Challenging Perpetuity: The Potential of Anaerobic Biodegradation Under Sediment Caps

In situ capping is a widely used contaminated sediment treatment technology to mitigate risks to human health and the environment resulting from contaminated sediments. Regulatory agencies typically require long-term effectiveness monitoring (LTM) of sediment caps to ensure that contaminants are contained by the cap, with monitoring typically in perpetuity well beyond a typical LTM duration for other remedy types. An implicit assumption behind the LTM requirement is that organic contaminants persist in anaerobic sediment environments below the cap, and cap design and associated performance monitoring typically do not account for biodegradation of residual contaminated sediments under the cap. However, the colonization of capping materials by microorganisms capable of transforming organic contaminants has been documented in numerous studies, with enhanced degradation of some organic contaminants via biostimulation or bioaugmentation. Given the regulatory focus on assessing the potential for sediment-water contaminant release, the long-term impact of in situ degradation under sediment caps, including the effectiveness of cap amendments at encouraging biodegradation, is poorly documented. This analysis evaluates the pathways of anaerobic biodegradation for common organic contaminant classes, conservative assumptions that neglect consideration of anaerobic biodegradation in sediment cap design, and LTM; compiles degradation rates for polycyclic aromatic hydrocarbons (PAHs) under anaerobic conditions; and identifies implications for future research, regulatory frameworks, and cap design. This review focuses on PAHs due to the robust body of data on anerobic degradation rate kinetics, but the underlying concepts and resulting recommendations are broadly applicable to other organic contaminant classes managed using capping technology. Current understanding and quantification of anaerobic degradation rates are largely based on controlled laboratory studies, which limits the reliable application of these findings to field-scale remedial design. This literature review presents recommendations to improve understanding of in situ anaerobic biodegradation with specific consideration for LTM planning and cap modeling.