Quantifying Chemical Mass Flux Components for Sediment Remedial Design at MGP Sites
Michael J. Gefell, P.G., C.P.G., Anchor QEA, LLC
Co-Authors: Kevin Russell, Deirdre Reidy, Billie-Jo Gauley, and Mike Rury, Anchor QEA, LLC
Most manufactured gas plants (MGPs) are situated along waterways, such as marine shorelines, canals, rivers, lakes, and bays. Subaqueous sediment near some MGPs contains MGP-related materials that may pose a risk to ecological or human receptors. Remedial alternatives can be evaluated, designed, and implemented to address risk; a typical remedial action objective is to establish surface sediment and/or porewater concentrations that meet risk-based goals and are predicted to continue achieving those goals over the long term. To develop remedial options, it is important to understand the mass flux of chemicals into the surface sediment from subsurface sediment. In addition, if engineered capping is selected as a component of the remedy (with or without prior dredging), designing the remedy requires quantitative chemical mass flux data for multiple key components—any of which can be important and should not be overlooked. The dissolved chemical mass flux, which is the product of porewater discharge rate and chemical concentrations, must be quantified to predict the effective lifespan of potential chemical isolation cap designs. The temporal variability of porewater discharge should be considered in evaluating a long-term average value for remedial design. Porewater concentrations can be measured by a variety of methods. However, if non-aqueous-phase liquid (NAPL) is present in sediment (which is often the case at MGPs), care must be taken to avoid including NAPL in porewater samples, to avoid producing analytical results that are unrealistically high. Also, if NAPL is present, the NAPL mass fluxes associated with NAPL advection, gas ebullition-facilitated transport, and post-cap sediment consolidation (squeezing) should all be considered in remedial design. This presentation highlights the importance of each mass flux component, methods to measure them, potential pitfalls, and economical innovations to quantify them by combining empirical data, modeling, and estimation techniques to achieve successful sediment remedies.
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.