Assessing nonpoint-source uranium pollution in an irrigated stream-aquifer system

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Uranium (U) mobilization in arid/semi-arid environments by irrigation and fertilization presents significant environmental and health risks.
• Elevated U, often co-occurring with selenium (Se) and nitrate (NO3), requires diligent monitoring and management strategies.
• Understanding U transport in stream-aquifer systems is crucial for assessing pollution dynamics.

Purpose of the Study:

• To develop and apply a numerical model for assessing uranium pollution in an irrigated stream-aquifer system.
• To quantify uranium levels and identify contributing factors in Colorado's Lower Arkansas River Valley (LARV).
• To establish a baseline for evaluating future best management practices (BMPs) for uranium mitigation.

Main Methods:

• Developed a distributed-parameter numerical model coupling MODFLOW for groundwater/stream flow and RT3D-OTIS for reactive uranium transport.
• Applied the model to a 552 km2 region in Colorado's LARV over a 14-year period.
• Calibrated the model using PESTPP-iES iterative ensemble smoother (iES) software against observed uranium concentrations.

Main Results:

• The model revealed substantial and variable uranium levels across the LARV, identifying potential hotspots.
• Uranium concentrations exceeded the US EPA chronic standard (30 μg/L) in groundwater across 44% of the region and along the river by an average factor of 2.9.
• Simulated average uranium concentrations (aquifer: 124 μg/L, river: 60 μg/L) closely matched measured values (aquifer: 112 μg/L, river: 62 μg/L).

Conclusions:

• Geological composition, irrigation practices, and riparian landscapes are key factors influencing uranium distribution.
• The developed model provides a valuable tool for assessing uranium pollution in irrigated regions.
• Findings offer a baseline for evaluating BMPs and a transferable methodology for other affected areas.