Elevated Radium Activity in a Hydrocarbon-Contaminated Aquifer

Abstract

Hydrocarbon spills that reach the subsurface can modify aquifer geochemical conditions. Biogeochemical zones typically form proximal to the source zone that include iron (Fe(III)) and manganese (Mn(III/IV)) (hydr)oxide reduction, with potential to release associated geogenic contaminants to groundwater. Here, multi-level monitoring systems are used to investigate radium (²²⁶Ra, ²²⁸Ra) activities in an aquifer contaminated with a mixture of chlorinated solvents, ketones, and aromatics occurring as a dense non-aqueous phase liquid in the source zone. ²²⁶Ra activities are up to 10 times higher than background 60 m downgradient from the source zone, where pH is lower, total dissolved solid concentrations are higher, and conditions are methanogenic. Correlations indicate that Fe and Mn (hydr)oxide reduction and sorption site competition are likely responsible for elevated Ra activities within the dissolved phase plume. ²²⁶Ra activities return to background within the Fe(III)/SO₄^2--reducing zone 600 m downgradient from the source, near the middle of the dissolved phase plume. Geochemical models indicate that sorption to secondary phases (e.g., clays) is important in sequestering Ra within the plume. Although maximum Ra activities within the plume are well below the U.S. drinking water standard, elevated activities compared to background emphasize the importance of investigating Ra and other trace elements at hydrocarbon-impacted sites.

Keywords: DNAPL; groundwater; natural attenuation; radionuclides; redox; trace elements.