Elevated Radium Activity in a Hydrocarbon-Contaminated Aquifer

Hydrocarbonspills that reach the subsurface can modifyaquifergeochemical conditions. Biogeochemical zones typically form proximalto the source zone that include iron (Fe(III)) and manganese (Mn(III/IV))(hydr)oxide reduction, with potential to release associated geogeniccontaminants to groundwater. Here, multi-level monitoring systemsare used to investigate radium (Ra-226, Ra-228)activities in an aquifer contaminated with a mixture of chlorinatedsolvents, ketones, and aromatics occurring as a dense non-aqueousphase liquid in the source zone. Ra-226 activities are upto 10 times higher than background 60 m downgradient from the sourcezone, where pH is lower, total dissolved solid concentrations arehigher, and conditions are methanogenic. Correlations indicate thatFe and Mn (hydr)oxide reduction and sorption site competition arelikely responsible for elevated Ra activities within the dissolvedphase plume. Ra-226 activities return to background withinthe Fe(III)/SO4 (2-)-reducing zone 600 mdowngradient from the source, near the middle of the dissolved phaseplume. Geochemical models indicate that sorption to secondary phases(e.g., clays) is important in sequestering Ra within the plume. Althoughmaximum Ra activities within the plume are well below the U.S. drinkingwater standard, elevated activities compared to background emphasizethe importance of investigating Ra and other trace elements at hydrocarbon-impactedsites. This study reports geochemicalconditions resulting in elevatednaturally occurring radium in groundwater near the source zone ofan organic chemical mixture, with implications for human and ecosystemhealth if extensively mobilized downgradient to aquatic systems anddrinking water sources.

Automated summary

Hydrocarbon spills in groundwater can lead to changes in geochemical conditions, potentially releasing geogenic contaminants. This study found that radium activities were up to 10 times higher in an aquifer contaminated with a mixture of chlorinated solvents, ketones, and aromatics. The elevated radium activities were likely due to iron and manganese (hydr)oxide reduction and sorption site competition within the dissolved phase plume. However, within the Fe(III)/SO4 (2-) reducing zone downgradient from the source, radium activities returned to background levels.