Petroleum Hydrocarbon

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1 Figure 1. Conceptualization of vapor transport-related natural source zone depletion processes at a petroleum release site. Source: American Petroleum Institute (API) Quantification of Vapor Phase-Related Natural Source Zone Depletion Processes. Publication No. 4784, May 2017; Reprinted with permission. Petroleum Hydrocarbon Natural Source Zone Depletion This article considers the natural processes occurring subsurface at petroleumcontaminated sites that achieve, on their own, part of the required site remediation.

After an oil release into the subsurface environment, petroleum hydrocarbon constituents in the oil (also known as light non-aqueous phase liquid or LNAPL) undergo various natural degradation

Natural source zone depletion (NSZD) is a term used to describe the collective, naturally occurring processes that result in mass losses of LNAPL.

1 NSZD is occurring on all petroleum release sites. At most release sites, it is a sustained, naturally occurring process that ultimately mineralizes the petroleum products.

Best practice is evolving to measure the rate of NSZD and incorporate the results into decision-making. Quantifying NSZD Quantifying site-specific NSZD rates is important for various reasons.

2 After an oil release into the subsurface environment, petroleum hydrocarbon constituents in the oil (also known as light non-aqueous phase liquid or LNAPL) undergo various natural degradation processes, including dissolution, volatilization, and biodegradation. Natural source zone depletion (NSZD) is a term used to describe the collective, naturally occurring processes that result in mass losses of LNAPL. These subsurface processes physically biodegrade the LNAPL directly and through mass transfer of chemical components to the soil moisture and groundwater. 1 NSZD is occurring on all petroleum release sites. At most release sites, it is a sustained, naturally occurring process that ultimately mineralizes the petroleum products. It behooves cleanup professionals to account for it and implement a synergistic remediation strategy that melds NSZD with active cleanup methods. Best practice is evolving to measure the rate of NSZD and incorporate the results into decision-making. Quantifying NSZD Quantifying site-specific NSZD rates is important for various reasons. First, NSZD forms an important part of the LNAPL Conceptual Site Model (LCSM). The LSCM is the representation of the physical, chemical, and biological processes that control the transport, migration, and impacts of contamination. Within the LCSM, NSZD supports interpretation of contaminant delineation and concentration trends. Second, NSZD rates typically range from 200 to 3,000 gallons of LNAPL being naturally degraded per acre per year and can be larger than mechanical remedies such as oil recovery systems. Third, measured NSZD rates can also form the basis for remediation technology selection, design, and optimization. An example would be using the NSZD rate as the numeric mass removal limit (i.e., shutdown point) for another more energy intensive remedy. Throughout the remediation life cycle, measured NSZD rates can be used for a variety of decision-making purposes, ranging from technology selection to determining the timing for remediation system shutdown or site closure. Traditional methods of NSZD monitoring have focused on tracking biodegradation processes only in the groundwater (e.g., aerobic respiration, sulfate reduction, and methanogenesis). This monitoring is associated with a remedy called monitored natural attenuation. 2 The groundwater portion of NSZD processes can be estimated using well-established, traditional means by measuring the consumption and production of the associated reaction chemicals in groundwater samples. Regarding the expression of NSZD in the soil gas, our understanding has recently improved via emerging research. 3 A large advance occurred with respect to quantifying the significant amount of gas predominantly methane (CH 4 ) that can be produced. At a site in Bemidji, MN, for example, Figure 2. Gradient method monitoring approach. Source: American Petroleum Institute (API) Quantification of Vapor Phase-Related Natural Source Zone Depletion Processes. Publication No. 4784, May 2017; Reprinted with permission.

the expression of NSZD in soil gas was shown to account for greater than 70 percent of the hydrocarbon biodegradation that occurs in the subsurface.

Within the hydrocarbon-impacted soil, typically within the zone of water table fluctuation, methanogenesis occurs and generates CH 4. CH 4 is subsequently transported up into the vadose zone (i.e., above groundwater) along with smaller amounts of carbon dioxide (CO 2 ) and volatile organic compounds (VOCs).

Typically, CH 4 and VOCs are oxidized in the subsurface and only CO 2 is emitted from ground surface.

3 the expression of NSZD in soil gas was shown to account for greater than 70 percent of the hydrocarbon biodegradation that occurs in the subsurface. 4 A new conceptualization of these vapor phase-related NSZD processes that are occurring at petroleum release sites is shown in Figure 1 on the opening page of this article. Within the hydrocarbon-impacted soil, typically within the zone of water table fluctuation, methanogenesis occurs and generates CH 4. CH 4 is subsequently transported up into the vadose zone (i.e., above groundwater) along with smaller amounts of carbon dioxide (CO 2 ) and volatile organic compounds (VOCs). Within the vadose zone, where they meet atmospherically supplied oxygen (O 2 ), the CH 4 and VOCs are oxidized to form CO 2 and heat. Typically, CH 4 and VOCs are oxidized in the subsurface and only CO 2 is emitted from ground surface. In summary, NSZD processes occurring within the subsurface manifest themselves as changes to both the groundwater and soil gas. The total NSZD rate can be best quantified by summing the petroleum hydrocarbon mass loss from both phases. Because of the recent improvements in our understanding of the soil gas portion of NSZD, the gaseous portion will be the principal focus of this article. Measuring the NSZD Gaseous Expression Three primary methods exist to quantify the gaseous component of NSZD; these are the gradient, passive flux trap, and dynamic closed chamber methods. New NSZD monitoring methods continue to emerge, including an approach that uses biogenic heat and thermal gradients. Due to their limited applications, however, these emerging methods are not discussed in this article. Method choice is a site-specific judgment based on data quality, objectives, and site conditions. Prior to monitoring using any method, it is important to review site conditions to assess the challenges and determine the appropriate monitoring approach, limitations, and uncertainties. The expression of NSZD in soil gas is complicated by concurrent, nonpetroleum-related processes occurring subsurface that also consume O 2 and create CO 2. These background processes include contributions from plant material and microbes present in surficial and deeper soils containing natural organic matter. Therefore, correction is needed to subtract these effects prior to using the data to estimate NSZD rates. Two options to eliminate the contributions of nonpetroleum-related sources include: 1. Install measurement locations in a nearby uncontaminated setting with similar surface and subsurface conditions. Calculation of NSZD rates using this approach involves subtracting the soil gas flux measured at the background location from the total flux at each survey location atop the LNAPL footprint. 2. Use radiocarbon-14 ( 14 C) analysis on the collected CO 2. The use of 14 C provides an alternative, more accurate means to isolate the NSZD-derived CO 2 efflux without the need to monitor outside areas. After elimination of effects due to nonpetroleum-related processes, the corrected flux results can then be used to Figure 3. Schematic (left) and photo (right) of a passive carbon dioxide flux trap. Source: E-Flux, LLC, Reprinted with permission.

estimate a NSZD rate. A NSZD rate is typically expressed as a hydrocarbon degradation rate per unit area, such as in grams per square meter per day (g/m 2 /d).

4 estimate a NSZD rate. A NSZD rate is typically expressed as a hydrocarbon degradation rate per unit area, such as in grams per square meter per day (g/m 2 /d). Using O 2 or CO 2 flux input, the NSZD rate is calculated by multiplying the background corrected gas flux, typically expressed as micromoles per square meter per second (µmol/m 2 /s), by the molar ratio of hydrocarbon degraded in a representative mineralization reaction as shown for octane in Equation 1: 2 C 8 H O 2 > 16 CO H 2 O (1) Stoichiometry is used to determine a mass-based NSZD rate from soil gas flux data. The mass-based unit can be converted to a volume-based unit (e.g., gallons per acre per year, or gal/acre/yr), using the LNAPL fluid density. Gradient Method The gradient method uses O 2 or CO 2 concentration profiles in soil gas and select soil properties (e.g., effective soil gas diffusion coefficients) to estimate the flux of gases through the vadose zone using Fick s first law. 5 The change in concentration of these gases with depth within un-impacted soil above the petroleum hydrocarbons is used as a basis for estimating the flux using Equation 2: flux = D v eff dc/dz (2) where D v eff is the effective diffusion coefficient of the gas of interest in the vadose zone soils and dc/dz is the vertical concentration gradient of the gas being used to estimate NSZD rates. A typical gradient method NSZD monitoring setup primarily includes a nested set of soil vapor probes installed above the LNAPL-impacted soils (see Figure 2). Soil gas samples are collected from the probes using industry-standard procedures and analyzed using a field landfill gas meter, for example. Passive Flux Trap A passive flux trap is a flow-through, static chamber fitted with a chemical trap that has historically been used to measure soil-surface CO 2 efflux. The passive flux trap method was recently adapted for NSZD monitoring 6 and subsequently commercialized and further refined by E-Flux, LLC. The E-Flux CO 2 trap employs a dual-sorbent design to collect CO 2 leaving the subsurface (see Figure 3). The traps are installed in the shallow ground surface (i.e., 1 3 inches deep) and left in place for a 1 2 week timeframe. Over this time, CO 2 derived from LNAPL degradation (i.e., fossil fuel-derived CO 2 ) migrating upward from the subsurface to the atmosphere, is collected inside the receiver pipe by a bottom caustic sorbent element. An upper sorbent element captures atmospheric CO 2 (i.e., modern CO 2 ) to avoid crosscontamination of the lower sorbent element, which is solely Vapor Intrusion, Remediation, and Site Closure A Decade of Progress and New Challenges December 5-6, 2018 Phoenix, AZ With nearly half of US states recently releasing new guidance on VI, this conference comes at the perfect time to bring together scientists, engineers, regulators and attorneys to address the new guidance, new technology, and best practices in remediation and mitigation solutions as well as toxicology, risk assessment, real estate transactions and communication. Call for Abstracts now open! Share your knowledge and present at the premier conference on VI. Submissions due May 24 to vapor@awma.org. Abstracts should demonstrate innovative, scientific approaches to the investigation and mitigation of the vapor intrusion pathway. The following topics are suggested for platform presentations and panel discussions: Vapor Intrusion Investigation Advancements in assessment methods Laboratory issues associated with VI Predictive modeling Toxicology and risk assessment Advancements in discerning background indoor air contributions Regulatory Considerations, Mitigation Strategies, Risk Management and Legal Issues USEPA guidance for hydrocarbons Evolution of state guidance documents VI mitigation and remediation strategies Operation, maintenance and monitoring Risk management considerations Make your plans now to present, sponsor, and attend this valuable conference! Learn more at

The CO 2 efflux is estimated by dividing the blankcorrected mass of CO 2 sorbed on the lower element by the deployment duration and the cross sectional area of the received pipe.

5 used for the NSZD estimate. The trap deployment time is limited so as not to allow either the top or bottom elements to become saturated with CO 2. After the deployment period, the trap is retrieved and shipped to a laboratory for analysis of CO 2. The CO 2 efflux is estimated by dividing the blankcorrected mass of CO 2 sorbed on the lower element by the deployment duration and the cross sectional area of the received pipe. Dynamic Closed Chamber A dynamic closed chamber (DCC) system is an active, specially adapted, direct measurement approach to estimate soil gas efflux at the ground surface. Historically, the DCC has been used primarily for ecological carbon monitoring purposes, and was recently adapted for NSZD monitoring. 7 Figure 4 shows an example DCC system, the LI-COR 8100A automated soil flux system and its typical setup for NSZD monitoring. A chamber is set on a soil collar shallowly embedded in the ground surface in order to capture vapor efflux from the soil. The DCC system then continuously pumps a small circulation of vapor in a closed loop between the chamber and an external non-destructive gas analyzer that monitors the increase in CO 2 concentration. To minimize errors associated with pressure differential inside and outside of the chamber, the DCC is fitted with a pressure-equalizing vent. 8 DCC has been demonstrated to be a consistent efflux measurement method and is used as a reference method for comparison with other measurement techniques. Figure 4. LI-COR 8100A DCC apparatus and setup. Source: American Petroleum Institute (API) Quantification of Vapor Phase-Related Natural Source Zone Depletion Processes. Publication No. 4784, May 2017; Reprinted with permission.

Through continuous circulation and in-line measurement of the CO 2 concentration by the non-destructive infrared gas analyzer (IRGA), the temporal increase in CO 2 is recorded.

6 Through continuous circulation and in-line measurement of the CO 2 concentration by the non-destructive infrared gas analyzer (IRGA), the temporal increase in CO 2 is recorded. The CO 2 efflux is estimated using a curve fitting routine on the time-series CO 2 concentration data and dividing the results by the cross sectional area of the soil collar. The DCC collects rapid CO 2 efflux measurements over a period of approximately two minutes. A series of multiple measurements are typically recorded during each field event. Summary NSZD is an important, natural degradation process for remediation of subsurface petroleum hydrocarbons and occurs sustainably at most petroleum release sites. Measuring the rate of NSZD can be very helpful in implementing an effective remediation strategy at these sites that optimizes the sole use of NSZD or combined use of both NSZD and active cleanup methods. Several approaches to measuring and quantifying NSZD rates at petroleum release sites are now available. Existing guidance is available to help practitioners implement NSZD. 9 em Tom Palaia, P.E., is with CH2M (now Jacobs), Denver, CO. Tom.Palaia@CH2M.com. References 1. Ng, G.-H.C.; Bekins, B.A.; Cozzarelli, I.M.; Baedecker, M.J.; Bennett, P.C.; Amos, R.T. A mass balance approach to investigating geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN; Journal of Contaminant Hydrology 2014, 164, 1-15; doi: 2. Natural Attenuation for Groundwater Remediation; NRC Committee on Intrinsic Remediation, Water Science and Technology Board, Board on Radioactive Waste Management, Commission on Geosciences, Environment, and Resources, National Academy Press, Washington, DC, Amos, R.T.; Mayer, K.U.; Bekins, B.A.; Delin, G.N.; Williams, R.L. Use of Dissolved and Vapor-Phase Gases to Investigate Methanogenic Degradation of Petroleum Hydrocarbon Contamination in the Subsurface; Water Resources Research 2005, 41 (2), W Molins, S.; Mayer, K.U.; Amos, R.T.; Bekins, B.A. Vadose zone attenuation of organic compounds at a crude oil spill site: Interactions between biogeochemical reactions and multicomponent gas transport; Journal of Contaminant Hydrology. 2010, 112, Johnson, P.C., P. Lundegarg, and Z. Liu Source Zone Natural Attenuation at Petroleum Hydrocarbon Spill Sites: I. Site-Specific Assessment Approach. Ground Water Monitoring and Remediation 26: pp McCoy, K.; Zimbron, J.; Sale, T.; Lyverse, M. Measurement of natural losses of LNAPL using CO2 Traps; Groundwater 2014; doi: /gwat Sihota, N.J.; Singurindy, O.; Mayer, K.U. CO2-Efflux Measurements for Evaluation Source Zone Natural Attenuation Rates in a Petroleum Hydrocarbon Aquifer; Environ. Sci. Technol. 2011, 45, LI-COR Biosciences Inc. Soil flux chambers, LI-8100, 2016; 9. Quantification of Vapor Phase-Related Natural Source Zone Depletion Processes; Publication No. 4784; American Petroleum Institute (API), May New Source Review (NSR) Manual A&WMA s best-selling publication! Prevention of Significant Deterioration and Nonattainment Area Permitting The long-awaited new manual on this critical topic is based on over 25 years of rules, changes, lessons learned and solutions developed by renowned experts. The New NSR Manual is based on the 2002 Reform Rule and focuses on collecting and explaining existing policy and decisions. Chapters cover: PSD Applicability Best Available Control Technology Air Quality Analysis Impact Analysis Nonattainment Area Requirements Permit Writing and Appeals and Enforcement with a section on the history and development of the rules. A single-user license package is $269 Member and $349 Nonmember. Multi-user licenses and government rates are also available. Principal authors include: John Evans, N.C. Department of Environmental Quality Eric Hiser, Jorden Hiser & Joy Gale Hoffnagle, TRC David Jordan, ERM Gary McCutchen, RTP Environmental Associates, Inc. Ken Weiss, ERM Published online in an interactive, hyperlinked and searchable format, the 300-page Manual is a living document and will be revised as updates become available. Order online at

Presentation Based on: Other References:

Presentation Based on: Other References: M EASUREMENT O F HYDROCARBON NATURAL ATTENUATION RATES IN SOILS USING PASSIVE CO 2 FLUX TRAPS Presentation Based on: McCoy, K., Zimbron, J., Sale, T. and Lyverse, M. (2014), Measurement of Natural Losses