，便成为一项亟需重视的严峻问题。本次研究采集了印度卡纳塔克邦东部的地下水样品共46份（n=46），其铀浓度范围为1.9~2744 μg/L；其中分别有78%和66%的采样点超过了美国环境保护署（United States Environmental Protection Agency, USEPA）与印度原子能监管委员会（Atomic Energy Regulatory Board, AERB）的限值标准。此类高浓度铀污染主要为地质成因。研究人员对同一批次样品开展了δ238U与铀活度比(234U/238U)的测定，以解析控制地下水铀循环的氧化还原过程。δ238U同位素等值线图（isoscape）的数值区间为-0.95‰至0.3‰，跨度达1.3‰，这表明研究区整体地下水环境呈氧化状态，仅在研究区南部存在局域性的铀还原环境。地下水中(234U/238U)的值介于1.2至5.4之间，这反映出α反冲效应、234U优先淋滤以及花岗岩类岩石中高铀区域溶解等过程的动态相互作用。通过对地球化学与野外参数开展统计分析，研究区可划分为三种截然不同的氧化还原环境：氧化环境、过渡环境以及六价铀（U(VI)）还原环境。将铀同位素组成、其他氧化还原指标与统计分析相结合的研究方法，可为解析数据受限的大型区域内控制地下水铀循环的地下环境提供一套稳健的研究路径。