Using Flux Chambers to Quantify Ebullition Facilitated NAPL and Contaminant Transport
Volume 10, Issue 4 | June 2022


Applied NAPL Science Review

Using Flux Chambers to Quantify Ebullition Facilitated NAPL and Contaminant Transport

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 Flux Chambers to Quantify Ebullition Facilitated NAPL and Contaminant Transport 


Amy L. Corp, P. Chem, EP, Anchor QEA, LLC 

Accurately quantifying gas ebullition and related mass transport of NAPL and other contaminants from sediment to the overlying water column is an important issue at some contaminated sediment sites. This article summarizes procedures for building and using flux chambers to collect technically defensible data.

Introduction

Gas ebullition is gas bubble formation and growth in sediment, followed by sediment fracture and the subsequent upward migration of gas bubbles through sediment to the surface water column. Migration of gas bubbles through nonaqueous phase liquid (NAPL) or other organic contaminants in sediments may result in the transport of NAPL/contaminants from sediments to surface water (i.e., ebullition-facilitated transport [EFT]). This can be an important transport mechanism for contaminated sediment sites, moving isolated contaminants into the biologically active zone (Viana and Rockne 2021, 2022 ; Fendinger et al. 1992; Viana et al. 2012, 2018; Yuan et al. 2009). Also, at sites where sediment capping is planned as part of the remedy, ebullition should be evaluated to understand potential impacts to cap design such as contaminant transport and potential gas buildup (which could potentially destabilize a remedial cap).

Flux chambers can be used to accurately quantify EFT rates of NAPL and other contaminants from sediment to the overlying water column. Quantifying the rate of gas production and NAPL/contaminant flux over a representative range of conditions allows for the calculation of annual EFT chemical loads to surface water. These annual loads can then be compared to other contaminant transport mechanisms that may be present at a site and used in remedial design evaluations. ASTM E3300 (ASTM 2021) provides further information on calculation of annual EFT chemical loads.

Near-bottom measurements (i.e., using flux chambers) have proven to better represent gas and contaminant flux than surface-based measurements (Rockne et al. 2011; Viana et al. 2012; Maeck et al. 2014; Zhu et al. 2015). While other methods can be used to measure EFT, this article focuses on the use of near-bottom flux chambers.

Flux Chamber Components

An example flux chamber configuration is illustrated in Figure 1.


Figure 1. Illustration of gas ebullition conceptual model and flux chamber schematic.

The flux chamber is composed of a solid frame that sits on the sediment surface when deployed. A funnel (made of materials such as polycarbonate or stainless steel) traps any upward moving gas bubbles that originate within the chamber footprint. The gas bubbles then pass into the glass wool chamber, where the NAPL/contaminants are sorbed onto glass wool and the gas continues to flow upward. The gas is stored in the tubing that runs to the water surface with a valve on the end that allows for gas collection from a vessel while the chamber is still deployed. The chamber is also outfitted with a control sampler of glass wool, which is affixed to the outside of the flux chamber and is not impacted by ebullition below the chamber. The objective of the control sampler is to provide an estimation of the NAPL/contaminant concentrations in the water column adjacent to the flux chamber that may originate from sources other than gas ebullition.

Flux Chamber Field Procedures

Flux chamber locations and deployment timing should be selected to capture a range of gas ebullition conditions within a site. ASTM E3300 (ASTM 2021) provides further information on determining the location of sampling devices, when these devices should be deployed, and the duration of the deployment.

Figure 2. Flux chamber prior to deployment.

Deployment

Flux chambers should be deployed in a manner that limits sediment disturbance and ensures trapped gas is vented from the system. This objective can be achieved through venting the chamber in the water, slowly lowering it to the mudline, and anchoring it with weights lowered down to the frame. Use of divers is discouraged to avoid potential sediment disturbance. Depending on water depth and clarity, an underwater camera should be used to confirm the submerged flux chamber is properly seated on the sediment surface (i.e., in an upright position).

Maintenance

Flux chambers should be inspected (with the underwater camera if necessary) on a daily basis to ensure correct position (i.e., chambers are not tipped over) and to collect or vent gas. Gas can be collected to measure gas volume or analyze gas composition, or it may be vented. This periodic release of the gas is important to ensure that too much gas does not accumulate in the tubing and back up into the glass wool chamber. Additionally, the temperature and atmospheric pressure should be measured at the time of gas collection. If a flux chamber is found to be tipped over, retrieve and replace with a new flux chamber.

Retrieval and Sample Processing

Flux chamber retrieval should be conducted in a way that minimizes sediment or chamber disturbance to the extent practicable. Gas should be collected (if required) or vented prior to flux chamber retrieval. Once the flux chamber is retrieved from the water, the glass wool compartment and control sampler should be removed and stored for processing. The inside (bottom) surface of the funnel should be cleaned with wipes (with solvent or without, based on the analytical method to be used), and the wipes should be added to the glass wool for sample extraction. Samples should be sent to a laboratory for extraction and analysis for NAPL mass (gravimetric determination) and organics (e.g., polycyclic aromatic hydrocarbons, n-alkanes and isoprenoids, and polychlorinated biphenyls). The sample results can then be used in the calculation of an annual load as described in ASTM E3300 (ASTM 2021).


Figure 3. Glass wool following retrieval of the flux chamber.

Conclusion

NAPL and contaminant transport facilitated by gas ebullition can affect the performance of a sediment remedy flux chambers have been successfully used at several sediment sites to collect quantifiable measurements of NAPL/contaminants and gas transported via ebullition. This method provides technically defensible data that can be used in annual chemical load calculations.

A Word of Caution

Flux chambers are a time-intensive method and may not be practicable for all sites. Disturbances to chambers may occur in busy waterways or those with rapid surface water flow, which may impact the ability to accurately measure EFT. Important considerations when using flux chambers at a site include location, number of chambers, number of sampling events, sediment bed temperature, changes in surface water elevation (e.g., tidal conditions), potential disturbances (e.g., vessels and aeration systems), and how to apply results to a larger site area.

These shared best practices are general recommendations, and project-specific considerations may warrant modifications to the methods discussed or consideration of alternate methods.

References 

ASTM, 2021. Standard Guide for NAPL Mobility and Migration in Sediment – Evaluating Ebullition and Associated NAPL/Contaminant Transport. Designation: E3300.

Fendinger, N.J., D.D. Adams, and G.E. Glotfelty, 1992. “The Role of Gas Ebullition in the Transport of Organic Contaminants from Sediments.” Science of Total Environment 112 (2−3):189−201.

Maeck, A., H. Hofmann, and A. Lorke, 2014, “Pumping Methane Out of Aquatic Sediments – Ebullition Forcing Mechanisms in an Impounded River.” Biogeosciences 11:2925–2938.

Rockne, K.J., R. Kaliappan, and G. Bourgon, 2011. Sediment Gas Ebullition Study, Grand Calumet River, Western Branch, Reaches 1 and 2, USACE-Chicago District, 234 pp.

Viana, P., K. Yin, and K. Rockne, 2012. “Field Measurements and Modeling of Ebullition-Facilitated Flux of Heavy Metals and Polycyclic Aromatic Hydrocarbons from Sediments to the Water Column.” Environmental Science and Technology 46(21):12046–12054.

Viana, P., K. Yin, and K. Rockne, 2018. “Comparison of Direct Benthic Flux to Ebullition-Facilitated Flux of Polycyclic Aromatic Hydrocarbons and Heavy Metals Measured in the Field.” Journal of Soils and Sediments 18:1729–1742.

Viana, P.Z. and K.J. Rockne, 2022. “Assessment and Control of Ebullition Facilitated NAPL and Contaminant Transport in Sediment.” Applied NAPL Science Review 10(3). May 2022.

Viana, P.Z. and Rockne, K. J., 2021. “Fundamentals of Ebullition Facilitated NAPL and Contaminant Transport.” Applied NAPL Science Review 9(6). September 2021.

Yuan, Q., K.T. Valsaraj, and D.D. Reible, 2009. “A Model for Contaminant and Sediment Transport via Gas Ebullition Through A Sediment Cap.” Environmental Engineering Science 26(9):1381−1391.

Zhu, T., D. Fu, C.T. Jafvert, and R.P. Singh, 2015. “Modeling of Gas Generation from the River Adjacent to the Manufactured Gas Plant.” RSC Advances 5:9565–9573.


Research Corner

Forecasting Groundwater Contaminant Plume Development Using Statistical and Machine Learning Methods

Elizabeth McConnnell
Master of Science
Colorado State University

Abstract:

A persistent challenge in predicting the fate and transport of groundwater contaminants is the inherent geologic heterogeneity of the subsurface. Contaminant movement has been primarily modeled by simplifying the geology and accepting assumptions to solve the advection- dispersion-reaction equation. With the large groundwater quality datasets that have been collected for decades at legacy contaminated sites, there is an emerging potential to use data- driven machine learning algorithms to model contaminant plume development and improve site management. However, spatial and temporal data density and quality requirements for accurate plume forecasting have yet to be determined. In this study, extensive historical datasets from groundwater monitoring well samples were initially used with the intent to increase our understanding of complex interrelations between groundwater quality parameters and to build a suitable model for estimating the time to site closure. After correlation analyses applied to the entire datasets did not reveal compelling correlation coefficients, likely due to poor data quality from integrated well samples, the initial task was reversed to determine how many data are needed for accurate groundwater plume forecasting. A reactive transport model for a focus area downgradient of a zero-valent iron permeable reactive barrier was developed to generate a detailed, synthetic carbon tetrachloride concentration dataset that was input to two forecasting models, Prophet and the damped Holt’s method. By increasing the temporal sampling schedule from the industry norm of quarterly to monthly, the plume development forecasts improved such that times to site closure were accurately predicted. For wells with declining contaminant concentrations, the damped Holt’s method achieved more accurate forecasts than Prophet. However, only Prophet allows for the inclusion of exogenous regressors such as temporal concentration changes in upgradient wells, enabling the predictions of future declining trends in wells with still increasing contaminant concentrations. The value of machine learning models for contaminant fate and transport prediction is increasingly apparent, but changes in groundwater sampling will be required to take full advantage of data-driven contaminant plume forecasting. As the quantity and quality of data collection increases, aided by sensors and automated sampling, these tools will become an integral part of contaminated site management. Spatial high-resolution data, for instance from multi-level samplers, have previously transformed our understanding of contaminant fate and transport in the subsurface, and improved our ability to manage sites. The collection of temporal high-resolution data will similarly revolutionize our ability to forecast contaminant plume behavior.


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
GWSDAT
ITRC LNAPL Resources
ITRC DNAPL Documents
SERDP-ESTCP
Sustainable Remediation Forum

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