Oil-Particle Aggregates and In Situ Deposited NAPL Sediments
Volume 9, Issue 2 | March 2021

Applied NAPL Science Review

Oil-Particle Aggregates and In Situ Deposited NAPL 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
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.

Oil-Particle Aggregates and In Situ Deposited NAPL Sediments

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

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

What are oil particulate aggregates (OPAs) and In situ Deposited NAPL (IDN) sediments? How does their presence and their degree of encapsulation affect the risk profile of NAPL impacted sediments?

Oil-particle aggregates (OPAs) develop from solid particles attaching to and/or penetrating into the surface of a non-aqueous phase liquid (NAPL) bead suspended in the water column (Figure 1). The OPA is composed of both solids (e.g., minerals) and oil. During suspension in the water column, solid material continues to cover the oil droplet and the OPA increases in density, until its density eventually is greater than the water it is suspended in and the OPA deposits on the bottom of the water body. As the outer solid material encapsulates the oil droplet, a physical barrier is formed that mitigates further coalescing with other oil droplets (Khelifa et al., 2005).

Figure 1. Schematic of OPA Formation

In situ deposited NAPL (IDN) sediments result from the deposition of OPAs on the sediment bed at the bottom of a water body. IDN sediments are typically formed from NAPL that is less dense than water (i.e., light non-aqueous phase liquid; LNAPL). IDN sediments may occur spatially over large areas, on the order of acres; this spatial distribution reflects the current movements within the water body relative to a known LNAPL discharge location (typically an outfall). This discharge can occur continuously or intermittently over a long period of time. In addition to discharges, the formation of OPAs and IDN sediments have been documented at oil spills sites (Dollhopf et al., 2014; Bragg and Owens, 1994). However, since oil spills are typically of shorter duration (less than one year) and the net sedimentation rates in depositional zones of water bodies are typically a few centimeters per year, any IDN strata formed from oil spills will typically be very thin (i.e., on the centimetre or sub-centimeter scale).

Most IDN sediments are composed of more finely-grained particles, such as clay, silt, and/or fine sand. Internally, the oil-phase is present in the smaller pores of the sediment as discrete and isolated droplets (Johnson et al., 2020). IDN sediments are characterized by stratification (Johnson et al., 2018a), which signifies the depositional origin of the sediment; the IDN strata thickness may range from meters to less than 1 millimeter (Figure 2). As illustrated in Figure 2, the deposition of OPAs at this site (i.e., the darker bands) was intermittent. The stratification of the IDN sediment documents that the depositional conditions at this location were temporally changing as evidenced by the lenses of non-OPA deposition (i.e., the lighter bands). These changes likely resulted from changes in operational and/or sedimentation patterns.

Figure 2. X-ray Tomography of a Laminated IDN Sediment

The formation of IDN sediments of an appreciable thickness requires several critical conditions including (1) a longer-term discharge of LNAPL to the surface water; (2) waves and/or turbulence to break-up the LNAPL into discrete droplets that become suspended within the water column; and (3) sufficient suspended solid particles in the water column to adhere to and/or penetrate the oil bead, such that deposition of the OPA formed results. Properties of the droplets (e.g., size) are dependent upon several key variables, such as water turbulence, oil viscosity, water salinity, and oil/water interfacial tension (Zhao et al., 2014). Smaller oil droplets become dispersed within the water column and may be transported over large distances in the water body before deposition, rather than depositing as localized IDN sediments.

Total petroleum hydrocarbon (TPH) concentrations from IDN sediment sites can range from 3,000 to over 110,000 milligrams per kilogram (mg/kg). In advectively emplaced NAPL in upland soils, TPH concentrations >50,000 mg/kg (conservatively) would result in mobile NAPL during their laboratory mobility testing. However, due to their different emplacement mechanism and the higher porosity of sediments compared to upland soils, it is common that IDN intervals containing elevated TPH concentration levels indicates that the NAPL is immobile in laboratory mobility testing, even with high-speed centrifuge testing (Johnson et al., 2018b).  Since the oil beads are discontinuously distributed in the IDN sediment and the solids coating them inhibit the formation of a continuous phase of NAPL in the pore network of the sediment, advective NAPL mobility is also constrained.

The degree of OPA encapsulation is dependent on the size of the solid particles forming the OPA. OPAs formed through the adherence to and/or penetration by clay and fine silt generally result in full encapsulation of the entire oil droplet, because many particles are required to increase the OPA density to the point where it deposits. In contrast, the adherence of only a few larger solid particles to an oil droplet (i.e., partial encapsulation) is can result in an increase of the OPA density to the point where it deposits.

The potential for the NAPL droplets in an IDN sediment to coalesce to form a continuous NAPL phase and/or to interact with the surrounding porewater is directly related to the degree of OPA encapsulation. Within fully encapsulated IDN sediments, the NAPL-phase occurs as isolated droplets within the IDN sediment matrix; the oil-phase can generally be considered immobile and have limited interaction with the surrounding porewater.  As a result, these sediments pose a lower environmental risk, despite containing a NAPL-phase. IDN sediments composed of partially encapsulated droplets may be of greater environmental concern, as porewater interactions with the IDN NAPL-phase will increase and the NAPL may have a greater potential for movement. Laser Induced Fluorescence (LIF) and Solid Phase Extraction (SPE) rods have been used as a measure of the degree of OPA encapsulation (Johnson et al., 2021), with IDN sediments formed from partially encapsulated OPAs having a much higher fluorescence response than sediments formed from fully encapsulated OPAs.

A Word of Caution
The systematic study of IDN sediments formed from OPAs is a relatively new area of study, with limited publications in the peer reviewed literature. As such, the understanding of IDN sediments is evolving over time.

Bragg, J.R., and Owens, E.H., 1994. “Clay–oil Flocculation as a Natural Cleansing Process Following Oil Spills––Part 1: Studies of Shoreline Sediments and Residues from Past Spills”. Proceedings of the 17th Arctic and Marine Oilspill Program (AMOP) Technical Seminar, Environment Canada, Ottawa, Ont., p. 1–23.

Dollhopf, R.H., Fitzpatrick, F.A., Kimble, J.W., Capone, D.M., Graan, T.P., Zelt, R.B., and Johnson, R., 2014. “Response to Heavy, Non-floating Oil Spilled in a Great Lakes River Environment: A Multiple Lines-of-evidence Approach for Submerged Oil Assessment and Recovery”. Proceedings, 2014 International Oil Spill Conference, Savannah, GA, May 7-9, 2014, p. 434–448, accessed January 10, 2015, at http://ioscproceedings.org/doi/pdf/10.7901/2169-3358-2014.1.434.

Johnson, J.A., Edwards, D.A., Blue, D., and Morey, S.J., 2018a. “Physical Properties of OPA Containing Sediments”. Soil and Sediment Contamination: An International Journal. 27:8, p. 706-722. Doi: 10.1080.15320383.2018.150642

Johnson, J.A., Edwards, D.A., Blue, D., and Morey, S.J., 2018b. “NAPL Mobility of OPA-containing Sediments”. Soil and Sediment Contamination: An International Journal. 27:8, p. 736-747. Doi: 10.1080/15320383.2018.1513990

Johnson, J.A., and Blue, D., 2020. “NAPL Mobility and Migration in Water Body Sediments – Contrasts with Land-Based Soils”. Applied NAPL Science Review. 8:4, p. 1.

Johnson, J.A., Mamonkina, I., Ruiz, C.E., Blue, D., and Schroeder, P.R. Application of Solid Phase Extraction (SPE) Media Rods to Assess Degree of NAPL Encapsulation in In Situ Deposited NAPL Sediments”.  Submitted for publication to: Soil and Sediment Contamination: An International Journal.

Khelifa, A., Hill, P.S., and Lee, K, 2005. “The Role of Oil-sediment Aggregation in Dispersion and Biodegradation of Spilled Oil”. In Al-Azab, M., El-Shorbagy, W., and Al-Ghais, S., eds., Oil Pollution and its Environmental Impact in the Arabian Gulf Region: Chapter 10, p. 131–145.

Zhao, L., Torlapati, J., Boufadel, M., King, T., Robinson, B., and Lee, K., 2014. “VDROP:  A Numerical Model for the Simulation of Droplet Formation from Oils of Various Viscosities”, Chemical Engineering Journal. 253, p. 93–106.

Research Corner


Songjing Yan
Doctor of Philosophy
Texas Tech University


Passive sampling technology is an emerging approach of using sorbents to obtain freely dissolved concentrations of target compounds in air or aquatic environment. This research focuses on using passive sampling technology to determine and monitor hydrophobic organic contaminants (HOCs) in sediment porewater using solid phase micro-extraction (SPME) with polydimethylsiloxane (PDMS) fibers. The traditional way to obtain porewater concentrations is to convert bulk sediment concentrations. Compared to conventional techniques, passive sampling technology has several advantages. It’s efficient and easy to process. It has less impacts on the surroundings and it can provide lower detection limits. More importantly, passive sampling method can directly obtain sediment porewater concentration which is regarded as a good indicator of bioaccumulation and chemical activity. Therefore, it is essential for risk management. In addition, passive samplers have the capability to capture the concentrations that change over time and don’t need to be corrected for organic carbon or lip species on a temporal or spatial scale. Due to the above advantages, passive sampling approach is a promising method to monitoring pollutants in aquatic environment, especially in contaminated sediment management and remediation. In this dissertation, three applications of passive sampling technologies in HOCs contaminated sediment management were explored based on in situ pilot studies. The SPME PDMS method was employed at two different polychlorinated biphenyl (PCB) contaminated sediment sites, Hunter’s Point Navy Shipyard (San Francisco, CA) and Columbia Slough (Portland, OR). The spatial representativeness of passive sampling method was explored and compared with bulk sediment measurement by developing spatial semivariogram models. The SPME PDMS fibers were used to monitor the concentration change with time after application of activated carbon to the sediment surface as an in situ treatment at the Hunter’s Point site. The ability of passive sampling to assess site transport conditions was also explored. An analytical model was developed to estimate groundwater upwelling velocities and effective diffusion coefficients using the rate of release of performance reference compounds. The results indicate that passive sampling approach is a viable and promising tool for evaluating exposure and risk of HOC contaminated sediment management, the effectiveness of in situ remediation and for characterizing site transport characteristics.

 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

API LNAPL Resources
Env Canada Oil Properties DB
EPA NAPL Guidance
ITRC LNAPL Resources
ITRC DNAPL Documents
RTDF NAPL Training
RTDF NAPL Publications

ANSR Archives

ANSR Archives

Coming Up

In coming newsletters, look for more articles on NAPL movement in sediment in 2021. Moving forward we are planning articles on surfactant injection case studies, bioremediation, and natural source zone depletion. Let us know if you have article ideas or would like to see articles on other topics.


The ASTM Standard Guide for NAPL Mobility and Migration in Sediment – Sample Collection, Field Screening, and Sample Handling (E3268-20) is now available!

API has published the “API LNAPL Transmissivity Workbook Training Video” to assist with baildown test interpretation and identification of frequently encountered problems.

For 2021 the International Petroleum Environmental Conference (IPEC) is running a series of Virtual, Instructor-Led Presentations bringing petroleum environmental professionals together in an engaging format with high quality technical presentations, followed with discussion and Q & A. Spring topics include Brine, Produced Water Management, Waste Management and Pollution Prevention, and Legal and Regulatory.

Check them all and join us on the ANSR LinkedIn page for discussion or to share your own tips and tricks!

Upcoming ITRC Training – Learn More Here.
  • April 6: PFAS Roundtable (AFFF and Treatment Technologies)  
  • April 8: Characterization and Remediation in Fractured Rock  
  • April 13: Harmful Cyanobacterial Blooms (HCBs) Strategies for Preventing and Managing – Session 1  
  • April 27: Optimizing Injection Strategies and In Situ Remediation Performance  
  • April 29: Harmful Cyanobacterial Blooms (HCBs) Strategies for Preventing and Managing – Session 2 
  • May 11: Connecting the Science to Managing LNAPL Sites 3-Part Series: Build upon your Understanding of LNAPL Behavior in the Subsurface (Part 1)
  • May 13: TPH Risk Evaluation at Petroleum-Contaminated Sites
  • May 20: Sustainable and Resilient Remediation
  • May 25: Integrated DNAPL Site Characterization
  • May 27: Connecting the Science to Managing LNAPL Sites 3-Part Series: Develop your LNAPL Conceptual Site Model and LNAPL Remedial Goals (Part 2)
  • June 1: Vapor Intrusion Mitigation Session 1: Conceptual Site Model for Vapor Intrusion Mitigation, Public Outreach, Rapid Response, Remediation & Institutional Controls
  • June 8: Connecting the Science to Managing LNAPL Sites 3-Part Series: Select/Implement LNAPL Technologies (Part 3)
  • June 10: PFAS Roundtable (Human and Eco Health Effects, Site Risk Assessment, Risk Communication and Stakeholder Perspectives)
  • June 15: Vapor Intrusion Mitigation Session 2: Active Mitigation, Passive Mitigation, Installation/ OM&M/Exit Strategy

Upcoming Conferences
Upcoming Conference Abstract Deadlines

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