Subsurface delineation of hydrocarbon through laser-induced fluorescence. Recently a client invited C2REM to visit a steam injection DNAPL remediation project in central California. (NOTE: The project itself is quite remarkable and may be the subject of a future article). During this visit we were introduced to a recent technology presently receiving a lot of attention. The product is known as the Rapid Optical Screening Tool (ROST®) and has been acquired by Fugro Geosciences, Inc. ROST®is a laser-induced fluorescence sensor deployed along with standard Cone Penetration Testing (CPT) equipment.

The product is a limited intrusion device used to delineate hydrocarbon sources (at the soil/CPT interface) from non-impacted media down to residual concentrations in soils, both above and below the water table. The ROST® system uses an ultraviolet laser source coupled with an optical detector to measure fluorescence (a technique known as Laser-Induced Fluorescence Spectroscopy). The laser emits a continuous pulse through a CPT as it is being advanced. This laser pulse is received at the optical detector (referred to as the sapphire window) and is correlated with known fluorescent characteristics for petroleum products. This system generates a fluorescence vs. depth curve, which can be used to produce a 3-dimensional picture of the extent of impacted soil and/or aquifer zone(s).

At the steam injection project discussed above, ROST® technology was utilized to delineate creosote. The client stated that the ROST® technology was more accurate, efficient and safer than the previous sampling done by conventional soils and groundwater borings. At this particular site, the ROST® system had a good correlation with soils and groundwater analytical results from grab samples (using conventional boring and sampling methods at noncontinuous intervals).

The ROST® technology has also been verified by the U.S. EPA through their environmental technology verification ETV program and the results were published in a verification statement dated February 1997. In this statement, U.S. EPA concluded that the ROST® system is an emerging technology worthy of consideration for site investigations where aromatic hydrocarbons (e.g., petroleum, oils, lubricants, and coal tars) are suspected. The technology offers a number of advantages over conventional drilling and sampling technologies for the purpose of screening a site to determine the nature and extent of contamination. U.S. EPA's ETV statement indicates that the typical operation process cost is approximately $4,200 - $5,300/day and can produce, under normal circumstances, approximately 300 feet /day of data (approximately $14-$20/foot).

Possible limitations and issues which should be considered prior to utilizing this technology includes:

Dissolved phased hydrocarbons will not typically be detected by this system (i.e., as materials migrate to the groundwater many constituents will dissolve and no longer be detectable, however nonaqueous phased constituents can be observed).

The limitations of the cone penetrometer equipment, primarily hydraulics (dependent largely on the subsurface stratigraphy).

As this system is not quantitative, agencies will require conventional sampling and analysis techniques in addition to the ROST® system for risk assessments and/or when specific clean up requirements are established for remediation. However, far fewer monitoring wells or sampling locations should be required.

Concerns with variation in stratigraphy and/or unidentified fluorescent interference can cause problems with clearly delineating subsurface conditions.

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