Determination of Radon Concentration in Soil: Testing Methods, Standards, and Applications

Soil radon concentration determination is a critical geotechnical and radiological test that measures the activity concentration of radon-222 in soil gas, expressed in becquerels per cubic meter (Bq/m³). This testing is mandated by standards such as GB 50325 and guided by ISO 11665-11 and ASTM D5756 to evaluate the radon potential of building sites, inform anti-radon engineering design, and protect occupants from long-term radiation exposure. Accredited third-party laboratories perform these measurements using driven-probe soil gas sampling coupled with scintillation cells, pulsed ionization chambers, or electret ionization chambers.

Technical Overview

Radon-222 is a colorless, odorless, radioactive noble gas produced by the decay of radium-226 within the natural uranium-238 decay chain. Because uranium and radium are ubiquitous trace elements in rocks and minerals, radon is continuously generated in the subsurface and migrates through soil pores and fractures toward the surface. When the gas accumulates in enclosed spaces such as basements and ground floors of buildings, chronic inhalation of its short-lived decay products becomes a confirmed health hazard, recognized by the World Health Organization as the second leading cause of lung cancer after tobacco smoking.

Determination of radon concentration in soil quantifies the activity concentration of radon-222 in soil gas at a defined depth beneath the ground surface. In China, this measurement is a mandatory pre-construction requirement under GB 50325 (Standard for Indoor Environmental Pollution Control of Civil Building Engineering), which links soil radon survey results directly to the level of anti-radon and waterproofing engineering measures required for the building. Internationally, ISO 11665-11 defines test methods for soil gas radon, while ASTM D5756 addresses the determination of radon in soil gas at low concentrations.

Under GB 50325, survey results are classified into graded response levels: sites at or below approximately 10,000 Bq/m³ generally require no special anti-radon measures; concentrations between 10,000 and 20,000 Bq/m³ trigger ground-floor anti-cracking construction requirements; ranges between 20,000 and 30,000 Bq/m³ additionally require first-grade foundation waterproofing; higher classes between 30,000 and 50,000 Bq/m³ require comprehensive structural radon protection for Class I civil buildings; and sites at or above 50,000 Bq/m³, or with soil surface radon exhalation rates at or above 0.05 Bq/(m²·s), require the most stringent combined engineering controls and, where necessary, supplementary exhalation rate testing or soil replacement.

Testing Samples

Soil Gas from Undisturbed Natural Ground

The primary sample medium is soil gas extracted in situ from natural, undisturbed soils within the footprint of the proposed foundation and the surrounding engineering geological investigation area. Sampling locations are selected to represent the dominant lithologies, fracture zones, and geological structures present on the site.

Soil Gas from Backfill and Engineered Fill

Where construction platforms involve imported fill, excavated and re-compacted material, or replaced soil layers, soil gas samples are collected from the fill horizon to verify that the emplaced material does not introduce elevated radon potential after remediation of a high-radon site.

Soil Gas from High-Risk Geological Settings

Sites underlain by granitic bedrock, uranium-bearing shires and phosphorites, karst carbonate terrain, or active fault zones typically warrant denser sampling, as these settings are associated with elevated soil radon and enhanced gas migration pathways.

Testing Items

The core deliverable of a soil radon survey is a set of point-by-point and site-level radiological indicators, typically including:

• Radon-222 activity concentration in soil gas at each sampling point, expressed in Bq/m³
• Arithmetic mean soil radon concentration of the survey area and the maximum measured value
• Identification and statistical treatment of anomalous points, with supplementary measurements where a single point greatly exceeds the site average
• Soil surface radon exhalation rate in Bq/(m²·s), where required by the classification outcome under GB 50325
• Sampling metadata, including depth, coordinates, soil description, and meteorological conditions affecting data interpretation

Testing Methods

Driven-Probe Soil Gas Sampling Procedure

The standardized approach follows the procedure specified in the annexes of GB 50325. Sampling points are laid out on a grid covering the foundation investigation range, typically at intervals not exceeding 15–20 m with a minimum of 16 points for a standard site, and with at least five points for small single-building plots. At each point, a hardened steel probe or pre-drilled hole penetrates to a typical depth of 0.6–0.8 m below grade. A fixed volume of soil gas is drawn through the probe, a particulate filter, and a desiccant column to condition the gas before it is delivered to the measurement device. Dead-volume air in the sampling train is purged before each reading to ensure the analyzed gas originates from the target depth.

Scintillation Cell (Lucas Cell) Method

In the scintillation cell method, soil gas is drawn into a zinc sulfide (silver-activated) coated cell where alpha particles emitted by radon and its decay products scintillate on the phosphor surface. A photomultiplier tube counts the light pulses, and the count rate is converted to activity concentration using a calibration factor traceable to a radon reference chamber. This grab-sampling technique provides rapid single-point results suitable for site surveys.

Pulsed Ionization Chamber and Continuous Monitor Method

Electronic continuous radon monitors based on pulsed ionization chambers or passively pumped electrostatic collection onto semiconductor (silicon surface-barrier or PIN diode) alpha detectors permit time-resolved measurement at each point. Alpha spectroscopy distinguishes the 6.0 MeV line of polonium-218 from thoron (radon-220) progeny, suppressing thoron interference, which is a recognized bias in soil gas measurement. Multiple counting cycles at each point allow assessment of reading stability and improve statistical confidence.

Electret Ionization Chamber Method

Electret ionization chambers integrate ionization produced by radon decay inside a small chamber and report an average concentration from the change in electret voltage. They are valued for their independence from external power, compact size, and suitability for short-duration soil gas measurements, provided temperature dependence and humidity effects are controlled through the desiccant train and correction factors.

Quality Assurance and Data Evaluation

Reliable results depend on instrument calibration against a certified radon chamber, pre- and post-survey background and leak checks, duplicate points at a defined proportion, and avoidance of sampling immediately after heavy rainfall or ground freezing, since soil moisture and pressure changes strongly influence soil gas radon. Survey reports present the point distribution map, statistical summary, comparison against the GB 50325 classification bands, and corresponding engineering recommendations.

Applications

• Pre-construction site classification and anti-radon design input for civil building engineering under GB 50325
• Geogenic radon potential mapping and regional radon hazard zoning for land-use planning
• Environmental impact assessment and baseline surveys for development on granitic, karst, or mineralized terrain
• Verification of soil replacement, ventilation layers, or foundation sealing works on high-radon sites
• Geological and geothermal studies using soil gas radon as a tracer for faults and deep fluid migration
• Supporting investigations of elevated indoor radon in existing buildings and post-remediation effectiveness confirmation

FAQ

What soil radon concentration is considered acceptable for a building site?

No single universal limit exists, but under GB 50325 the key threshold is approximately 10,000 Bq/m³. Sites at or below this level generally require no dedicated anti-radon engineering, while progressively higher bands trigger anti-cracking measures, enhanced foundation waterproofing, and comprehensive structural radon protection for sensitive building occupancies.

How many sampling points are needed and at what depth?

A grid layout with spacing not exceeding roughly 15–20 m and a minimum of about 16 points is typical for a full site, with fewer points permitted for small individual building footprints. Soil gas is normally extracted from a depth of approximately 0.6–0.8 m, below the zone most affected by atmospheric dilution, seasonal drying, and surface disturbance.

How long does a soil radon survey take?

Individual point measurements typically require a few minutes of gas extraction and counting with a portable monitor, so a standard site of 16 to 40 points can usually be completed within one working day, including probe installation, purging, and repeated counting cycles for stability.

What conditions can distort soil radon results?

Heavy recent rainfall, saturated or frozen ground, sharp barometric pressure changes, and strong winds can suppress or enhance soil gas radon readings. Standard practice is to record weather conditions, avoid sampling immediately after significant precipitation, and apply desiccant conditioning to manage humidity effects on the detector.

How does soil radon concentration differ from radon exhalation rate?

Soil radon concentration describes the radon activity per unit volume of gas within soil pores, whereas exhalation rate quantifies the radon flux released per unit area of soil surface per second, in Bq/(m²·s). Both parameters are defined in GB 50325, and exhalation rate testing is required as a supplementary measurement when soil radon concentration reaches the highest classification band.

Which standards govern soil radon determination?

The principal references are GB 50325 for civil building engineering site assessment, GB/T 14582 for standard radon measurement methodology in environmental settings, ISO 11665-11 for soil gas radon test methods, and ASTM D5756 for the determination of radon in soil gas. Instrument calibration should be traceable to recognized radon reference facilities, and testing should be conducted by accredited third-party laboratories with documented quality management systems.