Sampling tools described in previous articles 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 liquid (NAPL) body. At sites where petroleum releases have occurred, defining the extent of the NAPL body is critical to developing a conceptual site model and evaluating various exposure pathways. As a NAPL body can serve as a source of dissolved contaminants in groundwater and 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, thereby decreasing 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 toward 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 had decreased the vertical extent of the NAPL body near the source area. Results from the LIF study indicated that the NAPL body had 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.
LIF has traditionally been used to define petroleum and coal tar related NAPL bodies; however, recent advances have made it feasible to also delineate a chlorinated solvent NAPL body with this method. Regardless of the contaminant source, LIF serves as a valuable rapid assessment tool that is available to define the extent of a NAPL body.
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exposure pathway • laser-induced fluorescence • lif • non-aqueous phase liquids • napl • exposure risks • remediation • environmental investigation