Transport Mechanism #2 – Concentration Gradient Diffusion (rock & soil)

Even if no air is moving through the soil into a house, radon can enter through a process known as concentration gradient diffusion whereby high concentrations of radon can passively diffuse through the rock/soil and through the foundation below grade. 

In this transport mechanism, the radon will move from higher concentration to lower concentration areas of the house via:

• Cracks and holes in the foundation (molecular diffusion of gas through a mixture of gases).
• Solid concrete wall or slabs (molecular diffusion through a porous solid).

The highest radon concentrations will be nearest to the radium – the source. In uniform soil, concentrations will generally decrease as the distance from the radium increases. The more porous the soil, the faster the radon will diffuse bringing more uniformity to the soil concentrations. The tighter the soil, the higher the radon concentrations will be near the radium source, but the concentration will drop more rapidly away from the source. Radon concentrations decrease quickly as the surface of the ground is approached. At ground level, the atmospheric concentration is very low, thus the concentration gradient is largest in that direction.  If the surface covering has a low resistance to radon diffusion, then radon transport by diffusion will increase. For this reason, soil gas radon concentrations are often higher after a substantial rain or snowfall, or beneath concrete patios, driveways, parking lots, and slab floors.

Diffusion, however, is not usually the primary entry route for radon into buildings. If diffusion through the solid concrete and cracks and holes in the concrete were the only transport mechanism, a typical concrete slab would reduce the flux from bare earth by a factor of 25 to 50. It would take only 1/3 to 2/3 of a cubic foot of 1,000 pCi/L soil gas per hour entering the building through cracks and holes to equal the amount entering by diffusion through a 1,600 square foot slab (assuming an emanation rate from the concrete surface of .02 to .04 pCi/M2/sec and an emanation rate from the earth under it of 0.7 pCi/M2/sec).