NAPL Mobility and Migration in Water Body Sediments – Contrasts with Land-Based Soils
Volume 8, Issue 4 | December 2020

Applied NAPL Science Review

NAPL Mobility and Migration in Water Body Sediments – Contrasts with Land-Based Soils

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
Dr. Randall Charbeneau, PE, University of Texas
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., ExxonMobil Environmental & Property Solutions
Natasha Sihota, Ph.D.  Chevron
Kyle Waldron, Marathon Petroleum

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.

NAPL Mobility and Migration in Water Body Sediments – Contrasts with Land-Based Soils 

Jeffrey A. Johnson, Ph.D.
NewFields Companies, LLC

Douglas Blue, Ph.D.
Imperial Oil Environmental and Property Solutions

Approaches applied to evaluate non-aqueous phase liquid (NAPL) movement in land-based soils may have limited applicability at water body sediment sites. This article discusses: (1) the technical elements that require consideration in developing a NAPL Conceptual Site Model (CSM) for a sediment site and (2) how these differ from soils at a land-based site.

The character and potential movement (i.e., pore-scale mobility and/or NAPL body scale migration) of NAPL in water body sediments is different than in soils, due to the potential variability in NAPL emplacement, differences in the hydrologic conditions, and the variability in physical and chemical characteristics commonly present in the sedimentary environment. These differences require consideration in developing a NAPL CSM for a sediment site. Table 1 outlines some of the key differentiating characteristics between soils and sediments.

Compared to sediments, the movement (i.e., mobility and/or migration) of NAPL in soil settings is relatively well understood.  Standardized guidance and test methods currently exist for assessing NAPL mobility and migration at soil sites, from organizations such as ASTM International (Guides E2531 and E2856), the Interstate Technology and Regulatory Council (ITRC, 2018), and the American Petroleum Institute (API, 2001 & 2016).  Screening values for estimating the immobile NAPL saturation in soils (i.e., the residual NAPL saturation in vadose zone soils) have been available for at least two decades (Brost & DeVaull, 2000).

Development has been initiated on standardized guidance and test methods for assessing NAPL mobility and migration at sediment sites. The first in a series of ASTM Guides (Guide E3248) in this area was published in 2020; it focused on the development of Conceptual Models (CMs) for different NAPL emplacement mechanisms in sediments.  This guide provides a foundational basis for further Guides on this subject, since NAPL emplacement is a key factor that controls the potential for the movement of NAPL in sediment.   

At soil sites, NAPL movement occurs solely by advection. NAPL is released to the subsurface and moves downwards through the unsaturated zone by gravity. If a light non-aqueous phase liquid (LNAPL), it moves down to the water table where it moves laterally downgradient, typically in the direction of the hydraulic gradient. If a dense non-aqueous phase liquid (DNAPL), it moves downwards through the unsaturated and saturated zones to accumulate on a less permeable layer, where it will flow in the direction of the slope of the lithologic contact.  

In addition to advection, at sediment sites there are two other emplacement mechanisms. These emplacement mechanisms influence the distribution of NAPL at the pore scale and ultimately affect the potential for movement of the NAPL phase.  

The three NAPL emplacement mechanisms in sediment are briefly discussed below (these are discussed in more detail in ASTM Guide E3248) and conceptualized in Figure 1.   

Advective – NAPL is released in the soil environment and then migrates into the surface water environment, where it can directly discharge into the water body or migrate beneath the sediment-water interface. Unconfined LNAPL may seep into the surface water body forming sheens. Confined LNAPL and DNAPL may migrate beneath the sediment-water interface and under the water body. In this case, the NAPL may become stabilized due to low horizontal gradients and/or it may migrate vertically, in some cases into the surface water body. Migration into the surface water body may be mitigated if a less permeable sediment layer occurs above the NAPL phase. In these cases, the NAPL pressure might be insufficient to overcome the pore entry pressure of the less permeable upper layer of sediment.  

In Situ Deposited LNAPL – the discharge of LNAPL to a water body (e.g., via a sewer pipe, industrial outfall or other source) forms a separate phase sheen or layer on the surface of the water. When agitated by wind, waves, or propeller wash, the separate phase LNAPL layer will break down into discrete “oil beads or globules”. These “beads and globules” become suspended in the water column, where interaction can occur with the suspended solids. In the water column, solids will adhere onto (or penetrate into) the oil bead. The resulting discrete entity that is composed of both oil and solids is termed an oil-particle aggregate (OPA). The OPA remains suspended in the water column until its density exceeds the density of the surrounding water, at which point deposition occurs. Over time, sedimentation of OPAs will form a distinct layer at the sediment-water interface; sediment composed of OPAs are termed In Situ Deposited NAPL (IDN) sediments. Oil is present in the sediment, but it is encapsulated in the smaller pores. Thus, the NAPL will not readily interact with the surrounding pore water and its movement will be inhibited (Johnson et al., 2018). 

DNAPL Surface Flow – DNAPL is discharged to a water body (e.g., via a sewer pipe or industrial outfall), where it sinks until it finds a competent sediment surface. The DNAPL will then flow along the sediment surface (primarily due to gravitational forces), until it reaches an area with no slope, or it collects in a bathymetric low point. Solid particles will be incorporated in the DNAPL as it migrates.  Since the DNAPL forms a continuous phase, movement is possible. Because the NAPL is directly present at the sediment surface, it may be prone to movement due to erosive forces. 

Figure 1 – Summary of NAPL emplacement mechanisms for sediments

Studies conducted since the 1980s have provided a good understanding of the movement of NAPL in soils, but these types of investigations have just begun for NAPL-impacted sediments. Investigations performed so far have revealed that the application of NAPL characterization methodologies used for soils may not be appropriate for use at sediment sites. The significant differences in the physical and chemical characteristics of sediments (i.e., porosity, water content, dry bulk density, salinity, etc.) relative to soils adds new complexities to the understanding of NAPL movement in sediments. These complexities can include NAPL movement by erosion (e.g., propeller wash) or ebullition; NAPL movement through these mechanisms needs to be assessed separately from advective NAPL movement in sediments. It is anticipated that as characterization activities increase at sediment sites, consistent characterization approaches will be developed to better understand the potential for NAPL movement in sediments.

A Word of Caution 

In the past couple of decades, great strides have been made in developing guidance and methods to ascertain the potential for NAPL movement in upland soils. In contrast, the development of consensus investigation methodologies to collect and analyze data for NAPL movement in sediment is still in its nascent stages.  However, a number of ASTM guides addressing this issue are currently under development. Until these are finalized, practitioners should be aware that the application of upland NAPL characterization methodologies may not be appropriate for use at sediment sites.

References 

American Petroleum Institute (API), 2001. “Methods for Determining Inputs to Environmental Petroleum Hydrocarbon Mobility and Recovery Models,” API Publication Number 4711, 2001.

American Petroleum Institute (API), 2016. “API LNAPL Transmissivity Workbook: A Tool for Baildown Test Analysis – User Guide,” API Publication Number 4762, 2016.

ASTM E2531. “Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Non-aqueous Phase Liquids (LNAPL) Released to the Subsurface.”

ASTM E2856. “Standard Guide for Estimation of LNAPL Transmissivity.”

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

Brost, E. J., and DeVaull, G.E., 2000. “Non-Aqueous Phase Liquid (NAPL) Mobility Limits in Soil”, API Soil & Groundwater Research Bulletin Number 9, June 2000.

Interstate Technology and Regulatory Council (ITRC), 2018. “Light Non-Aqueous Phase Liquid Site Management: LCSM Evolution, Decision Process, and Remedial Technologies (LNAPL-3),” 2018. www.itrcweb.org

Johnson, J. A., Edwards, D. A., Blue, D. and Morey, S. J., 2018. “NAPL Mobility of OPA-containing Sediments,” Soil and Sediment Contamination: An International Journal, October 2018, pp.736-747.

Research Corner

Thank you to Dr. Tom Sale of the Colorado State University, Center for Contaminant Hydrology, for providing access to selected graduate level NAPL research.

Processes governing the performance of oleophilic bio-barriers (OBBs) – laboratory and field studies 

Laura Tochko
Master of Science
Colorado State University 

Abstract:

Petroleum sheens, a potential Clean Water Act violation, can occur at petroleum refining, distribution, and storage facilities located near surface water. In general, sheen remedies can be prone to failure due to the complex processes controlling the flow of light non-aqueous phase liquid (LNAPL) at groundwater/surface water interfaces (GSIs). Even a small gap in a barrier designed to resist the movement of LNAPL can result in a sheen of large areal extent. The cost of sheen remedies, exacerbated by failure, has led to research into processes governing sheens and development of the oleophilic bio-barrier (OBB). OBBs involve 1) an oleophilic (oil-loving) plastic geocomposite which intercepts and retains LNAPL and 2) cyclic delivery of oxygen and nutrients via tidally driven water level fluctuations. The OBB retains LNAPL that escapes the natural attenuation system through oleophilic retention and enhances the natural biodegradation capacity such that LNAPL is retained or degraded instead of discharging to form a sheen. Sand tank experiments were conducted to visualize the movement of LNAPL as a wetting and non-wetting fluid in a water-saturated tank. The goal was to demonstrate 1) the flow of LNAPL as a non-wetting fluid in sand, 2) the imbibition of LNAPL as a wetting fluid on the geocomposite, and 3) the breakthrough of LNAPL after saturating the geocomposite to the point of failure (sheens in the surface water). Dyed diesel was pumped through a tank with sand and geocomposite and photographed to document movement. Diesel was the non-wetting fluid in the sand and moved in a dendritic pattern. Diesel was the wetting fluid on the geocomposite and uniformly imbibed horizontally across the geocomposite before breakthrough to the overlying sand layer. A second set of laboratory experiments was designed to estimate the aerobic and anaerobic OBB degradation rates of LNAPL in field-inoculated sediment. Unfortunately, due to a flaw in the experimental design, the mass balance could not be completed, and degradation rates were not calculated. The setup was designed to emulate field conditions as best practically possible and to observe the effects of water table fluctuations, different loading rates, and iron. The effluent pumping system designed to remove water in the water fluctuation columns also inadvertently removed LNAPL, creating a mass balance discrepancy for the aerobic columns. Though degradation rates could not be calculated from this experiment, the experiment did visually document the changing redox conditions in the columns, such as formation of a black precipitant (likely iron sulfides) around LNAPL. Ideally, the limitations of this experimental design can be addressed for future research to eventually resolve degradation rates for OBBs. The success of a 3.8 m by 9.3 m demonstration OBB at a field site on a tidal freshwater river resulted in replacing the demonstration OBB with a 3.8 m by 58 m full-scale OBB. The construction event provided a unique opportunity to sample the demonstration OBB after a four-year deployment. The sampling results advanced the mechanistic understanding of how OBBs work to reduce LNAPL releases at GSIs. Sampling revealed the material was suitable for field LNAPL loading rates and was not compromised by field conditions such as ice scour or sediment intrusion. LNAPL analysis showed no LNAPL on the geocomposite or in the underlying upper sediment (0-10 cm). Diesel range organic (DRO) concentrations in the low 1,000s of mg/kg were observed in the sediment 10-20 cm below the geocomposite. LNAPL composition analysis suggests that the majority of the compounds are polar in the lower sediments (10-20 cm), providing a line of evidence that petroleum liquids have been oxygenated. Microbial data show the average number of bacterial 16s transcripts in the geocomposite is larger than in the sediment layers, confirming that the geocomposite is suitable substrate for microbe growth. The observation of ferric iron suggests that ferric/ferrous iron cycling may play a role in degradation processes, where the ferric iron acts as a “bank” of solid-phase electron acceptors. This sampling event suggests that LNAPL biodegradation rates in and below the OBB are comparable to the LNAPL loading rates. 

 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.

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