Applied NAPL Science Review

Using a Tiered Framework for NAPL Movement Evaluations

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

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.

Using a Tiered Framework for NAPL Movement Evaluations

Dusty R.V. Berggren, Jacobs

When performing a NAPL movement evaluation, a tiered testing approach may be used to arrange  multiple lines of evidence into a decision framework that can efficiently achieve project objectives and gain stakeholder agreement on interpretation of results.

When non-aqueous phase liquid (NAPL) is discovered at a site, site characterization and NAPL movement evaluations are performed to assist in developing a conceptual site model (CSM). This process is described for sediment sites in ASTM E3248: Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection and summarized in Reyenga 2021a. No single line of evidence (LOE) can provide the necessary information to delineate the NAPL body or determine if the NAPL is mobile or migrating. Therefore, several LOE need to be incorporated to form a well-supported CSM. Use of an evaluation framework, along with good professional judgement, will provide a sound technical basis for CSM development.

Tiered frameworks have been widely accepted by industry and the professional community (McNally et al. 2020, USEPA 2007, Ingersoll et al. 1997, etc.). The tiered approach is often formatted either as a decision tree between individual tests or multiple groupings (tiers) of evaluations with a decision point between each tier where a line of testing can be refined or discontinued based on previous results and evaluation objectives. There are usually at least three tiers, with each subsequent tier typically involving more representative, complicated, costly, and/or -time-consuming evaluations than the previous tier.

There is no standard format for these evaluations – their strength is tied to how well-defined the evaluation objectives are and the professional judgement used for sample selection, test results interpretation/integration and the evaluation of calculation results. The following sections outline the development of a tiered approach for site characterization and a NAPL movement evaluation in sediments, although the principles generally apply to upland sites as well.

Step 1: Asking Questions

• If NAPL is present at the site:
  • where is it located and at what magnitude/saturations?
  • is it mobile at the pore scale?
• If NAPL is mobile, is it migrating at the body scale?
• If NAPL is migrating, what is the risk of it coming into contact with any sensitive receptors?

If the questions can be arranged in a manner where they build on one another as in the example above, the tiers begin to form naturally between points of evaluation.

Step 2: Selecting Evaluation Methods

Once the objectives have been established, the set of tests, observations, or inputs necessary to definitively answer each question can be selected. Several of the available tools are presented and discussed in ASTM E3282 Standard Guide for NAPL Mobility and Migration in Sediments – Evaluation Metrics and are outlined in Table 1.

Table 1: Summary of Available Evaluation Methods Arranged by Tier

Evaluations associated with earlier tiers (e.g., NAPL presence/absence tests) typically consist of field observations or binary field tests that are then used to select where more quantitative measurement or sample collection for laboratory testing would be most useful; one approach for doing this is presented in ASTM E3281 Standard Guide for NAPL Mobility and Migration in Sediments – Screening Process to Categorize Samples for Laboratory NAPL Mobility Testing. Intermediate tiers (e.g., NAPL mobility tests) usually involve time-consuming laboratory tests, but they provide more quantifiable results. Within a tier, testing can begin with overly conservative approaches protective of project goals, followed by testing that is more representative of site-specific conditions. Later tier evaluations (e.g., NAPL migration calculations) are frequently calculation based, critically analyzing information from previous tiers to establish the theoretical bounds of what may happen with the NAPL at the site. This tier may use a weight of evidence approach to draw conclusions from the information available.

Step 3: Forming a Tiered Evaluation Framework

Selected evaluations can be organized into a tiered decision framework. An example is provided in Figure 1.

Figure 1 – Example of Tiered NAPL Movement Evaluation Approach (Reprinted, with permission, from ASTM E3282-21 Standard Guide for NAPL Mobility and Migration in Sediment – Evaluation Metrics, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.  A copy of the complete standard may be obtained from ASTM International, www.astm.org.)

In this tiered framework, if any one of the tiers/decision points results in a definitive negative response (e.g., NAPL is not present at the site), certain “off ramps” in the evaluation process may be taken. This is one advantage of the tiered approach: simple cases can be completed relatively quickly and at lower cost without completing the entire evaluation process. Built-in evaluation points also allow more complex cases to be completed more efficiently by using what is learned in previous tiers (or decision points) to refine how subsequent tiers are handled. For example, mobility evaluations may be performed first on “worst-case” (highest saturation) samples representing each of the strata where NAPL is found at the site first. If these are not mobile, testing of sediment samples with lower NAPL saturations may not be necessary.

A Word of Caution 

Tiered testing programs are designed to be responsive to what is learned during the evaluation. However, be careful to not cut off a line of evaluation before it is definitively answered for all likely site conditions. For example, a NAPL mobility evaluation may be performed on a highly saturated silty lens within a sediment that indicates the NAPL is immobile, but another location at the site may have NAPL in a sandy lens at a lower saturation that is mobile. In another instance, if the integrity of an “undisturbed” sample intended for mobility analysis is compromised, the disturbed matrix structure may indicate mobility, while the NAPL is immobile in-situ or vice versa. Additionally, evaluations may consider potential for changing site conditions due to erosion or climate change, especially in migration calculations. Ultimately it is the practitioners’ responsibility to ensure that the evaluation program is sufficient for the site conditions. 

References

ASTM E3248 – 20. 2020. “Standard Guide for NAPL Mobility and Migration in Sediment – Conceptual Models for Emplacement and Advection.” 

ASTM E3281 – 21. 2021. “Standard Guide for NAPL Mobility and Migration in Sediments – Screening Process to Categorize Samples for Laboratory NAPL Mobility Testing.”

ASTM E3282 – 21. 2021. “Standard Guide for NAPL Mobility and Migration in Sediment – Evaluation Metrics.” 

Byker, Marcus, and Groff, Kim. 2021. “Sediment Sample Collection and Field Screening to Inform NAPL Mobility and Migration Evaluations”. Applied NAPL Science Review Vol. 9 Issue 4, June 2021. 

Gefell, Michael J. 2021. “Estimating NAPL Hydraulic Conductivity and Migration Rate Based on Laboratory Test Results”. Applied NAPL Science Review Vol. 9 Issue 3, May 2021. 

Ingersoll, C. G., Dillon, T., and Biddinger, G. R., Ecological Risk Assessment of Contaminated Sediments, SETAC Press, Pensacola, FL, 1997.

McNally, Amanda, A. Fitzpatrick, D. Harrison Jr., A. Busey, S. Apitz. 2020. Tiered approach to sustainability analysis in sediment remediation decision making. Remediation, Vol. 31 Issue 1. 29-44.

Reyenga, Lisa. 2021a. “Evaluating Emplacement and Movement of NAPL in Sediment”. Applied NAPL Science Review Vol. 9 Issue 1, February 2021. 

Reyenga, Lisa. 2021b. “Typical Metrics for Evaluating Advective NAPL Movement in Sediments”. Applied NAPL Science Review Vol. 9 Issue 5, August 2021. 

USEPA. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water Volume 2 Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium. EPA/600/R-07/140, U.S. Environmental Protection Agency (2007).

Research Corner

Contaminant Flux through Capped and Uncapped Sediments  

Priscilla Z. Viana
Doctor of Philosophy in Civil Engineering  
University of Illinois at Chicago 

Abstract:

Cd, Cr, Pb, Ag, As, Ba, Hg, CH3Hg and CN transport through sand (25 cm), granular activated carbon (GAC, 2 cm), organoclay (2 cm), shredded tires (10 cm) and apatite (2 cm) caps was modeled by deterministic and Monte Carlo methods. Effective caps prevented above-cap concentrations from exceeding USEPA acute criteria at 100 yr assuming below-cap concentrations at solubility. Sand caps performed best under diffusion due to the greater diffusive path length. Apatite had the best advective performance for Cd, Cr and Pb. Organoclay performed best for Ag, As, Ba, CH3Hg and CN. Organoclay and apatite were equally effective for Hg. Monte Carlo analysis was used to determine output sensitivity. Sand was effective under diffusion for Cr within the 50% confidence interval (CI), for Cd and Pb (75% CI) and for As, Hg and CH3Hg (95% CI). Under diffusion and advection, apatite was effective for Cd, Pb and Hg (75% CI) and organoclay for Hg and CH3Hg (50% CI). GAC and shredded tires performed relatively poorly. Although no single cap is a panacea, apatite and organoclay have the broadest range of effectiveness. Cap performance is most sensitive to the partitioning coefficient and hydraulic conductivity, indicating the importance of accurate site-specific measurement for these parameters. This study also quantified the magnitude of organic and metal contaminant facilitated transport from the sediment to the water column due to gas ebullition at 14 urban waterway locations. The magnitude of the ebullition-facilitated measured fluxes indicates that gas ebullition is an important pathway for release of both polycyclic aromatic hydrocarbons (PAHs) and heavy metals from buried sediments in urban freshwater systems. Comparison of direct benthic release rates to ebullition facilitated rates suggests that total PAHs are released at significantly greater rates by biogenic gas production. Although the increase in release rate is not as great for metals, ebullition facilitated release rates are frequently greater than benthic release. Mechanistic and empirical models developed in this study may be used to predict in situ gas ebullition flux and ebullition-facilitated contaminant flux.

The primary objective of ANSR is the dissemination of technical information on the science behind the characterization and remediation of Light and Dense Non-Aqueous Phase Liquids (NAPLs). Expanding on this goal, the Research Corner has been established to provide research information on advances in NAPL science from academia and similar research institutions. Each issue will provide a brief synopsis of a research topic and link to the thesis/dissertation/report, wherever available.

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