Applied NAPL Science Review

Fundamentals of Ebullition Facilitated NAPL and Contaminant Transport

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
Reeti Doshi, National Grid

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.

Fundamentals of Ebullition Facilitated NAPL and Contaminant Transport

Priscilla Z. Viana, Ph.D., Arcadis U.S., Inc.

Karl J. Rockne, Ph.D., PE, BCEE, University of Illinois at Chicago

The migration of gas through sediment and surface water is called gas ebullition. Gas ebullition is caused by a combination of biological and physical processes in sediment. Gas ebullition can be an important mass transport mechanism of NAPL and other contaminants from the sediment to the overlying water column and air/water interface.

Under certain conditions, biogenic gases that are produced in sediment may rise to the sediment surface, and from there through the overlying water column to the air/water interface. This migration of gas through sediment and surface water is called gas ebullition. Ebullition can facilitate the transport of non-aqueous phase liquids (NAPLs) and other contaminants from the sediment to the surface water column, representing an important mass transport mechanism at some contaminated sediment sites (Fendinger et al. 1992, Viana et al. 2012, Viana et al. 2018, Yuan et al. 2009). The objective of this paper is to provide an overview of biogenic gas production in sediment, ebullition processes, and NAPL transport via ebullition. This information is contextualized with a summary of site-specific conditions and characteristics that affect gas production, gas ebullition, and associated NAPL and contaminant transport.

Biogenic gases are produced in sediments as end products from the biodegradation of organic matter by microorganisms in the sediment. The primary biogenic gas generated in sediments typically is methane, but other gases such as carbon dioxide are generated to a lesser extent. Gas production can occur anywhere in the sediment column where conditions favor this process.

Biogenic gases can be released from sediments via diffusion of dissolved gases in the porewater to the overlying water column (Kaliappan and Rockne 2015), or via bubble generation. It is important to recognize that the production of methane does not mean it will form a separate gas phase. This only occurs when the rate of production is sufficiently rapid for gas bubbles to grow, fracture the sediment, and then migrate upward into the overlying water column (Boudreau 2012, Zamanpour and Rockne 2018, Zamanpour et al. 2020). The main variables affecting the rate of production are:

• Availability of biodegradable organic matter (i.e., organic matter that can be easily broken down by sediment microorganisms), which can be from naturally occurring and/or anthropogenic sources,
• The presence of a population of microorganisms able to carry out the break down and production of methane and other end products from organic matter, and
• Sediment temperature. Because the process is dependent upon a biological process, the rate of gas production is greatly influenced by temperature. This is due to the increased rate of growth of the microorganisms that produce methane at greater temperatures.

The amount and availability of biodegradable sediment organic matter is thus critical, as it limits both the amount and rate of gas that can be produced at a site. For example, stormwater and wastewater from combined sewer outfalls typically contain large amounts of biodegradable organic matter that can stimulate gas ebullition in surface sediments (Viana et al. 2012). In contrast, the slower rate of biodegradation for some petroleum hydrocarbons (PHCs) may not be sufficient to drive gas ebullition alone in typical sediment environments (Zamanpour et al., 2020).

Gas production can vary seasonally with highest production typically occurring in spring and summer when sediment is warmest (Viana et al. 2012). Sediment thickness, water depth, and groundwater seepage affect sediment temperature and thus biogenic gas production rates.

Gas ebullition has been reported to originate primarily from shallow sediments (Adler et al. 2011, Chan et al. 2005, Viana et al. 2012, Wilkinson et al. 2015). This is typically thought to result from the:

• Presence of greater amounts of more biodegradable organic matter in shallow sediments,
• Greater sediment temperature in warmer months (although the converse is true in colder months), and
• Weaker sediment strength in poorly consolidated surface sediments that allows sediment fracture and growth of gas voids.

Sediment fracture can only occur when the pressure inside the growing bubble exceeds the sediment tensile strength and hydrostatic stresses of the sediment (Boudreau 2012). The compaction of deeper sediments results in greater sediment strength, which may prevent the fracturing of sediment and thus reduce the rate of gas bubble migration from deeper sediments into surface water. The presence of NAPL has complex effects on sediment characteristics. In some instances, NAPL has been shown to decrease the sediment strength, thereby indirectly affecting ebullition rates by favoring sediment fracture (Zamanpour and Rockne 2018). Therefore, site-specific conditions strongly affect the potential for ebullition to occur.

Ebullition-facilitated NAPL transport has been reported to occur via three mechanisms:

• NAPL accumulates on the surface of gas bubbles and is transported upward with the bubble;
• Volatile constituents of the NAPL partition into the gas phase and are transported upward; and
• Resuspension of sediment particles facilitated by gas bubble rise. Contaminants, entrained particles, and oil particle aggregates (OPAs) are released from the bubbles in a separate phase as they reach the water column (ASTM E3300-21, Johnson and Blue 2021, Johnson et al. 2018a, Johnson et al. 2018b, Viana et al. 2012).

It is through a combination of these processes that gas ebullition may facilitate NAPL transport from sediments to the water column in situations where NAPL is present within the sediment zone where gas ebullition occurs with sufficient magnitude and prevalence (Zamanpour and Rockne 2018).

Figure 1 illustrates how gas ebullition can enhance NAPL and contaminant transport from sediments into the water column. The main factors affecting gas ebullition rate are summarized in Table 1.

Figure 1 – Ebullition-Facilitated NAPL and Contaminant Transport

Table 1 – Main Variables Affecting Gas Production, Ebullition, and Ebullition-Facilitated NAPL/Contaminant Transport (content adapted from ASTM E3300-21)

Word of Caution

The root cause of gas ebullition is the transformation of biodegradable organic matter to methane and other gaseous end products carried out by microorganisms in sediment. It is important to recognize that biogenic production in sediments is not sufficient to cause gas ebullition. Instead, a combination of site-specific conditions, including rate of biological activity, presence of biodegradable organic matter, porewater chemistry, and sediment characteristics such as sediment strength and overburden pressure are necessary for the development of gas ebullition.

References

Adler, M., W. Eckert, and O. Sivan, 2011. Quantifying Rates of Methanogenesis and Methanotrophy in Lake Kinneret Sediments (Israel) using Pore-Water Profiles. Limnology and Oceanography 56(4): 1525–1535.

ASTM E3300-21. Standard Guide for NAPL Mobility and Migration in Sediment— Evaluating Ebullition and Associated NAPL/Contaminant Transport (Pending Publication).

Boudreau, B.P., 2012. The Physics of Bubbles in Surficial, Soft, Cohesive Sediments. Marine and Petroleum Geology 38:1–18.

Chan, O. C., P. Claus, P. Casper, A. Ulrich, T. Lueders, and R. Conrad, 2005. Vertical Distribution of Structure and Function of the Methanogenic Archaeal Community in Lake Dagow Sediment. Environmental Microbiology 7 (8): 1139–1149.

Fendinger, N. J., Adams, D. D., Glotfelty, D. E., 1992. The Role of Gas Ebullition in the Transport of Organic Contaminants from Sediments. Science of Total Environment 112 (2−3), 189−201.

Johnson, J. A. and D. Blue, 2021. Oil-Particle Aggregates and In Situ Deposited NAPL Sediments. Applied NAPL Science Review 9(2), March.

Johnson, J. A., D. A. Edwards, D. Blue, and S. J. Morey, 2018. Physical Properties of Oil-Particle Aggregate (OPA)-Containing Sediments. Soil and Sediment Contamination: An International Journal 27(8): 706-722.

Johnson, J. A., D. A. Edwards, D. Blue, and S. J. Morey, 2018. NAPL Mobility of OPA-Containing Sediments. Soil and Sediment Contamination: An International Journal 27(8): 736-747.

Kaliappan, R. and K. Rockne, 2015. Estimating Post-Capping GW-SW Exchange At The Grand Calumet River Using Streambed Temperature Profiles. In: Eighth International Conference on the Remediation and Management of Contaminated Sediments, January 13-15, New Orleans, Louisiana, 9 pp.

Viana, P., K. Yin, and K. Rockne, 2012. Field Measurements and Modeling of Ebullition-Facilitated Flux of Heavy Metals and Polycyclic Aromatic Hydrocarbons from Sediments to the Water Column. Environmental Science and Technology 46(21): 12046-12054.

Viana, P., K. Yin, and K. Rockne, 2018. Comparison of Direct Benthic Flux to Ebullition-Facilitated Flux of Polycyclic Aromatic Hydrocarbons and Heavy Metals Measured in the Field. Journal of Soils and Sediments 18: 1729-1742.

Wilkinson, J., A. Maeck, Z. Alshboul, and A. Lorke, 2015. Continuous Seasonal River Ebullition Measurements Linked to Sediment Methane Formation. Environmental Science and Technology 49: 13121-13129.

Yuan, Q., K. T. Valsaraj, and D. D. Reible, 2009. A Model for Contaminant and Sediment Transport via Gas Ebullition Through A Sediment Cap. Environmental Engineering Science 26 (9): 1381−1391.

Zamanpour, M. K. and K. J. Rockne, 2018. A Mechanistic Model for Gas Ebullition in the Presence of NAPLs in Sediments. In: World Environmental and Water Resources Congress, June 3–7, Minneapolis, Minnesota.

Zamanpour, M. K., R. S. Kaliappan, and K. J. Rockne, 2020. Gas Ebullition from Petroleum Hydrocarbons in Aquatic Sediments; A Review. Journal of Environmental Management 271: 110997.

Research Corner

Sediment Capping Effects on Gas Ebullition, Hyporheic Exchange and Benthic Microbial Community Structure  

Raja Shankar Kaliappan
Doctor of Philosophy in Civil Engineering  
University of Illinois at Chicago 

Abstract:

The research described in this dissertation added to the current body of literature on the effectiveness of active capping in mitigating ebullition facilitated contaminant fluxes and in lowering gas ebullition rates, during an active capping sediment remediation in the WBGCR. The three-year post-capping study provided a comprehensive dataset on ebullition rates and the influence of environmental parameters with reductions in gas ebullition rates of 84%, 63% and 61%. The post-capping gas fluxes were also strongly influenced by sediment temperature and water depth. Incubation experiments to assess gas production potential of the post-cap sediment layers showed that cumulative gas production was similar in the contaminated sediment (CSed) and new deposit (ND) layers, whereas the armor (GAL) and organoclay (OrgC) layers exhibited minimal gas production. These results provide further evidence that the CSed layer is ebullition active and thus continued ebullition is likely following capping. This study also evaluated active capping performance in mitigating ebullition-facilitated metal and PAH transport. Metal fluxes were lowered by 89-97% with re-suspension of surficial sediment being the primary mode of transport. PAH fluxes also fell sharply in the first year but increased to 60% of pre-capping levels in 2013 and followed again by a decrease in 2014. The rise and fall of PAH flux in 2013 and 2014 were accompanied by a rise and fall of sediment temperature although average gas fluxes were similar. This suggests that the higher temperatures stimulated increased gas production in the CSed layer thereby increasing PAH partition and transport for 2013. Increases in sediment temperatures could reactivate gas generation in the CSed layer, with subsequent potential for cap fracture, enhanced advective transport and lower design breakthrough times. The impact of capping on the sediment Archaeal community structure was evaluated by using phylogenetic analysis of 16 sRNA genes from pre- and post-capping sediment. The archaeal community structure was dominated by methanogens in both pre-and post-capping sediment. Capping resulted in a more diverse distribution of methanogens in the surficial zone, with evidence of methanogenesis occurring via the three major methanogenic pathways: hydrogenotrophic, acetoclastic and C1-methylotrophic methanogenesis where as pre-cap Archaea were dominated by acetoclastic and hydrogenotrophic methanogens. Field measured gas fluxes were also significantly correlated with Methanosaeta abundance in pre- and post-capping sediment, suggesting that acetoclastic methanogenesis controls gas production in the GCR. The analysis also revealed the presence of Ammonia Oxidizing Archaea (AOA) in increasing abundance with depth, suggesting a more important role for these newly discovered group in contaminated sediments. This research also explored the potential for using heat tracer methods to evaluate GW-SW interactions and the Darcy velocity in different layers of post-cap sediment. Two approaches utilizing amplitude/phase shift and forward modeling of temperature signal were used. The McCallum method provided a more comprehensive insight into the nature of GW-SW interaction in the top 25 cm, with GW fluxes strongly influenced by stream depth and storm events. The Darcy velocity were also found to decreased with depth in the OrgC and CSed layers, suggesting the presence of horizontal flow paths below the gravel layer. The higher velocities observed in the gravel layer suggest that the armor should not be viewed as additional protection against contaminant migration. Also, the higher seepage velocities can rapidly transport heat and nutrients to the subsurface thereby increasing the potential for gas production below the cap. Heat tracer methods if implemented properly can provide Darcy estimates with lower uncertainty compared to traditional methods.

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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