Recent research in light non-aqueous phase liquid (LNAPL) science has provided improved metrics to monitor performance. For example, LNAPL transmissivity (Tn) represents a unifying metric for LNAPL recovery across various technologies and natural sourcezone depletion (NSZD) (typically in units of gallons/acre/year) is used as a technology performance/evaluation metric. Although providing quantifiable information, the rates (i.e., recovery or natural loss) do not necessarily provide the level of risk reduction critical for site stewardship. Parsons, in collaboration with the client, is currently piloting alternative metrics to support prioritization of systems across its portfolio.
The management approach summary includes measurement of key plume migration (e.g. NAPL transmissivity, dissolved concentration trends) and depletion metrics (MNA/ NSZD) towards the development of Unified Performance Assessment Metrics (UPAMs) that normalize risk associated with these phases (e.g., NAPL, dissolved) to natural losses. The metric represents the potential distance the contaminant can migrate, thereby allowing for an easy and effective risk-to-receptor comparison. The methodology accounts for reductions of various technologies (e.g., physical barriers, pump and treat (P&T) systems, enhanced biodegradation) and provides a decision framework for choosing the right remedy or optimizing an existing one with regards to the remedial action objectives (RAOs).
The UPAM is being piloted at a large hydrocarbon-impacted site to evaluate the performance of existing physical remedies (slurry wall and P&T system) and MNA/NSZD measurements to address effectiveness in mitigating the LNAPL and dissolved phase risks to off-site receptors. Other factors, including safety, cost and implementability, were also considered. The initial model assessment indicated the long-term strategy is best served by alternate technologies, but existing P&T systems can be optimized on an interim basis while data gaps identified by the UPAM assessment are addressed.