124-5 Natural Attenuation Processes in Different Types of Aquifers Contaminated by Petroleum Hydrocarbons

Session: Fixing the Silent Leak: Identifying, Quantifying, Prioritizing, and Mitigating the Environmental and Health Impacts of Legacy Oil and Gas Drilling in North America

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Abstract:

While the fate and transport of hydrocarbons in intergranular aquifers have been extensively studied, these processes remain understudied in karst and fractured aquifer systems. This abstract presents a cross-site comparison of natural attenuation processes across three distinct hydrogeologic settings contaminated by petroleum hydrocarbons: an intergranular aquifer (“Vitanovac Site”, Serbia), a karst aquifer (“Lyda Site”, Bowling Green, KY), and a fractured basalt aquifer beneath the Red Hill Bulk Fuel Storage Facility ("Red Hill Site," Honolulu, HI). The natural attenuation processes observed at the Vitanovac Site are consistent with the established understanding of hydrocarbon fate and transport in intergranular aquifers. The dissolved hydrocarbon plume has persisted for more than three decades, gradually shrinking over time due to ongoing biodegradation. This is evidenced by elevated concentrations of metabolic byproducts (e.g., Mn²⁺ and Fe²⁺) and the depletion of key electron acceptors, including dissolved oxygen (O₂), nitrate (NO₃⁻), and sulfate (SO₄²⁻). In contrast, results from the Lyda Site suggest that advective transport processes primarily govern hydrocarbon contamination in karst aquifers. This is demonstrated by the rapid flushing of the contaminated well, where total petroleum hydrocarbon (TPH) concentrations in groundwater dropped from an initial 20.4 mg/L to below 1.4 mg/L following a precipitation-induced aquifer recharge event. Before the rainfall, the groundwater exhibited low concentrations of electron acceptors (O₂ and NO₃⁻) and elevated levels of biodegradation byproducts (Mn²⁺ and Fe²⁺). Although biodegradation processes were evident, their intensity did not reach the levels typically observed in intergranular aquifers. The results of initial analysis from the Red Hill Site confirm the occurrence of biodegradation processes in this fractured aquifer. At the Red Hill site, TPH concentrations remained concentrated in a limited number of impacted wells across the site. The structural complexity of this aquifer contributes to the persistent and spatially isolated TPH exceedances, as flow and transport are likely influenced by the orientation and connectivity of fracture networks. Based on the analysis of redox-sensitive compounds in groundwater, redox conditions within this site ranged from aerobic to methanogenic (observed in the well in proximity to the source of contamination). Overall, these results indicate the crucial role of aquifer porosity, hydraulic conductivity, and flow dynamics in governing the processes that contribute to the reduction of hydrocarbon contamination in groundwater.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10939

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