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LIF technology brings subsurface contamination to light
When fuel, coal tar or oil leaks or spills, it can pose a significant threat to groundwater. Unfortunately, designing a remediation program can be a bit of a guessing game when the contaminants disperse irregularly underground. There are, however, some oft-overlooked technologies available to screen sites more accurately than traditional methods have allowed. The time has come to start looking into these instruments in order to get the full picture of sites and better protect groundwater.
To locate and map out the subsurface contamination from coal tar, creosotes or other nonaqueous phase liquids (NAPLs), contractors traditionally have taken physical soil samples. Using drilling rigs or direct-push platforms (e.g., Geoprobe systems or cone penetrometer technology), the contractors bore multiple holes. While drilling down, they take samples at intervals, sometimes every 5 ft; when finished with one hole, they pick another location to do more sampling.
When collecting samples from concentrated areas of contamination, contractors often can tell the presence of crude oil, fuel or other petroleum-based substances from the appearance and smell of the soil. To determine the magnitude of contamination or the type of substance, the samples must be sent to a lab for analysis. It can take several days or weeks to see results (i.e., too late to act on the information in the field).
Although this method has been accepted by regulators and used by contractors for decades, it often results in an incomplete picture of the subsurface. Because the process is expensive and time-consuming, sampling contractors take shortcuts, which lead to gaps in the data. They may choose to sample only the soil directly above or below the ground- water table, where they believe all the contamination to be located, or try to save money by taking samples at fewer intervals than needed to properly define the contamination. Furthermore, sampling is associated with other problems, such as poor recovery, compression of soils, hole slough, smearing and depth inaccuracies.
After gathering the data from the soil samples, consultants generate a plan for remediation if needed. It is fairly common, however, for remediation activities to discover uncharted areas of contamination. In such an instance, a remobilization of sampling crews is necessary to create a more accurate depiction of contaminant distribution. It is not unheard of for this to recur, drawing out the time to complete the project and driving up costs.
The reason sampling usually does not detect all contamination during the first field effort is because oil, fuel and other NAPLs tend to disperse irregularly in the subsurface. This occurrence goes against the popular belief that these substances typically are contained in floating “pancake”- shaped layers at the groundwater surface. In reality, contaminants often are distributed in many narrow seams and soil fractures, and sometimes end up trapped as far as 20 to 30 ft below the groundwater surface.
Lighting the Way
To map out the distribution of NAPLs with better accuracy, some sampling contractors have adopted direct-push tools that use laser-induced fluorescence (LIF), a technology invented in the early 1990s and since verified by the U.S. Environmental Protection Agency. LIF allows contractors to conduct more soil readings in less time because, rather than taking physical samples, LIF optical screening tools (OSTs) use light to gather information in real time as the probe is pushed into the ground.
In simple terms, LIF acts as a design tool painting a detailed picture of where coal tar, creosote or fuel has leaked and flowed since release. The results are presented in colorized logs displaying the type and depth of contaminants through- out each hole that is logged. If sitewide context is desired, all the logs from a site can be combined with geographic coordinates to create 3-D conceptual site models (CSMs) using software available from a number of vendors. The CSMs clearly illustrate the distribution of NAPLs in the subsurface and show engineers exactly how to remedy the site correctly on the first attempt.
LIF technology takes advantage of the inherent fluorescence of polycyclic aromatic hydrocarbons (PAHs) found in oils, fuels and other NAPLs. Bunker fuel, for instance, appears pale orange under visible-wavelength excitation, and coal tar emits red. On the other hand, light NAPLs (e.g., crude oil and diesel) do not fluoresce well under visible-wavelength light, instead reacting to ultraviolet (UV) excitation light and emitting blue-green light.
Because of this phenomenon, different types of LIF instruments have been developed. These include the UV optical screening tool (OST) for detecting light NAPLs and the tar-specific green optical screening tool for use with dense coal tars and creosotes. The tool used on a job depends on the type of NAPL expected to be found. Otherwise, representative NAPL samples typically can be sent to LIF vendors for free analysis.
Generally, an LIF instrument consists of a steel probe with a sapphire window built into the side. Laser light is delivered to the window as the probe is driven into the ground. Any PAHs from fuel or oil outside the window are excited by the laser light and fluoresce. A fiber-optic cable returns any fluorescence to the surface, where it is recorded and displayed in real time. Above ground, the contractor and onsite consultants can watch as the contaminant log develops, immediately reacting to the result and determining the next logging location according to the results.
Whereas the most liberal sampling plans analyze the soil once every 4 ft or so, LIF instruments read and store measurements approximately once every inch the entire time the OST is being pushed into the ground. This ability to quickly analyze every inch of the subsurface allows OSTs to discover small but important seams and fractures of contamination, which often are missed by sampling.
Not only do LIF instruments produce more data per hole than sampling, but they also work more efficiently. Rather than stopping at various depths to collect soil, an OST probe continuously collects data while being pushed into the ground. Furthermore, LIF instruments can typically log between 300 and 500 ft per day, versus an average of 100 to 200 ft per day by sampling. The contractor need not wait for lab results either, as LIF immediately identifies the type and relative concentration of fuel and oil in the subsurface.
Because no contaminated soils are brought to the surface, LIF is considered a green technology. In addition, contractors no longer need to worry about exposure to contaminants brought up from the subsurface, and there is no investigation-derived waste to manage.
While sampling is still the most commonly used screening method, LIF is becoming more popular. In fact, many contractors are finding it to be integral in performing effective remediation due to the numerous benefits. Some consultants are starting to use LIF instruments as primary screening tools rather than as a backup to sampling tools.
No matter the type of release, engineers and con- sultants are likely to benefit from LIF. Sampling often generates more questions than answers, but LIF instruments provide a more accurate picture of the underground distribution of NAPLs. Then, if remediation is needed, the process is more likely to go quickly and stay within a reasonable budget— suddenly not too much to ask.