Laser-Induced Fluorescence

Description

Laser-induced Fluorescence (LIF) is a method for real-time, in-situ, field screening of hydrocarbons in undisturbed subsurface soils and groundwater. The technology is intended to provide highly detailed, qualitative to semiquantitative information about the distribution of subsurface petroleum contamination. LIF sensors are deployed as part of integrated mobile cone penetrometer (CPT) systems that are operated by highly trained technicians familiar with the technology and its application.

Typical Uses

LIF can detect gasoline, diesel fuel, jet fuels, fuel oil, motor oil, grease, and coal tar in the subsurface. This screening data can be used to guide an investigation or removal action or to delineate the boundaries of a subsurface contamination plume prior to installing monitoring wells or taking soil samples.

There are currently two major LIF systems available, the SCAPS and Rapid Optical Screening Tool (ROST™) systems. The Site Characterization and Analysis Penetrometer System (SCAPS) LIF system is one of several CPT-mounted sensors developed through a collaborative effort of the Army, Navy, and Air Force under the Tri-Services Program. The ROST™ system was developed by Loral Corporation and Dakota Technologies, Inc. The SCAPS LIF is available only through the U.S. Army Corps of Engineers (USACE) and the US Navy. ROST™ is available commercially through Fugro, Inc. Both systems, while differing in some respects, are very similar in their theories and methods of operation.

Click here to see a brief comparison of the two systems.

Theory of Operation

The method uses a fiber optic-based LIF sensor system deployed with a standard 20-ton cone penetrometer (CPT). Light at a specific wavelength generated from a laser is passed down a fiber optic cable to a sapphire window in the tip of the CPT rod string as it is advanced into the subsurface. The laser light excites two- or three-ring aromatic compounds, or polycyclic aromatic hydrocarbons (PAH), in the soil adjacent to the sapphire window, causing them to fluoresce. The relative response of the sensor depends on the specific analyte being measured because of the varying ratios of PAHs in each hydrocarbon mixture. The induced fluorescence from the PAHs is returned over a second fiber to the surface where it is quantified using a detector system. The peak wavelength and intensity provide information about petroleum product type or potential interferences. The intensity of the fluorescence is used as an indicator of the relative contaminant concentration.

System Components

Both the SCAPS and the ROST™ sensing devices are part of an integrated system consisting of a cone penetrometer (CPT) truck designed for direct push sampling, the laser, and a detector. The LIF sensor is used in conjunction with the stratigraphic sensing devices contained on the CPT truck. CPT and standard penetrometer testing are widely used in the geotechnical industry for determining soil strength and soil type from measurements of tip resistance and sleeve friction on an instrumented probe.

The forward portion of the truck-mounted laboratory is the push room. It contains the rods, hydraulic rams, and associated system controllers. The weight of the CPT truck provides a 20-ton static reaction force for advancing the probe rod into the ground, which allows the hydraulics to advance threaded-end steel rod into the ground at about 1 meter per minute. The rods, sensing probes, and sampling tools can be advanced to 50 meters or more in soil. As the rods are withdrawn, grout can be injected through probe rods to seal the push hole. The trucks are also fitted with a self-contained decontamination system that allows the rods and probe to be steam cleaned as they are withdrawn from the push hole, through the steam cleaning manifold, and back into the CPT push room.

The rear portion of the truck contains the laboratory in which components of the LIF system and onboard computers are located. The primary differences between the two systems are the laser and the detector systems. The SCAPS LIF system uses a pulsed nitrogen laser that produces light at a fixed wavelength of 337 nanometers (nm), which is passed down a fiber optic cable inside the probe rods to the sapphire window in the sensor tip of the CPT rod string. Fluorescence stimulated in the in situ soil “sample” by the laser is collected by a second fiber and returned to an optical multichannel analyzer (OMA) spectrograph, where it is dispersed spectrally on a photodiode array (PDA) detector. This arrangement allows for the rapid acquisition of spectral data, so that as the SCAPS LIF sensor is pushed into the soil, real-time plots are generated of depth versus maximum fluorescence intensity and the wavelength at which the maximum intensity occurs.

By contrast, the ROST™ system uses a tunable dye laser that can produce light at variable wavelengths chosen by the operator, typically 290 NM The detection system for the ROST™ consists of a monochromator, a photomultiplier tube, and digital storage oscilloscope (DSO). The monochromator acts as a variable wavelength narrow bandpass filter. By acquiring fluorescence data at a series of wavelengths, the fluorescence technician can determine the wavelength of maximum intensity in the fluorescence spectrum. The light passing through the monochromator at this wavelength is then converted to an electrical signal by the photomultiplier. The signal from the photomultiplier is fed to the DSO, which displays the waveform plot (fluorescence intensity as a function of time following the excitation laser pulse).

Mode of Operation

Because of both the complexity of the LIF methods and the specialized requirements of operating a CPT, the operation of either the SCAPS LIF system or the ROST™ takes considerable experience. For this reason, both are designed to be operated by their own trained technicians, not the general public. The SCAPS CPT and LIF technology is not available for use by private citizens or corporations, but is available to state and federal agencies. The ROST™ is available commercially through the Fugro corporation.

Both systems are typically deployed with a three-person crew and a geologist. Two people are needed to handle the push rods and operate the hydraulic press, and the third person operates the sensor, including measurements of the calibration and control standards, and monitoring the real-time CPT geotechnical data and fluorescence response from the soils. Once the system has been calibrated by the operator, the CPT truck is set up over the designated location for a push. Continuing calibration checks may be performed using a calibration standard held against the sapphire window before and after each push.

The ROST™ can be operated in two modes. In the dynamic mode, the fluorescence intensity is plotted as a function of depth below ground surface in real-time while the sensor is being advanced in the subsurface. The ROST™ system operator chooses the excitation laser wavelength and fluorescence emission monitoring wavelength and they are held constant during the push. Once areas of significant contamination have been tentatively identified in the dynamic mode, the CPT is held at a fixed depth and the ROST™ can be operated in the static mode to identify the general class of contamination present. During the static mode, ROST™ can obtain wavelength-time-matrices (WTM) which represent a 3-dimensional plot of relative fluorescence intensity versus fluorescence lifetime versus wavelength. WTMs produce contaminant-class-specific three-dimensional figures that can be used to identify the type of fuel that is present by comparing them with an on-board library of WTM “fingerprints” from common contaminants. Normally, in the static mode, the excitation wavelength is held constant and the emission monitoring wavelength is varied.

As the rods are withdrawn, grout is injected through probe rods to seal the push hole. The rods and probe are steam cleaned as they are withdrawn from the push hole and back into the CPT push room.

An SOP is available by clicking here.

Target Analytes

LIF can detect gasoline, diesel fuel, jet fuels, fuel oil, motor oil, grease, and coal tar in the subsurface.

Performance Specs

Interferences

The in situ fluorescence response of the LIF sensor to hydrocarbon compounds is sensitive to a number of interferences, but variations in the soil matrix are the most pronounced. LIF sensitivity to petroleum hydrocarbons in soil has been shown to be inversely proportional to the available surface area of the soils. Sandy soils tend to have a much lower total available surface area than clay soils, so hydrocarbon compounds in sandy soils generally yield a higher fluorescence response than they do in clay-rich soils.

Although intended to specifically target petroleum hydrocarbons, the excitation energy produced by the LIF system's laser may cause other substances to fluoresce as well, which may cause interference problems. Many common fluorescent minerals such as calcite can produce a measurable LIF signal. Other man-made non-hydrocarbon fluorescent material may be found in the subsurface environment: deicing agents, antifreeze additives, and many detergent products are all known to fluoresce very strongly, for example. Naturally occurring organic matter, which include PAHs, also can fluoresce. In many cases it is possible for an experienced operator to differentiate between the fluorescent signatures of hydrocarbons and other interfering compounds.

Detection Limits

LIF data quality is sufficient for qualitative screening, and relative intensities may be considered quantitative screening level data only. Site-specific detection limits vary from levels of 50 to 1,000 mg/kg, but exact detection limits are difficult to determine and will vary between sites and petroleum products. For example, according to results published in an EPA Innovative Technology Verification Report, the SCAPS LIF detection threshold is approximately 100 to 300 mg/kg for total petroleum hydrocarbons as confirmed by EPA Method 418.1

The effective upper detection range of both LIF detectors depend on the specific hydrocarbon analyte as well as the particular matrix. Generally, the response curves generated during calibration remain linear until approximately 10,000 mg/kg, when the response trails off. The upper effective range may be extended to higher concentrations by the operator, but this results in decreased sensitivity at lower concentrations.

Calibration

LIF systems measure the relative intensity of fluorescence in soils caused by hydrocarbon contamination. It is critical that these measurements be accurate if the data is to be useful for project decision making. For this reason, both systems must be calibrated prior to use.

The SCAPS LIF sensor is calibrated using spiked soil samples representative of the site. Diesel fuel marine standard or other petroleum hydrocarbons with a fluorescence response appropriate for the site are used to spike the soil samples. The ROST™ system is calibrated with a proprietary blend of synthetic motor oil and other substances. In both cases, the calibration standards are run in triplicate at the beginning of each day and again if equipment is changed. After measurement, the average and standard deviation is computed for each sample and the sample is rerun if the standard deviation exceeded 20 percent. A calibration curve is generated by plotting the average of maximum fluorescence peak intensity versus the concentration of fuel product added to the calibration soil sample.

Sample Preparation

Because LIF is deployed on a direct push CPT rod and analysis is conducted directly in situ, no sample preparation is possible. Some site preparation may be required, however. Hard surfaces such as concrete or caliche may require drilling or cutting prior to advancing the CPT rod into the ground.

Quality Control

Even though they are not quantitative systems, the data generated by the LIF system must be of a known and acceptable quality if it is to be useful for project decision making. For this reason, it is critical that the quality of the data produced by the LIF system be determined and documented. There are several types of quality control checks that can be applied to assess whether the LIF systems are functioning properly and are producing accurate and useable data.

The SCAPS LIF sensor's response is checked using a quinine sulfate fluorescent standard before and after each push. This measurement is a check of system performance and provides a means for normalizing measurements. If the fluorescent intensity changed by more than 20 percent of the initial value determined during pre-push calibration, system trouble shooting procedures are initiated.

A system check using a reference solution is performed before and after each ROST™ push as well. The reference is a selected mixture of hydrocarbons in solution contained in a standard fluorescence cuvette that can be strapped onto the sensor tip outside the sapphire window. Both wavelength and intensity of the standard are monitored. If the wavelength differs by greater than 5 NM from the known value, a wavelength calibration is performed. If the intensity changes by more than 20 percent, system troubleshooting is required.

A clean sand blank may be measured pre- and post-push as part of the standard data collection procedure. The blank helps assure that the sapphire window does not become contaminated and that the sensor doesn't develop a “memory effect” from previous samples. If the clean sand blank LIF measurement varies beyond 50 percent of its pre-push calibration value, troubleshooting procedures must be initiated.

Finally, a qualitative assessment can be made by comparing subsurface contaminant cross sections generated from either SCAPS LIF or ROST™ data to borehole logs or cross sections prepared using conventional methods such as a hollow stem auger rig and sampling data generated using EPA-approved analytical methods.

Precision and Accuracy

Precision refers to the reproducibility of measurements of the same characteristic, usually under a given set of conditions. Accuracy refers to the degree of agreement of a measurement to the “true” value, as determined by traditional analytical methods. Both provide a measure of the LIF system's performance and can help determine how useful their data is.

Precision is usually assessed by comparing the results of duplicate analyses. However, because both LIF sensors' are in situ sensors, it is not possible to obtain true duplicate analyses. Instead, an estimate of the instrumental precision can be obtained for either system by evaluating the results from multiple measurements of their respective calibration check samples, which are analyzed before and after each push. During an ETV verification study of the SCAPS, the standard deviation of 20 check sample measurements was less than 1 percent of the mean count. The standard deviation of 20 check sample measurements during a corresponding ROST™ demonstration was 2.2% of the mean count.

Because neither LIF system provides quantitative results, accuracy is assessed qualitatively by measuring the agreement between “detect/nondetect” determinations made by the LIF and corresponding confirmatory laboratory samples. For example, if the laboratory result was above the LIF's detection limit and the average LIF data from the push at the corresponding depth exceeded the LIF fluorescence threshold, the results agree. If the average LIF data were below the threshold and the corresponding analytical data were above the corresponding detection threshold, the result was a “false negative,” which is the most serious error in terms of environmental sampling. The false negative rate for the SCAPS during the ETV demonstration was 4.9 percent. At least 90 percent of the samples analyzed during the ETV demonstration of the ROST™ agreed with the confirmatory results, and the false negative rate ranged from 3.3 percent to 10 percent, depending on the confirmatory method used.

Superfund Innovative Technology Evaluation (SITE) Demonstration

The SCAPS LIF and ROST™ systems were evaluated in 1994 during a demonstration conducted under EPA's SITE program. The two systems were demonstrated at three sites in the Midwest that had varying concentrations of coal tar waste and petroleum fuels, and a range of soil textures. A qualitative assessment was made by comparing subsurface contaminant cross sections generated from either SCAPS LIF or ROST™ data to cross sections prepared using conventional methods (hollow stem auger rig and EPA-approved analytical methods) for the three sites. For both systems, the chemical cross sections were comparable to the conventional methods. The chemical cross sections for both systems showed close agreement to the conventional method cross sections in identifying low, medium, and high zones of contamination. Generally, the relative fluorescence intensity was positively related to the total petroleum hydrocarbon (TPH) and total PAH concentrations. However, a good, quantitative correlation between the fluorescence intensity and individual or classes of petroleum compounds could not be found for either system. In only one case during the demonstration did either system produce a false negative where conventional samples indicated contamination in the hundreds of ppm range. The results of the SITE demonstration for both systems can be found in individual Innovative Technology Evaluation Reports.

State of California Certification

Technology field validation studies at nine sites were conducted for the state of California for the SCAPS LIF system. Between 16 and 45 CPT pushes, along with 3 to 8 confirmation soil sample borings, were completed at each site. For the 164 TPH analyses completed, there were 9, or 5.5 percent, false positives and 12, or 7.3 percent, false negatives. For the 164 total recoverable petroleum hydrocarbon analyses, there were 6, or 3.7 percent, false positives and 16, or 9.8 percent, false negatives.

The California Military Environmental Coordination Committee (CMECC) guidance lists LIF as a screening tool and indicates that it should not be used to generate definitive data.

Advantages

The primary advantage of using LIF systems is their ability to provide real-time chemical and geological information while in the field. This data can reduce and focus the amount of physical sampling and laboratory analysis, as well as optimize monitoring well placement.

Both systems are capable of achieving 200 to 300 feet of pushes in a 10-hour work day.

The vertical spatial resolution is near 2.0 cm, which allows small zones of contamination to be delineated that might be missed by conventional sampling protocols.

No drill cuttings are produced with the system, saving the logistical requirement of handling drums of cuttings and eliminating disposal costs.

The sample holes can be grouted as the push rod is pulled from the hole. Also, the push rod can be decontaminated remotely as it is retracted from the hole. All the decontamination fluids are containerized in the process.

During a SITE demonstration, three sites were characterized using the SCAPS LIF system and the ROST™ system at a cost of $20,000 and $41,200, respectively. These costs can be compared to the approximate $43,000 using conventional drilling methods and on-site analytical capabilities. Both systems cost less than the reference methods and produced almost 1,200 more data points in a real-time fashion, as opposed to waiting hours or days for the data.

Limitations

The operation of the SCAPS LIF system or the ROST™ system takes considerable experience. It takes many days and numerous projects to become familiar with the operation of the technology. They are provided as services by their respective vendors for this reason.

Although these sensors provide a relative degree of contamination that closely matches reference method data, little direct, quantitative correlation has been found to individual or classes of petroleum compounds.

The cost of one of these systems may be prohibitive on small-scale projects. They have primarily been used at large sites such as Department of Defense (DoD) and Department of Energy (DOE) facilities.

Some maintenance of the CPT tools and the LIF sensors is required and breakdowns can be expected on long-term projects. Downtime due to breakage of fiber optic cables and push rods, fogging of the sapphire window, and problems with the grout pump or decontamination unit may occur.

These systems can only be used where direct push is feasible, such as in unconsolidated sediments, and where a 20-ton CPT truck can gain access.

The sensors are limited to a depth of 50 meters because of attenuation in the optical fiber umbilical cord.

Minerals such as calcite, naturally occurring organic matter, and man-made chemicals also can fluoresce, which may cause interference problems.

Work performed by the ROST™ system at Dover Air Force Base indicated the potential for smearing and a memory effect on the sensor.

Cost Data

The estimated cost of sampling using the SCAPS LIF system varies between $12 and $20 per foot depending upon whether the operators provide a turnkey operation or the customer provides field deployment assistance such as permitting, site management, and development of work and health and safety plans. Under normal conditions, 200 to 300 feet of pushes can be advanced per day.

The ROST™ technology is available for use within the 48 contiguous United States for a cost of approximately $5,300 per day or site-specific footage rates, which includes a CPT rig provided by a commercial vendor such as Fugro Geosciences. Crew per diem and mobilization costs are additional and site specific. Concrete coring, grouting, permit fees, and distant travel costs or mobilization/demobilization costs vary with each deployment and thus are not included. This translates to a cost of under $20 per foot.

The main savings attributable to an LIF system is that it can substantially reduce the number of monitoring wells drilled at a site. In a large general site characterization effort, it can provide data in less time and far less expensively than conventional drilling and sampling. Investigation-derived wastes are minimized along with the expense of handling and disposal.

During a SITE demonstration, three sites were characterized using the SCAPS LIF system and the ROST™ system at a cost of $20,000 and $41,200, respectively. These costs can be compared to the approximate $43,000 using conventional drilling methods and on-site analytical capabilities. Both systems cost less than the reference methods and produced almost 1,200 more data points in a real-time fashion, as opposed to waiting hours or days for the data.

The cost of one of these systems may be prohibitive on small-scale projects. They have primarily been used at large sites such as Department of Defense (DOD) and Department of Energy (DOE) facilities.

Manufacturers and service providers listed below should be contacted directly for cost information.

Verification/Evaluation Reports

Verification of the performance of site characterization and field analytical technologies is conducted through a variety of programs. Evaluation and verification reports from EPA's Superfund Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program, EPA's Environmental Technology Verification Program (ETV) program, along with links to certification statements from California EPA's (CalEPA) California Environmental Technology Certification Program, are provided below.

Superfund Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program
The SITE Demonstration Program encourages the development and implementation of innovative treatment technologies for (1) remediation of hazardous waste sites and (2) monitoring and measurement. In the SITE Demonstration Program, the technology is field-tested on hazardous waste materials. Engineering and cost data on the innovative technologies are gathered so that potential users can assess the technology's applicability to a particular site. Data collected during the field demonstration are used to assess the performance of the technology, the potential need for pre- and post-treatment processing of the waste, applicable types of wastes and waste matrices, potential operating problems, and approximate capital and operating costs. The following reports from the measuring and monitoring program are available for laser-induced fluorescence:

• This report describes the demonstration and evaluation of the Site Characterization and Analysis Penetrometer System (SCAPS) Laser Induced Fluorescence (LIF) sensor developed by the Tri-Services (Army, Navy, and Air Force). The SCAPS LIF is designed to provide rapid real-time, relatively low cost analysis of the physical and chemical characteristics of subsurface soil to quickly distinguish contaminated areas from noncontaminated areas.

EPA's Environmental Technology Verification (ETV) Program
EPA's Environmental Technology Verification (ETV) Program verifies the performance of innovative technologies. ETV was created to substantially accelerate the entrance of new environmental technologies into the domestic and international marketplaces. ETV verifies commercialized, private sector technologies. After the technology has been tested, the companies will receive a verification report that they can use in marketing their products. The results of the testing also are available on the Internet. The following reports from the ETV program are available for laser-induced fluorescence:

• The Site Characterization and Analysis Penetrometer System (SCAPS) Laser-Induced Fluorescence (LIF) Sensor and Support System was verified for in-situ detection of petroleum hydrocarbons. The verification documents available consist of a verification report and verification statement.
  
  
• Fugro Geosciences, Inc.- Rapid Optical Screening Tool was verified for in-situ detection of petroleum hydrocarbons. The verification documents available consist of a verification report and verification statement.

California EPA's California Environmental Technology Certification Program
CalEPA's environmental technology certification program is a voluntary program that provides participating technology developers, manufacturers, and vendors an independent, recognized third-party evaluation of the performance of new and mature environmental technologies. Developers and manufacturers define quantitative performance claims for their technologies and provide supporting documentation; CalEPA reviews that information and, when necessary, conducts additional testing to verify the claims. The technologies, equipment, and products that are proven to work as claimed are given official state certification. The certification program is voluntary and self-supporting. Companies participating in the program pay the costs of the evaluation and certification of their technologies. Technologies that have been certified through this program are listed below. Links are provided to the web sites that provide the Certified Environmental Technology Transfer Advisory and Certification Notice for the technologies.

• Site Characterization Analysis Penetrometer System with Laser-Induced Fluorometry US Department of the Navy Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD),

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