When Groundwater Mixes: Commingled Plume Technical Guidance Document Released

What’s a commingled plume?

A commingled plume, or mixed plume, is when contaminants from two or more distinct discharges mix in the underlying groundwater. These commingled plumes are often more difficult and complicated to clean up than single plumes.

Where do they come from?

Commingled plumes are common in densely developed areas and can arise from multiple different scenarios. Here are two basic examples of how commingled plumes are formed:

Example 1: Neighboring gas stations both had fuel spills and the underlying groundwater is contaminated with gasoline from both facilities.

Example 2: A neighboring gas station and dry cleaner each had releases and the underlying groundwater is contaminated with both dry cleaning solvents and gasoline.

The sources can be spatially distinct (i.e. originated from different locations) or temporally distinct (i.e. occurred at different times).

Why is cleaning up commingled plumes difficult?

Every commingled plume is unique and presents its own challenges for cleanup. If the commingled plume has multiple contributors, also known as potential responsible parties (PRPs), it can be hard to determine the limits of each individual plume. This can make choosing a remedial strategy and allocating each PRPs “fair share” of the cleanup difficult.

In Example 1 above, the contaminants are similar and so the challenge for the investigator is delineating the individual plumes in order to assign responsibility between the PRPs.

In Example 2 above, the contaminants are different, so it will likely be easier for investigators to separate the individual contributing plumes; however, the PRPs may need to work cooperatively when designing remedial strategies so as not to hinder the cleanup of one or more of the differing contaminants.  

The Innovate Dual Approach of the New Guidance

The Commingled Plume Guidance uses a dual technical and administrative approach to help investigators.

Technical Approach

Technical techniques to delineate the individual plumes that are discussed in the Commingled Plume Guidance include:

• Environmental forensics, such as Fingerprinting, PIANO analysis, degradation and trace compounds, biomarker analysis, etc.

• Fate and transport modeling; and

• Statistical analysis.

A helpful checklist is included in the Guidance for the investigator that identifies the information or potential lines of evidence that could be collected to assist with plume delineation and source identification.  

Administrative Approach

The Commingled Plume Guidance provides advice on working with other PRPs, including benefits and challenges of working cooperatively to clean up the commingled plume, options for dealing with uncooperative PRPs, and potential resolution mechanisms.  

The Commingled Plume Guidance can be downloaded from the NJDEP website or by clicking below.

NJDEP Commingled Plume Guidance Document