Defining a NAPL Body with Laser-Induced Fluorescence

by Derrick Lingle
Feb 2, 2016

NAPL_title slide

Sampling tools described in recent posts (Vapor Pin & Vapor Well Sampling and When Should the Incremental Sampling Method be Used?) provide a way of evaluating the risk associated with a particular exposure pathway, such as vapor intrusion or direct contact. Another site characterization method that Envirologic utilizes is laser-induced fluorescence (LIF), which excels as a rapid assessment tool that delineates the vertical and horizontal extent of a non-aqueous phase liquids (NAPL) body. At sites where petroleum releases have occurred, defining the extent of the NAPL body is critical in developing a conceptual site model (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 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.

Sample NAPLv1

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.

Site Plan_NAPL

Envirologic has recently 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.

Sample NAPLv2

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 available for defining the extent of a NAPL body.

If you have questions about this sampling method, please contact Project Geologist, Derrick Lingle.

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exposure pathway  laser-induced fluorescence  lif  non-aqueous phase liquids  napl  exposure risks  remediation  environmental investigation 

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