Uranium in groundwater - A synopsis based on a large hydrogeochemical data set.

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Existing knowledge on Uranium (U) in groundwater relies heavily on field experiments, modeling, and isolated process studies.
• Uranium's complex redox chemistry interacts with multiple element cycles (sulfur, carbon, iron, manganese), complicating prediction of its natural occurrence.
• Predicting U concentrations in groundwater is challenging due to simultaneous interactions with various geochemical cycles.

Purpose of the Study:

• To predict the occurrence of Uranium in groundwater using basic hydrochemical data.
• To identify key hydrochemical parameters and redox conditions influencing Uranium mobilization and accumulation in aquifers.
• To assess the impact of agricultural practices on Uranium occurrence in groundwater.

Main Methods:

• Analysis of a large dataset comprising over 8000 chemical groundwater analyses from more than 2000 sampling locations.
• Statistical analysis to identify strong correlations between Uranium concentrations and hydrochemical parameters like electric conductivity and alkalinity.
• Evaluation of Uranium concentrations across different redox conditions (oxic, nitrate-reducing, iron-reducing, sulfate-reducing).

Main Results:

• A strong positive correlation was found between Uranium concentrations and electric conductivity and alkalinity, suggesting geogenic source weathering and desorption.
• Uranium concentrations were moderate under oxic conditions, highest under manganese and nitrate-reducing conditions, and virtually zero in iron- and sulfate-reducing environments.
• Agricultural practices like liming, fertilizer use, and irrigation were found to influence Uranium occurrence in groundwater.

Conclusions:

• Mineral weathering, particularly of carbonates, combined with Uranium mobilization under manganese and nitrate-reducing conditions, presents the highest risk for elevated Uranium levels.
• Low pH and low groundwater mineralization, typical of granitic catchments, are associated with a low risk of high Uranium concentrations.
• Effective aquifer management, especially in agricultural areas, is crucial for controlling Uranium levels in groundwater used for drinking water production.