Natural Source Zone Depletion Screening Methodologies
Volume 8, Issue 2 | June 2020

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Applied NAPL Science Review

Natural Source Zone Depletion Screening Methodologies

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
Stephen S. Boynton, PE, LSP, Subsurface Env. Solutions, LLC
Dr. Randall Charbeneau, PE, University of Texas
Mark Lyverse, PG, Chevron Energy Technology Company
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., ExxonMobil Environmental & Property Solutions

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.

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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.

Natural Source Zone Depletion Screening Methodologies

Lisa Reyenga
GEI Consultants, Inc.

Andrew Kirkman, PE
BP Corporation North America

Natural Source Zone Depletion (NSZD) is the collective natural processes that reduce the mass, saturation, and mobility of light non-aqueous phase liquid (LNAPL) in the subsurface. These processes include volatilization, dissolution, and biodegradation. NSZD of the LNAPL begins from the moment the release occurs and understanding it is a critical component of the conceptual site model (CSM). Where NSZD is being utilized as a deliberate remedy, it is referred to as monitored NSZD (MNSZD). Multiple approaches are available for the monitoring portion of MNSZD. These methodologies infer NSZD is ongoing based on the byproducts of biodegradation, such as efflux of carbon dioxide 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).

Qualitative or semi-quantitative evaluation of NSZD has been used to evaluate if NSZD processes are ongoing and is termed “NSZD Screening” (Jewell and Wilson 2011). It is typically easier to implement and less costly than quantitative methods to estimate NSZD rates.

Examples of where NSZD Screening can be used for LNAPL sites include:

• Supporting site closure where site-specific metrics (e.g. maximum extent practical for LNAPL recovery) have been met and understanding of the fate of the source is desired.
• Refining the CSM to support a remedy evaluation. For example, providing an improved understanding of subsurface mechanisms to support enhanced bioremediation performance estimates and to determine or design the applicable methodologies for quantitative NSZD rate estimates.
• Contributing to an ongoing monitoring program to monitor progress toward site-specific metrics. NSZD Screening confirms that NSZD processes are continuing over time while compliance monitoring of specific exposure routes is performed separately.

NSZD Screening methodologies include, but are not limited to:

• Soil Gas Screening;
• Groundwater Screening;
• Thermal Screening; and
• Screening for LNAPL Compositional Change.

Each methodology is introduced below and the relative advantages and limitations are summarized in Table 1.

Soil Gas Screening

Soil gas screening involves collecting soil gas readings for oxygen, methane, and carbon dioxide. Typically, the data can be collected with a field gas meter from pre-existing vapor monitoring points or groundwater monitoring wells. Evidence that NSZD is ongoing includes depleted oxygen and enriched carbon dioxide and/or methane concentrations in comparison to background (Jewell and Wilson 2011, Sweeney and Ririe 2017). It is necessary to use an activated carbon or charcoal filter when field screening for methane to reduce interference from volatile organic compounds.

Figure 1 – In soil gas screening NSZD is indicated by decreased concentrations of oxygen and increased concentrations of carbon dioxide and/or methane, relative to background.

Groundwater Screening

There are two approaches for NSZD Screening in groundwater. The most familiar is the use of geochemical parameters in groundwater. This method is primarily applicable to NSZD occurring through the dissolution pathway and is referred to as natural attenuation (NA) or the natural attenuation indicating parameters (NAIPs) method (CRC Care 2018, ITRC 2018). NA is a component of NSZD, so evidence of NA indicates that NSZD is ongoing.

Another groundwater screening option is the degassing method. Dissolved gases (e.g. nitrogen, argon) can be “stripped” from groundwater due to partitioning into gaseous methane bubbles produced as a product of methanogenic biodegradation. Evidence of depleted dissolved gases and enriched dissolved methane in groundwater indicates that NSZD is ongoing (Amos et al 2005).

Figure 2 -Dissolved inert gases partition into gaseous methane bubbles biogenically produced in groundwater. NSZD is indicated by a linear relationship between methane concentration and nitrogen depletion relative to background (modified from Amos et al 2005).

Thermal Screening

Thermal screening involves collecting a vertical profile of soil temperature. Typically, the data can be collected with inexpensive equipment (e.g. iButtons or thermistors) from pre-existing wells.  An increased temperature in the LNAPL source area in comparison to background indicates that NSZD is ongoing (Sweeney and Ririe 2014, API 2017, CRC Care 2018, ITRC 2018).

Figure 3 – In thermal screening, NSZD is indicated by evidence of an exothermic biodegradation zone where the soil temperature is elevated relative to background (ITRC 2018).

Screening for LNAPL Compositional Change

LNAPL samples are collected and analyzed in a laboratory via gas chromatography–mass spectrometry (GC/MS) or gas chromatography with flame-ionization detection (GC/FID) to screen for LNAPL compositional change. The depletion of soluble and volatile constituents (“weathering”) in the chromatograms indicates that NSZD is ongoing (CRC Care 2018).

Figure 4 – As LNAPL migrates radially the incremental weathering with distance from the center typically results in compositional change illustrated in the chromatograms.  Benzene peak is highlighted in red and an isoprenoid is highlighted in blue.

Table 1 – Summary of the advantages and limitations of each screening methodology.

A Word of Caution

A thorough understanding of the CSM and the regulatory environment is required to effectively implement NSZD Screening at a site. Screening data are useful to document that NSZD processes are ongoing, but NSZD Screening does not directly provide quantitative NSZD rates. Critical aspects of the CSM to interpret qualitative and quantitative results include soil profile and moisture content relative to soil gas diffusion behavior.  Source delineation vertically and laterally combined with water table information will identify where to direct measurements vertically and spatially.  The LNAPL composition will aide in understanding existing composition as well as dominant weathering mechanisms, such as biodegradation versus volatilization combined with biodegradation. Whether NSZD alone is sufficient to protect both health and the environment is dependent on a comprehensive CSM and risk assessment.

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 depletion, Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, Technical Report series, no. 44 August 2018. https://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, 2018. Light Non-Aqueous Phase Liquids (LNAPL) Document Update: Evaluating and Implementing LNAPL Remedial Technologies.  Interstate Technology Regulatory Council, LNAPL Update Team, pending publication March 2018. https://www.itrcweb.org/Team/Public?teamID=73

Jewell, Kenneth P. and John T. Wilson, 2011. A New Screening Method for Methane in Soil Gas Using Existing Groundwater Monitoring Wells. Groundwater Monitoring and Remediation. Vol 31, no 3, Summer 2011, pg 82-94.

Ng, C.G.H., B.A. Bekins, I.M. Cozarelli, M.J. Baedecker, P.C. Bennett, R.T. Amos, and W.N. Herkelrath, 2015. Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN, Water Resources Research. DOI:10.1002/2015WR016964

Sweeney, Robert E. and George Todd Ririe, 2014. Temperature as a Tool to Evaluate Aerobic Biodegradation in Hydrocarbon Contaminated Soil. Groundwater Monitoring and Remediation. Vol 34, no 3, Summer 2014, pg 41-50.

Sweeney, Robert E. and George Todd Ririe, 2017. Small Purge Method to Sample Vapor from Groundwater Monitoring Wells Screened Across the Water Table. Groundwater Monitoring and Remediation. Vol 37, no 4, Fall 2017, pg 51-59.

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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.

Biotic control of LNAPL longevity – laboratory and field- scale studies

Eric Douglas Emerson
Master of Science
Colorado State University

Abstract:
Natural source zone depletion (NSZD) is an emerging strategy for managing light nonaqueous phase liquids (LNAPLs). Unfortunately, little is known about NSZD rates over extended periods of time, where heterogeneous redox conditions and changing LNAPL saturations may influence processes governing losses. Understanding long-term rates is central to anticipating LNAPL longevity under both natural and engineered conditions. Herein, laboratory and field-scale modeling studies were conducted to evaluate LNAPL longevity. Laboratory studies evaluated loss rates as a function of total contaminant concentration under sulfate-reducing (SR) and methanogenic (MG) conditions. Biotic and abiotic loss rates were determined via tracking biodegradation products and hydrocarbons in column effluents and produced gasses over time. Furthermore, compositional weathering of LNAPL was evaluated. Loss rates with elevated sulfate averaged 39.8 mmole carbon/day/m3 (±9.1 mmole carbon/day/m3). Once sulfate in the soil was depleted to influent water sulfate concentrations of 20 mg/L, subsequent average loss rates were 39.7 mmole carbon/day/m3 (±19.6 mmole carbon/day/m3). Overall, loss rates with and without elevated sulfate were similar. Furthermore, results suggested that loss rates are independent of LNAPL concentration over the range of 9,000 to 37,000 mg/kg and redox conditions observed. Loss rates independent of LNAPL concentrations indicated that biologically mediated NSZD follows zero-order kinetics over the range of conditions evaluated. Column loss rates were compared to field-measured loss rates assuming an LNAPL thickness of three meters. Given this assumption, mean observed early- and late-loss rates are 1.38 and 1.41 μmole carbon/m²/sec, respectively. Assuming decane as a representative LNAPL, observed loss rates are equivalent to 7890 and 8060 L/hectare/year. A column was sacrificed at the completion of the study. Predicted mass losses of the study equate to approximately 1% total initial LNAPL mass lost. Total petroleum hydrocarbons (TPH) soil analysis of initial and final grab samples of column soil did not detect significant mass losses. Moreover, no significant shifts in the LNAPL composition were seen during the course of the study. Mass losses in this range are difficult to accurately quantify via soil-phase hydrocarbon analyses, thus highlighting the utility of the approach used herein. An LNAPL longevity model (The Glide Path Model) was applied at a field site using a zero-order rate model for biological NSZD. LNAPL Longevity ranged from 35 to 105 years using a mean NSZD rate, plus or minus factors of 2 and ½, respectively. Active recovery was shown to have little effect on the longevity of LNAPL.

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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