Quantifying timeframes for transitioning the management of a fresh release of LNAPL petroleum from active recovery to natural source zone depletion: Simulation of a new diesel release.

Natural Source Zone Depletion (NSZD) is typically evaluated for weathered petroleum releases, often after active recovery options have been exhausted. This study addresses a key knowledge gap: when do NSZD rates surpass what can be achieved by recovery methods (here skimming) following a fresh petroleum fuel release? Here we simulate a field case of an intensively investigated fresh release of light non-aqueous phase liquid (LNAPL) diesel into a shallow sandy vadose zone and groundwater, with active efforts to recover LNAPL mass and simultaneously detailed measurements of NSZD rates. To assess uncertainties and key processes we simulated a number of scenarios. Validated multiphase, multicomponent models were applied across three representative mass releases, accounting for full hydrocarbon partitioning, microbial dynamics, and biodegradation pathways. Scenarios with and without residual LNAPL in the vadose zone were compared. Results show that NSZD rates can exceed active recovery (skimming) rates within two years. However, for clean sites with minimal native hydrocarbon-degrading microbes, NSZD dominance was delayed by an additional 1-2 years due to the lag required for microbial community growth. Residual LNAPL in the vadose zone significantly enhanced NSZD, often beyond the range that wells could recover. The timeframes over which active recovery dominated mass removal prior to transitioning to NSZD mass removal being greatest is quantified and compared to that determined from the detailed field investigation. This study offers a quantitative, scenario-based platform to support decision-making on when NSZD can be a viable management strategy for new spills, with potential application to other fuel types and geologic settings.