there (Tanner 1980). In addition to this direct process, a gas is partitioned between the air and water in the pore. This partitioning is described by Henry's law in terms of the Oswald coefficient, K:

where Cw and Ca are the radon concentrations by volume (Bq m-3) in the water and air, respectively. The Oswald coefficient varies inversely with temperature. At 10 °C, KRn = 0.3; it increases to about 0.5 near 0 °C (Lewis and others 1987). If the soil or bedrock is completely saturated with water, all the available radon will be dissolved in the water.

Migration of radon in soil gas is controlled by two processes: molecular diffusion and advective flow. Diffusion is the process whereby molecules migrate toward regions with lower concentrations. Radon concentrations in soil gas are typically 40,000 Bq m-3 and concentrations 10 to 100 times this value are not uncommon. The main reason for this is that the radon atoms are confined within a small volume defined by the pore space between the soil grains. Thus, radon will preferentially diffuse toward regions that have lower concentrations, such as caves, tunnels, buildings, and the atmosphere.

Advective flow is controlled by pressure differences. Air will flow toward locations with lower pressure, and changes in atmospheric pressure can force air into or out of the ground. Very often, the air inside a building is warmer than air in the soil that is in contact with the building. This temperature difference causes a pressure gradient that draws air containing high concentrations of radon into the structure. Wind—as well as airflow from a fan, furnace, or fireplace—can also reduce pressures inside a building, compared with the pressures in the soil adjacent to the building foundation. These processes constitute the primary reason that radon enters and may be present in buildings at higher concentrations than in ambient air.

The water supply can also contribute to indoor radon. When water leaves a faucet, dissolved gases are released. This process is increased by mechanical sprays during a shower or by the heating and agitation that occur during laundering, washing, and cooking. The increase in the indoor radon concentration due to radon release from indoor water use is described by the transfer coefficient:

where (a) is the average increase of the indoor radon concentration that results from using water having an average radon concentration of w. The various sources of radon and the resulting radiation exposure pathways are shown in figure 1.2.

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