Natural source zone depletion (NSZD) is increasingly being considered as a risk-management option at sites impacted with light non-aqueous phase liquids (LNAPL). NSZD can be applied in isolation or in combination with active remediation techniques, depending on site-specific risk management requirements. A case study of the transition from active remediation to passive NSZD is presented for a petroleum impacted site in northwest Europe. This transition was supported by multiple lines of evidence/management options including: the introduction of institutional controls on groundwater and land development restrictions, the results from a residual-NAPL risk assessment, monitoring to establish that the LNAPL plume is reducing in size, LNAPL transmissivity assessment, a CO2 equivalent assessment of remediation options, and a LNAPL recovery diminishing returns model. Through application of local sustainable remediation principles consistent with ISO / SuRF-UK sustainable remediation frameworks and tools, regulatory approval was obtained for a partial closeout of the remediation system. By the final year of operation, NSZD rates in the portion of the site on which transition to NSZD has been agreed were over three times greater than active LNAPL recovery rates (12,000 L/ha/a for NSZD; 3,800 L/ha/a for active LNAPL recovery). At the remaining active remediation areas total fluids extraction currently out-performs NSZD and will be continued until a comparable point is reached when NSZD removal exceeds active remediation. At that point transition to NSZD alone will be considered as the most sustainable risk-based approach.

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