NAPL Mobility Assessment in Sediments at MGP Sites Using ASTM’s New Evaluation Metrics Guide
Michael J. Gefell, P.G., C.P.G., Anchor QEA, LLC
Co-Author: Tripp Fischer, P.G., Chief Science Officer, Brownfield Science & Technology, Inc.
Many former manufactured gas plants (MGPs) are situated along waterways, such as rivers, lakes, and bays. Surface-water bodies are ecological systems and may serve as resources for water supply, recreation, and transportation. If subaqueous sediment contains non-aqueous phase liquid (NAPL) related to an MGP or other source, NAPL is already in close proximity to potential receptors. Therefore, the potential for NAPL movement in sediment is a crucial component of the conceptual site model and informs what remedial options could be chosen for the site to reduce potential risks to human health and ecological receptors, as appropriate. Sediment presents a unique and challenging environment for evaluating NAPL mobility. Sediment physical and hydraulic properties—combined with NAPL emplacement mechanisms—influence the potential for NAPL advection. Previously, no industry standard framework was available to evaluate if NAPL in sediment was mobile or migrating. To establish a sound technical basis to evaluate the potential for NAPL movement via advection in sediment, ASTM developed Standard Guide E3282-21: NAPL Mobility and Migration in Sediments – Evaluation Metrics. Using key elements of the guide, this presentation highlights how one can evaluate whether MGP NAPL in sediment is mobile or immobile at the pore scale, and if mobile, whether NAPL is capable of migrating via advection upward to a receptor. We briefly summarize topics covered by the guide, such as: centrifuge and water-drive NAPL mobility test methods, including sequential (tiered) testing; an innovative weight-of-evidence (WOE) procedure to interpret whether NAPL is mobile or immobile at the pore scale under field conditions; another WOE process to interpret whether a NAPL body in sediment is capable of migrating to a potential receptor; calculations to quantify NAPL movement, if any; and case studies that illustrate how the guide can be used to evaluate NAPL mobility at MGPs and other sites.
Michael Gefell, PG, CPG, is a Principal Scientist at Anchor QEA. Mr. Gefell has over 32 years of experience in environmental site assessment and remediation. Primary areas of interest include: quantitative hydrogeology, NAPL mobility assessment, groundwater/surface-water interaction, innovative site characterization methods, fractured bedrock, modeling and remedial design. He is the Technology Editor and an Associate Editor of Groundwater journal, and received an ASTM Distinguished Service Award for his contribution to recent guidance documents regarding NAPL mobility in sediments. He holds a BA in Geological Sciences from Cornell University and an MS in Geology from the University of California, Davis.