Radon in Soil

Radon-222 is continuously generated wherever radium-226 exists. In soil and rock, ²²⁶Ra is embedded in the crystal lattices of minerals — substituting for barium, strontium, or calcium, or adsorbed onto grain surfaces.

When a ²²⁶Ra atom undergoes alpha decay (half-life 1600 years), it emits a 4.78 MeV alpha particle and recoils as a newly formed ²²²Rn atom. This recoil, driven by conservation of momentum, propels the radon atom in the opposite direction with an energy of about 86 keV.

The recoil range — the distance the radon atom travels before stopping — is typically 20–70 nanometers in mineral crystals (shorter in denser minerals like zircon, longer in low-density materials like clays). Whether the radon atom ends up in the pore space (where it can migrate) or stays trapped in the solid grain depends entirely on where the parent radium sat and in which direction the recoil occurred.

The emanation power (or emanation coefficient, ε) is the fraction of radon atoms generated in a material that successfully escape into the pore space. It is dimensionless, ranging from 0 (no escape) to 1 (complete escape), typically expressed as a percentage.

Only radon atoms born within one recoil range (~20–70 nm) of a grain surface, and recoiling in the right direction, can escape. For a spherical grain of radius r, the escapable shell volume is proportional to the surface area, so emanation decreases with increasing grain size.

Typical values: fine-grained soils (clays, silts) show emanation coefficients of 20–70%, while coarse sands and gravels may show 10–30%. Crystalline bedrock can be as low as 5–15%.

Grain size: Smaller grains have higher surface-to-volume ratio, increasing the fraction of radon that can escape. A 10 µm grain may have ε ~ 50%, while a 1 mm grain might be ~10%.

Moisture content: Water in pore spaces stops recoiling radon atoms more effectively than air, preventing them from re-entering adjacent grains. Moderate moisture (20–25% saturation) can double or triple emanation compared to dry soil. At very high saturation (>80%), water-filled pores block radon diffusion, reducing net transport.

Mineralogy and radium distribution: If radium is concentrated on grain surfaces (adsorbed or as coatings), emanation is higher. If radium is locked deep inside resistant minerals (zircon, monazite), emanation is lower.

Porosity and packing: Loose, high-porosity soils allow more radon to escape into interconnected pore spaces. Compacted or cemented soils reduce emanation.

Once in the pore space, radon decays with a 3.82-day half-life. In undisturbed soil at equilibrium, radon activity concentration in soil gas (C_soil) is determined by the balance between generation (from radium) and loss (by decay and transport).

For static soil with no advective transport, the steady-state radon concentration is approximately: C_soil = (ε × ρ_bulk × A_Ra × λ_Rn⁻¹) / n, where ε is emanation coefficient, ρ_bulk is bulk soil density (kg/m³), A_Ra is radium-226 specific activity (Bq/kg), λ_Rn is radon decay constant (2.1×10⁻⁶ s⁻¹), and n is porosity (fraction).

Typical soil gas radon levels range from 1,000 to 100,000 Bq/m³, often 10–1000 times higher than indoor air. High-radium soils can exceed 500,000 Bq/m³.

For most buildings, underlying soil is the largest radon contributor. The World Health Organization estimates that soil accounts for about 75% of indoor radon on average, with building materials (~15%), outdoor air (~5%), and water (~5%) making up the rest.

Soil provides both the radon reservoir (high concentration in pore spaces) and the driving force (pressure differentials pull soil gas into buildings). Even moderate-radium soils can supply high indoor radon if the soil is permeable and the building has a strong depressurization (stack effect, HVAC imbalance, wind).

By contrast, building materials typically contribute a smaller, steady background (5–40 Bq/m³) because they are dry, compacted, and emit radon primarily by diffusion into the room. Water-borne radon is significant only in high-radon groundwater areas.

Radon concentration in soil gas typically increases with depth. Near the surface, radon is lost by diffusion to the atmosphere and decay during transport, resulting in lower concentrations.

At depths of 0.5–2 meters, radon concentrations stabilize and often reach a maximum, representing equilibrium between generation, decay, and minimal atmospheric loss.

Below 2–3 meters, concentrations may plateau or even decrease if soil moisture increases (blocking diffusion) or if radium content declines with depth.

Seasonal variations are significant: winter measurements can be 2–5 times higher than summer due to frozen surface soil (reduced atmospheric loss), higher soil moisture in spring, and snow cover acting as a diffusion barrier.

Soil gas radon is measured by driving a hollow probe 60–120 cm into the ground, purging dead volume, and drawing a sample into a radon detector (scintillation cell, ionization chamber, or continuous monitor).

ASTM C1177 and ISO 11665-11 provide standard protocols. Key quality controls: probe depth (shallow measurements are unreliable), purge volume (3–5× probe volume), and equilibration time (a few minutes for most detectors).

Typical measurement depths: 0.6–1.0 m for residential assessments; deeper (1–2 m) for bedrock or low-permeability soils.

Soil gas radon alone does not predict indoor radon — permeability matters equally. A site with 50,000 Bq/m³ soil gas but low permeability may have lower indoor radon than a site with 10,000 Bq/m³ and high permeability.