When fuel, oil or tar has leaked or spilled, it can pose a significant threat to ground water. Unfortunately, designing a remediation program can be a bit of a guessing game when the contaminants disperse irregularly underground. However, there are some sensing technologies available to screen sites much more accurately than traditional sampling methods. It’s time to start looking into these instruments, so you can get the full picture of your site contaminant in order to better engineer a solution.
The Old Habits
To locate and map out the subsurface contamination from fuels, oils, coal tars, creosotes and other non-aqueous phase liquids (NAPLs), contractors traditionally have taken physical soil samples. Using drilling rigs or direct-push platforms, such as percussion-driven probes or cone penetrometer technology (CPT), the contractors sample multiple locations. They take samples at intervals, sometimes at spacings of every few feet. They often sample in a fixed grid pattern.When collecting samples from areas of contamination, contractors often can tell the presence of separate phase NAPL from the appearance and smell of the soil (organoleptic method). However, to determine the magnitude of contamination, or the type of substance, the samples must be sent to a lab for analysis, which can take several days or weeks to see results (too late to act on the information in the field).
Although the sampling/analysis 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 plans often are limited in their spatial extent, leading to significant gaps in the data. For example, investigators may choose (or be required) to only sample the soil at the ground water potentiometric surface; or, they may try to reduce costs, and take samples at far fewer intervals than they need to properly define the contamination. Furthermore, sampling suffers from problems such as poor recovery, compression of soils, slough and smearing.
After gathering the data from the soil samples, consultants then generate a plan for remediation, if needed. However, it is fairly common for the remediation activities to discover areas of contamination undetected during characterization. Then, a remobilization of sampling crews is necessary to create a more accurate depiction of contaminant distribution. It’s not unheard of for this occurrence to repeat itself, drawing out the time to complete the project and driving up costs.
The reason why sampling doesn’t usually detect all contamination during the first field effort is because oil, fuel and other NAPLs tend to disperse irregularly in the subsurface. This occurrence is contrary to the popular belief that light NAPLs typically are contained in floating, pancake- shaped layers at the ground water surface. In reality, NAPLs often are distributed, perched, trapped or contained in many narrow seams and soil fractures – sometimes as far as 20 feet to 30 feet below the ground water surface.
Lighting the Way
To map out the distribution of NAPL with better accuracy, consulting engineers have begun to appreciate the benefits of laser-induced fluorescence (LIF), a technology that was invented in the early 1990s, and has since been verified by the Environmental Protection Agency. LIF allows service contractors to take many more soil readings in much 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 logging tool that records a detailed record of where coal tar, creosote or fuel has leaked and flowed since release. The results are presented in colorized logs that show the type and depth of contaminants throughout each hole that is logged. If site-wide context is desired, all the logs from a site can be combined with geographic coordinates to create three-dimensional 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 what they need to know 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, such as jet fuel and gasoline, do not fluoresce well under visible-wavelength light, instead reacting to ultra-violet excitation light, and emitting blue-green light.
Because of this phenomenon, different types of LIF instruments have been developed. These include the ultra-violet optical screening tool for detecting light NAPLs, and the tar-specific green optical screening tool for use with dense coal tars and creosotes. The particular 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 with fiber optics 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 second fiber optic cable returns any fluorescence to the surface, where it is recorded and displayed in real time. Aboveground, the contractor and on-site consultants simply watch as the contaminant log develops, immediately reacting to the result, and determining the next logging location according to the results.
Whereas most sampling plans only analyze the soil on regular spacings, LIF instruments read and store measurements approximately once every inch the entire time the OST probe 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 typically can log between 300 feet and 500 feet per day. The contractor does not need to wait for lab results either, because LIF tentatively identifies the type and relative concentration of fuel, oil or tar in the subsurface.
Since 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.
The Benefits of LIF
While sampling still is the most commonly used screening method, LIF is becoming much more popular for NAPLs. In fact, many are finding it to be integral in performing effective remediation. When sampling starts generating more questions than answers, LIF instruments can develop a more accurate picture of the subsurface. Some consultants even are starting to use LIF instruments as primary screening tools, rather than having them as a backup to sampling tools.No matter the type of release, engineers and consultants are likely to benefit from LIF. LIF instruments provide a more accurate picture of the underground distribution of NAPLs. If remediation is needed, the process is more likely to go quickly and, of course, stay within a reasonable budget.
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