Seasonal Recharge and Replenishment of Sulfate Associated with Biodegradation of a Hydrocarbon Plume

Abstract

AbstractThe downward flux of sulfate in a weathered clayey aquitard to a hydrocarbon plume in ground water was investigated at a gas well site in Alberta, Canada. The dissolved hydrocarbon plume includes monoaromatic BTEX compounds and extends less than 20 m downgradient from a natural gas condensate source in silt‐ and clay‐rich deposits. The vadose zone at the site has high concentrations of water soluble sulfate, which is apparently derived from pyrite oxidation. Seasonal infiltration of snowmelt water, most notably during the “wet” spring of 2003 and also in 2004, was focused in an excavation and recharged the water table. This recharge replenished the contaminated zone with sulfate that was flushed downward from the vadose zone. Recharge in 2003 resulted in episodic increases in sulfate concentrations in the ground water condensate plume of up to 1000 mg/L. After episodic increases in sulfate concentrations within the core of the contaminant plume associated with recharge, concentrations then decreased to less than 1 mg/L, accompanied by isotopic enrichment of the residual sulfate in 34S. The monitoring data provide strong field evidence that bacterial sulfate reduction is a dominant terminal electron‐accepting process in the natural attenuation of the hydrocarbon plume in the ground water. This result suggests that engineered infiltration and replenishment of sulfate to petroleum hydrocarbon plumes in ground water might enhance biodegradation of these plumes.