Incorporating Natural Source Zone Depletion (NSZD) into the Site Management Strategy

Abstract

This chapter summarizes three petroleum remediation case study sites that incorporated natural source zone depletion (NSZD) into their site management strategy. The sites include: a light petroleum non-aqueous phase liquid (NAPL) pipeline release in an arid climate that transitioned to an NSZD-based monitored natural attenuation (MNA) remedy after several traditional remedial actions; a former refinery in a temperate climate that incorporated NSZD with phytoremediation; and a former industrial petrochemical facility in a tropical climate with waste dense petroleum hydrocarbon NAPL that transitioned to NSZD after several decades of groundwater extraction and treatment. The narratives provide examples of how NSZD was used as an effective and sustainable/resilient management option. Key points related to the use of NSZD on a broader range of site conditions are highlighted. The importance of acknowledging, accounting, and incorporating NSZD into remedies is emphasized because significant depletion rates are frequently observed to occur at petroleum NAPL release sites. The chapter demonstrates how NSZD can be incorporated into a conceptual site model and the implemented remedy. It provides details on the importance of site risk profile and regulatory framework as well as lines of evidence to support transition to a MNA remedy inclusive of NSZD.

13.1 Introduction

The purpose of this chapter is to demonstrate natural source zone depletion (NSZD) as an effective and sustainable/resilient management remedy component for low-risk profile sites containing petroleum hydrocarbon non-aqueous phase liquid (NAPL). It stresses the importance of acknowledging, accounting, and incorporating NSZD into remedies because significant depletion rates are frequently observed to occur at petroleum NAPL release sites. This adds to the case study literature available on the subject to help practitioners see how NSZD can be used on their sites (Interstate Technology and Regulatory Council [ITRC] 2018; Cooperative Research Centre for Contamination Assessment and Remediation of the Environment [CRC CARE] 2020a, b). NSZD phenomena and measurement methods are discussed in further detail in Chap. 5.

This chapter presents a narrative of three diverse case study sites from the United States that incorporated NSZD into their site management strategy, both in the conceptual site model (CSM) and in the implemented remedy. Since NSZD occurs on most, if not all, petroleum NAPL sites, the NSZD evaluation results can be used to refine any CSM. The site-specific human health/ecological risk profile and regulatory framework are significant drivers that determine how NSZD is incorporated into the site management strategy, most logically in a monitored natural attenuation (MNA) framework. MNA has been traditionally thought of as a dissolved phase remedy, but, as described in Chaps. 1, 5, and 9, natural attenuation processes occur throughout the contaminated zone including the petroleum NAPL. It is fitting that NSZD be included in an MNA remedy because it is the term used to describe the collective, naturally occurring processes of dissolution, volatilization, and biodegradation of NAPL.

Using case studies, this chapter focuses on the relevance of the risk profile and regulatory framework to how NSZD is applied at a site. For those sites where NSZD monitoring may be included as part of the MNA remedy, this chapter also provides ideas on how NSZD rates can be assembled with other lines of evidence to support the use of or transition to MNA in various remedy configurations. Finally, it also shows how the NSZD remedy component can be implemented and monitored.

This chapter documents how NSZD was applied at diverse case study sites. At each site, NSZD was an accepted component of the site management strategy to control and degrade in-situ petroleum hydrocarbons. This chapter uses a combination of case study illustration and call out boxes of key points to relate important concepts to various other types of sites.

The case studies were chosen to show how NSZD can be applied in a wide range of regulatory, geographic, hydrogeologic, geochemical, and release conditions. While it is impossible to cover all situations, the three case studies cover typical scenarios. More importantly, they show the type of background information considered, provide a narrative on how the information was used to obtain a decision document with NSZD accepted, and highlight approaches that are likely typical of NSZD-oriented remedial programs. Extrapolation from these cases to readers’ projects may help to facilitate more rapid integration of NSZD into remedial action plans and stakeholder acceptance. This chapter shows how NSZD can be used in a multitude of different ways. In doing so, it illustrates how adaptable and effective NSZD can be in the management and remediation of petroleum NAPL-contaminated sites.

The reader is advised to use discretion when interpreting the case studies and call out boxes, and to work closely with technical and regulatory teams when considering how to include NSZD in site management at their sites.

13.2 Overview of Case Study Site Setting

Three sites were selected to demonstrate a broad range in use of NSZD for site management.

Site A is a small petroleum light non-aqueous phase liquid (LNAPL) pipeline release site where NSZD-based MNA was applied as a treatment train transition remedy after excavation, soil vapor extraction (SVE), and LNAPL recovery.

Site B is a large former refinery site where a combination of NSZD with phytoremediation and hydraulic controls was used to manage LNAPL.

Site C is a medium-sized former industrial petrochemical facility with waste heavy petroleum hydrocarbon dense non-aqueous phase liquid (DNAPL) (no chlorinated compounds) where NSZD-based MNA was implemented as a transition remedy after several decades of groundwater extraction and treatment.

Table 13.1 further summarizes the petroleum releases that occurred and the general setting at each case study site. The CSMs are presented in Sect. 13.4 and timelines of events for each site are presented in Sect. 13.5.

Table 13.1 Summary of case study site conditions

13.3 Importance of Site Risk Profile and Regulatory Framework

The site human health/ecological risk profile is perhaps the critical determinator of remedial approaches considered and, ultimately, selected for use. The regulatory framework is perhaps the most significant factor that drives how NSZD is selected and incorporated into a decision document.

While the application of this remedy component is maturing, Box #1 shows there are specific stakeholder concerns that pose a challenge to obtaining approval for NSZD in decision documents. Inherent to these concerns is the perception of risk. The following will outline how the regulatory framework, and various stakeholders’ risk tolerances, will play important roles in the data requirements and how NSZD is incorporated into decision documents.

Box #2 highlights site risk profiles and regulatory frameworks that are more conducive to use of NSZD as a remedy.

13.3.1 Impacts of Regulatory Drivers and Risk Profile on NSZD Data Requirements

The regulatory environment and risk profile under which a site is managed can affect what data are needed to support the selection of NSZD as a component of the site remedy.

Five primary categories of regulatory drivers for the use of NSZD at petroleum NAPL sites, and NSZD’s effectiveness at addressing each of these drivers, are shown in Table 13.2. Data collection activities should be focused on the site-specific regulatory driver(s).

Table 13.2 Regulatory drivers and NSZD’s effectiveness

For some regulatory programs, an estimate of the time needed to meet cleanup goals is required. In other programs, the rate of source reduction may be of primary concern to regulatory agencies. In the context of the risk profile, if exposure pathways are complete (current) or operable (potential future), then ensuring that risk is mitigated will drive data needs. If the risk profile and regulatory framework allow for an institutionally controlled restricted future land use and/or the typically long remedial timeframes for residual petroleum hydrocarbon remediation (i.e., a risk management plan), then the use of NSZD as part of a risk management plan may generate its own set of data requirements.

The three case studies highlighted some of the data requirements that may need to be fulfilled when using NSZD. While most of the data required was typical of petroleum NAPL remediation projects, some was unique to NSZD. The reader is cautioned to carefully consider these requirements in relation to the project timing, logistics, budget, and planning.

13.3.2 Impacts of Project Phase on Incorporating NSZD into a Remedy

Project phase is a factor in how NSZD is applied in each regulatory environment. If the release is recent and the project is in the remedy-development phase, then measuring site-specific rates or using estimated NSZD rates based on the literature and institutional knowledge of previous projects may be useful. These rates can be used as a baseline mass removal rate to compare NSZD to other remedial approaches. Regulatory agency’s receptivity to literature-based rates, or rates based on institutional knowledge, however, must be considered, as well as the uncertainties associated to these values instead of site-specific rates. The merits of using literature-based rates, or rates based on institutional knowledge, must be evaluated against the cost and time demands associated with developing a site-specific estimate of the rate. NSZD monitoring may also be a stand-alone component of an MNA remedy, in which case it would be considered when detailed remedial option analysis is being completed.

At historical release sites in a later project phase, such as remedy implementation and optimization, NSZD may be proposed as a final step in a treatment train. For many stakeholders, the use of NSZD may be challenging, as it may have been perceived as a “do-nothing” approach. This misperception may impede a switch to MNA at later stages in the project and delay approval of, or provision of, decision documents. Active and consistent education of stakeholders about the merits and effectiveness of NSZD can be key for projects at this phase.

Finally, NSZD can be incorporated in a multi-component (coupled) remedy at any phase. For example, it may be considered after other remedial approaches fail to meet remedial goals for the project. Again, stakeholders’ perceptions and expectations would have to be carefully calibrated and addressed for a project to be successful.

13.3.3 Regulatory Pathways Taken at the Case Studies

The following is a summary of the regulatory drivers at each case study site, the time when NSZD was identified as a feasible component of the remedy, and the follow-up field activities performed to refine the CSM and support use of NSZD. They illustrate the effects of site risk profile and regulatory framework on decisions related to how NSZD was incorporated into site management. A more detailed description of the NSZD measurement methods is provided in Chap. 5.

Site A:

• Land use at time of release: commercial/industrial, groundwater available as an adjudicated resource.

• Description of original complete/operable exposure pathways: no complete (current) pathways, operable (potential future) pathways included drinking groundwater, vapor intrusion into unprotected buildings, or utility workers in a trench.

• 2015—governing authority issued a Cleanup and Abatement Order to cleanup groundwater to drinking water levels.

• Cleanup was implemented using excavation, LNAPL recovery, SVE, and semiannual groundwater monitoring. After operating the LNAPL recovery system for 11 years and the SVE system for 5 years, system mass removal rates diminished.

• Monitoring with no further LNAPL recovery was proposed as a management strategy.

• The monitoring request was denied by the regulatory authority because it “… would not meet water quality objectives as LNAPL acts as a continuing source to groundwater.”

• NSZD was identified as a viable component of the MNA remedy to address immobile LNAPL entrapped in saturated sediments in a way that balanced social, financial, and economic impacts.

• The sitewide NSZD rate was quantified by measuring surface carbon dioxide (CO₂) efflux and estimating ongoing source mass reduction. See Sects. 13.4.1 and 13.4.2 for additional details on the initial Site A NSZD monitoring program and results, and Sect. 13.6.1 for the routine NSZD monitoring performed to support the MNA remedy.

Site B:

• Land use at time of release: operating oil refinery.

• Description of original complete/operable exposure pathways: complete (current) pathway to ecological receptors in the river, operable (potential future) pathways included drinking groundwater, vapor intrusion into new unprotected buildings, or utility workers in a trench.

• 2003—original Consent Order was signed between the property owners and the regulatory agency.

• Required corrective action goals included:

  • o Active product recovery of in-well LNAPL to the maximum extent practicable and to mitigate further LNAPL migration to the adjacent river.

  • o Groundwater maximum contaminant level (MCL) or Tier 2 risk-based cleanup goal for constituents of concern without an MCL.

• As part of the MNA remedy already established for the dissolved phase, NSZD was identified as a feasible additional component to address sitewide remaining LNAPL impacts and to augment the portion of the remedy associated with the primary river receptor.

• LNAPL transmissivity was tested, and NSZD rates were measured. See Sects. 13.4.1 and 13.4.3 for additional details on the initial Site B NSZD monitoring program and results, and Sect. 13.6.2 for the routine NSZD monitoring performed to support the MNA remedy. The concept of LNAPL transmissivity is further discussed in Chaps. 2 and 12.

Site C:

• Land use at time of release: operating industrial petrochemical plant.

• Description of original complete/operable exposure pathways: complete (current) pathway to ecological receptors from sediment in, and groundwater seepage into, an adjacent canal; operable (potential future) pathways include drinking groundwater, vapor intrusion into new unprotected buildings.

• 1988—a Federal hazardous waste management program issued a Permit to operate the facility and established groundwater protection standards.

• The Permit required closure of the former surface impoundments, capping of canal sediments, and operation of a groundwater recovery system to recover contaminants and achieve site-specific risk-based groundwater protection standards.

• The impoundments were stabilized/closed, and the impacted canal sediment was capped to prevent ecological contact.

• The groundwater recovery system mass removal rates diminished over the ~28-year operation period. At about the same time, a major hurricane, a series of earthquakes, and a global pandemic significantly reduced system operability.

• MNA, inclusive of NSZD, was identified as a more resilient remedy to address residual petroleum DNAPL impacts trapped in saturated soil. Costs to operate and maintain the aging existing remediation system and dispose of the hazardous waste (including overseas shipping) had increased to over one million dollars per year.

• The CSM was refined using laser-induced fluorescence (LIF) delineation of the NAPL footprint, transmissivity tests modified for behavior of DNAPL on confining layers, and two rounds of NSZD measurements. See Sects. 13.4.1 and 13.4.4 for additional details on the initial Site C NSZD monitoring program and results, and Sect. 13.6.3 for the routine NSZD monitoring performed to support the MNA remedy. LIF technologies are further discussed in Chap. 8.

13.4 Integrating NSZD into the Conceptual Site Model

Because significant NSZD is frequently observed to occur at petroleum NAPL release sites, NSZD should be used to refine the CSMs. Box #3 calls out the multitude of ways NSZD measurements can be used in CSMs. When considering NSZD as a remedy component, it is important to understand the sensitivity of various elements of the CSM to the effects of NSZD and incorporate the NSZD-specific factors in all possible aspects of its development. Developing the CSM without considering NSZD can suggest that unmitigated risks are present (such as NAPL as a constant groundwater and vapor contaminant source) and timeframes for the completion of remediation can be much longer than estimates where NSZD rates are calculated and presented in the CSM. Including and estimating NSZD rates in the CSM may improve the assessment of risks and establishes a more defensible end date for the completion of remediation.

As described in detail in CRC CARE (2018, 2020a), petroleum NAPL type, site-specific lithology, moisture content, hydrogeology, site setting and climate, and extent of petroleum NAPL are all important factors necessary to support a CSM containing NSZD. Relevant information is provided for each case study site in Table 13.1 and described in more detail in this section.

Each of the case study sites are located mostly within non-residential areas where other petroleum infrastructure exists (e.g., pipeline corridor, refinery). The exception is Site B which is in a mixed residential area with vacant land to the north. Sites B and C have groundwater migration toward surface water bodies (a river and ocean, respectively).

As noted in Sect. 13.3.3, NSZD was identified as a feasible component of the remedies. This section describes how the NSZD measurements were performed and used to refine the CSM at each case study site. A simplified CSM with brief overview of salient site conditions is presented including how it was used as a basis to revise the remedy to be inclusive of NSZD.

13.4.1 Site-Specific Factors to Consider When Estimating NSZD Rates

Prior to performing the NSZD measurements, relevant components of the CSM were assessed to identify conditions that could affect NSZD rate estimates (CRC CARE 2018). These conditions, described in Table 13.3, were taken into consideration in the design of the NSZD measurement programs at each case study site.

Table 13.3 Site-specific factors considered to estimate NSZD rates at case study sites

Fig. 13.1

Site A: illustration of CO₂ efflux at site with pervious and impervious ground cover (blue shading indicates areas where CO₂ efflux occurs over pervious ground cover areas and was included in the geospatial integration to estimate the sitewide NSZD rate)

NSZD rates were estimated at the case study sites using different CO₂ efflux methods.

In Site A, the sitewide NSZD rate was quantified using one round of 19 DCC measurements distributed across the LNAPL footprint with passive (sorbent) flux trap with ¹⁴C measurements collected at 5 of the DCC locations. One round was deemed appropriate due to the monotonous climate and similar year-round subsurface temperature conditions. The results from the DCC and traps were analyzed/geospatially integrated separately to derive a range of NSZD rates. It is important to understand the range of NSZD rates present at the site; there is inherent variability with NSZD measurements that should be quantified.

In Site B, NSZD rates were measured using DCC and passive (sorbent) flux trap with ¹⁴C methods over spring and fall seasons. DCC measurements were collected from 106 locations over the LNAPL footprint and 18 background locations. Traps were deployed at 25 locations distributed across the LNAPL footprint. NSZD rates in the early fall in temperate climates are generally at the high end of the spectrum when subsurface temperatures are warmest and vice versa in the spring. The results from the DCC and traps were analyzed/geospatially integrated separately for each spring and fall event to derive a range of NSZD rates.

In Site C, two rounds of NSZD measurements using DCC and one round of soil gas sampling for ¹⁴C analysis were completed at 35 locations over the petroleum DNAPL footprint and 4 background locations outside the DNAPL footprint. Two rounds of DCC measurements spanning a couple weeks apart was deemed appropriate due to the monotonous climate and similar year-round subsurface temperatures. ¹⁴C analysis on samples from a barium carbonate (BaCO₃) field soil gas precipitation method was performed at all DCC locations because of the extreme variability in ground cover types and non-petroleum-related contributions to CO₂ efflux across the efflux survey area (i.e., areas within and proximal to a semipermeable landfill soil cap, arid and heavy vegetation, mangrove trees, and a surface water body). The ability to correct for background at all locations maximized the data quality of the survey.

Of note, while these NSZD measurement programs are similar, NSZD monitoring options vary significantly and should be based on site-specific conditions. The range of NSZD monitoring methods are discussed in more detail in Chap. 5. The fact that all of the case studies herein use CO₂-efflux-based programs should not be implied to mean that it suits all sites. If the site-specific indicator data do not include CO₂ efflux, then other methods (e.g., biogenic heat or chemical composition change) should be used instead.

13.4.2 Site A CSM: Pipeline Release, Semi-Arid Climate

The following summarizes the CSM for Site A (as shown on Fig. 13.2):

• The age of the jet fuel release is unknown (discovered in 1995)

  Fig. 13.2

  

  Site A: CSM depiction

• Primary constituents of concern are benzene and TPH-jet fuel (C9-22) in groundwater

• Elevated LIF responses limited to thin discontinuous, coarse‐grained intervals below the water table near the release area (see Chap. 8 for more details on LIF technology)

• Significant regional water table rise occurred after the initial release and submerged the LNAPL body—pore fluid saturation test results from intact cores show that LNAPL is trapped in soil pores at residual saturation in the water-saturated media. LNAPL accumulates in wells by slow seepage of submerged LNAPL through screens.

• The LNAPL source is depleting and weathering as collectively indicated by:

  • o The NSZD rate ranges between 2,700 and 4,900 L (L) (700–1,300 gallons [gal]) LNAPL degraded per year across the site (6,600–12,000 L per hectare per year [L/ha/year] [700–1,300 gal per acre per year (gal/ac/year)] equivalent). Note the areas of CO₂ expression varied between the data evaluation methods used to estimate the NSZD range (i.e., the DCC and trap results were integrated separately). The CO₂ efflux measured by DCC and traps at ground surface is derived from oxidized CH₄ generated by methanogenesis of the LNAPL present in both the capillary and saturated zones. The CH₄ by-product is directly outgassed through the saturated zone to the vadose zone through ebullition and is transported vertically upward to the ground surface.

  • o After implementation of the past remedial activities (see Sect. 13.5.1), the recoverability of in-well LNAPL became impracticable¹ (transmissivity value of 0.005 square meters per day (m²/day) (0.05 square feet per day [ft²/day])

  • o Dissolved phase benzene concentrations decreased >93% from 2007 to 2011

  • o Drinking water quality objective exceedances in groundwater are limited to the LNAPL source area and immediate surrounding.

• Aqueous biodegradation is evident through observation of predictable differences in electron acceptors, biodegradation by-products, and redox conditions between upgradient and source area groundwater

• The perimeter monitoring wells surrounding the LNAPL body and dissolved phase plume halo remained absent of in-well LNAPL and below water quality objectives since 2013 when mechanical systems were shutdown

• The NSZD rate is ~10 times higher than the last recorded historical mechanical remediation rate (380 L [100 gal] LNAPL removed per year)

• Under the de facto NSZD remedy since 2013, there have been no complete (current) or operable (potential future) exposures due to:

  • o Large depth of impacts (>5.5 mbg [18 ft bgs])

  • o Soil gas survey results indicate concentrations do not pose a significant indoor or ambient air health risk

  • o The portion of aquifer impacted by this release is in a groundwater use restriction area and a local water purveyor supplies potable water

  • o The nearest municipal water supply well is 850 horizontal m (2,800 ft) hydraulically upgradient of the dissolved phase contaminant plume edge and screened within an aquifer that is >110 vertical m (370 ft) deeper than the LNAPL.

13.4.3 Site B CSM—Former Refinery, Temperate Climate

The following summarizes the CSM for Site B (as shown on Fig. 13.3):

• Petroleum hydrocarbon releases originated over 100 years ago and consist of crude oil and refined products

  Fig. 13.3

  

  Site B: CSM depiction

• Primary constituents of concern are benzene, MTBE, and TPH middle range hydrocarbons (C9–18) in groundwater

• Groundwater migrates toward an adjacent river

• Underlying weathered limestone is the aquifer matrix forming the primary lateral groundwater migration pathway; it is underlain by a competent limestone that retards vertical migration of groundwater

• Interceptor trenches were installed to prevent offsite migration of LNAPL and dissolved phase contaminants to neighboring properties and adjacent river

  • o Treatment wetland is used to treat extracted groundwater before discharge to river

• Remediation is augmented with a phytoremediation plot to enhance rhyzodegradation of contaminants upland of the river

• LNAPL is depleted in zones of recovery, it has not been recovered from trenches since 2016

• Transmissivity values in wells and trenches are below the lower ITRC recoverability threshold metric of 0.01 m²/day (0.1 ft²/day) (see Chap. 12 for more details on transmissivity tests and recoverability endpoints)

• LNAPL seemed to be immobile only, trapped in soil pores as evidenced by negligible LNAPL recovery, lack of in-well LNAPL accumulation, and only intermittent observation of sheens and globules in water samples

• NSZD was associated to most of the NAPL depletion at the site; the estimated NSZD rate ranges between 220,000 and 265,000 L (58,000–70,000 gal) of LNAPL degraded per year (2,800–12,000 L/ha/year [300–1,300 gal/ac/year] equivalent) significantly exceeding historical mechanical remedial actions. Note the areas of CO₂ expression varied between the data evaluation methods used to estimate the NSZD range (i.e., the DCC and trap results were integrated separately). The large magnitude of NSZD demonstrates that it has been the primary process historically reducing LNAPL saturations thereby limiting LNAPL migration. The range in the unitized NSZD rate is indicative of the relatively large difference of area of CO₂ expression on ground surface between the two monitoring events (i.e., the larger the integrated area of expression, the smaller the unitized NSZD rate).

• Under the interim measures including de facto NSZD, there are no complete (current) or operable (potential future) exposure pathways due to:

  • o Onsite residential land use is prohibited by institutional control

  • o Land is fenced and secured to avoid direct contact

  • o There are no occupied buildings

  • o The portion of aquifer impacted by this release does not qualify (due to low yield) for potable use and a local water purveyor supplies potable water

  • o Risk to river receptors is mitigated by historical interim measures, demolition of the facility, and hydraulic controls (trenches, wetlands treatment, and phytoremediation plot)

13.4.4 Site C CSM: Former Petrochemical Plant, Tropical Climate

The following summarizes the CSM for Site C (as shown on Fig. 13.4):

• Site is in an area vulnerable to extreme events (e.g., hurricanes, earthquakes)

  Fig. 13.4

  

  Site C: CSM depiction

• Primary constituents of concern are BTEX and PAHs in groundwater

• The petroleum hydrocarbon release is >40 years old and is waste material from a past petrochemical industrial process (viscosity >100 centipoise [cP])

• Shallow groundwater discharges to a canal that is tidally connected to the ocean and contains mammalian ecological receptors

• The petroleum DNAPL (1.08 specific gravity) migrated vertically downward from the waste impoundment via gravity- and density-driven forces

• The underlying limestone acts as an aquitard; very little DNAPL is observed at this interface. Most petroleum DNAPL is trapped in the overlying interbedded alluvium

• Petroleum DNAPL and the small, dissolved plume halo extend onto a neighboring industrial property

• Historical groundwater extraction focused on the downgradient edge of the DNAPL body

• Like Site A, the extent of groundwater protection standard exceedances is limited to the DNAPL source area and immediate surrounding

• Recoverability of DNAPL is low. Transmissivity tests (calculations modified for DNAPL behavior on a confining layer) at wells indicate a range between 0.001 and 0.2 m²/day (0.01–2 ft²/day). One well reported a transmissivity value greater than the high end of the ITRC recoverability threshold metric (0.08 m²/day [0.8 ft²/day])

• A saltwater wedge transition zone facilitates control of plume migration through continuous supply of sulfate to support subsurface biodegradation at the hydraulically downgradient edge

• The NSZD rate ranges from 12,000 to 15,000 L (3,200–3,900 gal) petroleum DNAPL degraded per year (1,400–1,800 L/ha/year [150–190 gal/ac/year] equivalent). The warm 30 °C subsurface temperature facilitates NSZD of the weathered petroleum DNAPL. The tight range in the NSZD rate is indicative of the relatively small difference in the area and magnitude of CO₂ expression on ground surface between the two measurement rounds

• A 3-year plume stability evaluation demonstrated the effectiveness of NSZD and showed that the DNAPL and dissolved phase plume are not unacceptably unstable in terms of DNAPL presence/absence at the perimeter and changes in plume center of mass migration distances and directions over time.

  • o An exception was localized to a couple of wells within the pre-existing DNAPL footprint and only occurred because of an earthquake that compromised well-seal integrity

• There are no complete (current) or operable (potential future) exposure pathways due to:

  • o The industrial land is fenced and secured to avoid direct contact and future land use over the DNAPL and dissolved plume footprint is prevented by deed restrictions limiting intrusive activities

  • o There are no inhabited buildings that pose a risk of volatile organic vapor intrusion

  • o The aquifer portion impacted by this release is not potable due to high salinity and low yield, and a local water purveyor supplies potable water

  • o Risk to ecological receptors in the canal was mitigated by corrective measures (i.e., sediment cap).

13.5 Incorporating NSZD into the Site Management Strategy

As noted in Sects. 13.3.3 and 13.4, NSZD was identified as a feasible remedy component at each case study site and NSZD rates were quantified and woven into updated, better-defined CSMs. That was just the start of incorporating NSZD into site management. This section and the next discuss how these sites carried NSZD further into the remedy and decision document.

13.5.1 Comparison of NSZD Rates to Historical Remedial Actions

Box #4 shows results from various remediation projects comparing mass depletion rates from NSZD (13 sites) and mechanical remedial technologies (16 systems) (Palaia et al. 2021). It documents the significance of NSZD and its effectiveness in relation to mechanical systems such as skimming and aeration technology in their early- and later-stage operation. The results are supported by the literature. Garg et al. (2017) summarized published NSZD rates and reported the middle two quartiles between 6,500 and 26,000 L/ha/year (700–2,800 gal/ac/year) with a median of 16,000 L/ha/year (1,700 gal/ac/year). CRC CARE (2020a) demonstrated that NSZD significantly outperformed LNAPL recovery at four of the six studied sites and recorded average NSZD rates range of 1,900–8,400 L/ha/year (200–900 gal/ac/year) across six sites.

To directly compare NSZD and mechanical system recovery for the case study sites, timelines and cumulative removal curves were created. Figures 13.5, 13.6 and 13.7 present an overview of the remedial activities conducted at each site and the petroleum NAPL removal achieved including a rough approximation of the volume removed by NSZD during each period. In each case, it shows NSZD removed more mass of petroleum hydrocarbon than the other remedial technologies. These data provide a strong line of evidence to support use of NSZD as a primary component of a site management strategy.

Fig. 13.5

Site A: remedial action and historical mass removal timeline (based on a constant NSZD rate simplifying assumption)

Fig. 13.6

Site B: remedial action and historical mass removal timeline (based on a constant NSZD rate simplifying assumption)

Fig. 13.7

Site C: remedial action and historical mass removal timeline (based on a constant NSZD rate simplifying assumption)

The NSZD removal values plotted in Figs. 13.5, 13.6 and 13.7 are based on a constant NSZD rate simplifying assumption in agreement with Garg et al. (2017) who suggested that NSZD rates typically are pseudo zero order over time. On the contrary, Davis et al. (2022) presented estimates of site-specific gasoline and diesel NSZD rates that exhibited nonlinear trends over a period of 21–26 years. Further research is encouraged to elucidate the long-term behavior of NSZD processes under various scenarios and provide practical solutions to predict NSZD rates over time. For the case studies presented in this chapter, the annual NSZD rates were estimated using randomized values derived from the measured ranges noted in Sects. 13.4.2 through 13.4.4 and extrapolated throughout the remediation life cycle. For Sites B and C associated with facilities and known startup dates, assumed NSZD rates were ramped up over a 12-year period starting from the inception of the operation noted on Table 13.1. The NSZD rates were assumed zero over years zero to two (facility was new, assumes no releases had yet occurred), then 10% of the measured rates over the next 5 years (some releases began), and 50% of the reported rates over the next 5 years (as the release volume started to accumulate).

Due to the large size of Site B (~61 hectares [150 acres]) and its 76-year historical use as a refinery that likely processed over 3.8 million L (1 million gal) of oil per day and had multiple large releases, the cumulative LNAPL removed by NSZD is very large.

13.5.2 Multiple Lines of Evidence to Support Use of NSZD in Remedy Transition

To assess whether the case study sites were ready for transition to an NSZD-based MNA remedy, multiple lines of evidence were used to assemble the proper rationale to obtain regulatory approval. This is typical to cover the various requirements of a remedy and address the inherent uncertainty associated with environmental data. It is analogous to presenting a case in a court of law; ample evidence must be presented to demonstrate the effectiveness of NSZD and the ability of the MNA remedy to protect human health and the environment and meet regulatory requirements. Box #2 can be used as a starting point to help craft the rationale.

As discussed above, the site-specific NSZD rates were estimated to demonstrate to each respective regulatory agency that mass reduction rates are both significant and greater than rates achieved from historical physical removal. However, the case to proceed with a management strategy that includes NSZD is not made solely with an NSZD measurement and comparison to past remedial activities.

The following elements provided a well-founded basis for the inclusion of NSZD into the remedy (the relevant site, as detailed in the CSM Sects. 13.4.2 through 13.4.4, is noted in parenthesis):

• CSM of the nature/extent and fate/transport of the petroleum hydrocarbon release was well defined (each site)

• Past remedial efforts operated to practical limit of recovery (Sites A and C).

• Petroleum NAPL and dissolved phase migration is mitigated either by NSZD (Sites A and C) or by hydraulic controls (Site B)

• No complete (current) or operable (potential future) exposure pathways exist (each site)

• The remedial timeframe for NSZD is comparable to other remedial approaches (Sites A and B)

• Other than NSZD rates, ample evidence of petroleum NAPL weathering and biodegradation (Sites A and C)

Perhaps the most important element that supports the implementation of NSZD is elimination or mitigation of human health and/or ecological exposure. It is crucial to note that this can be accomplished using various means including NSZD processes, administrative and engineering controls, other remedial actions, demonstration of poor resource quality (e.g., low transmissivity or high total dissolved solids in groundwater), and physical separation (e.g., adequate oxygenated soil exists between NAPL and receptor to alleviate concern of vapor intrusion). The case studies used each of these means to eliminate exposure pathways.

In addition, the sustainability and resiliency of continued mechanical removal versus NSZD should be considered. Box #5 summarizes five sustainability factors and their associated relative ranking for NSZD and four remedial technologies that were employed at one or more of the case study sites. NSZD is considered an inherently sustainable and resilient remedial process as it:

• Is naturally occurring and effectively remediates NAPL at many petroleum sites;

• Requires minimal energy and non-renewable resource use,

• Generates minimal waste,

• Consists of nominal vulnerable infrastructure,

• Is resilient to periodic climate or natural upsets like hurricanes and earthquakes,

• Uses readily available methods/materials that can be supported solely by local labor, and

• Can be monitored by remote telemetry if using the soil gas or biogenic heat methods obviating the need for routine travel.

In particular, at Site C, due to multiple historical extreme natural events, the benefits of remedy resilience to prolonged power outages and inaccessibility were an important factor in accepting the transition to an NSZD-based MNA remedy.

Table 13.4 summarizes the primary lines of evidence (technical, sustainability, and risk reduction factors) used to support NSZD as a remedial component for each case study site. While a NSZD-based remedy can result in significant cost savings, an important consideration during remedy evaluation, cost was intentionally excluded herein due to its inconsistent consideration by stakeholders as a selection criterion. A NSZD-based remedy is inherently low cost, typically lower than other remedial alternatives, and that is a strong advantage. However, the decision to select it is also typically weighted heavily on technical, risk, and sustainability-related factors.

Table 13.4 Summary of supporting lines of evidence for NSZD in case studies

It is important to note that while NSZD was brought into later phases of the three case study projects and coupled or paired with other remedial technologies, it can also be implemented as the sole process in an MNA remedy. Box #6 calls out the baseline, stand-alone, coupled component, or treatment train transition options that exist for incorporation of NSZD in an MNA remedy.

13.6 Implementation of the NSZD-Based MNA Remedy

After assembling multiple lines of evidence to support the transition to NSZD for each case study site, regulatory agencies were engaged to discuss, finalize, and codify NSZD as a component of the MNA remedy in a decision document. The following sections narrate the evolution of approving NSZD as a remedy component at each site and the associated routine NSZD monitoring plan.

As part of the NSZD monitoring program, clear objectives and endpoints for the NSZD remedy are essential. Box #7 calls out example objectives, endpoints, and indicator data to determine the endpoints.

13.6.1 Site A: Transition to NSZD-Based MNA Remedy with Groundwater Monitoring

Following an update of the CSM incorporating NSZD, time-series trends were evaluated to confirm that after mechanical remediation ceased, contaminant concentrations in source area groundwater were decreasing and the dissolved phase and LNAPL plumes were not migrating. The regulatory agency accepted the NSZD rate measurements and approved a follow-up report that provided the proposed cleanup goals (drinking water quality), remedial timeframe of < 75 years (comparable to other alternatives), a contingency plan for biosparging in case NSZD does not meet expectations, and technical and economic rationale on why attainment of background water quality was not feasible with other alternatives.

Fundamental to the success of gaining endorsement of NSZD was the demonstration that NSZD was depleting constituents of concern in the LNAPL, refuting the agency’s initial statements that LNAPL would serve as a perpetual source to groundwater. LNAPL samples were analyzed using whole oil analysis to show the depletion of benzene and the soluble components of TPH-jet fuel compared to fresh fuel.

Additionally, passive diffusion bag samplers were used to evaluate the magnitude of petroleum biodegradates (aka., polar metabolites) in groundwater samples collected within the LNAPL footprint. This demonstrated that the fraction of total “normal” laboratory TPH results (i.e., low-flow groundwater samples analyzed without silica gel cleanup) consisting of original petroleum constituents was very small (i.e., < 10%). It also helped further support observations that the LNAPL was weathering and the remaining LNAPL footprint was largely weathering residuals rather than regulated constituents of concern. This supported classification of the site as a low-risk profile.

NSZD will be monitored annually using DCC and trap methods for rate measurements and groundwater sampling and analysis of constituents of concern for the first 5 years and every 5 years thereafter. The DCC survey and trap locations are consistent with the initial characterization event (with a small offset to avoid prior soil disturbances) to maximize comparability between monitoring events. Traps are installed at ~20% of the DCC survey locations. The NSZD performance monitoring also includes LNAPL sample analysis every 10 years to monitor continued depletion of benzene for as long as LNAPL remains in-well. The LNAPL samples are chemically analyzed using U. S. Environmental Protection Agency direct oil injection Method 8260 gas chromatography using mass spectrometry (GC–MS) with high-resolution chromatograms. In addition, the analysis of natural attenuation indicator parameters will be performed every 5 years on groundwater samples to assess electron acceptors and biodegradation by-products and affirm redox conditions conducive to biodegradation surrounding the submerged LNAPL.

NSZD was determined suitable for a future planned land use of commercial/industrial, with groundwater use restrictions for the duration of the MNA remedy by institutional controls. Institutional controls are the risk mitigation measure accompanying the NSZD remedy.

13.6.2 Site B: NSZD with Phytoremediation and Hydraulic Control

Due to the early date of historical releases (circa 1917), weathering (i.e., NSZD) had already depleted a large fraction of the petroleum LNAPL by the time remedial efforts were undertaken. LNAPL is no longer observed in site recovery trenches, and thin thicknesses of mobile LNAPL was only measurable in a few site wells.

Following the update of the CSM incorporating NSZD, further testing confirmed LNAPL transmissivity values below 0.01 m²/day (0.1 ft²/day) (the ITRC lower-end threshold metric for effective recovery⁴) and appropriate evidence of ongoing natural attenuation of dissolved phase constituents of concern. The timeframe to achieve cleanup goals in groundwater for each remedial alternative was estimated at generally < 50 years with estimates of > 100 years in isolated LNAPL areas. A corrective action plan was provided to the regulatory agency containing a proposed MNA remedy including NSZD paired with phytoremediation and limited additional short-term interceptor trench operation along the river. The regulatory agency approved the plan and issued a new Consent Order that included specific endpoints for interceptor trench operation (e.g., LNAPL transmissivity of <0.08 m²/day [0.8 ft²/days]) and contingency actions. The MNA remedy was implemented. Contingencies included an additional interceptor trench along the river and sub slab vapor barrier in case remedial expectations were not met.

NSZD rates are measured every 5 years using 25 passive flux traps co-located with the initial measurement event to evaluate performance and document long-term effectiveness. Groundwater sampling and analysis of constituents of concern is performed annually. The results of NSZD and groundwater monitoring are used to determine when interceptor trench operation is no longer necessary and/or sufficiently beneficial to continue and conversely, when contingency actions are needed.

NSZD was determined suitable for future planned non-residential land use. Surrounding land remains residential, and groundwater use is prohibited due to low yield. Interim groundwater containment and onsite Environmental Use Controls are the risk mitigation measures accompanying the portion of the site with the NSZD remedy.

13.6.3 Site C: Transition to NSZD-Based MNA Remedy with Groundwater Monitoring

NSZD was identified as a viable component of an MNA remedy after a significant hurricane knocked out the power at the site necessitating high-cost measures to maintain permit compliance. A search for more resilient and sustainable remedies ensued because the expense to supply generator power was excessive. Due to regulatory agency skepticism of NSZD at a petroleum DNAPL site, the CSM was updated using LIF, NSZD rate estimates, and a multiyear plume stability evaluation.

Further data were required by the regulatory agency to document DNAPL chemical weathering, DNAPL recoverability using transmissivity tests, and the effectiveness of the saltwater wedge in mitigating dissolved phase plume expansion. Results of the direct inject GC–MS analysis indicated very low and decreasing contents of the contaminants of concern in the DNAPL. Transmissivity tests confirmed values below the ITRC threshold for recoverability. Study of the saltwater wedge verified that an upward gradient associated with high deeper groundwater salinity limited both lateral and vertical migration. The wedge’s effect on plume attenuation (by acting as a source of sulfate) combined with natural attenuation indicator parameter data indicate biodegradation sustained by the adjacent ocean is occurring. These results combined with the evidence of NSZD in the source area provided evidence that lateral migration of the dissolved phase plume is not occurring.

Geospatial statistical analyses of groundwater sampling and petroleum DNAPL gauging results confirmed that the dissolved phase and petroleum DNAPL plumes, respectively, were stable without mechanical remediation. NSZD was determined to be more resilient to the impacts of extreme weather events (primarily high winds, flooding, and electricity outages) than other remedial alternatives.

Based on these results, the project team worked with the regulatory agency to prepare a permit modification containing NSZD and limited petroleum DNAPL recovery with an ITRC-based transmissivity endpoint metric of 0.01 m²/day (0.1 ft²/day). The NSZD rates will be monitored continuously and remotely with web-enabled instrumentation at five locations across the petroleum DNAPL footprint.

NSZD was determined suitable for a future planned land use of undeveloped open space or possibly as a solar farm. Surrounding land remains industrially zoned and groundwater remains unusable due to high salinity. Deed and site access restriction (i.e., fencing) accompany the NSZD-based MNA remedy to prevent contact with residual DNAPL and impacted groundwater.

13.7 Recommendations for Improved Practice

The use of NSZD as a site management remedy component is evolving. For example, NSZD has not been quantified at many heavy hydrocarbon and/or dense petroleum NAPL sites. NSZD will become more widely understood as it is observed to occur at more petroleum NAPL release sites. This will result in expanded use and acceptance in decision documents. As a result, the weight of evidence (i.e., the number of lines of evidence) needed to support the effectiveness of NSZD and justify NSZD as a remedy component may change. As would be done with any unfamiliar remedial option, engagement with stakeholders at the outset of NSZD consideration is prudent. This will help inform starting knowledge and primarily identify the amount and types of data that will be needed to justify the inclusion of NSZD as a sole component of the MNA remedy or part of a multi-component remedy.

To emphasize earlier statements, active and consistent education of stakeholders about the merits and effectiveness of NSZD is key. Stakeholders’ perceptions and expectations must be carefully calibrated and addressed for a project to be successful.

13.8 Summary: Incorporating NSZD Into the Site Management Strategy

The narratives of three diverse case study sites presented in this chapter provide examples of how NSZD was incorporated into site management strategies. At each case study site, NSZD was incorporated both in the CSM and the implemented remedial option. Key points related to use of NSZD on a broader range of site conditions were highlighted in call out boxes. The case studies were used to demonstrate NSZD as an effective and sustainable/resilient management remedy component for low-risk sites containing petroleum NAPL. This chapter stresses the importance of acknowledging, accounting, and incorporating NSZD into remedies because NSZD is observed to occur at most, if not all, petroleum NAPL release sites.

The reader is advised to use discretion when interpreting the case studies and call out boxes, and to work closely with technical and regulatory teams when considering how to include NSZD in site management at their sites.