Degassing Method Natural Source Zone Depletion Case Study
Volume 8, Issue 3 | September 2020


Applied NAPL Science Review

Degassing Method Natural Source Zone Depletion Case Study 

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
Mark Lyverse, PG, Chevron Energy Technology Company
Brent Stafford, Shell Oil Co.

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.

Degassing Method Natural Source Zone Depletion Case Study 

Lisa Reyenga
GEI Consultants, Inc.

Natural Source Zone Depletion (NSZD) is the collective natural processes that reduce the mass, saturation, and/or mobility of light non-aqueous phase liquid (LNAPL) in the subsurface. These processes include volatilization, dissolution, and biodegradation. NSZD of LNAPL begins from the moment the release occurs and understanding it is a critical component of the LNAPL conceptual site model (LCSM). Multiple approaches are available to monitor NSZD based largely on biodegradation indicators such as efflux of carbon dioxide (CO2) at the ground surface, gradients of gas concentrations or temperature in the subsurface, or changes in groundwater geochemistry (API 2017, CRC Care 2018, ITRC 2018). 

While the indicators for NSZD processes are observed in both groundwater and the vadose zone, the majority of the mass loss occurs through vertical soil gas flux through the vadose zone. The processes that drive the vertical gas transport are shown in Figure 1.  

Figure 1 – Conceptualization of vadose zone NSZD processes (API 2017) 

Most of the NSZD rate estimation methodologies (API 2017, CRC Care 2018, ITRC 2018) are designed around inferring NSZD rates from site data based on measuring these transport processes, assuming one-dimensional (1D) vertical transport of soil gases. For example, passive CO2 flux traps measure the total CO2 flux rate at the ground surface, and the gradient method is based on measurements of changes in the concentration of soil gases with depth. 

However, the effectiveness of the measurement methodologies is reduced where site conditions affect vertical gas transport such that the 1D transport model does not apply. These hydrogeologic conditions (e.g., saturated soils above the LNAPL source) should be identified during the development of the LCSM. NSZD processes are still occurring, but the NSZD measurement may not accurately quantify the actual rate. For example, the full CO2 efflux may not reach the ground surface over the source zone or changes in soil gas concentration with depth may not be apparent.

The degassing method for measurement of NSZD rates (described in detail in Amos et al, 2005) measures concentrations of dissolved gases in groundwater, so it is not dependent on vertical soil gas flux measurements. The degassing method is based on the observation that methanogenic biodegradation of LNAPL in the saturated zone generates methane (CH4) bubbles that strip other dissolved gases (e.g., nitrogen (N2and argon) from groundwater. NSZD rates are estimated based on the rate of CH4 production required to produce the observed depletion of these other dissolved gases. 

The degassing method is generally considered to represent a lower bound estimate of the NSZD rate. It only accounts for CH4 that dissolved into the groundwater, and it does not include direct outgassing and ebullition (Garg et al 2017)However, when the vertical transport of gases is inhibited, a greater portion of the CH4 may dissolve into the groundwater, and the degassing method under those conditions may provide a more accurate total NSZD rate.  

A case study to evaluate this hypothesis was performed at a site where LNAPL was confined and the vertical transport of gases was inhibited both by the LNAPL confining layer and a shallow perched groundwater zone above. NSZD screening (Reyenga and Kirkman 2020) was used to confirm that NSZD under methanogenic conditions was ongoing in the lower permeable unit.  

For the degassing method, groundwater samples were collected upgradient, downgradient, and within the LNAPL source zone and tested for concentrations of dissolved gases (CO2, CH4, N2). Figure 2 shows the results where CH4 concentrations in groundwater are linearly and positively correlated to N2 depletion from background. Figure 3 shows the calibration of site data to the model from Amos 2005.  

Figure 2 – Degassing case study results demonstrating relationship between dissolved methane concentration and nitrogen depletion in groundwater. 

Figure 3  Degassing case study results showing correlation between site data and the degassing model. 

The NSZD rates from the degassing method were compared to those measured from other methodologies. The rates from the degassing method were generally higher than those measured using passive CO2 flux traps, indicating that vertical transport of CO2 was inhibited. The rates were generally consistent with those measured using the biogenic heat method, where the biogenetic heat method could be utilized. 

The degassing method was determined to be an accurate methodology to estimate NSZD rates at this site and offers a valid alternative to the standard methodologies when site conditions are known or likely to inhibit the vertical transport of gases. 

A Word of Caution 

A thorough understanding of the LCSM is required to determine potentially effective methodologies for assessing NSZD and accurately interpreting the results to quantify NSZDThe degassing approach has been successfully utilized to estimate NSZD rates at a limited number of sites and is still considered an emerging approach. It may represent an underestimate of the full NSZD rate, depending on the portion of CH4 lost to direct outgassing. However, quantitative NSZD rate estimates from different methodologies are generally consistent within an order of magnitude (CRC CARE 2020).  When utilized appropriately, the degassing method appears to produce results within a similar accuracy range and can be utilized under conditions that challenge the standard methods based on vertical soil gas flux measurements. 

References 

Amos, R.T., K.U. Mayer, B.A. Bekins, G.N. Delin, and R.L. Williams, 2005. Use of dissolved and vapor-phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface. Water Resources Research, 41(2), doi:10.1029/2004WR003433. 

API, 2017. Quantification of Vapor Phase-related Natural Source Zone Depletion Processes. API Publication 4784, First Edition, May 2017.  American Petroleum Institute, 124 pp. https://www.techstreet.com/standards/api-publ-4784?product_id=1984357 

CRC Care, 2018, Technical measurement guidance for LNAPL natural source zone depletionCooperative Research Centre for Contamination Assessment and Remediation of the Environment, Technical Report series, no. 44 August 2018https://www.crccare.com/publications/technical-reports 

CRC Care, 2020, Australian case studies of light non-aqueous phase liquid (LNAPL) natural source zon depletion rates compared with conventional active recovery effortsCooperative Research Centre for Contamination Assessment and Remediation of the Environment, Technical Report series, no. 47 March 2020https://www.crccare.com/publications/technical-reports 

Garg, Sanjay, Charles J. Newell, Poonam R. Kulkarni, David C. King, David T. Adamson, Maria Irianni Renno, and Tom Sale, 2017. Overview of Natural Source Zone Depletion: Processes, Controlling Factors, and Composition Change.  Groundwater Monitoring & Remediation 37, no. 3/ Summer 2017/pages 62–81. 

ITRC (Interstate Technology & Regulatory Council). 2018. LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies. LNAPL-3. Washington, D.C.: Interstate Technology & Regulatory Council LNAPL Update Team. https://lnapl-3.itrcweb.org

Reyenga, Lisa and Andrew Kirkman. Natural Source Zone Depletion Screening Methodologies. Applied NAPL Science Review Volume 8 Issue 2, June 2020. 


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.

Exposing new compositional coverage of weathered petroleum hydrocarbons through a tiered analytical approach 

Jens Blotevogel
Master of Science
Colorado State University 

Abstract:
Petroleum hydrocarbon spills are a widespread source of contamination that may threaten ecosystem services and human health, especially due to modern society’s dependence on petroleum-based fuels. Remediation mainly relies on natural source zone depletion (NSZD) processes, which may generate partially oxidized transformation products of the spilled hydrocarbons through weathering or biodegradation processes. These byproducts containing one or more heteroatoms (N, S or O) – referred to as “polar hydrocarbons” – have increased water solubility and mobility in the environment. The unknown fate and toxicity of these complex mixtures of polar metabolites are causing growing concern. The objectives of this thesis were (1) to use a tiered analytical approach to investigate polar transformation products from various sources and (2) to identify common marker compounds that can be used for a more focused characterization of weathering processes at petroleum-contaminated sites. Previous studies have shown that the majority of weathered petroleum hydrocarbon compounds could not be detected by the GC-based analyses currently required by the United States Environmental Protection Agency due to their low volatility and high molecular weight. Therefore, standard methods may yield misleading characterizations of plumes and impede effective risk management. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), an emerging analytical technique in the field of “petroleomics” (the characterization of petroleum at the molecular level) offers unrivaled resolving power and mass accuracy; here it was used to determine the elemental composition of highly complex petroleum mixtures present in hydrocarbon-impacted sediment samples collected from field sites with varying redox and hydrogeological conditions. The tiered analysis revealed that GC-based techniques could only detect select nonpolar, low-molecular weight species (<C₃₀) present in the sediment samples, while FT-ICR MS assigned molecular formulas to tens of thousands of individual compounds with a wide range of chemical functionalities. Principal component analyses indicated that species belonging to the O₂, N₁, and HC heteroatom classes – corresponding to carboxylic acids, pyrrolic nitrogen compounds, and PAHs, respectively – may be potential marker compounds for plume characterization. FT-ICR MS results challenged existing site conceptual models and demonstrated the value of this technique as a forensic and source tracking tool. Toxic petroleum-derived N- and S-containing compounds were detected in background samples with “clean” GC chromatograms. Multiple core structures with characteristic double bond equivalents (DBE, a measure of aromaticity) and atomic H:C ratios were associated with unique sources of original spilled products. Asphaltenes, the most recalcitrant fraction of crude oil which is only detectable by FT-ICR MS, were unexpectedly discovered in samples from a former refinery, associating the contaminant plume with a different site owner. Finally, the distribution of polar hydrocarbons between hydrogeologically distinct zones demonstrated the impact of advective transport on the fate of water-soluble metabolites; a higher abundance of oxygenated products was found in an anoxic, low-permeability zone compared to a highly weathered oxic zone of high permeability, challenging previous expectations solely based on redox conditions. This thesis demonstrates the unique capabilities of FT-ICR MS to enable more comprehensive site characterizations than previously possible, consequently exposing many new unknowns about the fate and transport of polar petroleum metabolites. An important limitation of this technique is the semi-quantitative nature of the results due to preferential ionization; relative abundances of the identified elemental formulas do not directly reflect concentration. More research is also needed to inform toxicological studies and risk assessment of these polar metabolite mixtures. Nevertheless, FT-ICR MS can improve understandings of natural attenuation pathways and the long-term fate of the oxidized transformation products at petroleum hydrocarbon-contaminated sites, all in support of better site management. 
 

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
ASTM LCSM Guide
Env Canada Oil Properties DB
EPA NAPL Guidance
ITRC LNAPL Resources
ITRC LNAPL Training
ITRC DNAPL Documents
RTDF NAPL Training
RTDF NAPL Publications
USGS LNAPL Facts

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