Evaluating Emplacement and Movement of NAPL in Sediment
Volume 9, Issue 1 | February 2021

Applied NAPL Science Review

Evaluating Emplacement and Movement of NAPL in Sediment 

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., 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.

Evaluating Emplacement and Movement of NAPL in Sediment 

Lisa Reyenga
GEI Consultants, Inc. 

When does the potential for NAPL movement in sediments need to be evaluated? What are the objectives for these evaluations?  

Conceptual models for how NAPL is emplaced in sediment and implications on the potential for advective movement of NAPL (NAPL flow due to pressure/gravity) were discussed in the article NAPL Mobility and Migration in Water Body Sediments in the prior issue of ANSR. In this issue, we will discuss how to determine if a detailed evaluation of the potential for movement is warranted and the objectives of such an evaluation. This approach is outlined in more detail in the Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection (ASTM E3248). 

How to determine whether or not a NAPL Emplacement and Movement Evaluation is necessary is summarized as follows (also shown on Figure 1):

• Where NAPL is suspected in sediments, field screening for NAPL presence is performed.  
• If NAPL is not identified in sediment at the site (or in a portion of a larger site), then a NAPL movement evaluation is not warranted.  
• If NAPL is identified, then the three-dimensional extent of the NAPL zone is delineated.  
• In some cases, it may make sense for all the NAPL–impacted sediment to be removed without further evaluation. Generally, this is the case where the NAPL zone is relatively small, shallow, and it is practicable and cost-effective to fully remove the impacted sediment. 
• Otherwise, a NAPL Emplacement and Movement Evaluation is typically warranted to characterize the potential for movement as part of the Conceptual Site Model (CSM) to inform risk evaluations and potential remedy selection. 

Figure 1 – Process to determine the need for a NAPL Emplacement and Movement Evaluation (Reprinted, with permission, from ASTM E3248−20 Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.  A copy of the complete standard may be obtained from ASTM International, www.astm.org.)

The goal of the NAPL Emplacement and Movement Evaluation is to identify if the NAPL in sediment has the potential for advective movement that could cause an exposure to a receptor or to surface water. The evaluation can be conducted at the pore scale and the NAPL body scale, as summarized as follows (also shown on Figure 2):  

• If pore scale testing or modeling demonstrates that the NAPL is immobile at the pore scale, then it must be stable at the NAPL body scale (under the conditions evaluated) and no further evaluation is required. 
• If mobile NAPL is identified at the pore scale, further characterization to determine if it is stable or migrating on the NAPL body scale may be warranted.  
• If NAPL migrating at the NAPL body scale is identified, analytical or numerical modeling to determine if it has the potential to impact a receptor or surface water can be performed.

Figure 2 – NAPL Movement Evaluation Framework (Reprinted, with permission, from ASTM E3248−20 Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.  A copy of the complete standard may be obtained from ASTM International, www.astm.org.)

The term “immobile” identifies NAPL that is not moving in the pore (void) spaces of the sediment, while mobile identifies NAPL that can move within the pore spaces. The terms technically refer to mobility at a prescribed condition (e.g. hydraulic gradient, seepage rate, temperature), based on laboratory tests and/or modeling. However, the identification of immobile NAPL is generally assumed to mean under any reasonably foreseen in situ conditions, and the test program should be designed to be sufficiently conservative in practice.

If the NAPL is immobile at the pore scale, it is by definition stable on the body scale. However, if mobile NAPL is identified, it may be warranted to determine if it is stable or migrating on the body scale. A stable NAPL body is not expanding in any direction, though may include mobile NAPL that can redistribute within the NAPL body but not expand outside it. A migrating NAPL body is expanding or capable of expanding under reasonably foreseen in situ conditions.

As part of the CSM development, the NAPL Emplacement and Movement Evaluation characterizes the potential for movement to inform risk evaluations and potential remedy selection. Further data collection and evaluation may be required for remedy design.

A Word of Caution 

The NAPL Emplacement and Movement Evaluation described here is to evaluate the potential for advective movement of NAPL once it is emplaced in sediment. All laboratory testing as well as analytical or numerical modeling must be performed in a way that they are adequately representative of the overall site conditions.  The NAPL Emplacement and Movement Evaluation does not address the potential for movement due to other forces, such as ebullition or scouring. It is based upon successful implementation at multiple sites but does not address any specific regulatory context. A further series of ASTM guides addressing the implementation of the general methodologies described here are under development. Ultimately it is the practitioners’ responsibility to ensure protection of human health and the environment. 

References 

ASTM E3248−20 Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. 

Research Corner

Influence of capping material upon biodegradation of polycyclic aromatic hydrocarbons in sediment-cap systems 

Giovanna Pagnozzi
Master of Science
Texas Tech University 

Abstract:

Sediment capping is currently considered one of the most convenient and efficient risk containment strategy for contaminated sediments. Conventional capping consists of placing one or more layers of inert materials on top of the contaminated sediment, retarding the flux of the pollutants to the water body and shielding the sediments from erosion or resuspension. The introduction of adsorbent materials promotes sequestration of the contaminants onto the capping media, but aging or leaks can affect the efficiency of such a reactive-cap. In a bioreactive capping, the use of an adsorbent material suitable for microbial colonization, facilitates both sequesteration and biodegradation of the contaminant. The aim of study is to experimentally evaluate the extent to which capping media selection affects bioactivity in model capping systems. Bench top laboratory studies investigated biological activity in model systems consisting of conventional capping materials (granular activated carbon [GAC], organoclay, and sand), mineral media, pore water extracted from contaminated sediments, electron acceptors (oxygen, nitrate, sulfate and iron) and the electron donor, naphthalene (a model polycyclic aromatic hydrocarbon). Microcosms were prepared and inoculated with microbial enrichements prepared with contaminated sediments collected from a river adjacent to a former manufactured gas plant. Concentrations of naphthalene and nahAc gene (encoding a dioxygenase associated with aerobic biotransformation of PAHs), were monitored for 100. Experimental data were collected and modeled; the relative kinetic rates were used to evaluate efficiencies of the different capping materials. Results suggest that GAC was the most efficient of the capping materials tested. Data showed that naphthalene concentration decreases only in oxic sediment-cap systems during the observation time, and naphthalene decay was statistically significant in oxic microcosms prepared with GAC. Abundance of the nahAc gene was sustained in oxic microcosms prepared with GAC and sand. Within oxic sediment-cap systems, a relationship between the naphthalene mass in solution and the gene copy numbers was observed in the microcosms prepared with activated carbon. This suggests that the nature of the capping material affected the interaction between abundance of a catabolic gene (nahAc) and concentration of the substrate (naphthalene). 

 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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