2 BACKGROUND
2.1 About Radon-in-Water
Radon originates from the radioactive decay of
naturally occurring uranium and radium deposits.
Th
ese elements can be found in trace amounts in
almost all soils and rocks. Being a gas, radon can
escape from mineral surfaces and dissolve in ground
water, which can carry it away from its point of
origin. Radon is rarely found in large concentrations
in surface waters, due to its rapid dispersal into the
atmosphere.
High concentrations of groundwater radon prevail in
parts of New England, New Jersey, Maryland,
Virginia, and the mountainous western states of the
U.S. Typical groundwater sources average between
200 and 600 pCi/L of radon. Roughly 10 percent of
public drinking water supplies have concentrations of
over 1,000 pCi/L, and around 1 percent exceed
10,000 pCi/L. Smaller water systems are
disproportionally a
ff
ected by high radon. [Milvy,
EPA]
Radon was
fi
rst noticed in water supplies by J.J.
Th
omson, a pioneer in the science of radioactivity, in
the
fi
rst decade of the 1900s. [Hess, Frame] At
fi
rst,
scientists and doctors believed radioactivity to have
benign, even curative, e
ff
ects on the human body.
Early research linked high radon concentrations to
natural hot springs long thought to have miraculous
powers. But eventually, science came to understand
the dangers of radiation exposure, a
ft
er a number of
serious accidents and fatalities. [Caul
fi
eld]
In the 1950s airborne radon decay products emerged
as the probable cause of high incidences of lung
cancer among underground mine workers. Study of
environmental radioactivity revealed unusually high
groundwater radon concentrations in the vicinity of
Raymond, Maine. [Bell] In the 1960s, scientists began
to investigate the e
ff
ect of ingested and inhaled radon
gas, observing the uptake of radon by digestive
organs and its dispersal through the bloodstream.
[Crawford-Brown] By the 1970s, radon was widely
recognized as a major component of our natural
radiation exposure. By the late 1970s, Maine had
initiated a program to attempt to reduce public
exposure to waterborne radon, having discovered
cases in which groundwater concentration exceeded
1 million pCi/L. [Hess]
Federal action on the problem of radon in drinking
water picked up in the 1980s with a nationwide
program to survey drinking water supplies for
radioactivity and to assess the risk to public health.
Congress directed the Environmental Protection
Agency (EPA) to take action on radioactivity in
drinking water, and in 1991 the EPA o
ffi
cially
proposed a Maximum Contaminant Level (MCL) for
radon in public drinking water of 300 pCi/L.
Th
is
MCL may one day become binding on public water
supplies. [Federal Register, EPA]
2.2 Health Risks Due to Waterborne
Radon
Waterborne radon leads to health risk by two
pathways: inhalation of radon and its decay products
following the release of radon gas from water into
household air, and the direct ingestion of radon in
drinking water.
Th
e risk of lung cancer due to inhaled radon decay
products has been well documented through the
study of underground mine workers.
Th
e cancer risk
due to ingestion, primarily cancer of the stomach and
digestive organs, has been estimated from studies of
the movement of radon through the gastrointestinal
tract and bloodstream. Radon has not been linked to
any disease other than cancer.
Th
e cancer risk from
the inhalation pathway probably far exceeds that
from the ingestion pathway. [Crawford-Brown,
Federal Register]
In a typical house, with typical water usage patterns, a
waterborne radon concentration of 10,000 pCi/L will
yield an average increase to indoor air concentrations
of about 1 pCi/L.
Th
e 10,000:1 ratio, while not to be
considered a hard rule, has been veri
fi
ed through
theoretical models and empirical evidence. [Hess] In
a house with a high radon in water content, air radon
concentrations tend to rise dramatically with water
usage, especially in the vicinity of the water-using
appliance, but decline steadily a
ft
er the water usage
tails o
ff
. [Henschel]
In most houses, waterborne radon is a secondary
source of indoor radon, far exceeded by soil gas
in
fi
ltration. It is an exception, though not a rare one,
that waterborne radon is the major contributor to
elevated radon in air. A homeowner who has
Section 2
Background
13
