Managing NAPL Heterogeneity’s Hijinks
Volume 10, Issue 8 | December 2022

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.

Introduction

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:

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.

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

References

Abstract

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