Part 2 of Managing NAPL Heterogeneity’s Hijinks
Volume 11, Issue 1 | March 2023

Applied NAPL Science Review

Part 2 of Managing NAPL Heterogeneity’s Hijinks

Editor: Lisa Reyenga, PE

ANSR Scientific Advisory Board
J. Michael Hawthorne, PG, Board Chairman, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions (Retired)
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University
Reeti Doshi, National Grid
Mahsa, Shayan, Ph.D., PE, AECOM Technical Services

Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.

DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.

Part 2 of Managing NAPL Heterogeneity’s Hijinks

Randy St. Germain, Dakota Technologies, Inc.

In Part 1 of this series, we introduced heterogeneity and how it makes nonaqueous phase liquid (NAPL) characterization inherently difficult. Part 2 examines two relatively different scales of heterogeneity; larger scale heterogeneity (10s to 100s of feet) and smaller scale (inches to feet) because these scales are commonly encountered.

Large Scale – Homogeneous Soil Example

To make this more engaging let’s play a game where you are shown light NAPL (LNAPL) release characterization data, and then you define the associated LNAPL body shape.

Let’s first discuss the rare case where geologic heterogeneity is NOT influencing the LNAPL release. The reader might want to print this article out on paper and draw the LNAPL body outlines right over the data figures. Avoid peeking ahead at subsequent figures though, because they’ll contain the true LNAPL body “reveals”.

This example follows a typical LNAPL characterization sequence:

1. Examine evidence gathered from wells/logging data (assume “ideal” tools and methodologies were used).

2. Develop the LNAPL CSM (LCSM).

3. Attempt to explain the LCSM if at all possible.

The first release site (Figure 1) includes the following data:

  • No scale is shown (actual scale isn’t important for this)
  • Groundwater (GW) elevations
  • LNAPL point of release (POR)
  • Five “nests” of data including:
    • monitoring wells (left side of each nest) – dashes indicate the well screen – yellow fill indicates the gauged LNAPL thickness
    • soil permeability (center)
    • LNAPL logging data (right side) – red fill indicates formation LNAPL (intensity denotes observed abundance)

These data are typical of the “multi-purpose” approach to characterizing LNAPL in that the well installations not only generate the required dissolved phase GW data, but formation LNAPL data as well.

Figure 1. Large Scale Homogenous Soil Example.

Observations for Figure 1 include (see corresponding circled letters in the figure):

A. Site soil is remarkably homogenous and readily conducts fluids.
B. GW table surface is sloped down toward the left.
C. LNAPL just below the release point is present in the formation, above the GW surface.
D. LNAPL is present in the formation below the GW surface – deepest observations are directly below the release and the penetration tapers with lateral distance from the release point.
E. The monitoring well LNAPL data are consistent with formation LNAPL, only smaller. The single exception is nest #5 where LNAPL is present in the formation but was not observed in the well.

Now draw some dashed “LNAPL extent” lines for: 1. Well LNAPL and 2. Formation LNAPL.
With your extent lines drawn, let’s look at the “reveal” in Figure 2. The characterization data remain ghosted behind the LNAPL body for reference.

Wouldn’t you just know it? It’s the classic pot-bellied release model! And why not? That classic was developed to visually describe, in general terms, how gravity acts to move LNAPL bodies in the absence of any complications.

Figure 2. Large Scale Homogenous Soil Reveal.

Here is a list of observations and explanations that the average investigator might come up with having relied solely on the relatively narrow slices of information available:

A. Site soil is remarkably homogenous and readily conducts fluids.
B. GW table surface is sloped down toward the left.
C. LNAPL just below the release point is present in the formation, above the GW surface.
D. LNAPL is present in the formation below the GW surface – deepest observations are directly below the release and the penetration tapers with lateral distance from the release point.

The only mismatch between our LCSMs and reality is the relative size difference, which isn’t enough to significantly affect the risk analysis and subsequent remediation design should one be required. This exercise demonstrates that in the absence of heterogeneity, LNAPL characterization is relatively straightforward, perhaps even boring.

Large Scale – Heterogeneous Soil Example

Let’s now examine Figure 3, which consists of the same scenario with the lone exception of swapping over to a more complicated geology.

Figure 3. Large Scale Heterogenous Soil Data.

Typical observations of these data include:

A. Site soils are heterogeneous, differing randomly in their fluid permeability.
B. Formation LNAPL is perched in the vadose zone at some locations, nest #3 perched lower than nest #2.
C. Formation LNAPL is below GW at some locations(nest #3 and nest #4).
D. Formation LNAPL at the GW surface at nest #1.
E. In-well LNAPL is limited to nest #3 and greatly exaggerated in comparison to the formation.

OK, now draw the “LNAPL extent” lines and then let’s look at the reveal in Figure 4 below.

Figure 4. Large Scale Heterogenous Soil Reveal.

Yikes – I sure miss our classic release model. How were we ever supposed to guess that?!
Did I mention I do not like this game anymore?

Oh well… let’s autopsy this:

A. We correctly identified the perched LNAPL extent right that was gravity driven to the left – well before it reached GW.
B. An important release chimney (i.e., confined NAPL), straddled by nest #3 and nest #4, was missed. LNAPL pooled and eventually migrated downward, falling into the cavern (area of greater permeability) and pushing far below the GW’s surface.
C. LNAPL eventually filled the impermeable cavern’s ceiling, its buoyancy easily driving it upgradient against the relatively weak forces of GW flow.
D. As a result, LNAPL is now upgradient of the release point at GW surface but at such a low saturation it doesn’t get forced into Well #1.
E. Lone encounter of classic unconfined LNAPL at the water table.
F. LNAPL penetrated low permeability soils, going against intuition (Adamski 2004).
G. Well #3 punctured the cavern ceiling and confined LNAPL flowed into the well, exaggerating its presence.
H. Well #2 doesn’t contain LNAPL because the well screen didn’t span the LNAPL in the formation.

        • The formation adjacent to nest #3 contains LNAPL because the well provided a preferential pathway – the investigation itself allowed NAPL to travel where it wouldn’t have if left alone.
        • The LNAPL body is continuously connected but our LCSM shows it’s considerably separated.
        • The vast majority of the LNAPL body remains hidden to investigators – there doesn’t appear to be troubling levels of LNAPL outside that revealed in Well #3.
        • Straddling the GW surface with well screens is a common practice, but this carries the risk of missing significant trapped and perched LNAPL.

Our LCSM doesn’t represent reality very well, but not due to interpretation errors on our part. We were dealt only five narrow strips of information – simply not enough for us to overcome heterogeneity’s countless cloaks and mirages. (Stock 2011)

Small Scale – Heterogeneity Example

Consultants and regulators delineating NAPLs often sample the same location twice. Reasons include:

  • Wanting to validate an unproven logging tool’s response by obtaining co-located physical samples during a demonstration/validation project (Einarson 2016).
  • Sampling adjacent to a laser-induced fluorescence (LIF) response to determine if a response is truly representative of NAPL versus false positive fluorescence such as wood, calcite, or peat.
  • Sampling adjacent to a laser-induced fluorescence (LIF) response in order to relate the LIF response to NAPL saturation (via lab testing of co-located soil/NAPL samples).
  • Logging NAPL at the same location twice over a period of time in order to assess changes in NAPL that resulted due to applying a remedy (e.g., dual phase extraction or surfactant enhanced recovery).
  • Sampling adjacent to a LIF log that contained an unexpected response that indicated NAPL of a different type than the investigation’s target NAPL. For example, confirming that a diesel signature observed while logging coal tar at a former manufactured gas plant was in fact not coal tar but rather diesel from a previously undocumented diesel release.

To illustrate how heterogeneity introduces uncertainty into repeat sampling at fixed locations, let us conduct an exercise where we’ll repeat NAPL assessments at nest #4, just a foot or two away from the original boring log. Figure 5 contains a few of the many possible outcomes. The original boring log is shown in each panel, while the co-located log(s) are shown alongside in slate blue.

Figure 5. Repeat NAPL Logging at Nest #4.

Missed It by THAT Much

The repeat log traveled through different geologic features than the original. If significant time had passed between the logging events, many investigators will naively assume that the differences are due to LNAPL movement. Had the initial log been produced by an unfamiliar (untrusted) tool, investigators will often consider the initial log to be “faulty” because a physical core is inherently more trustworthy. Falsely assuming that the two repeat methods sampled the same soil horizons is frustratingly common and perhaps even the norm.

The Escalator

Preconceptions trick us into thinking every LNAPL encounter was an encounter with a horizontal “layer” of LNAPL even though borings routinely pass through LNAPL features that are wildly tilted, even vertical. In this panel the repeated boring was to the left, not right, of the initial log. Investigators might falsely attribute this to LNAPL migration, the LNAPL has “moved down” – possibly due to GW fluctuation. The LNAPL hasn’t moved down, it’s simply contained within a severely tilted seam of permeable soil.

Whistling Past the Graveyard

Same location as Missed It by THAT Much, but this time targeted sampling (not continuous) was used. The repeat event not only disputes nest #4 (the lower LNAPL is mysteriously gone) but the repeat sampling event also failed to report the two upper LNAPL encounters that would have helped identify the important nearby “chimney” – in other words the repeat generated a false negative of sorts.

Mind Bender

Illusions can also be produced when rod flex occurs. Prior to incorporation of real time inclinometers into their cone penetration test (CPT) probes, CPT operators would suffer from a loss of verticalness (ISO 2022) and their cones would occasionally emerge from nearby ground, during the penetration test!

In-situ logging tools such as LIF or membrane interface probes also employ flexible rods but they are operated without the aid of real time inclinometers. As a result, their data is generally biased “deeper” than the contaminants really are, because the length of rod in the ground often exceeds the probe’s true depth. Combine this depth mirage with the probe travelling laterally and you have the recipe for major hijinks. Only the original log in nest #4 (conducted vertically and straight) represents the LNAPL’s true depth, while the two “benders” resulted in illusions (shown dashed and faded) that would mislead investigators.

A Word of Caution

We have just illustrated a number of heterogeneity-induced mirages that all of us have (or will) fall victim to. Heterogeneity, combined with faulty assumptions (we’re human after all), is a surefire recipe for inaccurate NAPL data interpretations. In Part 3 we’ll discuss some practical measures that help to manage, but never eliminate, heterogeneity-induced illusions.


Stock 2011
Paul Stock, “Where’s the LNAPL? How about Using LIF to Find It?”, L.U.S.T.Line, pp.13-18

Adamski 2004
Adamski, Mark & Kremesec, Victor & Kolhatkar, Ravi & Pearson, Chris & Rowan, Beth. (2005). LNAPL in fine‐grained soils: Conceptualization of saturation, distribution, recovery, and their modeling. Ground Water Monitoring & Remediation. 25. 100 – 112.

Einarson 2016
Einarson, M., Fure, A., St. Germain, R., Chapman, S., and Parker, B., DyeLIF™: A New Direct‐Push Laser‐Induced Fluorescence Sensor System for Chlorinated Solvent DNAPL and Other Non‐Naturally Fluorescing NAPLs. Groundwater Monitoring & Remediation, 38, 3, (28-42), (2018).

ISO 2022
ISO 22476-1:2022 (Figure 3)
Geotechnical investigation and testing — Field testing — Part 1: Electrical cone and piezocone penetration test

Research Corner

Hydrothermal Technologies for Destruction of Per- and Polyfluoroalkyl Substances (PFASS) in Aqueous Film-forming Foam (AFFF) and AFFF-impacted Wastes

Shilai, Hao
Postdoctoral Research Associate
Colorado School of Mines


Per- and polyfluoroalkyl substances (PFASs) are a family of chemicals with at least one perfluoroalkyl moiety (CnF2n+1-) used in a variety of industries and consumer products since the 1940s. The ubiquity of PFASs in the environment, wildlife, and humans has raised significant concerns and calls for action globally. One of the major sources of PFAS contamination is the use of aqueous film-forming foam (AFFF), water-based mixtures of fluorinated and hydrocarbon surfactants that are applied to rapidly extinguish hydrocarbon and solvent-based fires. Groundwater and soil at many sites in the U.S. have been reported to be impacted by the historic use of AFFF. Current treatment technologies mostly focus on separation (e.g., activated carbon, membrane separation), and there remain few options for achieving destruction and defluorination of the full range of PFASs in AFFF-impacted matrices (e.g., groundwater and soil). Thus, there remains a critical need to develop a rapid, effective, and robust treatment method for the destruction of PFASs. This thesis advanced the understanding and application of hydrothermal alkaline treatment (HALT) technologies for the destruction of PFASs in contaminated environmental and concentrate matrices, and key findings provide guidance on the integration of HALT within treatment trains that will be applied for real-world site remediation. Initially, I evaluated the effectiveness of HALT for destruction and defluorination of PFASs identified in AFFFs produced by suppliers via electrochemical fluorination- (ECF) and fluorotelomerization (FT) processes. Quantitative and semi-quantitative high-resolution mass spectrometry was used to track a wide range of PFAS structures during treatment with HALT. Results demonstrated rapid degradation of all 109 PFASs identified in two AFFFs when the solutions were amended with alkali (e.g., 1-5 M NaOH) at near-critical temperatures and pressures (350 °C, 16.5 MPa). This includes perfluoroalkyl acids (PFAAs) and a range of polyfluoroalkyl precursors. Most PFASs were degraded to non-detectable levels within 15 min. Perfluoroalkyl sulfonates (PFSAs) were the most recalcitrant class of PFASs, but these were degraded to non-detectable levels within 30 min when treated with 5 M NaOH. 19F-NMR spectroscopic analysis and fluoride ion analysis confirmed that destruction was accompanied by near-complete defluorination of PFASs in both dilute and concentrated AFFF mixtures (total fluorine up to 0.36 M), and no stable volatile organofluorine species were detected in reactor headspace gases analyzed by gas chromatography with mass spectrometry (GC-MS) detection. The application of the HALT was then extended to AFFF-impacted groundwater and soil matrices. Results showed that 148 PFASs identified in the collected field samples (2 groundwater samples, 3 soils), including 10 cationic, 98 anionic, and 40 zwitterionic PFASs, were mostly degraded to non-detectable levels within 90 min when treated with 5 M NaOH at 350 °C. The near-complete defluorination, as evidenced by fluoride release measurements, confirmed the complete destruction of PFASs. Rates of PFSA destruction in groundwater samples were similar to those measured in laboratory water solutions, but reactions in soil were slowed, attributed to base-neutralizing properties of the soil (e.g., reaction with silicate minerals). Further, the degradation of PFASs in groundwaters and soils was found to be a function of reaction temperature, NaOH concentration, and reaction times. The dissolution of soil minerals during HALT presents a challenge to direct soil treatment applications and suggests the need for future research to optimize PFAS destruction while minimizing soil matrix reactions. To reduce the overall energy requirements for treatment and destruction of PFASs, I then examined application of HALT for treatment of PFAS concentrates produced by foam fractionation (FF) processes being developed for groundwater remediation. Results showed that all 62 PFASs identified in two FF-derived concentrates were degraded by >90% within 90 min when treated with 1 M NaOH at 350 °C; concentrations were reduced below the detection limit when treated with 5 M NaOH for the same reaction time. The foam concentrate matrix, including elevated dissolved organic carbon (DOC; up to 4.5 g/L) did not significantly affect reaction kinetics for the most recalcitrant PFSAs. Efforts were included to characterize and track organic constituents during treatment, with results showing partial reduction of bulk DOC, but complete degradation of 43 hydrocarbon surfactants identified in an ECF-derived AFFF concentrate. An initial analysis of energy requirements for an integrated process coupling FF with HALT was estimated to be ~0.7 kWh/m3 groundwater, with the HALT step being a negligible contributor to the overall treatment process due to the small volume of concentrate requiring treatment. Studies were also conducted using ultra-short chain perfluoroalkyl acids (PFAAs) to better understand the mechanisms responsible for PFAA destruction and defluorination during HALT. Reactions were conducted with trifluoroacetic acetate (TFA) and trifluoromethanesulfonate (TFMS; triflate). Results confirmed the destruction and defluorination through HRMS, nuclear magnetic resonance (NMR), and fluoride ion measurement. TFMS showed much lower reactivity (~95 fold lower) than TFA, consistent with measurements of longer chain analogues. The carbon atoms of TFA and TFMS were converted to a mixture of formate and dissolved carbonate species, and the sulfonate group of TFMS was converted stoichiometrically to sulfate. Experiments also show TFMS defluorination could be mediated by the addition of alternative nucleophiles (iodide, bisulfide) in place of hydroxide. Results support a proposed stepwise nucleophilic substitution mechanism that destabilizes the alkyl chain within PFASs, leading to complete defluorination and partial carbon mineralization. Overall, this thesis demonstrated that HALT is a rapid, effective, and robust treatment method for the destruction of the full range of PFASs identified in water-containing environmental matrices and concentrates. A proposed nucleophile substitution mechanism produces inorganic fluoride ion as the sole product with no evidence for formation of undesirable volatile fluorinated products that can be produced by other thermal treatment processes, including incineration. Therefore, results support a conclusion that HALT has significant potential for addressing remediation and industrial treatment needs at a growing number of sites.

Related Links

API LNAPL Resources
CL:AIRE Technical Guidance
Concawe LNAPL Toolbox
CRC CARE Technical Reports
CSAP MNA Toolkits
EPA NAPL Guidance
Groundwater Monitoring & Remediation
ITRC LNAPL Resources
ITRC DNAPL Documents
Sustainable Remediation Forum

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Coming Up

Lined-up for 2023 we have articles planned on enhanced-NSZD and microbiological tools for NAPL sites. Moving forward, we will also intersperse articles on other topics of interest such as PFAS, NSZD, bioremediation, NAPL forensics, and remediation case studies. Please contact us if you would like to submit an article or would like to see articles on other topics.


Upcoming CLU-IN and ITRC Training – Learn More Here:
  • April 13: ITRC PFAS Introductory Training
  • May 11: Sustainable Resilient Remediation (SRR)
Upcoming IPEC Training – Learn More Here:
  • March 23: Mechanisms for Success in In-Situ Bioremediation Programs and Hydrocarbon Destruction via Biostimulation Alone under Baseline Conditions…
  • April 13: Organo-Halide Destruction via Biostimulation Without Augmentation… and
    Another WOTUS Rule Change?
  • April 20: Molecular Biological Tools to Optimize Hydrocarbon Biodegradation and
    Corrosion Inhibiting Method for Remediating Hydrocarbon Releases at Pipeline, Tank Batteries & AST Sites
  • May 4: Hydrocarbon Pipeline Integrity and Changes to Hydrocarbon Components During Composting Study
  • May 18: Enhance VOC, Sorbed, Globule, NAPL Remediation Exposing Limiting Factors

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