Petroleum Hydrocarbon - AWMA

This article considers the natural processes occurring subsurface at petroleum-

contaminated sites that achieve, on their own, part of the required site remediation.

Petroleum HydrocarbonNatural Source Zone Depletion

Figure 1. Conceptualization of vapor transport-related natural sourcezone 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.

Natural Source Zone Depletion by Tom Palaia

em • The Magazine for Environmental Managers • A&WMA • April 2018


After an oil release into the subsurface environment,petroleum hydrocarbon constituents in the oil (also known aslight non-aqueous phase liquid or LNAPL) undergo variousnatural 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 ofLNAPL. These subsurface processes physically biodegradethe LNAPL directly and through mass transfer of chemicalcomponents 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 thatultimately mineralizes the petroleum products. It behoovescleanup professionals to account for it and implement a synergistic remediation strategy that melds NSZD with activecleanup methods. Best practice is evolving to measure the rateof NSZD and incorporate the results into decision-making.

Quantifying NSZDQuantifying site-specific NSZD rates is important for variousreasons. First, NSZD forms an important part of the LNAPLConceptual Site Model (LCSM). The LSCM is the representationof the physical, chemical, and biological processes that controlthe transport, migration, and impacts of contamination.Within the LCSM, NSZD supports interpretation of contaminantdelineation and concentration trends. Second, NSZD ratestypically range from 200 to 3,000 gallons of LNAPL being

naturally degraded per acre per year and can be larger thanmechanical 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 numericmass removal limit (i.e., shutdown point) for another moreenergy intensive remedy. Throughout the remediation lifecycle, measured NSZD rates can be used for a variety of decision-making purposes, ranging from technology selectionto determining the timing for remediation system shutdownor site closure.

Traditional methods of NSZD monitoring have focused ontracking biodegradation processes only in the groundwater(e.g., aerobic respiration, sulfate reduction, and methanogene-sis). 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 ground-water 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 (CH4)—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. PublicationNo. 4784, May 2017; Reprinted with permission.



the expression of NSZD in soil gas was shown to account forgreater than 70 percent of the hydrocarbon biodegradationthat occurs in the subsurface.4 A new conceptualization ofthese vapor phase-related NSZD processes that are occurringat petroleum release sites is shown in Figure 1 on the openingpage of this article.

Within the hydrocarbon-impacted soil, typically within thezone of water table fluctuation, methanogenesis occurs andgenerates CH4. CH4 is subsequently transported up into thevadose zone (i.e., above groundwater) along with smalleramounts of carbon dioxide (CO2) and volatile organic compounds (VOCs). Within the vadose zone, where theymeet atmospherically supplied oxygen (O2), the CH4 andVOCs are oxidized to form CO2 and heat. Typically, CH4

and VOCs are oxidized in the subsurface and only CO2

is emitted from ground surface.

In summary, NSZD processes occurring within the subsurfacemanifest themselves as changes to both the groundwater andsoil gas. The total NSZD rate can be best quantified by summing the petroleum hydrocarbon mass loss from bothphases. Because of the recent improvements in our under-standing of the soil gas portion of NSZD, the gaseous portion will be the principal focus of this article.

Measuring the NSZD Gaseous ExpressionThree primary methods exist to quantify the gaseous compo-nent of NSZD; these are the gradient, passive flux trap, anddynamic closed chamber methods. New NSZD monitoringmethods continue to emerge, including an approach that usesbiogenic 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 challengesand 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 O2 and create CO2. Thesebackground processes include contributions from plant material and microbes present in surficial and deeper soilscontaining natural organic matter. Therefore, correction isneeded 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 uncontaminatedsetting with similar surface and subsurface conditions.Calculation of NSZD rates using this approach involvessubtracting the soil gas flux measured at the backgroundlocation from the total flux at each survey location atopthe LNAPL footprint.

2. Use radiocarbon-14 (14C) analysis on the collected CO2.The use of 14C provides an alternative, more accuratemeans to isolate the NSZD-derived CO2 efflux withoutthe need to monitor outside areas.

After elimination of effects due to nonpetroleum-relatedprocesses, 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, www.soilgasflux.com; Reprinted with permission.



estimate a NSZD rate. A NSZD rate is typically expressed asa hydrocarbon degradation rate per unit area, such as ingrams per square meter per day (g/m2/d). Using O2 or CO2

flux input, the NSZD rate is calculated by multiplying thebackground corrected gas flux, typically expressed as micromoles per square meter per second (µmol/m2/s), by the molar ratio of hydrocarbon degraded in a representativemineralization reaction as shown for octane in Equation 1:

2 C8H18 + 25 O2 —> 16 CO2 + 18 H2O(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 peryear, or gal/acre/yr), using the LNAPL fluid density.

Gradient MethodThe gradient method uses O2 or CO2 concentration profilesin soil gas and select soil properties (e.g., effective soil gas diffusion coefficients) to estimate the flux of gases throughthe vadose zone using Fick’s first law.5 The change in concen-tration of these gases with depth within un-impacted soilabove the petroleum hydrocarbons is used as a basis for estimating the flux using Equation 2:

flux = Dveff dC/dz (2)

where D veff is the effective diffusion coefficient of the gas of

interest in the vadose zone soils and dC/dz is the verticalconcentration gradient of the gas being used to estimateNSZD rates.

A typical gradient method NSZD monitoring setup primarilyincludes a nested set of soil vapor probes installed above theLNAPL-impacted soils (see Figure 2). Soil gas samples arecollected from the probes using industry-standard proceduresand analyzed using a field landfill gas meter, for example.

Passive Flux TrapA passive flux trap is a flow-through, static chamber fittedwith a chemical trap that has historically been used to measure soil-surface CO2 efflux. The passive flux trapmethod was recently adapted for NSZD monitoring6 and subsequently commercialized and further refined by E-Flux,LLC. The E-Flux CO2 trap employs a dual-sorbent design tocollect CO2 leaving the subsurface (see Figure 3). The trapsare installed in the shallow ground surface (i.e., 1–3 inchesdeep) and left in place for a 1–2 week timeframe.

Over this time, CO2 derived from LNAPL degradation (i.e.,fossil fuel-derived CO2) migrating upward from the subsurfaceto the atmosphere, is collected inside the receiver pipe by abottom caustic sorbent element. An upper sorbent elementcaptures atmospheric CO2 (i.e., modern CO2) to avoid cross-contamination of the lower sorbent element, which is solely

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used for the NSZD estimate. The trap deployment time islimited so as not to allow either the top or bottom elementsto become saturated with CO2. After the deployment period,the trap is retrieved and shipped to a laboratory for analysisof CO2. The CO2 efflux is estimated by dividing the blank-corrected mass of CO2 sorbed on the lower element by thedeployment duration and the cross sectional area of the received pipe.

Dynamic Closed ChamberA dynamic closed chamber (DCC) system is an active, specially adapted, direct measurement approach to estimatesoil gas efflux at the ground surface. Historically, the DCChas 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 forNSZD monitoring. A chamber is set on a soil collar shallowlyembedded in the ground surface in order to capture vapor efflux from the soil. The DCC system then continuouslypumps a small circulation of vapor in a closed loop betweenthe chamber and an external non-destructive gas analyzerthat monitors the increase in CO2 concentration. To minimizeerrors associated with pressure differential inside and outsideof the chamber, the DCC is fitted with a pressure-equalizingvent.8 DCC has been demonstrated to be a consistent effluxmeasurement method and is used as a reference method forcomparison 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 CO2 concentration by the non-destructive infrared gas analyzer (IRGA), the temporal increase in CO2 isrecorded. The CO2 efflux is estimated using a curve fittingroutine on the time-series CO2 concentration data and dividing the results by the cross sectional area of the soil collar. The DCC collects rapid CO2 efflux measurementsover a period of approximately two minutes. A series of multiple measurements are typically recorded during each field event.

SummaryNSZD is an important, natural degradation process for reme-diation of subsurface petroleum hydrocarbons and occurssustainably at most petroleum release sites. Measuring therate of NSZD can be very helpful in implementing an effec-tive remediation strategy at these sites that optimizes the soleuse of NSZD or combined use of both NSZD and activecleanup methods. Several approaches to measuring andquantifying NSZD rates at petroleum release sites are nowavailable. Existing guidance is available to help practitionersimplement NSZD.9 em

Principal authors include: • John Evans, N.C. Department of Environmental Quality• Eric Hiser, Jorden Hiser & Joy• Gale Ho�nagle, TRC• David Jordan, ERM• Gary McCutchen, RTP Environmental Associates, Inc.• Ken Weiss, ERM

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References1. 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:http://dx.doi.org/10.1016/j.jconhyd.2014.04.006.

2. Natural Attenuation for Groundwater Remediation; NRC Committee on Intrinsic Remediation, Water Science and Technology Board, Board on RadioactiveWaste Management, Commission on Geosciences, Environment, and Resources, National Academy Press, Washington, DC, 2000.

3. 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), W02001.

4. 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 biogeochemicalreactions and multicomponent gas transport; Journal of Contaminant Hydrology. 2010, 112, 15-29.

5. Johnson, P.C., P. Lundegarg, and Z. Liu. 2006. Source Zone Natural Attenuation at Petroleum Hydrocarbon Spill Sites: I. Site-Specific Assessment Approach.Ground Water Monitoring and Remediation 26: pp 82-92.

6. McCoy, K.; Zimbron, J.; Sale, T.; Lyverse, M. Measurement of natural losses of LNAPL using CO2 Traps; Groundwater 2014; doi: 10.1111/gwat.12240.7. 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, 482-488.8. LI-COR Biosciences Inc. Soil flux chambers, LI-8100, 2016; https://www.licor.com/env/products/soil_flux/chambers.html.9. Quantification of Vapor Phase-Related Natural Source Zone Depletion Processes; Publication No. 4784; American Petroleum Institute (API), May 2017.

Tom Palaia, P.E., is with CH2M (now Jacobs), Denver, CO. E-mail: [email protected].

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