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Elevated Radium Activity in a Hydrocarbon-Contaminated Aquifer.
Amy K Wiersma1, Glen Hook2, Madeleine Mathews3
1Environmental Chemistry and Technology Program, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.
Environmental Science & Technology
|June 7, 2023
Summary
Hydrocarbon spills can increase radium (Ra) in groundwater by altering aquifer geochemistry. Iron and manganese reduction processes are linked to elevated Ra activities, highlighting the need for trace element monitoring at contaminated sites.
Area of Science:
- Environmental Science
- Geochemistry
- Hydrogeology
Background:
- Subsurface hydrocarbon spills alter aquifer geochemistry, creating biogeochemical zones.
- These zones, including iron (Fe(III)) and manganese (Mn(III/IV)) (hydr)oxide reduction, can mobilize geogenic contaminants.
- Radium (Ra) isotopes (226Ra, 228Ra) are naturally occurring radionuclides of concern in groundwater.
Purpose of the Study:
- Investigate radium activities in an aquifer impacted by hydrocarbon spills.
- Determine the relationship between hydrocarbon contamination, biogeochemical conditions, and radium mobilization.
- Assess the fate and transport of radium within the contaminated aquifer.
Main Methods:
- Utilized multi-level monitoring systems to measure radium activities in groundwater.
- Analyzed aquifer geochemical parameters including pH, total dissolved solids, and redox conditions.
- Employed geochemical modeling to understand radium sequestration mechanisms.
Main Results:
- Radium-226 (226Ra) activities were up to 10 times higher than background levels 60 m downgradient from the source zone.
- Elevated Ra activities correlated with lower pH, higher total dissolved solids, and methanogenic conditions.
- Fe and Mn (hydr)oxide reduction and sorption site competition were identified as likely drivers for increased Ra.
- Ra activities returned to background levels 600 m downgradient in the Fe(III)/SO42--reducing zone.
- Geochemical models indicated sorption to secondary mineral phases (e.g., clays) is crucial for Ra sequestration.
Conclusions:
- Hydrocarbon contamination significantly impacts radium behavior in aquifers.
- Biogeochemical processes, particularly Fe and Mn reduction, influence radium mobilization and attenuation.
- While Ra activities were below drinking water standards, elevated levels underscore the importance of monitoring trace elements at hydrocarbon-impacted sites.
- Sorption to secondary phases plays a key role in retaining radium within the aquifer.
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