Typical Metrics for Evaluating Advective NAPL Movement in Sediments
Volume 9, Issue 5 | August 2021

Applied NAPL Science Review

Typical Metrics for Evaluating Advective NAPL Movement in Sediments

Editor: Lisa Reyenga, PE

ANSR Scientific Advisory Board
J. Michael Hawthorne, PG, Board Chairman, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions (Retired)
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University

Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.

DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.

Typical Metrics for Evaluating Advective NAPL Movement in Sediments

Lisa Reyenga P.E. – GEI Consultants, Inc.

Once you’ve determined that NAPL is present in sediment in an aquatic environment and an advective NAPL movement evaluation is warranted, how do you evaluate if it is mobile or migrating? This article will introduce typical metrics considered to determine if NAPL is mobile or immobile at the pore scale, and if mobile, whether it is stable or migrating at the NAPL body scale.   

Where non-aqueous phase liquid (NAPL) is present in sediments in an aquatic environment (e.g. rivers, lakes), a NAPL movement evaluation may be warranted to assess if the NAPL is mobile or migrating. This evaluation is described in detail in the Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection (ASTM E3248) and summarized in Reyenga 2021.  

The potential for NAPL movement in the sediment is a critical component in the development of the conceptual site model (CSM) in order to determine the potential risks to human health and ecological receptors and inform remedial selection. It can help answer questions such as: 

• Will migrating NAPL reach sensitive receptors? 
• What rate of NAPL migration would a remedy need to contain or mitigate?  
• If the NAPL is not migrating, what is the potential risk from the NAPL, and is a remedy need? 

The answers to these questions can help focus remedies on areas that would yield the greatest benefit and lower the environmental footprint of the remedial action.  

The goal of the NAPL movement evaluation is to determine if NAPL is mobile or immobile at the pore scale, and if mobile, whether it is stable or migrating at the NAPL body scale. The term “immobile” identifies NAPL that is not advectively moving in the pore (void) spaces of the sediment, while “mobile” identifies NAPL that can move advectively within the pore spaces. If the NAPL is immobile at the pore scale, it is by definition stable on the body scale. However, if mobile NAPL is identified at the pore scale, it may warrant an evaluation of if it is stable or migrating at the body scale. A stable NAPL body is not expanding outside its existing footprint in any direction, though it may include mobile NAPL that can redistribute within the NAPL body. A migrating NAPL body is expanding or capable of expanding outside its existing footprint under current or reasonably foreseen in situ conditions (ASTM E3248, Reyenga 2021). 

These types of evaluations are particularly challenging in sediments. Methods typically used in upland environments that rely on monitoring well networks, are impractical to implement in sediments. Therefore, methods have been developed for sediments that primarily rely on data from sediment cores. 

Potential metrics and test methods for evaluating if NAPL is mobile or immobile at the pore scale are summarized in Table 1. Potential metrics for evaluating if a NAPL body is stable or migrating are summarized in Table 2. 

Table 1 Typical metrics and test methods for pore scale NAPL mobility evaluations, adapted from ASTM E3282. 

Table 2 Typical metrics for NAPL migration evaluations, adapted from ASTM E3282. 

Application of these metrics requires understanding of the overall site conditions, and additional site data are typically required to supplement them. Some examples of these supplemental data include, but are not limited to: 

• Examine the site-specific hydraulic gradient to determine the degree of conservatism in gradients induced in the centrifuge and water drive tests;   
• Establishment of a site-specific immobile saturation for use in evaluation of the saturation metrics in Tables 1 and 2 (e.g., change in NAPL Saturation, NAPL travel distance prior to depletion); and  
• Determining the pore entry pressure of sediments separating the NAPL body and the surface water body for calculation of the critical NAPL body thickness metric (Table 2).  

A single metric is typically not sufficient to determine if the NAPL is mobile or migrating. Multiple metrics are incorporated as a lines of evidence evaluation based on the site conditions and professional judgment (see ASTM E3282 for examples). When taken together, these lines of evidence provide a sound technical basis, including quantitative thresholds, to determine if NAPL is mobile or immobile at the pore scale, and if mobile, whether it is stable or migrating at the NAPL body scale.   

A Word of Caution 

The metrics provided directly or indirectly rely on evaluating the potential for advective NAPL movement in sediment cores, which includes laboratory testing of short intervals within those cores. By necessity, the results represent discrete intervals and may not be representative of the bulk behavior of the NAPL in the sediment. Care must be taken in the selection of the intervals for testing and the design of the tests to provide sufficient conservatism. Typically, this is done via utilizing the most impacted intervals of the most impacted cores for the evaluations, as well as incorporation of safety factors. Ultimately, it is the practitioners’ responsibility to ensure that the results are sufficiently representative to be utilized in the advective NAPL movement evaluation. The metrics only inform advective movement of NAPL in sediment, and do not address other mechanisms that could cause NAPL movement such as ebullition or scouring.  

References

ASTM E3248 – 20. 2020. “Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection.” 

ASTM E3282 – 21. 2021. “Standard Guide for NAPL Mobility and Migration in Sediment – Evaluation Metrics.” 

Cohen, R. M., and Mercer, J. W., “DNAPL Site Evaluation,” EPA/600/SR-93/022, U.S. Environmental Protection Agency, April 1993. 

Gefell, M. J., Russell, K., and Mahoney, M., “NAPL Hydraulic Conductivity and Velocity Estimates Based on Laboratory Test Results,” Groundwater, Vol 56, No. 5, August 2018, pp. 690-693. 

Gefell, Michael J. 2021. “Estimating NAPL Hydraulic Conductivity and Migration Rate Based on Laboratory Test Results” Applied NAPL Science Review Vol. 9 Issue 3, May 2021. 

Reyenga, Lisa. 2021. “Evaluating Emplacement and Movement of NAPL in Sediment” Applied NAPL Science Review Vol. 9 Issue 1, February 2021. 

Pankow, J. F., and Cherry, J. A., Dense Chlorinated Solvents and Other DNAPLs in Groundwater: History, Behavior, and Remediation, Waterloo Press, Portland, OR, 1996. 

Research Corner

Pragmatic Groundwater-Surface Water Model Coupling with Unstructured Grids  

Leland Scantlebury
Master of Applied Science 
University of Waterloo 

Abstract:

Faced with an array of water issues exacerbated by a rapidly changing climate, hydrologists and hydrogeologists have increasingly found themselves needing to simultaneously model the groundwater and surface water domains together. Historically, for convenience and due to computational limitations, they have been modeled separately, with tools evolving based upon the different needs and questions driving researchers and practitioners in each domain. The tools emerging to solve these new problems range from highly complex, fully coupled, parallelized software solutions requiring enormous computational resources, to comparatively simple combinations of existing models sharing fluxes between the domains. Both groups generally have utilized relatively inflexible representations of the surface-water domain, often with a fixed level of complexity that prevents explorations of model structural uncertainty and process algorithmic skill. In this thesis, a loosely coupled groundwater-surface water modelling framework is presented that allows for adjustable model complexity in both domains. This is accomplished through pairing MODFLOW-USG, a recent version of the industry-standard MODFLOW family of modular groundwater modelling codes that allows for unstructured model grids, with Raven, a state-of-the-art surface water modelling framework supports flexible representations of hydrologic processes, forcing interpolation, and spatial discretization schemes. The resulting software, compiled into a single executable, is aimed at modelling watersheds at the regional scale. Recharge estimated by Raven is directly entered into the MODFLOW-USG flow solution. River-groundwater interactions are handled through a novel sub-grid river package added to MODFLOW-USG, called the polyline boundary junction (PBJ) package. The PBJ method evaluates boundary conditions along individual segment locations within a grid’s dual Delaunay triangulation and geometrically distributes the resultant fluxes to the appropriate Voronoi and/or rectangular cells. Groundwater heads are interpolated along the segment to handle head-dependent flux calculations. The resulting river fluxes are added or subtracted from the Raven river channel water balance, allowing for a closed simulation of the hydrologic cycle. The new coupled Raven framework is demonstrated on the Alder Creek watershed in Southern Ontario and shown to produce physically realistic flows between the surface and subsurface domains.

The primary objective of ANSR is the dissemination of technical information on the science behind the characterization and remediation of Light and Dense Non-Aqueous Phase Liquids (NAPLs). Expanding on this goal, the Research Corner has been established to provide research information on advances in NAPL science from academia and similar research institutions. Each issue will provide a brief synopsis of a research topic and link to the thesis/dissertation/report, wherever available.

Related Links