discovered elevated air concentrations, and whose
house uses private well water, should test the water
for radon content to assess the water's contribution to
the airborne radon.
Th
is test ought to be done before
any attempt to mitigate soil gas in
fi
ltration,
particularly if other wells in the area have been found
to have radon. [Henschel]
2.3 Physical Properties of Waterborne
Radon
Radon gas is mildly soluble in water. But, being a gas,
it is volatile. It tends to leave the water upon contact
with air.
Th
is is known as aeration.
Th
e rate of radon transfer from water to air increases
with temperature, agitation, mixing, and surface area.
In household water usage, showers, baths,
dishwashers, laundries, and toilets all provide
adequate aeration to release a high percentage of the
water's radon content into household air. [Prichard]
In principle, the radon will continue to be released
from water as the aeration process continues, until a
state of equilibrium develops. According to Henry's
Law of dilute solutions, equilibrium will occur when
the water concentration and air concentration reach a
fi
xed ratio for a certain temperature.
Th
is ratio,
derivable from the Henry's Law constant for radon
dissolved in water, is known as the distribution
coe
ffi
cient or partition coe
ffi
cient.
For radon in water at 20 degrees C (68 F) the
distribution coe
ffi
cient is about 0.25, so radon will
continue to release from the water until the water
concentration drops to about 25 percent of the air
concentration. Remember that as the radon leaves the
water into the air it raises the air concentration and
lowers the water concentration. At lower
temperatures the distribution coe
ffi
cient increases,
rising to 0.51 at 0° C (32° F). At higher temperatures
the distribution coe
ffi
cient decreases, dropping to
about 0.11 at 100° C (212° F). An empirical
expression for the distribution coe
ffi
cient of radon in
water as a function of temperature can be found in
[Weigel].
2.4 Radon as a Tracer for Groundwater
movement
Soil and rock typically contain signi
fi
cant
concentrations of uranium and radium. Radon is
continually being created in the ground so that
groundwater o
ft
en has high radon content. By
contrast, open water contains very little dissolved
radium.
Th
at, together with the proximity of the
water surface, means that the background
concentration of radon in sea and lake water far from
land is very low.
Radon, then, with its 4-day half life, is an almost
perfect tracer for measuring and monitoring the
movement of ground water into lake and sea water
along the shore [Lane-Smith, Burnett].
While open water monitoring o
ft
en requires
continuous, fast-response radon measurement at
high sensitivity (as provided by the RAD AQUA
[www.durridge.com]), for ground water in situ it is
usually more convenient to use the RAD H
2
O.
2.5 Standard Methods for Radon in Water
Analysis
Several methods have been developed to measure
radon in water.
Th
ree of these are Gamma
Spectroscopy (GS), Lucas Cell (LC) and Liquid
Scintillation (LS).
Gamma spectroscopy seeks to detect the gamma rays
given o
ff
by radon's decay products from a closed
container of radon bearing water. While simple in
concept, this method lacks the sensitivity to detect
radon at the lower levels now considered important.
Th
e Lucas Cell method has been in use for decades
for laboratory analysis of radon-222 and radium-226
(via radon emanation).
Th
e LC method tends to be
somewhat labor intensive, using a complicated
system of glassware and a vacuum pump to evacuate
a Lucas (scintillation) cell, then bubble gas through
the water sample until the cell
fi
lls.
Th
e cell is then
counted by the usual technique. In the hands of a
skilled technician this method can produce accurate,
repeatable measurements at fairly low concentrations.
[Whittaker, Krieger (Method 903.1)]
Th
e Liquid Scintillation method dates to the 1970s. A
liquid scintillation cocktail is added to the sample in a
25mL glass LS vial.
Th
e cocktail draws the radon out
of the water, so that when it decays the alpha particles
scintillate the cocktail.
Th
e method uses standard LS
counters, which are highly automated and can count
several hundred samples in sequence without
intervention.
Th
e EPA has determined that the LS
method is as accurate and sensitive as the LC
method, but less labor intensive, and less expensive.
Section 2
Background
14
