This document has been adopted from portions of the Arcadis Standard Operating Procedures for Radon Measurement in Support of Vapor Intrusion Investigations. Durridge is not responsible for its content. For more information, please check out this link (starting on p. 12) or contact Arcadis.

Scope and Application

This document describes the standard operating procedures for using the DURRIDGE RAD7 electronic radon detector to collect radon measurements from below a building slab, for the purpose of investigating vapor intrusion. Radon can be used as a tracer gas for volatile organic compound (VOC) vapor intrusion. An attenuation factor can be calculated based on the ratio of indoor air to sub-slab concentrations of either VOCs or radon. Radon data can also be used to locate potential points of soil gas entry. It can be particularly useful to measure thoron (220Rn), which is a short-lived isotope of radon, since it is only found in significant concentrations near entry points, and the DURRIDGE RAD7 provides a rapid response. Detailed instructions for measuring sub-slab radon concentrations are provided below.

Personnel Qualifications

Field sampling personnel must have current health and safety training, including 40-hour Hazardous Waste Operations and Emergency Response (HAZWOPER) training, site supervisor training, site-specific training, first aid training, and cardiopulmonary resuscitation (CPR) training, as needed. Field sampling personnel must be well versed in the relevant standard operating procedures and possess the required skills and experience necessary to successfully complete the desired field work. Personnel responsible for leading vapor intrusion investigations must have previous experience.

Health and Safety Considerations

All sampling personnel should review the appropriate health and safety plan (HASP) and job safety analysis (JSA) prior to beginning work, to become aware of all the potential hazards associated with the job site and the specific task. The following are examples of hazards that are often encountered while conducting vapor intrusion assessments and radon measurements:

  • In residential buildings and neighborhoods, unfamiliar dogs can pose a hazard. Even though proper permission for sampling may have been secured, it is still possible to encounter persons suspicious of or hostile to the sampling team.
  • In occupied industrial buildings be aware of the physical hazards of ongoing industrial processes. Examples include moving forklifts and equipment pits.

If indoor air in the breathing zone (for example within a basement or crawl space) exceeds the NRC level of 30 pCi/L averaged over a year, exit the space as soon as possible and consult the health and safety officer. For more information please see

Initial Building Survey on Integrity of Building and Slab

An initial building survey should be carried out to assess the condition of the home. If a building survey has been completed in support of collecting indoor air samples, this survey will suffice for radon sample collection as long as the integrity of the building (basement slab, concrete walls, etc) is well detailed. Information on the heating, ventilation, and air conditioning system (HVAC) should also be detailed. If a building survey has not been previously completed, use the following steps:

  1. Confirm with the homeowner if an existing radon mitigation system is present in the building. If so, the project manager should be consulted for further instructions.
  2. Survey the building for cracks in the floor slab, cracks in the walls, floor sumps, floor drain, or other preferential pathways for vapor transport using the appropriate building survey. Record relevant information on a survey form and document the investigation with photographs as appropriate.
  3. Record the date, time, location, and other relevant notes on the sampling form or electronic field data collection device.
Equipment List

The equipment required for collecting radon measurements from below a building slab are presented below. It is assumed that a permanent or temporary sub-slab sample port has already been installed during the vapor intrusion investigation.

  • DURRIDGE RAD7 portable electronic radon detector
  • Appropriate-sized open-end wrenches (typically 9/16-inch) to access sample ports
  • Tygon tubing to connect sample ports to the RAD7 radon detector. (Swagelok fittings or other methods may be appropriate to affix tubing to the port. Staff should check this before heading out into the field.)
  • Camera
  • Chain-of-custody (COC) form
  • Sample collection log form
Sub-Slab Soil Vapor Radon Measurement Procedure

The DURRIDGE RAD7 radon detector is ideally suited for the measurement of radon in soil gas originating from beneath building foundations. The instrument operates using an internal sample cell with a solid-state, ion implanted, planar, silicon alpha detector. The instrument is capable of real time measurements from 0.1 – 20,000 pCi/L (4.0 – 750,000 Bq/m3). It is possible to detect radon below the slab in near real-time when operating the RAD7 in SNIFF mode, with a cycle time of 5 to 10 minutes for radon or 3 minutes for thoron. This allows the sampling staff to actively identify potential entry points within the basement. A 15 minute equilibration time per sub-slab port is recommended. Depending on the structure to be sampled, collecting samples from multiple locations will provide better coverage of the sub-slab conditions. The location of sampling points should also be guided by visual or interview evidence of differences in floor system construction in various areas of the structure. Examples of areas that should generally be treated as different zones for evaluation are:

  • Areas with different construction styles – for example basement vs. crawl- space vs. slab on grade
  • Areas constructed at separate times
  • Areas with differing flooring materials or wall construction materials
  • Areas with separate HVAC systems

Samples should be collected from either permanent sub-slab sample ports, or temporary ports installed during the vapor intrusion investigation. Sample ports should be allowed to sit undisturbed for at least 24 hours after installation prior to radon measurement to allow for equilibration. Radon samples may be collected directly after purging soil gas port volumes and sub-slab soil gas sampling for volatile organic compounds (VOSs). All air purged while inside a building should be collected in a Tedlar bag and exhausted outside the building. Alternately, if sub-slab sampling for VOCs is not necessary, the standard flow rate of the RAD7 with unrestricted flow (0.5 to 1.0 L/min) should be adequate to purge most sampling points early in the 15 minute equilibration time, so a separate purge period may not be necessary. Field staff should rely on the RAD7 manual for the operation and troubleshooting of the instrument. Sections of note are listed below:

  1. Setting up the RAD7 (Section 1)
    • Inspect all power cords.
    • Inspect desiccant to ensure it is ready for use (It should blue, not pink or gray).
    • Ensure that the RAD7 powers up properly.
  2. Starting a SNIFF test (Section 1.5)
    • Set the unit to SNIFF mode for both radon and thoron.
    • Each location should be sniffed for at least 15 minutes to give a reliable radon reading.
    • Record all readings on the sampling sheet and/or field book.
  3. Details of sniffing (Section 4.3)
    • Use a 5-minute cycle time, discard first two cycles to ensure the unit has been cleaned out between sample points.
    • Watch the current required to operate the pump, readings over 100 mA indicate tight soils and poor flow.
    • Record pumping rate.
  4. Background on radon (Section 3.11)
  5. Maintenance of the unit (Section 7)
    • The desiccant must be regenerated when it appears pink.
  6. Staff should sample each sub-slab sample port using the SNIFF test mode to look for radon. Ignore the first two 5-minute SNIFF test cycles and average the next two cycles to arrive at the concentration of radon below the building slab.
    • The RAD7 should be connected to the sample port using Teflon tubing and standard vapor intrusion sampling fittings.
    • Ensure that these connections can be properly made prior to heading into the field.
    • Capture the exhaust air from the RAD7 in a Tedlar bag and exhaust this air outside of the building.
  7. Upon completion of the test at each sample port, field staff should use the RAD7 radon detector to investigate potential entryways (i.e. cracks, sumps, pits, drains) and note the presence of elevated radon or thoron readings from any of these features.
    • Photograph any potential entry points as they are found
    • Note that such measurements may only be accurate to the extent that differential pressure moves gas into the building at the time of measurement.
Quality Assurance

Duplicate samples should be collected in the field as a quality assurance step. Generally, duplicates are taken of 10% of samples, but project specific requirements should take precedence. Ambient background samples outside the building should be taken with the RAD7 inlet tubing located several feet above the ground. These samples should be taken at least at the beginning and end of each monitoring date.


Lutes, C.C. , R. Uppencamp, L. Abreu , C. Singer, R. Mosley and D. Greenwell “Radon Tracer as a Multipurpose Tool to Enhance Vapor Intrusion Assessment and Mitigation” poster presentation at AWMA Specialty Conference: Vapor Intrusion 2010, September 28-30, 2010, Chicago, IL

Steck, D.J. “Exposure to Atmospheric Radon” Environmental Health Perspectives Volume 107, No 2, February 1999.

U.S. Environmental Protection Agency Office of Air and Radiation (6604J)
EPA 402-R-92-004 July 1992 (revised) “Indoor Radon and Radon Decay Product Measurement Device Protocols”

US EPA, 2006, “Assessment of Vapor Intrusion in Homes Near the Raymark Superfund Site Using Basement and Sub-slab Air Samples”, EPA/600/R-05/147.

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