- Direct-Push Technologies
- Fiber Optic Chemical Sensors
- Gas Chromatography
- Graphite Furnace Atomic Absorption Spectrometry
- High-Resolution Site Characterization (HRSC)
- Infrared Spectroscopy
- Laser-Induced Fluorescence
- Mass Flux
- Mass Spectrometry
- Open Path Technologies
- Passive (no purge) Samplers
- Test Kits
- X-Ray Fluorescence
Direct-push analytical systems are attachments designed to be used with direct-push platforms. These systems include a diverse and growing class of instruments that are adapted for in situ use as part of the direct-push tooling. Direct-push analytical instrumentation allows real-time or near real-time data to be generated in the field while sampling, without the many requirements associated with sample management and while generating minimal investigation-derived waste. This "all-in-one" approach potentially allows the user to conduct a more rapid and detailed assessment at a lower overall cost than they could achieve with more traditional methods such as drill rigs and off-site laboratories.
Some of the systems incorporate a sensor such as a laser-induced fluorescence (LIF) or x-ray fluorescence (XRF) directly into the probe that is advanced with the direct-push tooling into the subsurface. Others are sophisticated closed systems that retrieve volatile organic compounds (VOC) from the subsurface and route them into an integrated instrument for analysis. What all of these systems have in common is that they allow the user to quickly characterize a site in the field using relatively agile and minimally intrusive direct-push platforms.
Direct-push analytical systems can be used for practically any environmental purpose, including site assessment, site characterization, removal assessment, or even monitoring of natural attenuation. Practical uses for these systems are partly a function of the platforms themselves, in situ field analyses, and specifications of the particular system. General applications of direct-push platforms and in situ field analysis are described in more detail below; the characteristics and applications of several classes of direct-push analytical system are described in the following sections.
As described in the entry of the same name, direct-push platforms are highly versatile and efficient means of accessing and sampling sites with shallow subsurface contamination. Briefly, these platforms, which include both rotary hammer and cone penetrometer (CPT) rigs, advance samplers and analytical instrumentation into the subsurface by hydraulically pushing a string of rods into the ground. Rotary hammer units are deployed on a truck or van, although units have been developed for a wide variety of vehicles including small four-wheel all-terrain vehicles and several sizes of tracked vehicles. Rotary hammer units use a hydraulic ram with a rotary hammer, combining the static force of the platform and the percussive force of the hammer to advance the tool string into the subsurface. CPT rigs are more massive than rotary hammer units, typically weighing anywhere from 15 to 30 tons. A hydraulic ram that uses the static force of the vehicle alone is sufficient to advance the tooling into the ground for this reason. Both units can advance a variety of soil and soil gas samplers, groundwater samplers, geotechnical sensors, and analytical instrumentation into the subsurface. Many of the sampling and analytical systems originally developed exclusively for one platform or the other are being adapted for both, creating a great deal of cross-pollination between the two and allowing users a growing range of choices for using these direct-push platforms.
The very nature of the direct-push platforms leads to certain types of uses for direct-push analytical systems. By virtue of their size, weight, and limited height these platforms are agile in comparison to a drill rig and can be used to access hard-to-reach areas. They are often used inside buildings or in tight outdoor spots with limited overhead clearance. Little waste is produced. No drill cuttings are generated because the tooling is pushed into the ground. Conversely, direct-push platforms can only advance a sampler or instrument into unconsolidated soils and sediments and are limited in the depths they can achieve (between 60 and at most about 100 feet for a rotary hammer, and about 100 to 150 feet for a CPT under ideal conditions). For these reasons, these analytical systems are typically used to assess and characterize sites with shallow soil and groundwater contamination, and often are used on sensitive or hard to reach sites such as residential property or inside industrial buildings.
In situ field screening or analysis of any kind is limited in the precision and accuracy of the results that can be generated. Samples are analyzed directly in the subsurface or are brought to the surface to be analyzed. While there may be some inherent advantages to analyzing samples in situ (such as a reduced loss of VOCs or elimination of human error in sample handling), precision and accuracy of the sample results will vary from formal laboratory results because of the lack of controls on sample collection and analysis. For example, soils and sediments may not be in direct contact with the sensor window, sample volumes may be inexact when compared with a precision balance used in the laboratory, and sample collection may be affected by subsurface conditions such as extremely tight clays. Some direct-push analytical sensors such as LIF are not compound-specific but provide a measure of total contaminants in a class.
Direct-push analysis is well suited for application in a triad approach to conducting site characterization and removal monitoring, particularly voluntary cleanup and Brownfield sites. The triad approach uses on-site analytical tools in conjunction with systematic planning and dynamic work plans to streamline sampling, analysis, and data management conducted during site assessment, characterization, and cleanup. Field analysis in general and direct-push systems in particular are often used to speed collection and reduce costs on projects where the sites are large, a high volume of data points are needed, the sites are partly or totally inaccessible by a large drill rig, or to minimize sampling disturbances in sensitive habitats. Despite the inherent limitations of field analysis, careful use of direct-push platforms and analytical systems can reduce the overall uncertainty surrounding site contamination.
A variety of analytical instrumentation systems are presented in detail in the following sections. These sections provide information on systems for both organic and inorganic screening and analysis, including a general description and typical uses of the systems, the basic theory of operation, components, and information on target analytes and performance. The general advantages and limitations of the systems are also provided, along with links to the manufacturers and vendors.
A table summarizing and comparing the available technologies may be viewed by clicking here.
Fluorescence instrumentation refers to a general class of technologies in which a subsurface sample is bombarded with an ultraviolet (UV) light source that causes petroleum hydrocarbons in the sample to fluoresce. The resulting fluorescence is used to identify and measure the contaminant. The fluorescence is produced when molecules of a certain class of VOCs, known as polycyclic aromatic hydrocarbons (PAH), give off energy in an attempt to return to their "natural" or ground state after becoming excited by the energy from the UV source.
Fluorescence instruments were originally developed for use by CPT rigs but are now also deployed from rotary hammer rigs. Several energy sources, including lasers and mercury lamps, have been used in these instruments. This section will discuss the mercury (Hg) lamp fluorescence detector. The fiber-optic based LIF chemical detection system allows for real-time, in situ subsurface detection of fuel hydrocarbon contaminants. An encyclopedia devoted entirely to LIF may be viewed by following this link:
Source: Applied Research Associates, Inc. 1999
Like LIF, Hg fluorescence instruments are used to rapidly characterize a site accessible to direct-push platforms. With direct-push equipment, Hg fluorescence detectors (Figure 1) cut the time needed to delineate the extent of hydrocarbon plumes from fuel spills or leaking storage tanks. Hg instruments are used for site-specific relative screening of contamination levels.
Theory of Operation
As with LIF, a subsurface sample is bombarded with UV light from the Hg source, causing certain petroleum hydrocarbons to fluoresce. The sample absorbs the source energy, which results in the elevation of electrons from orbitals in the ground state to higher energy orbitals in an excited state. When the excited electrons return to the ground state, light energy is released as fluorescence emission spectra that can be measured and analyzed (Silverstein and others 1991). The absorption of UV source energy is dependent on the electronic structure of the organic molecule. Compounds consisting of double carbon bonds with weakly attached electrons (such as PAHs) can be identified using low-energy bombardment techniques. The source energy dictates which specific analytes and compounds can be detected (Keller and others 2001).
The resulting fluorescence is measured using an electronic detector and the reading is converted into a contaminant concentration.
A common commercial Hg system consists of a down-hole module and an up-hole controller. The down-hole module can be attached directly to CPT geotechnical instruments to provide simultaneous lithologic analysis and contaminant screening. A 254-nanometer Hg lamp is used to project UV light through a sapphire window in the side of the probe to excite the sample. The resulting fluorescence is returned as an optic signal to the controller module on the surface via a cable that extends inside the direct-push tool string. Unlike LIF, the signal light is converted to an electric current for amplification and signal conditioning, which reduces the hardware cost of the system and improves its ease of operation. Using the controller, the operator is able to adjust for background fluorescence levels in soils with naturally occurring fluorescent substances.
Mode of Operation
When the Hg fluorescence instrument is used with CPT equipment, it provides a continuous output of fluorescence over the entire depth of the investigation. This information can be viewed graphically in real time using a data acquisition system as the probe is advanced, providing an easily interpreted view of the plume (Figure 2). The continuity of the data reduces the time and effort required for data interpretation and presentation. Multiple profiles across a site can be used to develop a three-dimensional model of the plume during the site characterization, without the time or expense of off-site sample analysis or post-processing of the data.
|Source: Applied Research Associates, Inc. 1999|
An Hg system can be configured to detect a wide range of contaminants such as jet fuel, diesel, unleaded gasoline, home heating and motor oil, and by applying filters, coal tars and creosote.
Although intended to specifically target petroleum hydrocarbons, the excitation energy produced by the UV source may cause other substances to fluoresce as well, which may cause interference problems. Many commonly occurring fluorescent minerals such as calcite can produce a measurable signal. Other man-made non-hydrocarbon fluorescent material may be found in the subsurface environment, such as deicing agents and antifreeze additives. Many detergent products are known to fluoresce very strongly. Naturally occurring organic matter, which includes PAHs, also can fluoresce. An experienced operator may be able to differentiate between the fluorescent signatures of hydrocarbons and other interfering compounds.
Environmental conditions, including temperature and moisture may affect the performance of the instrumentation. The output of the Hg lamp is reduced in cold temperatures, resulting in lower sensitivity. It is recommended that the probe be stored in a warm place prior to testing. Moisture effects on sample results are measurable but are generally not significant.
Detection limits are soil and fuel dependent. Total petroleum hydrocarbon values as low as 100 parts per million (ppm) can be detected in sandy soils. Detection limits may be reduced in clays because clays have a greater surface area than sandy soils. More fuel is locked within the clay structure, obscuring a portion of the signal that is received by the detector. In addition, fuels with a lower proportion of PAHs may have higher detection limits, and additives such as dyes (sometimes used for taxation purposes) may obscure the response and raise detection limits.
The Hg fluorescence instrumentation is field calibrated before and after each test. Calibration is relatively simple. A special card that has a black surface and a gray surface is placed in front of the probe. The black surface gives no response to the detector, providing a baseline “zero” reading for the instrumentation. The gray surface gives a response that is about 50 percent of a full-scale output.
Quality control for down-hole analytical instruments consists of proper instrument calibration, and confirmatory samples.
The field calibration method is repeated after sampling as a calibration check, to determine whether any shift occurred in the detector during use. If a shift is detected in the instrument, the pre- and post-calibration results can be used to correct the data during post-processing. If the shift is very large, the instrument may have to be returned to the factory for evaluation. Confirmatory samples are collected by advancing samplers adjacent to the down-hole sample location. These samples are collected from intervals exhibiting varying levels of contamination (including non-detect) and sent to a formal laboratory for analysis. The analytical results are then compared to the in situ results to assess the technology's performance across the entire range of contaminant concentrations.
Precision and Accuracy
In general, fluorescence units are semi-quantitative and semi-qualitative in nature. Only the PAHs in fuel fluoresce when bombarded by the UV source; different fuel fractions are qualitatively identified by analyzing the wavelength at which they fluoresce to determine which PAHs are present in what ratios, and this information is compared to standards for various fuels. The correlation between fluorescence and hydrocarbon contamination in a given soil matrix is generally linear as represented in Figure 3. This relationship is used to convert the site-specific fluorescence levels into relative contamination concentration levels.
|Source: Applied Research Associates, Inc. 1999|
The permeable membrane interface probe (MIP) was developed to allow for near real-time evaluation of subsurface VOCs using rotary hammer units. The MIP is now used widely with CPT rigs as well. The MIP probe consists of a thin composite metal and a Teflon membrane impregnated into a stainless steel screen on the face of a probe. The probe is mounted on a standard direct-push rod (see Figure 4). A carrier gas line runs from the probe to the detector through the inside of the tooling, and can be connected to several types of detectors, including flame ionization detectors (FID), photoionization detectors (PID), or direct sampling ion trap mass spectrometers (DSITMS). The device allows the user to detect VOCs as it is driven to depth. VOCs are drawn through the system's semi-permeable membrane and carried to a detector at the surface where they are analyzed and measured. The MIP also incorporates a lithologic sensor and logs advancement speed. Use of the MIP allows investigators to identify the contaminant distribution and migration pathways in real time.
The MIP system may be used to characterize any site with shallow subsurface VOC contamination, including sites with fuel releases, chlorinated solvent releases, and dense non-aqueous phase liquid (DNAPL). The technology is only applicable to shallow contamination; standard components include a 60-foot gas carrier line, but with a custom-built gas carrier line the practical vertical limit of characterization is about 100 feet when deployed with a rotary hammer, and could be as deep as 150 feet if driven with a CPT rig. The system is used to simultaneously characterize the subsurface soils and sediments with chemical contamination. Using the MIP, electrical conductivity logs, and advancement speed logs, it provides information on contaminant distribution and migration pathways.
The MIP is typically used with a PID or FID detector (or both in series) to rapidly delineate the distribution of VOCs in the subsurface. These detectors have the advantage of providing rapid, continuous measurements of relative VOC concentrations in the subsurface. The data collected with the MIP can be used to accurately place a minimum number of conventional sampling points (soil bores and monitoring wells) for site characterization and monitoring.
|Source: U.S. Army Environmental Center 2000|
The DSITMS (Figure 5) may be used instead of the PID or FID as a high-level field screening technique to provide qualitative identification of specific compounds. The DSITMS is a versatile tool for fast on-site qualitative or quantitative measurement of organic compounds in air, water, soil, and wastes. The DSITMS is best suited for analyzing samples for the presence of VOCs in support of site activities requiring the analysis of large numbers of samples in a short period of time, or for routine quantitative monitoring of sampling locations that have been previously characterized.
Theory of Operation
In practice, the MIP membrane is heated to between 80° C and 125° C as it is advanced through the subsurface. VOCs present in the subsurface partition into the membrane and migrate through it by diffusion, rather than being pulled in by a vacuum or actively purged using an inert gas, as with the Hydrosparge system (described below). The VOCs move across the membrane into a helium carrier gas that flushes the back of the membrane and transports the VOCs to the aboveground detector. Unlike many analytical instruments, the system can operate in both the vadose zone and beneath the water table.
Relative concentrations of aromatic VOCs may be measured continuously using a PID during advancement of the probe, while a FID is used to detect less volatile, straight chain hydrocarbons; both detectors may be used in series. The PID and FID detectors are not compound-specific but measure the total response of all volatile compounds in the sample that can be ionized by the detectors and are present in sufficient concentrations to be detected by the detector. The PID and FID units are not quantitative; the electrical response of the detector to VOCs in the subsurface is registered in micro-volts (µV). The range of response to similar contaminants varies from site to site. For example, the detector response for gasoline-range organics has been found to vary from 4,000 to 50,000 µV per milligram per kilogram (mg/Kg) in soil, depending on the soil type (Geoprobe Systems 1996). For this reason, the relative responses of the detector may be used as a general indication of the concentration of VOCs present in the subsurface.
For qualitative identification of specific compounds, a DSITMS is used. The MIP is capable of multiple, discrete VOC measurements in a single penetration, and is a versatile tool for a fast on-site qualitative or quantitative measurement of organic compounds in air, water, and soil. Much progress has been made in recent years developing and validating methods for the screening and quantification of targeted VOCs in discrete water and soil samples. Detection limits are typically in the range of 1 ppm or less with little or no sample preparation required, and a sample analysis time of 2 to 3 minutes. The DSITMS field methodology (draft Method 8265) has been conditionally approved by the U.S. Environmental Protection Agency (EPA).
Subsurface lithology is determined by comparing readings from the integrated electrical conductivity sensor with the rate of advance through the subsurface, in much the same way as CPT sensors compare sleeve friction to tip resistance readings. Briefly, an electrical current is passed between two opposing poles of the conductivity sensor, and the conductivity of the soils is measured as the probe is advanced. Different classes of soils have different specific conductance, and this property is used to identify different soil layers during advancement. The speed of penetration is also logged and can be compared with conductivity to confirm the results, as the rate of advance differs by soil class.
The MIP system consists of the probe, a "trunkline" containing the power cable, data cable, and gas line, an electronic controller and data output system. An electrical conductivity sensor is also coupled with the MIP to provide a real-time log of subsurface soils based upon changes in their electrical resistivity. Components of this system include the sensor and data acquisition "stringpot" cable. The cabling runs through the inside of the tool string as well. An advancement speed gauge is attached to the hammer probe when used with a rotary hammer platform.
Mode of Operation
As the operator advances the MIP sensor into the subsurface, a log is displayed on the field computer's screen. This log provides information about total VOC contamination obtained from the detector(s). The real-time log (see Figure 6) also provides a depth and speed graph, conductivity log of the soils, and temperature log of the heated sensor.
MIP Data Log
|Source: Kejr, Incorporated 2001|
Running MIP logs on a grid or targeted pattern across an investigation area will provide a three-dimensional view of VOC distribution and lithology. Optional software allows construction of cross sections from the MIP and conductivity logs (see Figure 7).
|Figure 7. Source: Kejr, Incorporated 2001|
Information from both the MIP and electrical resistivity logs provides information on contaminant distribution and migration pathways. This information enhances development of an accurate site conceptual model, which can significantly reduce the cost of investigation activities.
An assortment of VOCs in the vadose zone and in groundwater may be measured and possibly identified, depending on the detector used with the system.
The PID and FID units are total VOC detectors. Because the MIP system does not incorporate a gas chromatograph (GC) for compound separation prior to analysis, the system cannot differentiate VOCs if these detectors are used. Any VOCs that are not targets of the investigation may interfere with the instruments' response. For example, it would not be possible to delineate a jet fuel release if diesel or gasoline had also been released into the subsurface.
The DSITMS does not use a GC for compound separation. A series of scans containing ions indicating the presence of VOC analytes is used to qualitatively identify analytes of interest. Multiple scans of standards containing the target analytes are integrated, calibrating the instrument to identify these compounds; however, ions with a similar mass and charge to the target analytes' characteristic ions could cause interference and artificially increase the reported concentrations.
A blank sample should be run for background subtraction, to ensure there is no carryover in the transfer line, any time samples are run having greater than 500 ppm of VOC contamination, and between analysis of samples from different sources. A system blank check should be performed before and after each set of in situ measurements when using the DSITMS detector.
The detection limit for typical chlorinated compounds using the PID and FID detectors is about 5 ppm. The DSITMS detector can detect hydrocarbons as low as 1 ppm in solution, depending on the subsurface conditions.
Calibration of the MIP system when equipped with the PID and FID detectors consists of response testing. Response testing demonstrates to the operator that the system is working and is generating a response to environmental contaminants. A standard containing a known quantity of fuel related hydrocarbons or chlorinated solvents is prepared and mixed into 0.5 liter of fresh water. The MIP probe is first immersed into a bucket of clean sand saturated with fresh water, in order to stabilize the baseline response of the detector to water (the PID is by nature sensitive to moisture on the ultraviolet lamp used to ionize target compounds). The probe is then placed into the water containing the standard for 45 seconds, and the response is observed.
Calibration of the DSITMS is run initially at project start up. A calibration curve is developed using laboratory-prepared standards of known concentrations bracketing the expected contaminant concentrations for a particular site. Critical operating parameters used to generate the calibration curve, including carrier gas flow rate of the MIP, membrane temperature, and DSITMS settings, should not be changed during sample analysis.
For the PID and FID detectors, response testing should be performed before each MIP log.
Externally-prepared calibration check standards should be run at the startup and at the end of each day of operation using the DSITMS to confirm the unit's calibration. An acceptable calibration compound such as perflourotributylamine should be used.
A minimum of 5 percent verification sampling and off-site analysis by EPA Method 8260B should be performed to confirm the results obtained at a particular site using the DSITMS. Locations and depths for verification sampling should be selected to include a range of contaminant concentrations from non-detect to the maximum concentrations detected by the DSITMS. PID and FID readings may also be compared to confirmatory samples such as these to provide a general indication of the concentrations observed at other locations during operations.
Precision and Accuracy
The PID and FID detectors do not provide quantitative results. The relative response of the PID and FID can vary as much as an order of magnitude depending on site conditions and soils.
The DSITMS has demonstrated the capability of meeting the precision and accuracy quality control (QC) performance criteria established for water analysis by Continuing Calibration Check - EPA Method 624 (40 Code of Federal Regulations [CFR] Part 136). Validation data collected from saturated soils and compared to data for samples collected by EPA Sampling Method 5035 and Analysis Method 8260B indicate that the system provides quantitative estimates of subsurface contamination distribution. Quantitative results appear to be less precise for vadose soils
Source: U.S. Army Environmental Center 2000
The Hydrosparge system shown in Figure 8 is similar to the MIP system in that it extracts VOCs from groundwater and brings them to the surface for analysis via a closed system. It differs from the MIP in a couple of respects. Unlike the MIP, the Hydrosparge is active and physically purges VOCs from the sample interval rather than allowing them to passively diffuse into the sampler. The Hydrosparge is only able to sample one discrete interval, as the probe must be retracted to expose the sampler and cannot be re-advanced (multiple depths may be characterized within the sampler screen). The Hydrosparge does not incorporate a lithologic sensor of any sort. Sampling intervals are selected based upon separate pushes with CPT sensors at the desired sample location. Standard CPT sensors are deployed in a probe that measures the resistance against the cone tip and friction against the cone sleeve; system software estimates the makeup of subsurface soils and sediments by measuring and comparing these mechanical forces.
The Hydrosparge system is designed to collect VOCs from groundwater for real-time analysis by analytical instrumentation in the direct-push vehicle on the surface. The Hydrosparge sampler is lowered through a commercially available direct-push groundwater sampler that has been advanced into the water table, where it sparges (purges) VOCs from the groundwater using inert gas; the VOCs are carried to the DSITMS detector on the surface for analysis. In this way, groundwater VOCs can be analyzed during advancement without retrieving the direct-push rods and handling or packaging samples, leading to increased efficiency and precision and reduced cost over traditional sampling methods.
As with the MIP, the Hydrosparge may be useful for characterizing any site with shallow groundwater VOC contamination (for example, fuel releases, or chlorinated solvent releases and DNAPL plumes). The system uses a DSITMS detector for sample analysis. As described previously, DSITMS is a method for the quantitative measurement, continuous real-time monitoring, and quantitative and qualitative preliminary screening of VOCs in water, soil, and air.
As with all direct-push technologies, this system is only applicable to shallow sites with unconsolidated soils and sediments. The Hydrosparge has been pushed to about 185 feet below ground surface (bgs) during testing, but 100 feet bgs is considered to be a realistic average maximum depth.
Theory of Operation
The Hydrosparge system in effect takes a step from the laboratory procedure for VOC analysis and performs this step in the subsurface. Common methods for VOC analysis, such as EPA Method 8260B, exploit the volatile nature of VOCs to remove them from their sample media, either soil or water, into a gaseous phase for introduction into the analytical instrument. In the case of Method 8260B, VOCs are driven from the sample matrix by passing an inert gas through it. VOCs are then passed through the analytical detector for quantitative and qualitative analysis.
The Hydrosparge system has adapted this approach for in situ analysis. A carrier gas is used to purge VOCs from the groundwater and transport them to the detector. The Hydrosparge uses helium as the carrier gas. The system must be closed to ensure that VOCs are not lost in transport from the surface and that a known quantity of sample volume is introduced into the detector if quantitative analysis is to be performed.
The Hydrosparge system integrates a customized, 2-inch CPT probe with a small sampling port, a Teflon transfer line for carrier gas, and an aboveground DSITMS detector in the truck. The Hydrosparge VOC sensor uses a commercially available HydropunchTM or PowerpunchTM direct-push groundwater-sampling tool to access the groundwater. Click here for a diagram of the system.
Mode of Operation
The HydropunchTM sampler is pushed to the desired depth and the push rods are retracted, exposing the screen to the groundwater. The water level is then allowed to come to equilibrium, which generally takes about 15 to 20 minutes (but this is dependent upon the hydrogeologic condition). The in situ sparge module (see the link above for a diagram) is then lowered into the groundwater to operate about 1.5 feet below the groundwater surface. Using helium gas, the sparge module purges the VOC analytes in situ from the groundwater to the DSITMS system in the truck, where VOCs are analyzed in real time.
Click here to view a short QuickTime video of the Hydrosparge in operation (recommended for broadband Internet connections).
VOCs in groundwater.
The DSITMS does not use a GC for compound separation prior to analysis. Rather, a series of scans containing ions indicating the presence of VOC analytes is used to qualitatively identify analytes of interest. Multiple scans of standards containing the target analytes are integrated, calibrating the instrument to identify these compounds; however, non-target VOCs that generate an ion with the same mass and charge ratio as a target analyte may cause a positive interference.
General detection limits for DSITMS are typically in the range of 1 ppb or less with little or no sample preparation required. Sample analysis times are 2 to 3 minutes. Detection limits for the DSITMS used in conjunction with the Hydrosparge are typically in the low ppb range, with a linear quantitation range in the low ppm.
The Hydrosparge unit is calibrated by spiking a 250-milliliter flask of distilled water with known concentrations of VOCs of interest, inserting the in situ-sparge module into the flask and analyzing the resulting purge gas with the DSITMS. Calibration of the DSITMS is run initially at start up. A calibration curve is developed using laboratory prepared standards of known concentrations, bracketing the expected contaminant concentrations for a particular site. Critical operating parameters used to generate the calibration curve, including carrier gas flow rate and DSITMS settings, should not be changed during sample analysis.
Daily calibration, check standards, and performance evaluation standards are used to ensure data quality. Confirmation samples may be collected from a groundwater sampler immediately after removing the in situ sparge module; Method 8260B is recommended for analysis, as this method is quite similar to the Hydrosparge method.
Precision and Accuracy
The reliability of in situ, direct sparging of VOC analytes from groundwater in concert with the DSITMS has been successfully demonstrated at numerous sites, and the California Environmental Protection Agency (Cal/EPA) Innovative Environmental Technology Certification Program has certified the Hydrosparge for use at contaminated sites. Click here to access the certification notice and study.
According to Cal/EPA:
The technology was demonstrated to be a qualitative to semi-quantitative field screening method for TCE, benzene, and carbon tetrachloride and met the criteria of less than 5% false positives and negatives and had good correlation (R2 = 0.80). For PCE, toluene, and xylenes, the technology was demonstrated to be a qualitative field screening method and met the criteria of less than 5% false positives and negatives but had lower correlations (R2 < 0.80). For DCE, the technology was demonstrated not to meet the criteria of less than 5% false negatives but had good correlation (R2 = 0.80) and could be a qualitative field screening method for this analyte.
The Thermal Desorption VOC Sampler (TDS) is similar in principle and practice to the MIP and Hydrosparge systems, and is specifically geared toward characterization of vadose zone soils in situ. The TDS system is a closed system that draws VOCs directly from the subsurface for analysis by a surface detector. The direct-push rod is advanced with a special probe that collects a soil plug into a chamber where it is heated. An integrated pneumatic system transports purged VOCs to the surface for analysis by DSITMS. The system may be used to collect VOCs onto analytical traps for later analysis.
The TDS does not incorporate a lithologic sensor. Sampling intervals are selected based upon separate pushes with CPT sensors at the desired sample location. As described previously, standard CPT sensors are deployed in a probe that measures the resistance against the cone tip and friction against the cone sleeve; system software estimates the makeup of subsurface soils and sediments by measuring and comparing these mechanical forces.
As with the MIP and Hydrosparge systems, the TDS may be useful for characterizing any site with shallow subsurface VOC contamination from fuel releases, or chlorinated solvent releases. The TDS is designed for screening on-site soils only, and uses a DSITMS detector for sample analysis. As described previously, DSITMS is a method for the quantitative measurement, continuous real-time monitoring, and quantitative and qualitative preliminary screening of VOCs.
As with all direct-push technologies, the TDS system is only applicable to shallow sites with unconsolidated soils and sediments. CPT probes have been pushed to more than 150 feet bgs during testing, but 100 feet bgs is considered to be a realistic average maximum depth for CPT advancement.
Theory of Operation
Like the sparge systems, the TDS also takes a step from a laboratory procedure for VOC analysis and performs this step in the subsurface. Many laboratory and field analytical methods for VOC analysis, such as EPA Methods 8260B and 5021, drive the VOCs from their sample media into a gaseous phase for analysis by heating the sample. For both methods, an aliquot of sample (either soil, sediment or water) is removed from the sample vial that has been filled to eliminate any air pockets. The sample material is placed into a new vial and heated, driving the VOCs into the headspace of the new vial for removal and analysis by the detector.
The TDS has adapted this approach for in situ analysis. As the probe is deployed the sample chamber is filled, resulting in a sample of known quantity for quantitative analysis. The sample chamber itself is then heated to increase the mobility of the VOCs and an inert carrier gas transports them to the surface, where they adsorb onto an analytical trap. This trap is then heated to drive (desorb) the VOCs into the DSITMS detector for analysis. The carrier gas system is closed to ensure that VOCs are not lost in transport from the surface.
The TDS consists of a custom soil probe, carrier gas lines and supply, an analytical trap, and a DSITMS detector deployed from a direct-push platform. The sample probe incorporates an internal piston and a heated thermal sample chamber that is connected to the carrier gas lines (see figure 9 in the following section).
Mode of Operation
The operation of the TDS is based on the capture of a known volume of soil. The TDS is pushed to the desired ground depth and an interior rod retracts the penetrometer tip, which locks into the top of the sample chamber. The probe is then pushed further into the soil, collecting a 5-gram soil plug in the sample chamber. The soil plug is heated, releasing the VOC gases from the soil. The vapors are drawn to the surface by the carrier gas, where they are trapped on an adsorbent media. The trap is then thermally desorbed into the onboard, field-portable DSITMS, where VOCs are analyzed in near-real time.
After analysis, the soil plug is expelled from the sample chamber by reseating the piston into the drive position, and the sample chamber is heated and purged to remove any residual contamination before the process is repeated, allowing for screening of multiple depths during a single push.
Alternatively, the TDS may be used as a vapor sampler in the vadose zone by applying a vacuum to the transfer line, drawing soil vapors to the surface where they are trapped, desorbed, and analyzed by the DSITMS in near-real time.
For a QuickTime video of the direct-push TDS system in operation, click here (recommended for broadband Internet connections).
VOCs in vadose zone soils.
Moisture does not affect sample results until a sample is saturated, as the sample is dried and water vapor is purged from the sample prior to analysis. The TDS is designed to be used in the vadose zone only for this reason.
The DSITMS does not use a GC for compound separation prior to analysis. Rather, a series of scans containing ions indicating the presence of VOC analytes is used to qualitatively identify analytes of interest. Multiple scans of standards containing the target analytes are integrated to calibrate the instrument to identify these compounds; however, ions with a similar mass and charge to the target analytes' characteristic ions could cause interference and artificially increase the reported concentrations. According to the Cal/EPA certification, the TDS has applicability to field screening for the presence of known VOCs, and the identification of unknown substances when ions uniquely characteristic to those substances are present.
The transfer line is heated to prevent water vapor from condensing in the transfer line, but this is a potential interference if a large length of line is being used to analyze a deep sample during very cold conditions.
The ITMS, using the conditional EPA Method 8265, is capable of detecting most VOCs qualitatively and quantitatively in the sub-ppm range. The TDS has been certified as achieving detection thresholds for trichloroethene (TCE) and total dichloroethene (DCE) comparable to those of EPA Method 8260A (which has been superceded by EPA Method 8260B).
Calibration of the DSITMS is run initially at start up. A calibration curve is developed using laboratory-prepared standards of known concentrations bracketing the expected contaminant concentrations for a particular site. Critical operating parameters used to generate the calibration curve, including carrier gas flow rate and DSITMS settings, should not be changed during sample analysis.
Daily calibration, check standards, and performance evaluation standards are used to ensure data quality. Calibration standards should be run daily. Confirmation samples may be collected by pushing a soil sampler to the same sampling interval screened by the TDS and analyzed by EPA Method 8260B.
Precision and Accuracy
The reliability of in situ, thermal desorption of VOCs from the soil, in concert with the DSITMS has been successfully demonstrated at various sites. A number of field demonstrations have provided direct comparisons between the TDS and DSITMS combination, and the standard EPA-mandated procedures. Field studies for total DCE and TCE demonstrated that the TDS technology achieved less than 5 percent false negative results and less than 5 percent false positive results when compared to verification core samples analyzed by EPA Method 8260B. For the two analytes having the most performance data there appears to be a relatively good correlation between the TDS sample results and verification sample result (r2 =0.83 for DCE; r2 =0.97 for TCE). Cal/EPA has certified the TDS as a near real-time, qualitative to semi-quantitative, in situ subsurface field screening method for VOCs in the vadose or capillary zone. Click here to access the certification notice and study.
While much attention has been paid to analytical systems for organic analysis, two systems have been developed for deployment on direct-push platforms for screening inorganic contaminants in the subsurface. The first has adapted XRF technology to subsurface characterization. XRF is a well-established, non-destructive laboratory and hand-held field screening method for determining elemental concentrations at ppm levels. The second system has been built around a more recent technology, laser induced breakdown spectroscopy (LIBS). Both systems are used in the detection, identification, and delineation of heavy metal contaminants in the subsurface. The LIBS system operates in the unsaturated and capillary zones, while the XRF system may be applied in both the unsaturated and saturated zones.
Direct-push inorganic systems are applicable to a variety of sites with shallow heavy metal contamination in unconsolidated sediments, due to their multiple analyte detection capability. Direct-push deployment of metals sensors allows for high-resolution delineation of subsurface metals contamination, with no waste generation.
Theory of Operation
In XRF analysis, a process known as the photoelectric effect is used in analyzing samples. Fluorescent x-rays are produced by bombarding the subsurface soils with an x-ray source that has an excitation energy similar to, but greater than, the binding energy of the inner-shell electrons of the elements of interest present in the soils. Some of the source x-rays will be scattered, but a portion will be absorbed by these target elements. Because of their higher energy level, they will cause ejection of the inner-shell electrons, and the electron vacancies that result will be filled by electrons cascading in from outer electron shells. However, since electrons in outer shells have higher energy states than the inner-shell electrons they are replacing, the outer shell electrons must give off energy as they cascade down. The energy is given off in the form of x-rays, and the phenomenon is referred to as x-ray fluorescence (click to view a schematic diagram of the x-ray fluorescence process). Because every element has a different electron shell configuration, each element emits a unique x-ray at a set energy level or wavelength that is characteristic of that element. The elements present in a sample can be identified by observing the energy level of the characteristic x-rays, while the intensity of the x-rays is proportional to the concentration and can be used to perform quantitative analysis. In other words, qualitative analysis is performed by observing the energy of the characteristic x-rays. A quantitative analysis is performed by measuring the intensity of the x-ray.
The LIBS sensors use high-power pulsed lasers to generate plasma in the soil. The output of the laser beam is focused on the surface of the soil outside the probe. This causes a breakdown of the soil and the formation of a high temperature plasma spark. For a brief time, this plasma spark emits light.
The wavelengths of the light, or the constituent colors, are indicative of the elements present in the soil; specific wavelengths correspond to specific metals. The brightness of the light at a metal's wavelength indicates how much of that metal is present. A spectrometer breaks this light into its constituent colors, much like the action of a prism. This information is analyzed onboard the direct-push rig to obtain qualitative and quantitative data from the characteristic signatures from each of the metals detected. An example of the readout is presented below in Figure 10. The analysis is similar to that performed by an inductively coupled plasma analyzer. The advantage of LIBS is that little or no sample preparation is required to obtain useful results and the technique is readily portable to the field.
For a more detailed discussion of the theory behind XRF and other applications of the technology, visit the XRF section.
The XRF system shown in Figure 11 consists of a probe containing an x-ray tube configured to focus its beam through a boron carbide window, a detector and preamplifier, a collimator, a boron carbide window, and standard CPT sensors for soil lithology and moisture. A high-voltage cable delivers power down the inside of the direct-push tool string to the x-ray tube, and a detector cable returns the signal to the multichannel analyzer and computer in the truck on the surface.
The fiber-optic LIBS system (Figure 12) is configured with a laser in the direct-push rig, a fiber-optics transmission system extending down the inside of the tool string, and a custom probe combining the lens assembly for the LIBS laser with standard CPT sensors for soil lithology and moisture. The downhole LIBS configuration houses the laser and fiber-optic components within the probe, along with standard CPT sensors.
Standard CPT sensors measure the resistance against the cone tip and friction against the cone sleeve; system software estimates the makeup of subsurface soils and sediments by measuring and comparing these mechanical forces. Click for a schematic of the complete fiber-optic LIBS system or the downhole LIBS.
Mode of Operation
The XRF sensor is advanced to a selected sampling depth at which point an x-ray source in the probe tip bombards the surrounding soil. The source may be either an x-ray tube or a radioisotope. X-ray tubes are more powerful and achieve lower detection limits, but a radioisotope source generates a more consistent output and may deliver more precise and accurate results. The operator must take care and follow all applicable regulations when shipping and handling a radioisotope source. Radioactive materials handling licenses may be required by the state or locality in which it is used.
Metal atoms present in the soil are excited and emit fluorescent x-rays with an energy that is characteristic for the specific elements. These emitted x-rays are detected at the probe tip and provide an individual peak for each type of metal present in the soil. These signatures are identified and quantified in real time onboard the direct-push rig.
For a QuickTime video of the direct-push XRF system in operation, click here (recommended for broadband Internet connections).
The LIBS probe is advanced into the subsurface with standard direct-push tooling. The output of the laser beam is focused on the surface of the soil outside the probe and emits periodic pulses during advancement, causing a breakdown of the soil and the formation of a high temperature plasma spark. The wavelengths of light (indicative of the elements present in the soil) are picked up by a detector in the probe that relays the electronic signal to the spectrometer on the surface in the truck, which breaks this light into its constituent colors, much like the action of a prism. This information is analyzed onboard the direct-push rig to obtain qualitative and quantitative data from the characteristic signatures from each of the metals detected. The resulting LIBS data are used to generate detailed graphics depicting the metals concentration as a function of depth.
The LIBS system also can be deployed in a stand-alone system, without the CPT but with a backpack or cart-mounted system, to analyze surficial soil samples or grab samples. The unit is designed to be used above ground, and proper care must be taken to prevent accidental discharges between probes and during handling. Operators should wear appropriate skin and eye protection during use.
Matrix effects can cause a great deal of variation in inorganic sample analyses. Physical matrix effects result from variations in the physical character of the sample soils, such as particle size, uniformity, homogeneity, and condition of the surface. Metals analyses by both XRF and LIBS are especially susceptible to heterogeneity of contaminant distribution in the soil, which is a common problem with inorganic contaminants. The contaminants are not removed from the matrix as with formal laboratory procedures. The option of homogenizing the sample is not available for in situ analysis.
There are several other interferences that can affect the ability of these sensors to detect and quantify elements in a sample. Moisture content above 20 percent may cause problems for XRF analysis (since moisture alters the soil matrix for which the XRF has been calibrated), but these problems are manageable and XRF may be used in the saturated zone. By contrast, LIBS is very sensitive to moisture and does not perform well in saturated soils and sediments. Because typical x-ray penetration depths range from 0.1 to 1 millimeter (mm), the window of the XRF probe should be in direct contact with the sample. Chemical matrix effects also can hamper XRF analysis as x-ray absorption and enhancement phenomena. Finally, the resolution of the detector may cause problems in analyzing some elements. If the energy difference between the characteristic x-rays of two elements (as measured in eV) is less than the resolution of the detector in eVs, the detector will not be able to resolve the peaks. In other words, if two peaks are 240 eVs apart, but the resolution of the detector is 270 eV, the detector will have difficulty in differentiating those peaks.
XRF can detect heavy metals at levels below 100 ppm, up to the full depth allowable by the direct-push rig. A field test of the LIBS system at a national laboratory successfully measured a range of chromium concentrations from 30 ppm (background) to 1,200 ppm. In general, LIBS is a more sensitive technology than XRF and can achieve lower detection limits, often in the single ppm range. Laboratory tests of the LIBS system were used to perform quantitative analysis of lead in soils. From this data, it was determined that the minimum LIBS detection limit for lead in soils was 10 to 40 ppm.
The XRF is calibrated by analyzing standard reference materials (SRM) from the National Institute of Standards and Technology containing varying concentrations of the analytes of interest. Additional calibration standards may be created by spiking clean soils with known concentrations of one or more of the target analytes at concentrations representative of what is expected at the site. The calibration samples are placed in a sample chamber attached to the sample probe, which protects the operator from x-ray exposure during analysis. The calibration samples are measured with three to five replicates each for a 100-second acquisition time. A calibration curve is then created by performing a linear least-squares fit to the SRM and prepared samples if used.
The LIBS is typically calibrated in the laboratory prior to going into the field. A site-specific suite of calibration standards is prepared by spiking clean soils from the site with known concentrations of standards. Using site soils allows the operator to tailor the calibration to the soil conditions found on site. A "zero" sample of clean soil is analyzed along with prepared samples containing a range of contaminant concentrations expected to be encountered at the site. A "nominal" calibration consisting of a software generated generic calibration may be performed if relative response data is acceptable and an accurate lower detection limit is not necessary.
XRF and LIBS are field screening methods and do not eliminate the need for traditional laboratory analyses. Soil samples must be collected from an adjacent location at a percentage (typically 5 to 10 percent) of field screening locations and submitted to an off-site laboratory for analysis. These results are used to recalibrate the sensor in the case of XRF, and to confirm that the sampling results are accurate in both cases. Field data may be mathematically corrected after the fact based upon the confirmatory sample results if a clear trend is apparent for a given analyte or analytes. Calibration check samples may be analyzed by retrieving the XRF probe from the soil periodically and re-analyzing a calibration standard. The purpose of the calibration check is to assess whether the instrumentation's response has remained constant during operation.
Precision and Accuracy
During the previously-described field test of the LIBS system, measurements of chromium concentrations from 30 ppm (background) to 1,200 ppm correlated highly with data collected from other soil borings taken in the test location.
The Explosive Sensor (ES) probe detects total concentrations of explosives contamination in the subsurface soil. The ES uses electrochemical sensors that detect the presence of certain chemical compounds characteristic of explosive compounds. The probe also incorporates geotechnical sensors (tip resistance and sleeve friction sensors) for determining soil lithology. By combining the ES and geotechnical sensors, the probe collects soil classification and contaminant concentration versus depth information during the penetrometer push. The ES is currently in the precommercial stage, and is available for adoption by eligible agencies of the Federal government or for commercialization.
The probe incorporates an external pyrolyzer system used to transform explosive compounds into electroactive vapors and a pneumatic system to transport these vapors from the soil to the electrochemical sensors inside the probe. Various other materials present in the soil may contain some of the same compounds characteristic of explosives. Although unable to differentiate between specific explosives compounds, the ES is equipped with electrochemical sensors to differentiate between compounds containing organic nitrogen and inorganic nitrogen, and to distinguish explosives compounds from other compounds, such as common fertilizers. A diagram of the system is provided below in Figure 13.
The ES is used for assessment of sites contaminated with energetic materials such as the explosives trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), Cyclotetramethylenetetranitramine (HMX), their manufacturing intermediates, and subsequent breakdown products. Characterization of these sites is often very time- and cost-intensive because contaminant distribution is generally widespread over large areas. Ammunition plants typically cover many thousands of acres. The traditional methods to measure TNT and other explosives in the environment include collection, transportation of samples, and extraction, coupled with sophisticated laboratory analysis (Yinon and Zitrin 1981). Direct-push analytical instrumentation offers cost relief and time savings for such large-scale sites.
As with all direct-push technologies, the ES is only applicable to shallow sites with unconsolidated soils and sediments. CPT probes have been pushed to over 150 feet bgs during testing, but 100 feet bgs is considered to be a realistic average maximum depth for CPT advancement. Furthermore, the ES may only be used to characterize vadose zone contamination.
Theory of Operation
Explosive compounds such as TNT can be thermally decomposed into vapor. In order to decompose explosives in situ, the soil is heated to release the nitrogen-containing gases that are characteristic of explosives. The analyte vapor must be electrochemically active and must come into physical contact with the active electrode of the sensor. The nitric oxide (NO) gas sensor cannot directly measure a response from solid TNT because of its low vapor pressure. For this reason, explosive compounds must be vaporized from the soil matrix for analysis.
Vaporization is achieved using a heater located on the outside of the probe, in close proximity to the sensor, to thermally degrade TNT and other explosives that are generally found in crystalline form in the soil. During operation, the wire is heated briefly to about 900°C. The surrounding soil (from 0.15 to 0.25 centimeter [cm] from the filament) is warmed from 125°C to 150°C. The explosives in the soil sublime, and the heat dissociates the NO2 groups that are weakly bound to the main portion of energetic molecules. The resulting vapor contains carbon monoxide (CO), carbon dioxide (CO2), water (H2O) and nitrous oxide (NO2). The NO2 molecules are further dissociated into NO and oxygen. The NO fragment, nitric oxide, is the dominant gaseous product.
The electrochemical sensor measures a change in current as result of contact by the NO2 ion with the sensor electrode. The voltage response is a direct function of the total concentration of explosive compounds.
The explosives probe contains, in addition to standard CPT geotechnical sensors, an electrochemical sensor that is responsive to NO2, an internal pneumatic system, associated power supply and signal conditioning electronics inside the probe, and output monitors in the truck. The probe is designed for air to be continuously pumped through the output ports of the probe and into the soil matrix adjacent to the probe, collected through the vapor inlet ports, directed over the sensor, and drawn to the surface where it is vented to the outside.
A CO sensor is included in the probe design. Multiple sensors permit discrimination of organic nitroaromatic compounds from inorganic nitrogen compounds that do not give off CO compounds when dissociated by heat. The heater consists of a 20-cm length of 0.010-inch-diameter platinum wire wound in two loops around a ceramic inset in the probe. It is located between the output and inlet ports so that the heater wire is offset from the soil surface by 0.15 inch. A commercial zirconium oxide ceramic paint is used as an electrical insulator between the platinum wire and the probe's surface.
Mode of Operation
The probe is pushed to the full depth of interest at a constant rate of 2 cm per second to collect soil stratigraphy data using the CPT sensors. After pushing to the maximum depth, the probe is retracted about a foot, releasing a sacrificial sleeve that protects the pyrolyzer unit during the downward push, exposing the wire heater, the module's vapor delivery, and the sampling ports. Sensor readings are then made at discrete depths during retraction. Each reading consists of a short stabilization period of 0.5 to 1 minute, a 30-second activation of the heating elements to thermally degrade explosives in the surrounding soil, followed by another stabilization period of about 30 seconds while the sensor recovers. The probe is then retracted to the next sampling point. The probe may be pushed to the full depth allowed by the umbilical cable length. The minimum depth at which the sensor is able to operate is 4 inches bgs.
Explosives (TNT, RDX, HMX, their manufacturing intermediates, and subsequent breakdown products) in the vadose zone.
An electrochemical CO sensor is used with the ES to differentiate between compounds containing organic nitrogen and inorganic nitrogen, and to distinguish explosives compounds from other compounds, such as common fertilizers.
Matrix interferences are a concern because explosives are relatively insoluble crystalline compounds that tend to be dispersed very heterogeneously. Solid TNT is generally not uniformly distributed but instead exists in soils as localized particles. Under these circumstances, samples collected from nearly identical site locations and depths can have significantly different contamination levels.
Laboratory results indicated that the probe is very sensitive to low concentrations of TNT in dry soil. The calculated lower detection limit based on the signal equivalent to three times the noise is 0.5 ppm.
The standard method of operating the ES in the field consists of calibrating the stratigraphy, electrochemical sensors, and air flow prior to pushing the probe. A known concentration of NO in nitrogen gas is used to calibrate the explosives electrochemical sensor.
After initial sensor measurements indicate explosives contamination in a particular area, a direct-push soil sampler is used to obtain 18-inch cores at depths suspected of having explosives contaminants. The verification soil samples may be analyzed in the field using an immunoassay field test procedure or sent to an off-site laboratory for confirmatory analysis by EPA Method 8330A.
Precision and Accuracy
The response of the probe is linear over a broad range of concentrations. Although not exact, the explosives sensor probe readings correspond well with the laboratory results obtained by EPA Method 8330 and the immunoassay field method. Some of the discrepancies between the three methods may be attributed to the heterogeneity of explosives in environmental samples. The ES is designed to be a rapid site screening tool to distinguish areas of contamination from areas without contamination, and functions as a semi-quantitative tool.
Direct-push analytical systems share a number of advantages over traditional investigative methods, including drilling and collection of samples for off-site analysis (regardless of the sampling platform). These advantages are inherent to in situ analytical systems and direct-push platforms. In addition, the individual analytical systems described above have their own particular strengths. The following advantages apply to some or all of the systems described above.
- Generation of (near) real-time data, which often allows completion of site characterization in one mobilization
- Investigation flexibility due to real-time data generation
- Greater throughput than traditional sampling methods, which allows for a greater amount of data to be generated and overall site uncertainty to be minimized
- The ability to access hard-to-reach or sensitive areas, or areas with limited overhead clearance (for example, power lines, inside buildings)
- Generation of little to no investigation-derived waste
- Vertical sampling capabilities to about 60 feet bgs for rotary hammer rigs and 100 feet bgs for CPT rigs
- The ability to characterize subsurface contamination at multiple intervals (or continuously) during one push (Hg fluorescence, MIP, TDS, LIBS, XRF)
- Simultaneous generation of lithologic logs (MIP)
- Sampling capabilities in both unsaturated and saturated zones (MIP, XRF)
Direct-push analytical systems have limitations that are important to keep in mind when considering their application for site characterization, including:
- Detection limits can be higher and precision and accuracy lower than with traditional analytical methods
- Some systems may be semi-quantitative (Hydrosparge and MIP in unsaturated soils) or semi-qualitative (MIP/PID and MID/FID configurations and Hg fluorescence) depending on the target contaminants and applications
- Because experienced operators are generally required, it may not be possible to lease the systems; many may be offered as contract services only
- Practical vertical sampling limits are about 100 feet bgs for rotary hammer rigs and 150 feet bgs for CPT rigs
Studies indicate that direct-push analytical systems may provide significant savings over conventional site assessment and characterization methods. Studies performed by the U.S. Department of Energy for CPT-mounted systems indicate that 25- to 35-percent savings may be achieved. Cost information varies greatly among the different technologies as well as for projects of different scope. The following contractors and vendors may be contacted for more detailed cost assessments.
Verification of the performance of site characterization and field analytical technologies is conducted through a variety of programs. Evaluation and verification reports for several of the technologies described in this encyclopedia entry are provided below.
The reliability of in situ, direct sparging of VOC analytes from groundwater in concert with the DSITMS has been successfully demonstrated at numerous sites, and Cal/EPA has certified the Hydrosparge for use at contaminated sites. Click here to access the certification notice and study.
Cal/EPA has certified the TDS as a near real-time, qualitative to semi-quantitative, in situ subsurface field screening method for VOCs in the vadose or capillary zone. Click here to access the certification notice and study.
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