Natural Source Zone Depletion (NSZD): from process understanding to effective implementation at LNAPL-impacted sites

Abstract

This paper describes the proceedings of a Special Session on Natural Source Zone Depletion (NSZD) at the AquaConSoil conference, held virtually in June 2021. It spans research across Europe, the United States of America and Australia. NSZD has been described as the ‘combination of processes that reduce the mass of LNAPL light non-aqueous phase liquid) in the subsurface’. LNAPL NSZD research and investigations have been focused on a range of hydrocarbon products, such as gasoline, diesel, jet fuel, as well as crude oil. Key NSZD processes include aerobic biodegradation, fermentation and methanogenesis of LNAPL constituents, dissolution of LNAPL constituents into groundwater and volatilization of LNAPL constituents into the unsaturated zone. In turn the generated methane, carbon dioxide and dissolved/volatilized constituents can be cycled and biodegraded in the unsaturated and saturated zones. Importantly these physical, chemical, and biological processes can act without human intervention to reduce the NAPL mass and toxicity. Over time NSZD can both reduce LNAPL mass, and change its chemical composition resulting in risk reduction, and ultimately source depletion. Methanogenesis of organic materials has long been recognized in municipal landfills and natural anoxic environments, such as peat and wetlands. Recognition of similar processes in LNAPL source zones in the past decade along with high rates of aerobic biodegradation observed in unsaturated zones above LNAPL-impacted areas, has significantly revised the conceptual model of LNAPL source zone behaviour and persistence. Several NSZD monitoring approaches have been developed and are being applied in field studies. While the quantitative NSZD rates derived can vary between techniques, they all demonstrate that NSZD LNAPL removal can exceed that delivered by engineered LNAPL recovery techniques, particularly for mature LNAPL bodies.