Environmental Engineering Reference
In-Depth Information
mulation and transport. Heavy rains may also affect radon transport into
buildings by causing a piston-like displacement of soil gas around building
perimeters, forcing soil gas to flow inward. Frozen ground and concrete
roadways may cause a similar phenomenon. In tropical climates, rainy
weather contributes to elevated radon levels indoors.
Several investigators have proposed that transient atmospheric pressure
changes associated with meteorological conditions can affect radon transport
into buildings. Studies conducted in Florida have shown significantly
increased indoor radon concentrations associated with semidiurnal atmo-
spheric pressure changes in slab-on-grade houses built on low permeability
soils. Peak concentrations occurred when other sources of house depressur-
ization or pressurization were small; i.e., when houses as a whole are under
neutral pressure relative to the outdoors.
In general, radon transport into buildings increases with increasing neg-
ative pressurization relative to the outdoors. Such negative pressurization is
produced naturally by the stack effect and increased wind speeds. The stack
effect increases with increasing temperature differentials between the inside
and outside of buildings. As a consequence, highest radon concentrations in
U.S. housing are thought to occur in the winter heating season when the
thermal stack effect is the strongest. However, in some studies, highest
concentrations were observed during summer months, ostensibly due to
occupant operation of heating/cooling systems and strong seasonal varia-
tion in radon emanation potentials in the soil (soil gas radon concentrations
were higher in summer than winter at several sites studied).
Natural forces are not the only cause of building depressurization. In
mechanically ventilated schools and other large buildings, depressurization
occurs when more air is exhausted from building spaces than is brought into
the building through outdoor intakes. In dwellings, depressurization may
result from the use of heating systems that require chimneys to exhaust flue
gases from furnaces and fireplaces. Depressurization in basements can result
from leaky furnace fan housings and cold air returns, and in crawlspaces
from leaky air returns. Mechanical depressurization can significantly affect
the flow of soil gas into buildings and, as a consequence, increase indoor
radon levels.
B.
Groundwater
Though most radon is transported into buildings through pressure-driven
soil gas flows, groundwater serves as a limited, but sometimes significant,
radon source in some geographical/geological regions. Radon has been
reported to occur in well water, with concentrations ranging from a few
100 pCi/L to approximately 30,000 pCi/L. Highest concentrations are asso-
ciated with drilled wells, particularly in areas with granitic bedrock. Radon
is released from water when (1) temperature is increased, (2) pressure is
increased, and (3) water is aerated. Optimum conditions for radon release
and exposure occur during showering. Water with a radon concentration of
