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.

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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.

Managing NAPL Heterogeneity’s Hijinks
Randy St. Germain, Dakota Technologies, Inc.
Heterogeneity, we’re all at least loosely familiar with the term. Most of us don’t bat an eye when we hear it, even though it should get our blood pressure up a bit when we do. In this three part series we’ll explore NAPL heterogeneity, have an honest look at the surprising degree to which it affects our work, and some of the things we can do to manage its inescapable influence.


Anyone involved in non-aqueous phase liquid (NAPL) characterization eventually runs headlong into bewilderment caused by spatial heterogeneity of NAPL in soil. NAPL’s spatial and chemical complexity is particularly troublesome when we’re attempting to characterize NAPL and just the NAPL alone, i.e. the source term.

Typical scenarios include:
• Testing of various methods capable of rapidly screening for NAPL – and contrasting their results against the “gold standard” lab-based methods such as gas chromatography (DRO, GRO and TPH), the correlation between them is often quite poor
• Applying analytical analysis to soil core samples in order to validate an indirect NAPL-specific logging technology such as laser induced fluorescence (LIF), the correlation is often terrible
• Employing a NAPL-indicating method before and/or after an attempt at remedy has been applied, in order to assess the efficacy of the remedy’ attempt to impact the NAPL in-situ in some fashion, and the numbers are all over the place – some show promise, others show the problem got much worse after the remedy

Much of the issue is due to NAPL’s nature. Unlike the aqueous or gaseous phase contaminants that it exudes, NAPL itself isn’t appreciably transported by “the wind” of groundwater advection and diffusion. NAPLs literally have to crawl, fall, get lifted, or even pushed through small pore spaces in order to migrate. Thus, a release site’s NAPL body is far more localized and discretely distributed than the larger more broadly distributed plume of aqueous and gaseous phases. This fact naturally makes NAPL the more challenging target for characterization, because its smaller and its structure is decidedly more localized and complex. Think of NAPL as the “fire”, a rich concentrate that sources the more diffuse “smoke” (the aqueous and gaseous phases).

There are sites throughout the industry where the standard lines of NAPL evidence (soil cores subjected to visual examination, shake tests, dyes, glove staining, etc.) appear to correlate very poorly with other lines of evidence, especially lab analyses. The term poor correlation intuitively triggers a healthy dose of scientific skepticism, and evaluation of which of the conflicting lines of evidence “got it wrong”.

Commonly it turns out that the lines of evidence were collected appropriately and none of them are “wrong” – they simply differed. The various methods being compared were almost always being applied to different soil samples – which contained wildly differing NAPL impacts or non-NAPL phases – so the methods were justifiably in disagreement. The true cause was simply NAPL heterogeneity – most often spatial, but sometimes includes heterogeneity of the chemical composition of the NAPL or discernment of NAPL vs its non-NAPL constituents of concern (CoCs).

Recognizing the influence that heterogeneity has on the evaluation of NAPL impacts to soil means that we have to accept that any single NAPL measurement does not define the nearby soil’s condition. Soils surrounding NAPL will contain sorbed, gaseous, or aqueous CoCs that were emanating from the NAPL, but they won’t necessarily contain NAPL. Move over just a foot in any direction, even an inch, and the CoC content will be different, often orders of magnitude different. It’s all maddeningly variable and introduces significant (and often unacknowledged) uncertainty into site characterization, risk assessment, and ultimately remedial design.

Uncertainty may make decisions especially uncomfortable for “NAPL hunters” (site investigators in general) who tend to be perfectionists when it comes to developing their CSM. They’re generally very diligent in their work because if they’re not, they won’t succeed. There are field veterans that have apparently tilted at this heterogeneity windmill enough in the past that they’ve learned to just roll with it and accept its effects, but most of us are still unsettled by its influence.

The beginners in our ranks? Well, the majority of those happy-go-lucky folks have yet to appreciate heterogeneity’s negative impacts and don’t seem to get too ruffled by heterogeneity… yet. Sure, they see evidence of it all the time when they’re coring and sampling and gathering NAPL data. But they usually get to let the more senior staff back at the office deal with the inevitably ugly r-squared’s that the regulators won’t like to see in the CSM data – and will want to get explanations for.

The Root Cause

The fact that NAPL’s spatial heterogeneity is due almost entirely to heterogeneity of the soil and water in which it travels has been known for a long time. Nonetheless, this fact wasn’t adequately recognized for the first decade or two of petroleum release characterization. The geologists among us never forgot though, they’ve catalogued and even marveled at soil heterogeneity for centuries. For them, heterogeneity is often fascinating to observe and document because it provides valuable insights into how soil strata were deposited centuries or eons ago. Geotechnical engineers on the other hand aren’t quite as delighted by soil complexity because they have to risk consequential failure if and when they get their craft wrong. Karl von Terzaghi, the father of soil mechanics (a discipline which long predates our NAPL characterization discipline) described soil heterogeneity’s challenge this way, almost a century ago:

This quote was made in 1936, well before NAPL the remediation site characterization field existed. Reading it for the first time immediately took me back to the early years when LIF was being validated in a large, controlled Superfund Innovative Technology Evaluation (SITE) study (EPA 1995). Heterogeneity’s potential to cause problems had been recognized by the planning team (an experienced and capable group) going into the study, and controls for its potential influence were written into the work plan. But its negative impacts still managed to taint the interpretation of the study’s data. Who would have thought heterogeneity would be so problematic and so difficult to account for, even knowing its potential for hijinks going in.

That SITE study experience, along with innumerable others since, taught us that von Terzaghi’s “Natural soil is never uniform.” was practically universal in its relevance and nearly always more severe than anticipated, at least as far as NAPL is concerned. We gradually implemented ever more controls on our work, including the occasional co-located whole core examination, where NAPL’s heterogeneity can be viewed its “natural habitat” (Figure 1.) and usually revealing a spectacularly heterogeneous distribution across the open core’s cut face.

Figure 1 – Soil core with visual light and UV-induced fluorescence of gasoline NAPL – an ultra-high resolution technique.
You can verbally explain heterogeneity until you’re blue in the face but only high-resolution data like this make it intuitively obvious enough for people to accept that unless comparisons are made using the exact same soils, any two or more measurements in this core are likely to yield dramatically different results, simply because the soils all vary significantly with respect to their NAPL content. The exception is the NAPL-free soils, which are more homogenously impacted due to diffusion’s assistance in evening things out.

Now take this example of severe heterogeneity within a single core (which is quite common by the way) and extrapolate it to any attempt to core and sample soils from the same location for direct comparison with each other (when comparing two in-situ NAPL-sensing technologies or during a before/after remedy study for instance). It’s easy to recognize that your chances of sampling the same (i.e., truly representative) soils between the two sampling events is essentially zero. This is the great challenge for anyone characterizing NAPL over time or across methodologies.

What’s Next?

Part 2 of this series we will look at examples of the impacts heterogeneity has on NAPL studies and how it can cause us to draw faulty conclusions.

Part 3 of this series we will discuss ways to assess NAPL heterogeneity, address the uncertainty it introduces, and list several controls that can be asserted to help reduce its impact to the final work product.

A Word of Caution

At this point you’re probably thinking “wow, the author seems overly concerned about heterogeneity”. I know, it’s just that I’ve spent decades explaining why NAPL data doesn’t corelate well between differing methods and over repeated measurements using the same method, and it’s most often been due to heterogeneity. Many of the points I’m trying to make in this series are personal opinions, and I caution you to arrive at your own conclusions as to how much heterogeneity impacts your team’s work.


Research Corner

Anastasia Nickerson
Doctor of Philosophy
Colorado School of Mines


Poly- and perfluorinated alkyl substances (PFASs) are a class of recalcitrant environmental contaminants used in a variety of industries and consumer products. Use of aqueous film-forming foams (AFFF) at military bases and airports is one significant source of PFAS contamination to groundwater and communities. AFFF formulations are composed of diverse PFAS classes, including anionic, zwitterionic, and cationic structures. Many of the polyfluorinated substances have been shown to transform to the perfluorinated substances in the environment. Despite years of research concerning the mostly perfluorinated anionic substances, the fate and transport of the zwitterionic and cationic PFASs remain largely unknown. The objective of this dissertation was to develop a better understanding of the transport and transformation of PFASs at AFFF-impacted sites, with a particular focus on zwitterionic and cationic compounds. The first research objective was to develop a soil extraction method to enhance the recovery of all PFASs. The second research objective was to conduct a comprehensive site characterization via high spatial resolution sampling of soil and groundwater samples with estimated concentrations of all detected PFASs. The final research objective was to simulate biosparging of an AFFF-impacted soil in column experiments to understand changes in PFAS transformation and release from source zone materials altered by remediation. The results indicated that a combination of strongly basic followed by strongly acidic extraction conditions were needed to achieve sufficient recovery of all PFASs from soils. The site characterization showed that the majority of the polyfluorinated mass remained near the source zone despite decades since release, and the majority of these compounds were zwitterionic or cationic. The third research effort showed that biotransformation of polyfluorinated precursors occurred in both O2-sparged and N2-sparged soil columns, and higher concentrations of certain zwitterionic PFASs eluted from O2-sparged columns shortly after start of sparging. The findings from this dissertation will allow for a more comprehensive view of the PFASs in the subsurface and how they move and change with time. These efforts will benefit remedial plans and site investigations at AFFF-impacted sites.

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Coming Up

Lined-up for 2023 we have two more articles in this series on NAPL heterogeneity. We also 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 ITRC Training

  • December 19: Evaluating Plant Uptake Pathways of Chemical Contaminants in State Models for Risk Assessments of Contaminated Urban Gardening Sites
  • January 24: Soil Background & Risk Assessment
  • January 31: 1,4-Dioxane: Science, Characterization & Analysis, and Remediation
  • February 7: Optimizing Injection Strategies and In situ Remediation Performance
  • February 14: Vapor Intrusion Mitigation (VIM-1) – Part 1
  • February 21: Vapor Intrusion Mitigation (VIM-1) – Part 2
  • March 2: Strategies for Preventing and Managing Harmful Cyanobacteria Blooms – Part 1
  • March 9: Strategies for Preventing and Managing Harmful Cyanobacteria Blooms – Part 2

Upcoming IPEC Training

  • 2023 Schedule TBD

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