Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 8.5
FIELD SCREENING WITH CONE PENETROMETER AND SENSORS/PROBES

8.5 FIELD SCREENING WITH CONE PENETROMETER AND SENSORS/PROBES

There are a variety of sensors and probes that can be used to optimize sampling and analyses at contaminated sites. Geotechnical sensors can provide an indication of where historical fill materials could be present, and they can be used to refine information for processing geophysical data. In addition, geotechnical methods like cone penetrometer technology (CPT) can be used to obtain detailed geologic information, and high-resolution sensors/probes on on direct push platforms (DPT) can be used to delineate a water table and even to predict where vertical gradients could be present.

Other types of probes and sensors used in combination with CPT are designed specifically to target the identification of contamination in the subsurface, like the membrane interface probe (MIP) or fluorescence tools that look for hydrocarbons. These instruments are generally stacked together such that the maximum amount of information for a particular portion of a site is collected as efficiently as possible. These tools can be extremely valuable, but are also selective in terms of the type of data they can generate and the requirements for collecting data under controlled conditions.

Mobilization costs for sensors and probes with CPT can be significantly more than other sampling/analysis strategies, especially for small numbers of samples. However, for large numbers of samples appropriate sensors or probes in combination with CPT may be more cost effective and a valuable assistance to site delineation efforts.

8.5.1 DESCRIPTION OF CONE PENETROMETER TECHNOLOGIES

CPT techniques include samplers and analytical devices (typically a steel cone) that can be deployed into the subsurface using a direct push platform. Direct push platforms use hydraulic pressure to push a steel rod into the ground. This creates a small borehole by pushing soil out of the way as opposed to removing soil as in drilling. The same method is used to advance the sampling devices, geotechnical sensors, or analytical sensors associated with CPT. The sensors (attached to the tip of the cone) are connected to electrical cable running inside the hollow push rods, enabling the collection of data by a computer acquisition system at the surface. As such, “real-time” data can be collected using CPT techniques.

Two platforms are used for direct push technology (DPT). The first advances the tool string by the weight of the truck and supplemental steel weights and is known as a CPT. The second uses the weight of the truck aided by a rotary hammer and is known as the rotary hammer system. The CPT system is usually mounted on a 10-30 ton truck, while the rotary hammer system is mounted on pick-up trucks. The use of the name cone penetrometer system for the larger platform is misleading since CPT technologies can be deployed from both the CPT and rotary hammer platform.

The two platforms differ in scale of application and, to some extent, in the types of instruments and tools that they deploy. The devices developed for these platforms are: samplers for soil, soil gas and groundwater; geotechnical sensors for soil texture and hydraulic conductivity; and chemical sensor sampling techniques to detect petroleum, volatile organic compounds, metals and explosives. The sensors are connected to data acquisition devices mounted on the trucks.

CPT techniques are utilized for field screening only. Use CPT data to guide the placement of boreholes and selection of sampling locations. Collect samples for laboratory analysis to verify contaminant plume extent and contaminant concentrations.

8.5.2 CONE PENETROMETER DATA VERIFICATION

Always verify interpretation of CPT data by correlating it to site data. Correlate CPT data for soil type to lithological samples collected from the site to allow accurate interpretation of the CPT data (USEPA 2005).

Correlate conductivity (resistivity) data collected by CPT techniques to samples collected from the site. Conductivity varies with grain size but also with soil water content and ionic strength of the pore water or groundwater. Ionic strength of the groundwater can change due to contaminant content. Dense non-aqueous phase liquids (DNAPL) have a very low conductivity and can thus be detected by conductivity measurements (USEPA 2004c and 2005). Light non-aqueous phase liquids (LNAPLs) can also be detected; however, other methods are more efficient in locating LNAPL plumes (USEPA 2005). Because conductivity is influenced by soil type, water saturation, solute type, solute concentration and presence of non-aqueous phase liquids, the interpretation of the data is not straight-forward and requires calibration against site samples.

8.5.3 CPT ADVANTAGES AND LIMITATIONS

Cone penetrometer technology is deployed through either of two platforms of direct push technology (DPT).The advantages of direct push technologies (DPT) are:

  • Deployment of in situ instruments allows rapid, real-time collection of data, which may be used to guide further drilling and sampling efforts, avoiding laboratory turn-around times and remobilization.
  • Sampling and data collection may be faster than with traditional drill rigs.
  • DPT does not generate large quantities of soil cuttings, thus reducing the amounts of investigation derived waste generated during the course of an investigation.
  • Installation of micro-wells or pre-packed wells is substantially lower in cost than installation of permanent monitoring wells using traditional drill rigs.
  • The rotary hammer system is deployed using smaller rigs, often pick-up truck mounted, and are therefore more mobile than traditional drill rigs. Smaller rigs can often access buildings or difficult to reach off-road locations. Also small rigs can be used in areas with overhead wires or other overhead hazards.

Cone Penetrometer Technology (CPT techniques) have the following limitations:

  • CPT techniques can be used only in unconsolidated formations. Hard layers, partially cemented sediments, rocks and boulders limit the penetration. However, rotary hammer systems have rotary capability and can be used to penetrate concrete or other thin hard layers.
  • CPT can often be advanced to depths greater than 100 feet, but cannot advance boreholes as deep as traditional augers can.
  • Vertical changes in formation density limit the method. Hard layers encountered under soft layers may cause refusal, bending or breaking of the drilling rod.
  • When the CPT system is mounted on trucks that weigh 20 to 30 tons, they are limited to locations with firm ground (the rotary hammer system is mounted on pick-up trucks with fewer limitations).

8.5.4 OTHER CPT INSTRUMENTS

Piezocones can be used to determine the hydraulic conductivity of subsurface soils and the depth to groundwater. These data can be used to identify potential contaminant pathways in the subsurface, or to aid in the selection of sampling locations (USEPA 2005).

Available chemical analytical sensors include: laser-induced fluorescence (LIF) probes and fuel fluorescence detectors (FFD), which can identify PAHs; and membrane interface probes (MIP) and SCAPS Hydrosparge™ systems for the detection of volatile organic compounds in soil and groundwater (USEPA 2004 and 2005).

USEPA evaluation of chemical sensors used with CPT has revealed that the sensor output does not correlate well with results obtained from laboratory methods (USEPA 1998; USEPA 1995). The HEER Office considers chemical data collected with CPT techniques as qualitative. In addition, the chemical sensors often detect classes of analytes rather than specific analytes.

Induced Fluorescence Tools

There are two basic delivery systems that can be used to detect hydrocarbons in the subsurface. One is a laser-induced fluorescence (LIF) set of tools and another is the fuel fluorescence detection (FFD) systems. Both provide a method for real-time, in situ, field screening of hydrocarbons in subsurface soil and groundwater. The technologies are intended to provide highly detailed, qualitative to semi-quantitative information about the distribution of subsurface petroleum contamination. LIF and FFD sensors are generally deployed as part of integrated mobile CPT systems that are operated by highly trained technicians familiar with the technology and its application. See ASTM, 2010 for a standard describing characterization of petroleum contaminated sites with LIF.

LIF and FFD systems can, with relative degrees of success depending on the tool configuration, detect gasoline, diesel fuel, jet fuels, fuel oil, motor oil, grease, and coal tar in the subsurface. The data can be used to guide an investigation or removal action or to delineate the boundaries of a subsurface product contamination plume prior to installing monitoring wells or taking soil samples.

There are currently four major induced-fluorescence systems available to private sector clients: the rapid optical screening tool (ROST) systems, the ultraviolet optical screening tool (UVOST), the tar-specific green optical screening tool (TarGOST), and FFD. Also, 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, but it is only available for federal facility projects. The ROST system was developed by Loral Corporation and Dakota Technologies, Inc., and is available commercially through Fugro, Inc. The UVOST and the TarGOST are available commercially from several vendors including Dakota Industries. All of these systems, while differing in some respects, are similar in their theories and methods of operation.

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.
  • 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.

Limitations:

  • The operation of the fluorescence system takes considerable experience. It takes many days and numerous projects to become familiar with the operation of the technology. Operation of the technologies is provided as services by their respective vendors for this reason.
  • Although these sensors provide information regarding the 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 the large, truck-mounted versions of these systems may be prohibitive for small-scale projects. However, recent advancements in the delivery systems and laser electronics are making fluorescence systems capable of tackling almost any size job more economically.
  • Some maintenance of the CPT tools and the LIF sensors is required, and breakdowns can be expected on long-term projects. For example, 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. 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. Smearing and a memory effect on the sensor may occur when pushing through fine-grained sediments such as clays.

Figure 8-12 Membrane Interface Probe with Conductivity Probe Tip

A MIP is a semi-quantitative field screening device that can detect VOCs in soil and sediment. It is used in conjunction with a direct-push platform (DPP), such as a CPT testing rig or a rig that uses a hydraulic or pneumatic hammer to drive the MIP to the depth of interest to collect samples of vaporized compounds. The probe captures the vapor sample, and a carrier gas transports the sample to the surface for analysis by a variety of field or laboratory analytical methods. Additional sensors may be added to the probe to facilitate soil logging and identify contaminant concentrations. The results produced by a MIP at any location are relative and subject to analytic verification.

MIP technology is capable of sampling VOCs and some SVOCs from subsurface soil in the vadose and saturated zones. It is typically used to characterize hydrocarbon or solvent contamination. Its ability to rapidly locate and identify contaminants reduces uncertainty in management decisions associated with costly cleanup projects, such as those commonly involving source zones of dense non-aqueous-phase liquid (DNAPL) and light non-aqueous-phase liquid (LNAPL). MIP technology uses heat to volatilize and mobilize contaminants for sampling. Heating the soil and/or groundwater adjacent to the MIP’s semi-permeable membrane volatilizes the VOCs, which then pass through the probe’s membrane and into a carrier gas for transportation to the ground surface.

The MIP consists of a small polymer (tetrafluoroethene) port, or membrane, that is permeable to gas but impermeable to liquid. The port is secured onto a steel block that also contains a resistive heater coil and a thermocouple, allowing the temperature of the membrane to be controlled and monitored. The heater coil heats the soil near the membrane to 80 to 125 ºC (160 to 232 ºF), which allows VOCs in the soil and groundwater to partition across the membrane in saturated or unsaturated soil. The subsurface temperature needs to be at or above the boiling point of the target compound(s). Nitrogen is the most commonly used carrier gas, but helium has been used in some applications. The carrier gas sweeps across the back of the membrane, entrains the VOC sample, and carries the VOC to the detection device located at the surface.

Typically, the MIP probe includes a tip that measures soil or water conductivity at a known distance below the membrane. The conductivity measurements can help correlate contamination to known soil stratigraphy. The probe conductivity measurements cannot identify the specific type of soil (based on grain size) distribution that is encountered unless the conductivity measurements can be compared to actual site soil core data. In the absence of on-site data, the MIP conductivity measurements identify changes in the soil’s electrical behavior that can be related to changes in stratigraphy or groundwater quality. Analytical devices commonly used with an MIP include gas chromatography (GC)-grade detectors (e.g., photo-ionization [PID], flame ionization [FID], electron capture [ECD], and dry electrolytic conductivity [DELCD] detectors) that establish the presence of VOC vapor, dissolved phase LNAPL, or DNAPL in soil. These detectors may be deployed singly or in line depending upon the site’s contamination. PIDs are best used for detecting aromatic compounds, such as BTEX (benzene, toluene, ethylbenzene, and xylene isomers). FIDs are used to detect petroleum hydrocarbons (straight and branched chain alkanes). ECDs and DELCDs are used to identify chlorinated hydrocarbons (e.g., PCE, TCE, dichloroethene, carbon tetrachloride).

Speciation of the contaminants can be accomplished either by collecting the off-gas on carbon or Tenax traps and subsequently desorbing the contaminants into a GC/mass spectrometer (MS), or by direct injection into an on-site ion-trap mass spectrometer (ITMS). Since the ITMS lacks a GC, its ability to resolve complex mixtures of contaminants is limited. See ASTM 2007b for a standard describing use of DPT for volatile contaminant logging with the MIP.

Advantages

  • Real time data.
  • Limited investigation derived waste.

Limitations

  • MIPs provide screening-level data that need to be supplemented with analytical soil or groundwater data to fully support human health risk assessments or remediation decisions.
  • Determining the depth at which the sample was taken when the sampler is in a near-continuous operating mode and the push rate is variable can be difficult. Compounds may be found in the subsurface for which the detectors were not calibrated.
  • As with all direct push devices, MIP is only useful for deployment in unconsolidated matrices. Speciation with the ITMS can be problematic when the gas stream contains a complex mixture of chemicals. In many cases, the detection limit of MIP equipment for specific contaminants is above the detection limit required for human health risk assessment.
  • ITMS-MIP overestimates contaminant concentrations for most vadose zone soils when compared with validation results, and it underestimates contaminant concentrations for clay-type vadose zone soils.