Natural Source Zone Depletion of Petroleum Hydrocarbon NAPL

Abstract

In the last decade, it has become widely recognized that petroleum found in soil and groundwater in the form of non-aqueous phase liquid (NAPL) is depleted by naturally occurring microbial communities. Losses of petroleum NAPL via natural processes are referred to as natural source zone depletion (NSZD). The natural loss rates of petroleum NAPL are large enough that they can often be the primary component of a site management strategy. Losses of NAPL through NSZD processes provide by-products such as CO₂, CH₄, and heat. As such, based on consumption of O₂, production of CO₂ and CH₄, generation of heat, or changes in petroleum NAPL chemical composition over time, a variety of methods have been developed to measure NSZD rates. Each method has advantages and limitations. Therefore, care is needed to select the method that best fits site conditions and site- and project-specific data quality objectives.

5.1 Overview of NSZD Process

Inadvertent releases of subsurface petroleum hydrocarbons in the form of light non-aqueous phase liquids (LNAPLs) and non-chlorinated dense non-aqueous phase liquids (DNAPLs), collectively termed “petroleum NAPL,” are a common occurrence in the industrial world. Natural mechanisms in the subsurface depleting petroleum NAPL in the source zone are referred to as natural source zone depletion (NSZD) (ITRC 2009, 2018). NSZD begins right after a release. Volatilization and dissolution occur, but biodegradation eventually predominates as intrinsic microorganisms acclimate and use the petroleum NAPL as a growth substrate (ITRC 2018).

Significant NSZD is frequently observed to occur at petroleum NAPL release sites through a combination of direct-contact biodegradation of the petroleum NAPL body itself, biodegradation of solubilized hydrocarbons at the oil/water interface, volatilization of petroleum NAPL constituents followed by biodegradation in the vadose zone (as further discussed in Chap. 6), and, to a lesser extent, dissolution of petroleum NAPL constituents and aqueous biodegradation in the saturated zone (Johnson et al. 2006; ITRC 2018; CRC CARE 2018).

Evolution of biodegradation reactions in the petroleum NAPL source zone is dependent on available electron acceptors (Baedecker et al. 2011). Biodegradation of petroleum NAPL constituents occurs across the entire thickness of the smear zone (i.e., from unsaturated, to partially, to fully saturated) by naturally occurring microorganisms (ITRC 2018). Biodegradation occurs via a multitude of mechanisms in both aerobic and anaerobic conditions. Aerobic biodegradation occurs where ample oxygen (O₂) is present. Over time, depletion of O₂, manganese (Mn⁺⁴), nitrate (NO₃), and iron (Fe⁺³) leads to sulfate reduction and methanogenesis becoming the primary drivers of biodegradation (Bekins et al. 2005; Atekwana and Atekwana 2010; Molins et al. 2010; Irianni Renno et al. 2016; Garg et al. 2017; Smith et al. 2022). Following Fig. 5.1, methanogenesis is central to NSZD. While methanogenesis can be slow, it has the remarkable advantages of not being limited by the availability of an electron acceptor (API 2017). Methanogenesis is a multi-step syntrophic process that is intricate, and, as a metabolic phenomenon, two groups of organisms work together to degrade complex organics (Garg et al. 2017).

Fig. 5.1

Conceptual model of primary NSZD process (modified from Amos et al. 2005; Irianni-Renno et al. 2016; Karimi Askarani et al. 2018). The primary heat source occurs in Region 2, and heat generation through other processes (e.g., methanogenesis) is negligible

As illustrated in Fig. 5.1, under anaerobic conditions in Region 3, the consumption of hydrocarbons by methanogenic organisms produces methane (CH₄) and carbon dioxide (CO₂). Gas bubbles form from local exceedances of aqueous-phase gas solubilities in the saturated zone due to produced CH₄ and CO₂. Buoyancy forces can overcome capillary forces when the gas bubbles become sufficiently large leading to upward ebullition and outgassing of gases including CH₄ and CO₂ along with volatile organic compounds (VOCs) into the saturated zone (Amos et al. 2005; Ramirez et al. 2015; Garg et al. 2017). In the overlying vadose zone, the upward advection–diffusion-driven flux of CH₄ is encountered by a downward flux of O₂ from atmosphere (Region 2). Where the CH₄ and O₂ meet in the vadose zone, CH₄ is converted to CO₂ by methanotrophs and generates heat via the exothermic oxidation reaction (Stockwell 2015; Irianni Renno et al. 2016; Garg et al. 2017). Additionally, downward flux of O₂ may be consumed periodically by VOCs instead of CH₄ (Davis et al. 2005; Sookhak Lari et al. 2019).

Direct-contact biodegradation occurs in the immediate proximity to the petroleum NAPL, within pores with oil where air-phase porosity is present (e.g., top of an LNAPL body). By-product gases from this reaction are directly outgassed to the vadose zone and do not enter the aqueous phase. Research conducted at the Bemidji crude oil site attributed over 80% of the observed carbon efflux to direct-contact biodegradation and outgassing phenomena (Ng et al. 2015; ITRC 2018).

5.2 Measuring NSZD Rates

Measurement of NSZD rates is central to develo** a conceptual understanding of petroleum NAPL sites, develo** rational remedies, and tracking progress to clean up. First, it is critical to establish the purpose for which NSZD data will be used (i.e., establish data quality objectives). Initially, site characterization and conceptual site model (CSM) development may be the primary driver for measuring NSZD. With time, motives for resolving NSZD rates can move to support remedy selection decision and monitoring progress to clean up. Moreover, selection of an NSZD measurement scope and method relies on various other aspects of the petroleum NAPL CSM. Elements of the CSM, such as extent of NAPL, depth to groundwater, water table fluctuation, moisture content, lithology, and others, can impact the NSZD measurement method and rates. Key elements of the CSM and how they relate to NSZD measurement are described in detail in CRC CARE (2018).

Temporal and geospatial dynamics in the subsurface affect fluxes of O₂, CO₂, CH₄, and the associated heat. Incidentally, NSZD rates measured using these parameters may be variable in time and space. For instance, NSZD rates have been found to increase with temperature (Kulkarni et al. 2022b). NSZD rates can also vary from point to point with differences in the petroleum NAPL vertical distribution, condition (i.e., unconfined or confined), and proximity to the periphery of the NAPL body (CRC CARE 2018). For instance, as shown in Fig. 5.2, consumption of O₂ or production of CO₂ is low with a flat rate of change in concentration over depth within the vadose zone in the background location. Due to the absence of CH₄ and VOCs, no NSZD-related heat is generated at this location. In contrast, over the petroleum NAPL footprint where NSZD is occurring, there is significant O₂ consumption and CO₂ production with a rapid change in concentration over depth. Also, the presence of CH₄ and VOC provides strong heat signals in the subsurface. Hence, understanding the NAPL distribution is a crucial first step in specifying the locations for NSZD measurements.

Fig. 5.2

Hypothetical soil gas concentration profiles and biogenic heat at background and high NSZD rate area (modified from CL: AIRE 2014; CRC CARE 2018)

The simplified conceptualization of processes governing NSZD (Fig. 5.1) shows the possible fluxes of gasses and heat generated for the methanogenesis-dominated case. Accordingly, following CRC CARE (2020) the most frequent methods for quantifying NSZD rates from the biogases focus on:

• (1) vertical profiles of CO₂, O₂, CH₄, and VOCs concentrations in the vadose zone

• (2) near surface CO₂ efflux, and/or

• (3) temperature gradients in the vadose zone.

In addition, the NSZD rate can be quantified via observed changes in petroleum NAPL chemical composition over time (Ng et al. 2014 and 2015; DeVaull et al. 2020). This chapter includes a description of the chemical compositional change method but excludes discussion of dissolved-phase methods. Details of those generally well-established methods are covered in detail in various other available literature (CRC CARE 2018; ITRC 2009; NRC 2000).

In the following sections, methods for quantifying NSZD rates are summarized to provide a basis for quantifying NSZD rates including:

• (i) Concentration gradient

• (ii) Passive flux trap

• (iii) Dynamic closed chamber

• (iv) Soil temperature

• (v) Chemical composition change.

Table 5.1 summarizes key attributes of these NSZD rate measurement methods (Tracy 2015; CRC CARE 2018; Karimi Askarani 2019). They are discussed in more detail below.

Table 5.1 Summary of NSZD rate measurement characteristics (based on Tracy 2015; API 2017; CRC CARE 2018; Karimi Askarani 2019)

Overall, these data can be used to (i) demonstrate that NSZD is occurring; (ii) quantify rates of NSZD; (iii) incorporate NSZD into the CSM, and (iv) evaluate NSZD as a component of the remedy. As mentioned above, NSZD rate measurements inherently involve both temporal and geospatial variability in results and are affected by site conditions in the CSM. As such, site-specific characteristics and project goals should be carefully considered when selecting, designing, and implementing NSZD measurement methods (API 2017; ITRC 2018; CRC CARE 2018).

5.2.1 Soil Gas Methods

High levels of CH₄ and CO₂ above petroleum NAPL bodies and, uptake of O₂, provide important lines of evidence for NSZD. Here, CH₄ and CO₂ produced through methanogenic processes within the petroleum NAPL smear zone are transported via diffusion and advection upward through the vadose zone. Where CH₄ encounters O₂, it is oxidized to CO₂. Hence, NSZD rates can be quantified by measuring the flux of the gasses. Notably, background correction is required for all soil gas flux-based methods to isolate the gas flux associated with NSZD (J_NSZD) from natural soil respiration (J_Background). Following Sihota et al. (2011):

$$J_{{{\text{NSZD}}}} = J_{{{\text{Total}}}} - J_{{{\text{Background}}}}$$

(5.1)

After background correction, the gas flux (J_NSZD) can be converted to an NSZD rate using stoichiometry following CRC CARE (2018) as:

$$R_{{{\text{NSZD}}}} = \left[ {\frac{{J_{{{\text{NSZD}}}} m_{r} {\text{MW}}}}{{10^{6} }}} \right] \times \frac{{86{,}400\,{\text{s}}}}{d}$$

(5.2)

where $R_{{{\text{NSZD}}}}$ is the total hydrocarbon degraded or NSZD rate (g/m²/d), $J_{{{\text{NSZD}}}}$ is the background-corrected soil gas flux measurement (micromoles per square meter per second (μmol/m²/s)), $m_{r}$ is the stoichiometric molar ratio of hydrocarbon degraded to CO₂ produced (unitless), and MW is the molecular weight of a representative hydrocarbon in the petroleum NAPL (g/mol).

It is important to note that the equations above use CO₂ efflux as the basis for the NSZD rate calculation. If site conditions do not allow for complete conversion of all CH₄ and VOCs emitted from the NSZD processes to be converted to CO₂, (i.e., O₂ diffusion into the subsurface is slowed by semi- or impervious ground cover or excessive soil moisture), the use of these equations will result in an underestimate of the NSZD rate. To accurately quantify the NSZD rate in this case, additional measurements of CH₄ and/or VOC efflux are required, and equations specific to CH₄ stoichiometry and VOC mass loss must be derived for the site. The total NSZD rate for the site will then equal the summation of the stoichiometric contributions from both CO₂, CH₄, and VOC mass loss.

5.2.1.1 Concentration Gradient Method

In the vadose zone, O₂ and CO₂ concentration profiles change due to underlying NSZD processes. Using background-corrected soil gas concentration gradients (i.e., slope of the concentration versus depth profile) of either O₂ or CO₂, and assuming that diffusion is the primary transport process, Fick’s equation can be used to estimate gas fluxes and correspondingly NSZD rates (Johnson et al. 2006; Lundegard and Johnson 2006; ITRC 2009). The gradient method is based on Fick’s first law of diffusion and used to estimate the gas flux as:

$$J_{diff} = D_{v}^{{{\text{eff}}}} \left( {\frac{{{\text{d}}c}}{{{\text{d}}z}}} \right)$$

(5.3)

where $J_{diff}$ is the steady-state diffusive flux (g/m²/s), $\frac{{{\text{d}}c}}{{{\text{d}}z}}$ is the soil gas concentration gradient (g/m³/m), and $D_{v}^{{{\text{eff}}}}$ is the effective vapor diffusion coefficient (m²/s).

The key assumptions of the gradient method include (i) diffusion is the primary governing process for gas flux, (ii) the vadose zone is homogeneous and isotropic with respect to diffusion coefficients, and (iii) it is a steady-state condition. The gradient method can be applied to any soil gas parameter, but is most commonly used with CO₂. If the concentration gradient of another gas such as CH₄ or O₂ is used, then the stoichiometry in the NSZD rate Eq. 5.2 must be modified accordingly.

Measurement Procedure—As illustrated in Fig. 5.3, application of the concentration gradient method includes the following:

• (1) install multilevel vapor sampling probes at petroleum NAPL-impacted and background locations,

• (2) measure O₂, CO₂, CH₄, and VOC concentrations in the sampling probes,

• (3) immediately after the soil gas concentration measurements, conduct an in-situ tracer test to estimate an effective diffusion coefficient (Johnson et al. 1998),

• (4) calculate concentration gradients (i.e., slope of the concentration profile) at background and petroleum NAPL-impacted locations to resolve background-corrected gradients, and

• (5) estimate a point in time NSZD rate based on the fluxes of gases.

Fig. 5.3

Schematic of the concentration gradient method implementation (following Johnson et al. 2006)

Details of the key steps involved in measuring soil gas gradients and estimating NSZD rates using the concentration gradient method are provided in CRC CARE 2018.

Challenges, Considerations, and Feasibility—The effective diffusion coefficient ($D_{v}^{eff}$) varies significantly with changes in soil water content and geology (Tillman and Smith 2005; Wealthall et al. 2010; Kulkarni et al. 2020). This will affect the derived NSZD rate from measured soil gas concentration profiles. Therefore, it is recommended to perform monitoring during dry weather and perform synoptic soil gas concentration measurements and diffusivity tracer tests (API 2017). Moreover, given a non-uniform vadose zone, $D_{v}^{{{\text{eff}}}}$ and soil gas concentration profiles should be measured within each unique geologic area and depth interval where gradients are present at the site. As the gradient method offers instantaneous flux on the period of measurement, additional measurements at different times of the year are required to ascertain the variability (API 2017). Gradient method can also be costly due to the high level of effort required for installation and collection of samples (CRC CARE 2018). Considering the aforementioned factors, the concentration gradient method is suitable for the sites with relatively uniform, non-stratified vadose zones (>1.5 m below root zone) (CRC CARE 2018).

5.2.1.2 Passive Flux Trap Method

Historically, flux traps were configured as chemical traps for collecting and measuring CO₂ gas venting from the subsurface to the atmosphere (Humfeld 1930; Edwards 1982; Rochette and Huchinson 2005). The historical flux trap method was modified for NSZD monitoring using a polyvinyl chloride (PVC) pipe at grade with two caustic sorbent elements (Zimbron et al. 2014; McCoy et al. 2015). As seen in Fig. 5.4, the CO₂ leaving the subsurface is absorbed by the bottom element and is converted into solid phase carbonate. The top element is used to collect atmospheric CO₂ to prevent it from reaching the bottom element. The absorbent elements are analyzed using an ASTM International method to quantify CO₂ efflux from the subsurface (McCoy et al. 2015).

Fig. 5.4

Schematic of the passive flux trap method implementation (following www.soilgasflux.com)

Following CRC CARE (2018), the key assumptions inherent to this method include (i) CO₂ migrates vertically into receiver pipe and (ii) trap results are time-integrated over a typical multiday to several week period.

Measurement Procedure—The passive flux trap method is comprised of the following general steps:

• (1) deploy the passive flux traps and leave them on-site for approximately two weeks or as needed to meet logistical needs or sorbent limitations,

• (2) retrieve the traps and return them to a specialty laboratory,

• (3) perform carbon and radiocarbon (¹⁴C–CO₂) analysis,

• (4) evaluate the background (i.e., modern) and NSZD (i.e., fossil fuel) fractions of carbon, and

• (5) calculate the NSZD rate.

Details of the key aspects of deploying passive CO₂ flux traps and using their results to estimate NSZD rates are provided in CRC CARE 2018.

Challenges, Considerations, and Feasibility—Passive flux traps provide integral measurement of CO₂ efflux over the period of deployment. Based on challenges associated with passive flux trap method, this method is best suited for unpaved sites during dry periods (API 2017). Passive flux traps are unable to obtain a representative measurement of CO₂ efflux when installed on top of or through an impervious ground cover (e.g., an asphalt paved or concrete surface). As such, the method is not well suited for sites with large impervious areas or areas with highly compacted, low-permeability surface soil (API 2017; CRC CARE 2018). Wind is another potential challenge for this method as it might cause positive or negative air pressures that could bias CO₂ efflux through the traps (Tracy 2015). Therefore, it may be necessary to monitor wind speeds at sites with excessive winds and consider the correction for wind effects. Noteworthy, E-Flux redesigned the CO₂ traps to provide more accurate result with respect to this source of error (E-Flux, LLC 2015). In the case of precipitation during deployment, preferential flow can develop due to the rain cover preventing wetting of underlying soil (Johnson et al. 2006; Maier and Schack-Kirchner 2014). Hence, it is recommended to evaluate deployment duration and schedule a time to avoid heavy rainfall events.

5.2.1.3 Dynamic Closed Chamber Method

Chamber methods have been used to assess shallow soil respiration for more than 80 years (Norman et al. 1997; Rochette and Hutchinson 2005). Recently, dynamic closed chambers (DCC) have been applied for NSZD monitoring (Sihota et al. 2011). As shown in Fig. 5.5, a soil gas flux measurement chamber is placed on a PVC collar (e.g., LI-COR 2010). Soil gas accumulating in the chamber is circulated by a small pump between it and an infrared gas analyzer (IRGA) where CO₂ concentrations are measured approximately every 2 s. The rate of accumulation of CO₂ over the total measurement period (roughly 90 s) inside a known chamber volume is used to estimate the CO₂ efflux. A pressure equilibration device is fitted to the chamber to minimize measurement artifacts that could be caused by a pressure differential between the chamber and its surroundings. For the DCC method, it is prudent to collect total CO₂ efflux measurements in a background (unimpacted) location for use in comparison with those collected over the petroleum NAPL footprint.

Fig. 5.5

Schematic of the dynamic closed chamber method implementation (following www.licor.com)

Following CRC CARE (2018), the key assumptions of this method include (i) gas flux is vertically uniform in the subsurface, and (ii) the discrete CO₂ measurement (or multiple measurements) is representative of site conditions.

Measurement Procedure—Implementation of the DCC NSZD measurement method consists of the following general steps:

• (1) install collars and allow re-equilibration,

• (2) perform soil gas efflux survey using the portable chamber and IRGA,

• (3) (optional) collect soil gas samples and send them to laboratory for analysis of ¹⁴C,

• (4) perform data validation,

• (5) quantify the NSZD fraction of the measured total CO₂ efflux using background and/or ¹⁴C–CO₂ results, and

• (6) calculate NSZD rate.

Details of the key steps in designing and implementing a DCC-based CO₂ efflux monitoring program and estimating site-wide NSZD rates using the DCC method are provided in CRC CARE 2018.

Challenges, Considerations, and Feasibility—Considering the challenges, the DCC method is best suited to unpaved sites during dry periods with a relatively uniform background gas efflux. Diurnal and seasonal fluctuations in background CO₂ efflux, shallow soil water content, wind, impervious and compacted ground cover have a significant impact on the representativeness of DCC measurements. Hence, it is necessary to avoid rainfall events and impervious areas and monitor wind speed to correct the results for elevated wind speeds (API 2017). Since the DCC measures short-term CO₂ efflux subjected to diurnal and seasonal fluctuations, multiple measurements are required to understand the range of plausible NSZD rates. Using DCC in the areas with active carbon cycling in the root zone is also challenging. Background CO₂ efflux associated with root zones and natural organic matter respiration are best resolved using a background location. For sites with more complex background levels of CO₂ efflux, ¹⁴C analysis can be used or DCC measurements can be collected during colder time of the year, when root zone/respiration activity is at a minimum (Sihota and Mayer 2012; API 2017; Crann et al. 2016; Reynolds 2019; Wozney et al. 2021).

5.2.2 Soil Temperature Methods

As mentioned above, observed elevated soil temperatures in the vadose hydrocarbon oxidation zone overly the petroleum NAPL footprint can be used to estimate NSZD rates (Sweeney and Ririe 2014; Stockwell 2015; Warren and Bekins 2015; Sale et al. 2018). Quantifying the amount of heat generated by NSZD in terms of J/s/m² or W/m² requires an estimate of the thermal gradient and the enthalpy of oxidation (Sale et al. 2018; Karimi Askarani et al. 2018; Kulkarni et al. 2020). Two methods of using soil temperatures to estimate NSZD rates are described in this chapter section.

5.2.2.1 Background-Corrected Method

Background-corrected approaches for quantifying NSZD rates using subsurface temperatures have been advanced by Sweeney and Ririe (2014), Warren et al. (2015), Sale et al. (2018), and Karimi Askarani et al. (2018). In this method, soil temperature measurements taken at discrete depths by thermocouples (Fig. 5.6) are used to obtain vertical vadose zone soil temperature profiles in both background and atop petroleum NAPL-impacted zones. Temperature at any depth in the impacted area is a function of heat generated through NSZD processes and heat from other sources (e.g., surface heating and cooling). Abiding by the assumption that all heat sources/sinks, except for the heat generated through NSZD processes, are similar in background and impacted locations, temperatures associated with NSZD can be simply estimated as (Stockwell 2015; Sale et al. 2018; Karimi Askarani et al. 2018):

where $T_{{{\text{NSZD}}}}$ is the component of temperatures associated with NSZD (°C), $T_{{{\text{Imp}}}}$ is the measured temperatures at an impacted location (°C), $T_{{{\text{Back}}}}$ is the measured temperatures at the background location (°C), $i$ is a fixed time (s), and $z$ is a fixed vertical position (m).

$$\left. {T_{{{\text{NSZD}}}} } \right|_{z}^{i} = \left. {T_{{{\text{Imp}}}} } \right|_{z}^{i} - \left. {T_{{{\text{Back}}}} } \right|_{z}^{i}$$

(5.4)

Fig. 5.6

Conceptual model and implementation schematic of the background-corrected soil temperature NSZD measurement method. The colors in the subsurface represent interpolated background corrected temperatures measured by multilevel thermocouples in the subsurface through a transect

Based on Fourier’s first law, the heat flux associated with the NSZD process is expressed by (Hillel 1982):

$$q = - K\frac{\partial T}{{\partial z}}$$

(5.5)

where q is the heat flux due to conduction (W/m²), K is the thermal conductivity (W/°C/m), and $\frac{\partial T}{{\partial z}}$ is the change in temperature with respect to distance in the vertical direction (°C/m).

As shown in Fig. 5.6, performing an energy balance on a one-dimensional vertical reference volume, NSZD heat flux is obtained by (Karimi Askarani et al. 2018):

$$\dot{E}_{{{\text{NSZD}}}} = \dot{E}_{{{\text{Top}}}} - \dot{E}_{{{\text{Bottom}}}} + \frac{{{\text{d}}E_{{{\text{Sto}}}} }}{{{\text{d}}t}}$$

(5.6)

where $\dot{E}_{{{\text{Top}}}}$ is the energy flux at the top of the reference volume (W/m²), $\dot{E}_{{{\text{Bottom}}}}$ is the energy flux at the bottom of the reference volume (W/m²), $\dot{E}_{{{\text{NSZD}}}}$ is the energy produced through NSZD over the height of energy balance element (W/m²), and $E_{{{\text{Sto}}}}$ is the stored energy over the height of energy balance element (J/m²). The background-corrected methods developed by others neglect the energy storage $\left( {\frac{{{\text{d}}E_{{{\text{Sto}}}} }}{{{\text{d}}t}}} \right)$ in Eq. 5.6 to simplify the calculations.

Equation 5.6 is used to determine the NSZD rate using the following equation:

$$R_{{{\text{NSZD}}}} = \frac{{ - \dot{E}_{{{\text{NSZD}}}} {\text{MW}}}}{{\Delta H_{r} }}$$

(5.7)

where the $R_{{{\text{NSZD}}}}$ is in (g/m²/s), $\Delta H_{r}$ is the enthalpy released during oxidation of the NAPL (6770 kJ/mol for complete mineralization of 1 mol of decane), and ${\text{MW}}$ is the molecular weight of a representative hydrocarbon in the petroleum NAPL (g/mol).

Following Karimi Askarani et al (2018), the key assumptions of the soil temperature method include (i) all factors controlling surface heating and cooling at impacted and background locations are sufficiently similar, (ii) soil thermal properties are constant through time with uniform unique values, (iii) heat conduction is the dominant mechanism of heat transfer, and (iv) horizontal transport of heat is considered negligible.

Measurement Procedure—Implementation of the background-corrected soil temperature method includes the following general steps:

• (1) install soil temperature monitoring devices in both background and petroleum NAPL-impacted locations and record temperatures at various depth intervals within the hydrocarbon oxidation zone at least daily,

• (2) quantify soil thermal properties (thermal conductivity),

• (3) perform background correction to isolate NSZD-related heat, and

• (4) calculate NSZD rate.

Challenges, Considerations, and Feasibility—Considering the following challenges with the background-corrected method, this approach is suitable for long-term NSZD monitoring at select key locations atop petroleum NAPL at sites with uniform hydrogeology, uniform ground cover, absence of heat sinks/sources (e.g., fluid-filled utility lines), and a water table deeper than 1.5 m, and background areas (absent of petroleum NAPL) that are representative of petroleum NAPL-impacted locations. Infiltration of precipitation can not only affect soil thermal properties by changes in soil water content but also play a role as a heat sink through cold seasons. Hence, it is recommended to modify soil thermal property values to incorporate these intermittent effects, if significant. In addition, backfilling and sealing the borehole equipped with thermometers can limit the infiltration. Soil heterogeneity may also result in a variable thermal gradient—heterogeneity can be addressed by using volumetric average of derived thermal property values from each unique lithology in the NSZD calculation (Warren and Bekins 2018). In the case of presence of external heat sources/sinks (e.g., pipelines), temperature monitoring should be performed at least 10 m away from external heat sources/sinks to avoid confounding background correction. Ideally, in-situ, site-specific, thermal soil properties should be measured (Karimi Askarani et al. 2021; Kulkarni et al. 2022a). Notably, given periodic temperature data (e.g., daily) continuous NSZD rates can be estimated and averaged over time to obtain time-integrated and instantaneous NSZD rate.

5.2.2.2 Non-background-Corrected Method

To estimate the NSZD rate using the background-corrected method, background locations need to be largely similar to the petroleum NAPL-impacted locations in terms of all factors controlling surface heating and cooling through time (Karimi Askarani and Sale 2020). Flawed background correction can lead to fictitious negative NSZD rates and or elevated NSZD rates in areas where no petroleum NAPL is present. Alternatively, Karimi Askarani and Sale (2020) advanced a computational method to transform subsurface temperatures to NSZD rate without background correction, known as “single stick” in the literature. In this method, soil temperature measurements at discrete depths are only needed from petroleum NAPL-impacted areas. As shown in Fig. 5.7, at an impacted location, temperature at any point is a function of surface heating and cooling ($q_{s}$), subsurface heat source associated with NSZD process ($q_{ss}$; i.e., primarily being generated via the conversion of CH₄ and O₂ to CO₂ and H₂O in the vadose zone) and position of subsurface heat source ($x^{\prime}$). Simplistically, given temperatures at two points, heat transport equations for each point can be written with $q_{s} \,{\text{and}}$ $q_{{{\text{ss}}}}$ as unknowns. The system of equations can be solved for $q_{s} {\text{and}}$ $q_{ss}$.

Fig. 5.7

Conceptual model of non-background-corrected method (following Karimi Askarani 2019)

In more detail, given a media with two primary heat sources ($q_{s}$ and $q_{{{\text{ss}}}}$), independent soil thermal properties of position and direction, and negligible horizontal heat fluxes, the governing conductive heat transfer equation (Eq. 5.8, Carslaw and Jaeger (1959); Jury and Horton (2004); Hillel (2013)) is stated as:

$$\frac{{\partial^{2} T}}{{\partial x^{2} }} = \frac{1}{\kappa }\frac{\partial T}{{\partial x}}$$

(5.8)

where T is temperature (°C), t(s) is time, x(m) is spatial coordinate, and $\kappa = K/\rho c$ is thermal diffusivity (m²/s).

Resolving Eq. 5.8 for temperatures associated with $q_{s}$ and $q_{{{\text{ss}}}}$ yields:

$$\begin{aligned} T\left( {x,t} \right) - T_{0} & = \frac{{2q_{s} }}{{\text{\rm K}}}\left\{ {\sqrt {\frac{\kappa t}{\pi }} {\text{exp}}\left( { - \frac{x^{2}}{4\kappa t}} \right) - \frac{x}{2}{\text{erfc}}\left( {\frac{x}{{\sqrt {4\kappa t} }}} \right){ }} \right\} \\ & \quad + \frac{{q_{{{\text{ss}}}} }}{\rho c}\left\{ {\sqrt {\frac{t}{\pi \kappa }} {\text{exp}}\left( { - \frac{{\left( {x - x\prime } \right)^{2} }}{4\kappa t}} \right) - \frac{{\left| {x - x\prime } \right|}}{2k}{\text{erfc}}\left( {\frac{{\left| {x - x\prime } \right|}}{{\sqrt {4\kappa t} }}} \right){ }} \right\} \\ & \quad + \frac{{q_{{{\text{ss}}}} }}{\rho c}\left\{ {\sqrt {\frac{t}{\pi \kappa }} {\text{exp}}\left( { - \frac{{\left( {x + x\prime } \right)^{2} }}{4\kappa t}} \right) - \frac{{\left( {x + x\prime } \right)}}{2k}{\text{erfc}}\left( {\frac{{\left( {x + x\prime } \right)}}{{\sqrt {4\kappa t} }}} \right){ }} \right\} \\ \end{aligned}$$

(5.9)

where $T_{0}$ is initial temperature (°C), c (J/kg/°C) is heat capacity, and ρ (kg/m³) is sediment density. Given measured subsurface temperature ($T\left( {x,t} \right)$), Eq. 5.9 can be solved in a two-equation, two-unknown system for $q_{s}$ and $q_{ss}$, following an iterative approach to determine $x\prime$. Derived calculations and procedures are explained in detail in Karimi and Sale (2020).

Following Karimi Askarani and Sale (2020), the key assumptions in this method are (i) surface and subsurface heat sources are represented as planar features, (ii) all processes controlling surface heating and cooling are included in surface heating and cooling term ($q_{s}$), and (iii) the position of subsurface heat source ($x\prime$) varies spatiotemporally.

Measurement Procedure—Implementation of the non-background-corrected soil temperature method to estimate NSZD rates consists of the following general steps:

• (1) install the soil temperature monitoring devices atop petroleum NAPL-impacted locations and record temperatures at various depth intervals within the hydrocarbon oxidation zone at least daily,

• (2) quantify soil thermal properties (thermal conductivity),

• (3) compute the NSZD-related heat, and

• (4) calculate NSZD rate.

Challenges, Considerations, and Feasibility—This advanced computational approach is suitable for long-term NSZD monitoring at select key locations atop petroleum NAPL sites with a water table depth greater than 1.5 m with the majority of the area absent of heat sinks/sources (e.g., fluid-filled utility lines). This method is applicable to sites with paved/impervious ground surface and unavailable background locations as there is no need for background correction. Infiltration of precipitation, heterogeneity, and presence of external heat sources are the challenges that need to be considered and addressed as explained in the background-corrected method. This method is computationally complex and is not publicly available. A promising aspect of the soil temperature methods is being able to obtain continuous NSZD rates that can be averaged over time to obtain a time-integrated NSZD rate.

5.2.3 Petroleum NAPL Chemical Composition Change

The chemical composition of petroleum NAPL changes over time as the weathering processes of dissolution, volatilization, and biodegradation constantly collectively act to deplete it. Therefore, the composition of petroleum NAPL can be evaluated over time as a direct measurement of petroleum NAPL losses. Biodegradation is identified as a significant contaminants of concern (COC) mass loss mechanism from the petroleum NAPL (Ng et al. 2014, 2015). Chemical analysis of petroleum NAPL can be performed to evaluate both bulk petroleum NAPL losses and losses of individual chemicals. Monitoring of losses of individual chemicals of concern is of high relevance to NSZD monitoring because none of the other methods are chemical-specific. Being chemical-specific, the results from the chemical composition change method can be used to evaluate the effectiveness of NSZD to reduce risks associated with toxicity to human health and the environment.

COC-specific NSZD rates in petroleum NAPL (R_COC-NAPL) can be calculated following CRC CARE (2018):

$$R_{{\text{COC-NAPL}}} = \frac{{{\text{d}}m_{{{\text{COC}}}} }}{{{\text{d}}t}}\rho_{{{\text{NAPL}}}} D_{{{\text{NAPL}}}}$$

(5.10)

where R_COC-NAPL is the COC-specific NSZD rate (g/m²/d), ${\text{d}}m_{{{\text{COC}}}}$ is the change in chemical content in the petroleum NAPL normalized for a conservative marker in the NAPL (g/g), t is time (d), $\rho_{{{\text{NAPL}}}}$ is the petroleum NAPL density (g/m³), and $D_{{{\text{NAPL}}}}$ is the specific volume of in-situ petroleum NAPL (m³/m²).

It is important to note that Eq. 5.10 is presented in a way that results in a unit of measurement common to the other NSZD measurement methods (i.e., g/m²/d). Use of the chemical compositional change method can be used in a way that simply tracks reductions in $\frac{{{\text{d}}m_{{{\text{COC}}}} }}{{{\text{d}}t}}$ for specific chemicals of concern. For this reason, the routine of measurement of $\rho_{{{\text{NAPL}}}}$ and $D_{{{\text{NAPL}}}}$ is considered optional.

The key assumptions in using petroleum NAPL compositional change to estimate a COC-specific NSZD rate include (i) a time-series regression of the chemical content of the petroleum NAPL is representative of its NSZD rate, (ii) in-well petroleum NAPL samples collected after purging are representative of the petroleum NAPL in the surrounding formation, and (iii) R_COC-NAPL is inclusive of losses due to dissolution, volatilization, waterborne biodegradation, and direct-contact oil biodegradation (CRC CARE 2018).

Measurement Procedure—Use of the petroleum NAPL compositional change method consists of the following general steps:

• (1) well purging and petroleum NAPL sampling from in-well or soil samples at various time periods (e.g., several years apart),

• (2) laboratory analysis of petroleum NAPL for COCs, conservative markers, and fluid density,

• (3) calculation of COC and marker loss in petroleum NAPL and normalization of COC change in mass fraction over time,

• (4) (optional) laboratory analysis of intact soil cores for capillarity and pore fluid saturation,

• (5) (optional) estimation of petroleum NAPL-specific volume, and

• (6) (optional) calculate the NSZD rate.

Details of the key aspects of using chemical compositional change in petroleum NAPL as a way to estimate chemical-specific NSZD rates are provided in CRC CARE 2018.

Recently, Devaull et al. (2020) advanced a quantitative analysis method that is consistent with constituent marker methods (Douglas et al. 1996). It consists of prescribing steps for (i) selecting the most appropriate conserved marker constituents based on measured composition data, (ii) doing parameter transformations, and (iii) performing regression analyses on time-series data sets.

Challenges, Considerations, and Feasibility—To accommodate for an anticipated high variability in the chemical quality of the petroleum NAPL, an adequate number of samples is necessary to observe the range in COC-specific NSZD rates. Calculation of the NSZD rate using multiple conservative markers is recommended that may be prudent to assess the variability in rate estimates (CRC CARE 2018). A possible constraint to this approach is the long data records (e.g., multiple years to decade) that are needed to get to resolve an NSZD rate. The petroleum NAPL compositional change method is suitable at the sites with a need for detailed understanding of NSZD related to rates of attenuation of a COC(s) that drive decision making on the remedial efforts. Overall, further research is needed to improve the calculation of depletion rates using petroleum NAPL composition data.

5.2.4 Emerging Science and Future Vision

5.2.4.1 Emerging Science

As presented in this chapter, there are ample available NSZD measurement methods; the results can be combined with other site characterization data collected for the CSM to improve understanding and remedial application of NSZD. All methods are credible but have advantages and disadvantages under certain site conditions. Regardless, all methods point to the widespread occurrence of petroleum NAPL NSZD at rates that often exceed depletions achievable with mechanically engineered remedies. As addressed in detail in Chap. 13, a current challenge is to determine the multiple lines of evidence needed to support a decision document that incorporates NSZD into a remedy that is regulatory compliant and protective to human health and the environment. As was the case in the early years of the natural attenuation remedy, the regulatory response to the emergence of NSZD science has been timid, and practitioners slow in bringing NSZD into both the CSM and remedial action at petroleum NAPL sites. Without doubt, however, NSZD is a part of the remedial solution for many (if not all) sites with petroleum liquids in soil and groundwater.

The future of NSZD-related questions being addressed today includes:

• What are the key factors governing NSZD processes and how might they be optimized to increase rates?

• What is the best approach to tracking the performance of NSZD-based remedies through time?

• How can NSZD be used to meet the modern challenges of resiliency, sustainability, and environmental and social governance?

Nascent tools for resolving the above questions are in development. These include but are not limited to cryogenic coring (Kiaalhosseini et al. 2016; Trost et al. 2018), molecular biological tools (further discussed in Chap. 10), and three-dimensional arrays of real-time sensors (e.g., temperature, ORP, pH, conductivity, and water level) that are linked to a cloud-based data storage, analytics, visualization, and reporting platform (Sale et al. 2021a, b). As this journey continues, more will be developed to help us with modern management of subsurface petroleum NAPL.

5.2.4.2 Future Vision

Applying new technologies, such as Internet of Things (IoT) and sensors, to the field of vapor, soil, and groundwater monitoring makes long-term monitoring of contaminated sites more feasible. They offer benefits in terms of real-time data enabling faster responses to adverse conditions, lower cost, improved safety, and a smaller environmental impact (Karimi Askarani and Gallo 2020; Blotevogel et al. 2021). For example, multilevel subsurface sensor-based monitoring is evolving at a rapid rate and providing ever-growing larger valuable data sets of temperature, soil oxidation reduction potential (ORP), and water levels at contaminated sites (Karimi Askarani and Sale 2021). Eventually, through cloud computing, these data sets will be managed and transformed into real-time actionable outputs such as NSZD rates and petroleum NAPL source zone depletion timeframe estimates. While IoT and cloud computing provide connection of sensors, platforms, and objects for data transmission and data transformation, synergistically, artificial intelligence and machine learning will capture the interrelationships of all this data and provide a better understanding of NSZD processes and establish the appropriate application of NSZD in managing petroleum NAPL sites (Karimi Askarani and Sale 2021).

5.2.5 Summary and Conclusion

Hundreds of thousands of sites are affected by historical releases of petroleum NAPL, including crude, fuels, lubricants, heating oil, creosote, and petrochemical wastes. In the past decade, NSZD—primarily the naturally occurring process of direct-contact oil biodegradation—has garnered considerable scientific and regulatory interest. This increased interest arises from the growing recognition that natural processes can lead to petroleum NAPL mass losses from the subsurface at rates rivaling the depletion rates achieved with mechanically engineered remedies. In this chapter, advanced methods used to measure NSZD rates based on gas flux, biogenic heat, and petroleum NAPL chemical composition are documented. The challenges and considerations with each method are provided to highlight site conditions that might control NSZD rates and help practitioners select the best approach to fit their CSM.