A roadmap to understanding key knowledge gaps in natural source zone depletion

There are a number of key aspects and challenges to the adoption of natural source zone depletion (NSZD) of oil-contaminated soil and groundwater. NSZD involves the interplay of multiple natural partitioning, transport and biodegradation processes that collectively lead to contaminant mass and risk reductions. Here we build an expanded conceptual site model of the NSZD processes that drive and influence NSZD rates, and emphasize the least-understood aspects and major challenges. By elaborating on these least-known topics and remaining challenges, we underscore the need for multidisciplinary research efforts to better address these challenges and to deliver improved management and environmental outcomes. Furthermore, we explore emerging issues and complexities surrounding NSZD, such as additives and emerging contaminants co-mingled with petroleum compounds in source zones, the potential to automate the measurement and quantification of NSZD to gain management efficiencies, and the capability of simulation platforms to capture and predict all key NSZD features. We discuss challenges to its adoption and uptake and how advancements in science can address many of the challenges. Ultimately, our study serves as a roadmap to guide future research endeavours aimed at unravelling the complexities of NSZD, and ensuring its reliable measurement. This will benefit efforts to effectively mitigate environmental risks and enable the broader use of NSZD as a management option for petroleum-impacted sites. This Review explores key aspects and challenges to adoption of the natural source zone depletion (NSZD) of subsurface petroleum hydrocarbons.