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Membrane Interface Probe (MIP)

Description

A MIP is a semi-quantitative, field-screening device that can detect volatile organic compounds (VOC) in soil and sediment. It is used in conjunction with a direct push platform (DPP), such as a cone penetrometer testing rig (CPT) 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. (For non-air regulatory purposes VOCs are defined by the EPA Terminology Reference System as organic compounds that readily pass off by evaporation.) 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.

Typical Uses

MIP technology is capable of sampling volatile organic compounds and some semi-volatile organic compounds 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). EPA has developed a dynamic field process in which MIPs may be used to produce reliable estimates of the contaminated mass, which is crucial to achieving cost-effective cleanups. Additional information on the use of the dynamic field process is in Using Dynamic Field Activities for On-Site Decision Making: A Guide for Project Managers.

Theory of Operation

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.

System Components

The MIP is mounted on a DPP, which drives the probe into the soil and estimates the probe’s depth.

MIP with Conductivity Probe Tip 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 125º C (160 to 232º F), 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 onsite 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 a MIP include gas chromatography (GC)-grade detectors (e.g., photo-ionization (PID), flame ionization (FID), electron capture (ECD), and dry electrolytic conductivity (DELCD)) 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. PID detectors are best used for detecting aromatic compounds, such as BTEX (benzene, toluene, ethylbenzene, and xylene isomers). FID detectors are used to detect petroleum hydrocarbons (straight and branched chain alkanes). ECD and DELCD detectors are used to identify chlorinated hydrocarbons (e.g., perchloroethene, trichloroethene, 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 onsite ion-trap mass spectrometer (ITMS). Since the ITMS lacks a GC, its ability to resolve complex mixtures of contaminants is limited.

Another approach to analyzing vapor samples collected by MIP that is under development is a DPP-delivered halogen-specific detector, which can be positioned immediately behind a MIP. As of July 1, 2007, this probe was not commercially available. However, a newly designed version of the probe, which is expected late in 2007, will offer higher spatial resolution for delineation of DNAPL source terms and lower sensor acquisition and operating costs. It also can be operated in concert with other chemical and physical sensors.

Mode of Operation

All necessary point-installation permits for digging, coring, drilling, and groundwater monitoring should be obtained prior to mobilizing equipment to the field. Prior to initiating any intrusive subsurface activities, the proposed sampling locations should be cleared and all utility lines in the investigation area should be marked. Care should be taken not to cross-contaminate deeper aquifers by puncturing an aquitard underlying the contaminated groundwater or DNAPL source.

The MIP is pushed into the ground at a rate of about a minute per foot. The push strategy depends upon the data quality objectives, soil matrix, and the chemical species that are expected to be present. For example, benzene in a sand might allow continuous sampling, while a less volatile compound in a clay matrix may require a push and hold strategy that provides more thorough heat transfer to the soil matrix. The manufacturer of the probe recommends a push and hold strategy. The time it takes for the carrier gas to transport the sample to the surface varies with the length of the carrier tubing. The detector and carrier tubing can become saturated when driving the probe through an LNAPL or DNAPL. While the carrier tubing usually can be cleared by continuous carrier-gas purging, in some instances, the probe has to be pulled and the tubing replaced.

The carrier gas can be injected directly into a measuring device. Some contractors offer logs from three-detectors, including PID, ECD, and FID, as part of their normal DPP/MIP service. When a greater degree of speciation is required, an ITMS, GC, or GC/MS may be used, as discussed above.

At the conclusion of subsurface investigations, each sampling location that is not used to install a groundwater monitoring point or well should be properly sealed with bentonite chips or pellets, grout, or other appropriate material to eliminate any potential for contaminant migration to the groundwater.

For details on mode of operation, including QC procedures, see MIP SOP.

Target Analytes

Target analytes typically sampled with MIP technology include VOCs, such as BTEX and halogenated hydrocarbons. Some semi-VOCs also can be sampled.

Performance Specifications

DPP/CPT rigs are generally capable of surveying 75 m (250 ft) or more of subsurface per day and hence are far cheaper to use than obtaining similar stratigraphic information and samples for laboratory analysis with a conventional drill rig. Because the MIP is usually advanced at a rate that allows the soil matrix to be heated, a more modest 37 to 62 meters (120 to 200 feet) per day is typical. It generally takes about 75 seconds for the carrier gas (nitrogen) to travel through 200 feet of inert tubing to reach the detectors. About 20 samples per day can be analyzed when GC/MS is used as the analytic device.

Detection Limits

The MIP’s detection limits depend on the soil type, temperature, and detector used. PIDs used to detect benzene, toluene, and ethylbenzene (BTE) have a detection limit of about 1 ppm. ECDs to detect chlorinated hydrocarbons with a nitrogen carrying gas have a detection limit of nearly 2.5 ppb. DELCDs to detect chlorinated hydrocarbons with nitrogen as the carrying gas have a detection limit of nearly 1 ppm.

Calibration

The MIP is calibrated by inserting the probe into a sand or water standard prepared in advance with known concentrations of the VOCs of concern. For information on preparing calibration standards see MIP SOP (CLUIN disclaimer policy http://www.cluin.org/usenotice.cfm)

Sample Preparation

While no sample preparation is needed, when MIP is deployed from a DPP, hard surfaces, such as concrete or caliche, may require drilling or cutting prior to advancing the probe into the ground.

Quality Control (QC)

Several types of quality control checks can be applied to assess whether the MIP systems are functioning properly and are producing accurate data that will be useful for project decision-making. One of the most important steps is calibration with clean sand-blank measurements taken pre- and post-push as part of the standard data collection procedure. This step ensures there is no carry-over from the previous push.

To ensure that the membrane itself is functioning correctly the manufacturer's SOP states: "A probe membrane is considered in good working condition as long as two requirements are met: 1) The butane sanity test result is greater than 1.0E+06 uV response, 2) Flow of the system has not varied more than 3 mL/min from the original flow of the system (a flow meter or bubble flow meter should be kept with the system at all times). If either one of these requirements are not met, a new face must be installed."

A qualitative assessment may be conducted by comparing subsurface contaminant cross sections generated from MIP data to borehole logs or cross sections prepared using dual tube direct push soil sampling techniques coupled with onsite GC or GC/MS confirmation data.

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 MIP system's performance and can help determine how useful the data are.

Precision is usually assessed by comparing the results of duplicate analyses. However, because MIP samples are taken in situ, it is not possible to obtain true duplicate samples. Instead, an estimate of the instrumental precision can be obtained for the entire system by evaluating the results from multiple measurements of their respective calibration check samples, which are analyzed before and after each push.

Because MIP analytical detection systems do not provide fully quantitative results, accuracy is assessed qualitatively by measuring the agreement between detect and non-detect determinations made by the MIP and by corresponding confirmatory laboratory samples. Interpretation of MIP data produced by total detectors is best done by comparing relative responses rather than absolute values.

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 ion trap mass spectrometer (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 (Myers 2002).

Cost Data

The major costs of operating a MIP system involve the direct push platform and labor, MIP operating system, and detectors. Depending upon the equipment used, the cost of a MIP system with crew can range from $2,000 to $4,000 per day. If speciation of contaminants (e.g. MTBE vs. benzene or PCE vs. TCE) in the off-gasses from the MIP detector is required, a mobile lab with portable GC or GC/MS can be brought to the site. A GC can typically process 30 to 40 samples per day. Typically, speciation adds an additional $2,000 to $3,000 per day to the cost. Speciation also can be accomplished by collecting the off-gasses on carbon traps for analysis at an off-site lab. The cost per carbon trap collection for off-site analysis is about $175.

Verification/Evaluation Reports

A variety of programs exist to verify the performance of site characterization and field analytical technologies. Evaluation and verification reports from EPA's Superfund Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program, EPA's Environmental Technology Verification Program (ETV) program, and the Department of Defense’s (DoD) Environmental Security 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 remediation of hazardous waste sites and monitoring and measurement. Innovative technologies are field-tested on hazardous waste materials, and the engineering and cost data from the tests are published for potential users to use to 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. No reports are available for MIP.

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 domestic and international marketplaces. ETV verifies commercialized, private-sector technologies. After the technology has been tested, the companies receive a verification report that they can use to market their products. The results of the testing also are available on the Internet. No reports are available for MIP.

DoD Environmental Security Technology Certification Program

The goal of DoD’s Environmental Security Technology Certification Program is to demonstrate and validate promising, innovative technologies that target DoD's most urgent environmental needs. These technologies provide a return on investment through cost savings and improved efficiency. 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 the Certification Notice for the technologies.

Tri-Service Site Characterization and Analysis Penetrometer System Validation of the Membrane Interface Probe
Myers, K., W. Davis, and J. Costanza
U.S. Army Corps of Engineers, ERDC/EL TR-02-16, 62 pp, 2002

References

Accelerated VOC Source Investigation Pairing SCAPS/MIP with EPA Triad, Marine Corps Base Camp Pendleton
Collins, K.
Conference on Accelerating Site Closeout, Improving Performance, and Reducing Costs Through Optimization, San Diego, California, 42 pp, (ppt)

Defining TCE Plume Source Areas Using the Membrane Interface Probe (MIP) abstract
McAndrews, B.; K. Heinze, and W.DiGuiseppi
Soil and Sediment Contamination (formerly Journal of Soil Contamination), Volume 12, Number 6, November-December 2003, pp. 799-813

Sensor Technologies Used During Site Remediation Activities – Selected Experiences
U.S. EPA, EPA 542-R-05-007, 110 pp 2005

Site Characterization Technologies for DNAPL Investigations
U.S. EPA, EPA 542/R-04/017, 165 pp, 2004

Vapor Intrusion and Ambient Air Study Final Results Report Armen Cleaners Ann Arbor, Michigan
U.S. EPA, 139 pp, 2006

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Page Last Modified: November 20, 2007