Use of Dissolved-Phase Plume Stability to Evaluate LNAPL Body Stability with GWSDAT
Volume 11, Issue 3 | August 2023

Applied NAPL Science Review

Use of Dissolved-Phase Plume Stability to Evaluate LNAPL Body Stability with GWSDAT

ANSR Scientific Advisory Board

Lisa Reyenga, PE, Board Chair, GEI Consultants, Inc.
J. Michael Hawthorne, PG, 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
Chis Marks, Ph.D., Arizona Department of Environmental Quality
Kammy Sra, Ph.D., Chevron
David Edgerton, USEPA

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.

Use of Dissolved-Phase Plume Stability to Evaluate LNAPL Body Stability with GWSDAT

Lisa Reyenga, P.E., GEI Consultants, Inc.

LNAPL body stability is a prerequisite to a risk-based approach to LNAPL management. Dissolved-phase stability is a key line of evidence for LNAPL body stability, and it can be efficiently evaluated using the free Groundwater Spatiotemporal Data Analysis Tool (GWSDAT).


Evaluating LNAPL body stability is critical to understand potential risks and the potential need for engineered LNAPL remediation. A stable LNAPL body does not have the potential to create new risks as the result of LNAPL migration into new areas. Stable LNAPL bodies are potential candidates for a risk-based and/or natural remediation approach to LNAPL management.

During its initial release, LNAPL will move downward due to gravity, and if there is sufficient volume and head, it will eventually encounter the water table. The LNAPL will penetrate the water table and spread radially until a balance is achieved between the LNAPL head pressure and the capillary entry pressure in the soil and natural degradation at the edges of the LNAPL body. At this point, the 3-dimensional extent of the LNAPL body will not change in the future, barring additional releases or significant changes to subsurface conditions. This process typically takes a few months to several years (ITRC 2018, CL:AIRE 2014).

Evaluation of LNAPL body stability typically requires multiple lines of evidence (Table 1). Further details on lines of evidence for LNAPL body stability can be found in ITRC 2018 and Hawthorne 2013.

Minimum RequirementsAdditional Lines of Evidence
  • General age of the release(s)
    • LNAPL footprint evaluation
  • Dissolved-phase plume evaluation
  • Site setting, current and future land use, potential receptors
  • LNAPL release details
  • Site geology and hydrogeology
  • LNAPL body spatial distribution/extent
  • LNAPL physical and chemical properties
  • LNAPL mobility assessment
  • Natural degradation processes

Table 1. Lines of evidence of LNAPL body stability

A dissolved-phase plume stability evaluation is included in the minimum requirements for conducting an LNAPL body stability evaluation because a stable or shrinking dissolved-phase plume indicates a stable or shrinking LNAPL body. However, an expanding dissolved-phase plume may be caused by a shrinking, stable, or expanding LNAPL body and does not provide a definitive line of evidence in the LNAPL body stability evaluation (CL:AIRE 2014). The amount of time it takes for the dissolved-phase plume to stabilize is affected by other factors such as the solubility of individual COCs, groundwater gradient and velocity, and biodegradation rates. Therefore, additional time may be required for dissolved-phase plumes to stabilize after the LNAPL body has stabilized. The condition of the LNAPL body for each stability condition of the dissolved-phase plume is summarized in Table 2.

Dissolved-phase plume stability is also a reliable indicator of LNAPL body stability in situations where the full three-dimensional extent of the LNAPL body cannot be characterized due to site access restrictions (e.g., under buildings). If the dissolved-phase plume is fully delineated and stable or shrinking, the LNAPL body source is also stable or shrinking.

Dissolved-Phase Plume Stability ConditionLNAPL Body Stability Condition                 
StableShrinking or Stable
ExpandingShrinking, Stable, or Expanding

Table 2. Potential LNAPL body stability conditions based on the observed dissolved-phase plume condition

Groundwater Spatiotemporal Data Analysis Tool

The Groundwater Spatiotemporal Data Analysis Tool (GWSDAT) is a free, open-source, user-friendly web application to perform dissolved-phase plume stability evaluations. GWSDAT includes tools such as:

  • Time-series plots for each location and constituent provided;
  • Mann-Kendall Trend evaluations (e.g., increasing, decreasing, stable);
  • Threshold evaluations (e.g., above or below screening levels);
  • Spatial visualizations of concentrations; and
  • Estimations of plume mass, area, and average concentration over time.

GWSDAT is an accessible and efficient tool to perform dissolved-phase plume evaluations at the plume scale and/or for individual wells as lines of evidence in the LNAPL body stability evaluation.

Case Study

The Site was the location of a pipeline release of LNAPL. The data set includes 10 years of dissolved-phase concentrations of benzene from 2007 through 2016. During this time hydraulic recovery was conducted to remove LNAPL mass, but no active compositional change remedies were utilized to preferentially remove benzene from the LNAPL.

Figure 1 presents a spatial animation showing the interpreted concentration in the aquifer for each year of monitoring. The model interpolates through both space and time, so concentrations can be interpreted even where a location was not included in a particular monitoring event. Individual concentration data points are shown next to the well location. The red line indicates the extent of the plume corresponding to a benzene concentration of 5 µg/L (e.g., the Tier 1 screening threshold). The red cross indicates the location of the center of mass of the plume. The animation provides an excellent visual tool to illustrate the shrinking plume.

Figure 2 presents a series of graphs showing the calculated plume mass, area, and concentration over time using GWSDAT. These graphs illustrate a steady decrease from 2007 through approximately 2013. At that point concentrations greater than 10 µg/L are largely absent and the plume mass, area, and concentration stabilize. There is an apparent expanding trend in plume area from approximately 2014 through 2016. However, this is an artifact of changes in the monitoring program rather than actual plume expansion. During this time, only the perimeter wells were monitored, and no data points were collected in the interior of the plume.

Figure 3 demonstrates the benzene concentrations over time for an example individual well. The concentration is on a log-scale, indicating that the benzene concentration is decreasing exponentially, consistent with natural attenuation. The Mann-Kendall p value (0.0864) is also shown on the graph. This result indicates a statistically significant decreasing trend.

Figure 1 – Spatial animation showing the interpreted concentration of benzene in the aquifer for each year of monitoring.  


Figure 2 – Benzene plume mass, area, and concentration over time estimated with GWSDAT

Figure 3 – Benzene concentration over time at an individual well

The shrinking benzene plume is a critical risk evaluation component for the Site. It confirms there are no potential new risks as the result of LNAPL or dissolved-phase migration into new areas, barring new releases or significant hydrogeologic changes at the Site.

A Word of Caution

Like any analytical model, the GWSDAT results are only as reliable as the data provided. Professional judgement must be utilized to ensure that the monitoring network is sufficient in both space and time to characterize the plume extent and behavior. Dissolved-phase plume stability may also vary, depending on the constituent. Evaluation of more than one constituent may be required to fully characterize the Site.


CL:AIRE. 2014. An illustrated handbook of LNAPL transport and fate in the subsurface. London: Contaminated Land: Applications in Real Environments (CL:AIRE).

Hawthorne, J. Michael. 2013. “LNAPL Body Stability, Part 1: Lines of Evidence.” Applied NAPL Science Review Vol. 3 Iss. 4, May 2013.

ITRC, 2018. Light Non-Aqueous Phase Liquids (LNAPL) Document Update: Evaluating and Implementing LNAPL Remedial Technologies.  Interstate Technology Regulatory Council, LNAPL Update Team, pending publication March 2018.

Research Corner

Remediation of Per and Polyfluoroalkyl Substances by Pilot-Scale Applications of Advanced Water Treatment Technologies

Charlie Jeffrey Liu
Colorado School of Mines

The widespread use of per- and polyfluoroalkyl substances (PFASs) and the associated long-range transport and recalcitrance in the environment has led to the contamination of water resources worldwide. Most conventional water treatment methods are ineffective against PFASs and while the removal of PFASs by advanced water treatment technologies including granular activated carbon (GAC), anion exchange resin (AER), nanofiltration (NF), reverse osmosis (RO), and UV-sulfite has been demonstrated in bench-scale laboratory studies, little information is available on the treatment performance of these technologies at the full-scale. The works in this thesis advance the understanding of real-world PFAS treatment by these technologies using pilotscale assessments operated under conditions and water matrices representative of full-scale treatment and provide guidance on the advantages and disadvantages associated with each technology.

The treatment of PFAS impacted groundwater by four GAC products was evaluated at a municipality near Colorado Springs, Colorado using a pilot-scale system operated for 7 months. Breakthrough of PFASs by GAC was influenced by perfluoroalkyl chain length and headgroup with breakthrough of longer chain PFASs and perfluorosulfonic acids (PFSAs) occurring later than shorter chain PFASs and perfluorocarboxylic acids (PFCAs), respectively. Greater adsorption capacity was found for GAC products containing high volumes of transport pores. A subsequent study compared a better performing GAC product evaluated from the prior study against three AER products in a similar groundwater for 13 months. For AERs, breakthrough of PFCAs followed compound chain length but breakthrough of PFSAs occurred at the same time for all AERs, possibly attributed to accumulation of metals from the source water in the AERs. Still, PFSAs exhibited greater adsorption affinity to AERs than PFCAs. AERs adsorbed 6-7 times more PFASs than GAC per unit media at breakthrough; however, based on volume of water treated at breakthrough, AERs and GAC performed similarly for PFCAs but better than GAC for PFSAs.

All AERs evaluated performed similarly. When media replacement is dictated by breakthrough of perfluorooctanoic acid (PFOA), similar operations and maintenance costs can be expected if AER media cost is ~3.5 times GAC media cost.

The impact of operating conditions, water matrix, and adsorption on rejection of PFASs by spiral-wound NF and RO membrane elements was evaluated. Membrane operating conditions did not have a significant impact on rejection of PFASs in a laboratory electrolyte matrix and was >98% by NF and >99% by RO. Rejection of the same PFASs present in a groundwater matrix by NF was lower, between 92-98%, and was attributed to water matrix effects. Adsorptive losses of longer-chain hydrophobic PFASs to the NF spiral wound membrane elements and the membrane system were observed but did not affect rejection of PFASs by NF. A subsequent study combined NF and UV-sulfite in a treatment train for the sequential removal and destruction, respectively, of PFASs in groundwater. Most PFASs were rejected to >95% by NF when operating at 90% permeate recovery. UV-sulfite treatment of the NF reject resulted in variable destruction of individual PFASs, with rates also being dependent upon pH and the identity and concentration of UV photosensitizer. Rates of PFCA degradation were greater than those measured for PFSAs and polyfluorinated PFASs and were independent of perfluoroalkyl chain length. In contrast, rates of PFSA degradation increased with increasing chain length. Collectively, >75% of the detected PFAS mass in the NF reject was destroyed after 4 h of UV treatment, increasing to 90% after 8 h of treatment.  Electrical energy per order magnitude requirements for the NF and UV treatment train were estimated to be <13.1 kWh/m3 for all PFCAs and 14.1 kWh/m3 for PFOS.

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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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.


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