LNAPL Remediation Technologies

There are a variety of Light Non-Aqueous Phase Liquid (LNAPL) remediation approaches. This article provides brief descriptions of various LNAPL remediation technologies with links for additional information, summarizes performance evaluation results, and presents the Interstate Technology and Regulatory Council (ITRC) framework for selecting LNAPL remediation technologies.

Related Articles

• LNAPL Conceptual Site Models
• NAPL Mobility
• Petroleum Hydrocarbons (PHCs)
• Vapor Intrusion - Separation Distances from Petroleum Sources

Contributors: Dr. Shahla Farhat, Dr. Thomas McHugh, and Dr. Phillip de Blanc

Key Resources:

• LNAPL Site Management: LCSM Evolution, Decision Process, and Remedial Technologies (LNAPL-3)

• Managing Risk at LNAPL Sites - Frequently Asked Questions, 2nd Edition

• Progress in Remediation of Groundwater at Petroleum Sites in California

Problem Definition

LNAPL releases can result in a range of adverse impacts, and an effective remediation approach needs to match the impact. The nature of the potential LNAPL concerns include:

• Risk to health and safety based on LNAPL composition. This includes the generation and migration of dissolved hydrocarbon plumes that can affect drinking water resources, and the generation of hydrocarbon vapors that can, in limited circumstances, create vapor intrusion risks. (See also: Vapor Intrusion - Separation Distances from Petroleum Sources)
• The migration of LNAPL itself (based on the LNAPL saturation) which expands the LNAPL source zone and, in some circumstances, can cause sheens on surface water at discharge points. (See also: NAPL Mobility)
• LNAPL-specific regulatory requirements (such as removal of LNAPL to the “maximum extent practicable”).
• Other physical or aesthetic impacts, such as odors or soil stability issues.

To address these concerns, general LNAPL remediation goals and technology-specific remediation objectives can be developed. General technology classes include:

• Mass Removal Technologies (e.g., hydraulic recovery of LNAPL)
• Mass Control Technologies (e.g., barrier wall)
• Phase Change Technologies (e.g., soil vapor extraction)
• Natural Source Zone Depletion (NSZD)

However, even with the wide range of remediation technologies that are available, LNAPL remediation can be challenging. Sale et al. (2018) described the problem this way:

Experience of the last few decades has taught us: 1) our best efforts often leave some LNAPL in place, and 2) the remaining LNAPL often sustains exceedances of drinking water standards in release areas for extended periods. Entrapment of LNAPLs at residual saturations is a primary factor constraining our success. Other challenges include the low solubility of LNAPL, the complexity of the subsurface geologic environment, access limitations associated with surface structures, and concentration goals that are often three to five orders of magnitude less than typical initial concentrations within LNAPL zones.

Because of these challenges, it is important to understand what LNAPL technologies can and can’t do to address different LNAPL concerns.

LNAPL Technologies

Figure 1. Dual Pump LNAPL Recovery System (Sale et al. 2018).

The ITRC (2018) presented an overview of remediation technologies (summarized below) based upon a survey of the LNAPL team’s experience and knowledge. Some are more innovative or have a better record of accomplishment than others. Note that enhanced aerobic biodegradation is not listed separately because aerobic biodegradation occurs during many of the other technologies including all aeration methods and In Situ Chemical Oxidation (ISCO).

• Excavation: LNAPL body is physically removed and properly treated or disposed. Accomplishes removal and/or reduction of all phases including LNAPL, adsorbed, dissolved, and vapor.

• Fluid Recovery (4 variations; excludes soil vapor extraction [SVE] or groundwater-only recovery):
  • Skimming: LNAPL is the only fluid removed using a pump or similar continuous mechanical device. There may be a small volume of incidental water recovery.
  • Vacuum Enhanced Skimming: LNAPL and vapor are the phases removed. LNAPL drawdown via skimming and vacuum induces an LNAPL gradient toward the recovery point. Also referred to as bioslurping or vacuum enhanced fluid recovery (VEFR).
  • Total Liquid Extraction: LNAPL and water are the fluids removed. Drawdown of LNAPL and groundwater induces an LNAPL gradient toward the recovery point. Groundwater drawdown may expose submerged LNAPL thereby increasing LNAPL mobility and recovery rate. Groundwater extraction may also provide hydraulic containment of potentially migrating LNAPL. Also referred to as dual pump liquid extraction (DPLE) (Figure 1)
  • Multi-Phase Extraction (MPE): LNAPL, water, and vapor are the phases removed. LNAPL drawdown, groundwater drawdown, and vacuum induce an LNAPL gradient toward the recovery point. Groundwater drawdown may expose submerged LNAPL thereby increasing the LNAPL recovery rate. Groundwater extraction may also provide hydraulic containment of potentially migrating LNAPL. Also referred to as dual-phase extraction (DPE) or two-phase extraction (TPE). MPE is also often used as a phase-change technology to expose submerged LNAPL (as well as LNAPL in the vadose zone) to volatilization and enhanced aerobic biodegradation with hydraulic LNAPL removal as a secondary remediation objective.

• Enhanced LNAPL Recovery (three variations):
  • Water Flooding (including hot water flooding): Water is injected to enhance the LNAPL gradient toward recovery wells. Hot water may be injected to reduce LNAPL viscosity and interfacial tension and further enhance LNAPL removal by hydraulic recovery.
  • Surfactant-enhanced Subsurface Remediation (SESR): A surfactant is injected to decrease interfacial tension and increase solubility. LNAPL and water are recovered hydraulically.
  • Cosolvent Flushing: A solvent is injected to increase LNAPL solubility and mobility. LNAPL and water are recovered hydraulically.

• In situ Thermal Remediation (four variations): (See also: Thermal Remediation)
  • Steam Injection: LNAPL is removed by forcing steam into the aquifer to vaporize and solubilize LNAPL, increase LNAPL recovery by reducing the viscosity and interfacial tension of LNAPL, and enhance the LNAPL gradient. Vapors, impacted groundwater, and LNAPL are recovered via vapor extraction and hydraulic recovery.
  • Thermal Conduction Heating: Soil is heated using heating elements to vaporize and solubilize LNAPL and increase LNAPL recovery by reducing the viscosity and interfacial tension of LNAPL. Vapors, impacted groundwater, and LNAPL are recovered via vapor extraction and hydraulic recovery.
  • Electrical Resistance Heating: Electrical current is used to heat soil and groundwater through subsurface electrodes to vaporize and solubilize LNAPL and increase LNAPL recovery by reducing the viscosity and interfacial tension of LNAPL. Vapors, impacted groundwater, and LNAPL are recovered via vapor extraction and hydraulic recovery.
  • In situ Smoldering: This is an emerging technology with very limited project experience. In situ smoldering is initiated through a short duration, low energy “ignition event.” Combustion is sustained by the addition of air through a well to the target treatment zone. The energy of the reacting contaminants is used to pre-heat and initiate combustion of contaminants in adjacent areas, propagating a combustion front through the LNAPL-impacted zone in a self-sustaining manner, provided a sufficient flux of oxygen is supplied.

Figure 2. Air Sparging/Soil vapor extraction system (AS/SVE).

• Air Sparging/Soil Vapor Extraction (AS/SVE): AS injects air into the saturated zone to solubilize (enhanced through turbulence), aerobically biodegrade, and volatilize LNAPL constituents from the submerged and overlying portions of the LNAPL body where airflow occurs. SVE also enhances aerobic biodegradation and volatilization of the LNAPL constituents from the vadose zone and collects vapors created by AS (Figure 2). AS or SVE can also be used individually if conditions are appropriate. A significant portion of the LNAPL depletion accomplished by AS/SVE is due to enhancement of aerobic biodegradation. LNAPL mass removal in the vadose zone is sustained during SVE because it can draw continuous LNAPL from the saturated zone into the vadose zone via capillary processes (wicking). (See also: Soil Vapor Extraction (SVE))

• Biosparging/Bioventing: Similar processes to AS/SVE, except air/oxygen is injected more slowly with the main goal being stimulation of aerobic biological degradation of LNAPL in the saturated and unsaturated zones. Various configurations are possible including inducing airflow into and through the unsaturated zone by extraction of soil vapors.

• In situ Chemical Oxidation (ISCO): LNAPL is depleted by enhanced solubilization and chemical destruction through the addition of a chemical oxidant into the LNAPL zone (e.g., hydrogen peroxide and persulfate). Chemical oxidation typically requires the addition of catalysts, stabilizers, and/or activators to control the rates of oxidation. Oxidation reactions with LNAPL can be vigorous and require controls, for example for off-gas. (See also: In Situ Chemical Oxidation (ISCO))

• Enhanced Anaerobic Biodegradation: Enhanced anaerobic biodegradation involves supplying electron acceptors other than oxygen (e.g., nitrate and sulfate). Anaerobic biodegradation rates can also be enhanced by increasing the subsurface temperature.

• Natural Source Zone Depletion (NSZD): LNAPL is degraded via naturally occurring processes of biodegradation, volatilization, and dissolution. The predominant process is biodegradation, including direct LNAPL-contact biodegradation. Biodegradation produces gaseous products, such as methane and carbon dioxide and may ultimately completely mineralize the LNAPL. (See also: Natural Source Zone Depletion (NSZD))

• Activated Carbon: Activated carbon with electron acceptors or chemical oxidants is injected into the subsurface or placed into excavations to enhance biodegradation or destruction processes. The activated carbon adsorbs organic compounds and provides a physical substrate for biomass growth. The added reagents support enhanced (aerobic or anaerobic) bioremediation or destruction by chemical oxidation. (See also: In Situ Groundwater Treatment with Activated Carbon

• Phytotechnology: Phytotechnologies use plants to remediate or contain contaminants in the soil, groundwater, surface water, or sediments. Phytoremediation is generally considered a phase-change technology, enhancing subsurface biodegradation, but, to a lesser extent, can also be considered as a mass control technology if designed for hydraulic control.

• Physical or Hydraulic Containment: Subsurface barriers (e.g., sheet piles, French drains, slurry walls, groundwater extraction, trenches, and permeable absorptive barriers) are constructed to prevent, impede, or divert LNAPL migration. It is generally a mass control technology and does not significantly reduce LNAPL mass unless designed to do so (e.g., oleophilic biobarriers).

• In situ Soil Mixing (Stabilization): LNAPL is immobilized by reducing the permeability of the LNAPL zone through in situ mixing of amendments (e.g., addition of bentonite clay or cement). In addition to reducing LNAPL mobility, it also reduces the leachability and volatility of the LNAPL constituents.

The ITRC describes what each technology can do with regard to different LNAPL concerns (e.g., dissolved phase plume migration, vapor intrusion, LNAPL body migration, etc.). A quantitative look at LNAPL remediation performance for one particular concern, dissolved phase hydrocarbon plumes that exceed action levels, is presented below. (Note this is just one LNAPL concern and remediation performance will be different if a remediation technology is being applied to address a different concern.)

Expected Remediation Performance – Managing Dissolved Plumes

Figure 3. LNAPL Remediation Technology Performance for Benzene. Data points plotted on the X-axis of the chart represent concentrations before treatment (or the first-year monitoring concentration for MNA/NSZD sites). Data points plotted on the Y-axis of the chart represent concentrations after treatment ended (or the most recent year monitoring concentration for MNA/NSZD sites).

Farhat and Newell (2016) analyzed the performance of various LNAPL remediation strategies at 53 sites based on a data-mining effort. Their evaluation attempted to address two key questions: 1) How many orders of magnitude (OoM) reduction can be achieved with specific remediation technologies? 2) How many sites have achieved a reduction to Maximum Concentration Levels (MCLs) or below after remediation? The goal was to determine a statistical “success rate” of active LNAPL remediation (where success is defined as meeting the MCLs). Seven in situ remediation technologies were investigated for their ability to reduce benzene concentrations in groundwater:

1. ISCO,
2. Air sparging,
3. Mobilizing surfactants,
4. Solubilizing surfactants,
5. LNAPL recovery,
6. Pump and Treat (P&T), and
7. Natural Source Zone Depletion (NSZD).

Data were generally obtained from remediation projects that targeted high concentration source zone areas as opposed to barrier projects or plume remediation projects.

An Order-of-Magnitude (OoM) plot visualizing remedy effectiveness is presented in Figure 3. Each data point on the chart represents actual before and after concentrations for an individual project. From the location where the data point falls on the chart, the diagonal lines can be used to determine the OoM reduction achieved by the project based on the before and after treatment concentrations. The blue line toward the bottom of the chart represents the MCL of 0.005 mg/L for benzene and can be used to determine whether a project achieved the MCL after treatment. Size of the symbols represents the remediation timeframe.

Table 1. Evaluation of Remedy Effectiveness for In Situ Remediation Technologies at Hydrocarbon Sites

Remediation Technology	Number of Sites	Median Years of Remediation	Percent Reduction in Benzene Concentration a
			25th Percentile	Median	75th Percentile
Air Sparging	8	1.3	72%	99%	100%
ISCO	13	2.3	(Increase 2%)	58%	94%
LNAPL Recovery	7	4.0	15%	76%	81%
NSZD	10	3.8	18%	40%	85%
Pump and Treat	8	7.7	(Increase 2%)	56%	94%
Mobilizing Surfactant	3	0.1	36%	64%	77%
Solubilizing Surfactant b	3	0.4	(Increase x48)	(Increase x3)	70%
Notes: a A value of 0.001 mg/L was assumed for all concentrations reported as below the method detection limit. b Increase in concentration is feature of “Solubilizing Surfactant” that is pumped out and does not indicate poor performance. Definitions: ISCO = In Situ Chemical Oxidation. MNA = Monitored Natural Attenuation. NSZD = Natural Source Zone Depletion.

Key insights included:

• Of the 53 sites analyzed, only 32% achieved OoM reductions greater than 1.
• Only 13% of the sites indicated ≥ 2 OoM (i.e., ≥ 99%) reductions for benzene.
• Only 11% of sites indicated concentration reductions to below the MCL.

Comparing the different technologies (Table 1) revealed:

• Air sparging was the best performing remediation technology with a median 99% reduction in benzene concentration.
• Natural Source Zone Depletion (based on the change in groundwater concentrations for the wells in the source area at each site for several years) had slightly lower performance (40% reduction in benzene concentration over a median of 3.8 years) than the non-sparging technologies (which ranged from 56% to 76% reduction in benzene concentrations).

Three limitations of the study are: 1) only 53 sites were evaluated, 2) at some sites, the maximum site concentration may have been outside the direct impact zone of the remediation treatment, and 3) only benzene removal performance was considered. However, because these observations are comparable, both to similar studies of chlorinated solvent remediation performance (see Remediation Performance Assessment at Chlorinated Solvent Sites) and to the data mining analysis of McHugh et al. (2013), the results are likely to be representative of remediation performance at LNAPL sites.

Figure 4. Effect of remediation technology on source attenuation rate showing median attenuation rates for (1) sites where the technology was the only technology applied (green bar); (2) all sites where the technology was applied including sites where other technologies were also applied (red bar); and (3) sites where the technology was not applied but other remediation technologies were applied (blue bar). The error bars show the 95% confidence intervals for the median attenuation rates. The number of sites is shown at the bottom of each bar. Statistical difference compared to sites without the technology by Mann-Whitney U-test: *=statistical difference at p<0.05, **=statistical difference at p<0.01.

In 2013, the American Petroleum Institute funded a data mining study using the GeoTracker database that evaluated over 2,000 hydrocarbon remediation sites in California for the 10-year period from 2001 to 2011. For this study McHugh et al. (2013) evaluated remediation of both benzene and methyl tertiary-butyl ether (MTBE) and concluded:

An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type.

The performance of different remediation technologies over the 10-year study period (as expressed by first order source attenuation rates) of the McHugh et al. (2013) study are presented in Figure 4. In the figure, sites with a particular technology (red and green bars) are compared to sites where other technologies were applied but not that particular technology (blue bars). The study concluded that:

• SVE and air sparging appeared most effective for benzene and groundwater P&T most effective for MTBE.
• Median source half-lives for benzene and MTBE concentrations in source area monitoring wells were 3.9 years (based on all technologies at 4,765 sites) and 1.9 years (based on all technologies at 4,284 sites), respectively.
• Median concentration decreases of 85% for benzene (based on all technologies at 1,128 sites) and 96% for MTBE (based on all technologies at 1,109 sites) were observed when comparing the most recent concentration to the site maximum historic concentration.

While this study was an intense data mining exercise of hydrocarbon site remediation technologies, the authors did not evaluate the actual performance of these technologies at the sites. The study did not evaluate the duration, start date, or end date of a particular technology application, but represents a comparison of a large number of sites where the technology was used in some fashion to sites where it was not used.

ITRC Remediation Technology Selection Process

Figure 5. ITRC LNAPL Technology Screening Process.

Figure 6. Relationship between LNAPL State, LNAPL Concern, Technology Group, and Recoverability.

The ITRC (2018) developed a general framework for remedial technology process selection (Figure 5) in which remediation goals are established, remediation objectives are determined (see LNAPL Conceptual Site Models, coming soon), followed by a general screening process, and then a detailed screening. The ITRC observed that:

Technology selection is a stepwise and linear process for addressing each LNAPL concern; however, remedy selection is seldom linear. The initial focus in technology selection, therefore, should not be when (i.e., in what sequence) each concern is addressed or step is performed, but rather that each is addressed/performed sufficiently. If all concerns are addressed, then the regulating authority can be confident that a complete remedial strategy is being proposed, and the proposing entity can be confident that the proposal is likely to be effective and acceptable to regulators and stakeholders alike. However, some concerns ultimately may take precedence over others and thus dictate the order of technology implementation.

The ITRC LNAPL guidance provides a series of detailed tables to help with the general or preliminary screening of technologies based on the LNAPL remedial goal, LNAPL remediation objective, and applicable site conditions. Comprehensive technology summaries (provided in Appendix A of the ITRC guidance) are then relied upon for the detailed screening. After applicable technologies have been identified, a final evaluation is performed, typically based on four to six key factors from the following list of Technology Evaluation Factors:

• Remedial timeframe
• Safety
• Waste stream generation and management
• Community concerns
• Environmental factors (carbon footprint / energy requirements)
• Site restrictions
• LNAPL body size
• Regulations and Permits
• Cost

Figure 6 shows the relationship between LNAPL state (which includes residual, mobile, and migrating LNAPL), LNAPL concern (LNAPL saturation vs. composition), technology group (phase change, mass recovery, and mass control technologies) and LNAPL recoverability (as a function of transmissivity). Overall, a LNAPL Conceptual Site Model that includes the new technical understanding about LNAPL sites is instrumental in managing LNAPL sites, where LNAPL concerns drive remediation goals, goals drive remediation objectives, and the objectives form the basis to select and implement remediation technologies.

The best currently available information indicates that NSZD is occurring within all LNAPL source areas with LNAPL removal rates of 100’s to 1,000’s of gallons per acre per year (See also: Natural Source Zone Depletion (NSZD)) Therefore, implementation of an active remediation technology is most appropriate when this baseline LNAPL removal rate is not sufficient to achieve the remediation objectives.

See Also

• American Petroleum Institute LNAPL Portal

• An Illustrated Handbook of LNAPL Transport and Fate in the Subsurface, 2014. Contaminated Land: Applications in Real Environments (CL:AIRE). Report.pdf Also available from: CL:AIRE

• Interstate Technology and Regulatory Council (ITRC), 2009. Evaluating NSZD at Sites with LNAPL (LNAPL-1)

• Naval Facilities Engineering Command (NAVFAC), 2010. LNAPL Site Management Handbook.

• US Environmental Protection Agency (USEPA), 1995. Ground Water Issue: LNAPL, EPA 540/S-95/500.

• US Environmental Protection Agency (USEPA), 1996. How to Effectively Recover Free Product at LUST Sites, EPA 510-R-96-001.

• US Environmental Protection Agency (USEPA), 2005. A Decision-Making Framework for Cleanup of Sites Impacted with LNAPL, EPA 542-R-04-011.