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Defining a NAPL Body

May 24, 2022

During an environmental investigation, contaminant exposure risk is evaluated through several pathways. These exposure pathways include: drinking water, direct contact, groundwater/surface water interface (GSI), and particulate or vapor inhalation. A conceptual site model (CSM), which is used to visualize the various exposure pathways that are applicable to a facility, is constructed through site characterization via soil, soil gas, and groundwater analytical data, soil borings, etc. If the level of exposure risk is unacceptable at a facility, the pathway is deemed potentially complete and additional action is necessary.

In the environmental field, an assortment of tools are available to characterize a facility during an environmental assessment. Envirologic has utilized various specialized methodologies to evaluate contaminated sites and study potential exposure pathways. The following tools have been incorporated into the Envirologic toolbox for environmental investigations.

  • LIF NAPL Profile
  • Vapor Pin and Soil Gas Well Sampling
  • Incremental Soil Sampling
  • Pore Water Sampling

In this article, we are going to focus on one of the site characterization methods that Envirologic utilizes; laser-induced fluorescence (LIF), which excels as a rapid assessment tool that delineates the vertical and horizontal extent of a non-aqueous phase liquid (NAPL) body. At sites where petroleum releases have occurred, defining the extent of the NAPL body is critical in developing a CSM and evaluating various exposure pathways. As a NAPL body can serve as a source of dissolved contaminants in groundwater or as a source of vapors that can emanate to the surface, the primary goal at a site with NAPL is often to reduce the mass of the NAPL body and thus decrease exposure risks. Remediation techniques are more efficient and effective when the NAPL body is adequately defined. Understanding a NAPL body through a LIF study during the early stage of an environmental investigation can significantly reduce overall project costs.

The key component to LIF is that polycyclic aromatic hydrocarbons (PAHs), which are found in petroleum products, are very fluorescent. If the proper wavelength of energy is applied to a PAH, the molecule will in return release excess energy and fluoresce. A continuous LIF profile is generated by advancing a probe through the subsurface via direct push technology, such as a Geoprobe®. The LIF probe emits a laser signal that excites the PAH molecules in the NAPL body and then records the intensity of the PAH fluorescence. The fluorescence intensity will vary depending on the NAPL saturation adjacent to the probe, the fluorescence response is generally strongest near the center of a NAPL body and decreases towards the fringe of the body. Different petroleum products (gasoline, diesel, machine oil, etc.) will generate different response signatures, which can help distinguish two separate NAPL bodies.

Envirologic completed a LIF study to characterize the extent of NAPL at a former bulk petroleum storage facility. A total of 30 LIF borings were advanced across the site. Two LIF borings identified a previously unknown NAPL arm along the northern portion of the site. Additional LIF borings confirmed that previous remedial efforts have decreased the vertical extent of the NAPL body near the source area. Results from the LIF study indicate that the NAPL body has migrated from the source area to beyond the western property boundary. Soil sampling was then conducted across the NAPL body to evaluate exposure risks; the LIF study helped reduce the number of soil borings and analytical samples that were necessary to complete the exposure assessment.

Site Plan_NAPL

LIF has traditionally been used to define petroleum or coal tar-related NAPL bodies; however, recent advances have made it feasible to delineate a chlorinated solvent NAPL body with this method. Regardless of the contaminant source, LIF serves as a rapid assessment tool that is available to define the extent of a NAPL body.

There are often sampling needs as part of an environmental investigation; let Envirologic support your project using this or other tools. For a complete list of our services, please visit our website or contact a member of our team at (269) 342.1100 or

Posted in Blog, Environmental Investigation & Remediation

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