Q&A: Understanding Commingled PFAS Plumes

As PFAS investigations become more complex, environmental professionals are increasingly encountering situations where groundwater impacts from multiple sources overlap. These commingled PFAS plumes can create significant challenges for source attribution, liability allocation, and remediation planning. In this Q&A, Langan explores how advanced analytical tools and multiple lines of evidence can help support the evaluation of commingled PFAS plumes.

Key Takeaways

• Commingled PFAS plumes occur when groundwater impacts from multiple PFAS sources overlap.
• Standard PFAS analytical methods may not provide sufficient information to differentiate sources or support source attribution.
• Advanced analytical tools can help identify PFAS source characteristics and evaluate potential off-site contributions.
• Langan’s PFAS experts use a multiple-lines-of-evidence approach to support defensible PFAS investigations and remediation decisions.

What is a “commingled plume,” and why does it matter in PFAS investigations?

A commingled PFAS plume occurs when groundwater quality impacts from multiple sources overlap and produce a combined distribution of PFAS. These situations are increasingly common due to the widespread historical use of PFAS in firefighting foams, industrial processes, and household products. From a regulatory and legal standpoint, commingling complicates the identification of potentially responsible parties, the determination of an endpoint in relation to investigation and delineation activities, and the allocation of remediation costs. 

Because many PFAS sources release similar compounds, such as PFOA and PFOS, differentiating between the sources contributing to a PFAS plume is a technically complex but critically important task. The first step in evaluating if a commingled plume is present is determining if PFAS concentrations are migrating onto the property from an off-site (upgradient) source. Additionally, the prevalence of PFAS in urban and industrial areas often leads to regional PFAS groundwater quality impacts, which further complicates the issue.

Why can’t we rely solely on standard PFAS analytical results?

Traditional analytical methods (e.g., EPA Methods 537.1 and 1633) focus on a defined list of PFAS compounds and provide high-quality concentration data for those specific compounds. While certain patterns can be identified when comparing on-site data related to upgradient samples, these methods capture only a fraction of the total PFAS present and offer limited insight into the origin of those compounds. 

Additionally, these methods are often unable to directly quantify precursor compounds, which can transform into regulated PFAS over time. As a result, relying solely on routine analytical results can obscure differences between sources and limit the ability to support defensible source attribution or cost allocation decisions.

What advanced analytical tools are available to help differentiate PFAS sources?

A range of advanced analytical techniques can provide additional resolution beyond standard target analyte data. Pattern analysis compares relative concentrations of PFAS compounds to identify characteristic source “fingerprints,” while multivariate statistical methods can highlight clustering trends. Isomer analysis can distinguish between electrochemical fluorination and fluorotelomerization-based production processes for PFAS, particularly PFOS and PFOA, by evaluating the ratio of branched and linear isomers, and can provide information regarding the potential timing of PFAS releases. 

Additionally, the total oxidizable precursor assay converts oxidizable precursors into terminal acids to reveal hidden PFAS mass, total organic fluorine/extractable organic fluorine quantifies unidentified PFAS fractions, and high-resolution mass spectrometry identifies compounds associated with specific industrial or commercial formulations. Langan’s PFAS experts understand that every situation is unique. Careful planning must occur before collecting samples to identify the data that will help differentiate between site-related impacts and potential off-site contributions.

What limitations should be considered when interpreting results? 

Despite advances in PFAS forensic techniques, uncertainties remain. Many PFAS-containing formulations share similar compounds, and manufacturing processes have evolved, reducing the uniqueness of some signatures. Environmental processes like sorption, transport, and precursor transformation can also alter PFAS profiles and obscure source characteristics. Additionally, regulatory frameworks for PFAS forensics are still developing, and there is limited standardization in analytical approaches. 

As a result, conclusions regarding responsibility are typically probabilistic and should be presented within a clear framework of assumptions and uncertainty. 

Evaluating commingled PFAS plumes with Langan

In practice, PFAS source attribution relies on a multiple-lines-of-evidence approach to support data interpretation and conclusions from PFAS investigations. Langan’s team of PFAS professionals helps clients review and evaluate all available data, whether collected at the subject site or through off-site data sets, to provide a defensible argument that can reduce the burden of potential PFAS investigation and remediation.

Kyle Falk is a Project Geologist in Langan’s Doylestown office with over a decade of environmental consulting experience. He plays a key role in Langan’s PFAS Core Group, helping clients stay abreast of evolving regulations, manage PFAS impacts across their sites, and implement advanced remediation strategies. His PFAS-related expertise includes complex soil and groundwater delineation considerations, remedial evaluations, and waste disposal.

Commingled Plumes FAQs

Why are commingled PFAS plumes difficult to investigate?

Commingled PFAS plumes can be difficult to investigate because contamination from multiple sources may overlap within the same groundwater system, making it challenging to distinguish individual source contributions. Similar PFAS compounds may be present across different source areas, and environmental processes such as transport, sorption, and precursor transformation can further alter PFAS profiles over time. As a result, investigators often rely on multiple lines of evidence to support source attribution and data interpretation.

Can off-site sources contribute to a commingled PFAS plume?

Yes. Off-site PFAS sources can contribute to a commingled plume when contaminated groundwater migrates onto a property and mixes with site-related impacts. Evaluating potential upgradient sources, groundwater flow conditions, and regional PFAS groundwater quality impacts is often an important step in determining whether a commingled plume is present.

What information is needed to evaluate a commingled PFAS plume?

Evaluating a commingled PFAS plume typically requires groundwater quality data, hydrogeologic information, site history, potential source area information, and analytical results from both on-site and off-site locations. Advanced analytical tools such as pattern analysis, isomer analysis, and high-resolution mass spectrometry may also help investigators differentiate between potential PFAS sources and evaluate source contributions.