DURRIDGE RAD H2O User manual
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RAD H2O
Radon in Water Accessory for the RAD7!
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User Manual!
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INTRODUCTION
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e RAD H2O is an accessory to the RAD7 that enables you to measure radon in water over a concentration
range of from less than 10 pCi/L to greater than 400,000 pCi/L. By diluting your sample, or by waiting for sample
decay, you can extend the method's upper range to any concentration.
e equipment is portable and battery operated, and the measurement is fast. You can have an accurate reading
of radon in water within an hour of taking the sample. e RAD H2O gives results aer a 30 minutes analysis
with a sensitivity that matches or exceeds that of liquid scintillation methods. e method is simple and
straightforward. ere are no harmful chemicals to use. Once the procedure becomes familiar and well
understood it will produce accurate results with minimal effort.
It is assumed that the user has a good, working knowledge of the RAD7. If both the RAD7 and the RAD H2O are
new to the user, then time should be spent learning how to make good measurements of radon in air with the
RAD7 before embarking on radon in water measurements. Instructions for RAD7 operation with the RAD H2O
are given in this manual but, for more detail about the instrument and its use, the reader is referred to the RAD7
manual.
Grateful acknowledgment is made of the signicant contribution to this manual by Stephen Shefsky, who wrote
most of the original NITON RAD H2O manual, much of which is incorporated in this version. However, all
responsibility for the content now rests with DURRIDGE Company."
Introduction 2
TABLE OF CONTENTS!
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INTRODUCTION 2
TABLE OF CONTENTS 3
1 GETTING STARTED 5
1.1 Unpacking 5
1.2 General Safety Instructions 7
1.3 Taking a Look 8
Fig. 1 Aerating a 250mL water sample 8
Fig. 2 Aeration in progress 8
Fig. 3 RAD H2O Schematic 9
1.4 Running a Test 9
1.4.1 Preparing the RAD7 9
1.4.2 Collecting a Sample 10
1.4.3 Setting Up the Equipment 10
1.4.4 Starting the Test 11
Fig. 4 Aerator assembly 11
1.4.5 Finishing the Test 11
1.4.6 Interpreting the Results 11
Fig. 5 RAD H2O printout 12
2 BACKGROUND 13
2.1 About Radon-in-Water 13
2.2 Health Risks Due to Waterborne Radon 13
2.3 Physical Properties of Waterborne Radon 14
2.4 Radon as a Tracer for Groundwater movement 14
2.5 Standard Methods for Radon in Water Analysis 14
2.6 Mitigation Strategies 15
3 RAD H2O TECHNIQUE 16
3.1 The Closed Loop Concept 16
3.2 Desiccant 16
3.3 Purging the System 16
3.4 Background and Residuals 17
4 RESULTS 19
4.1 How Calculation Is Made 19
4.2 Decay Correction 19
Table of Contents 3
4.3 Dilution Correction 19
Fig. 6 Decay Correction Factors 20
5 ACCURACY AND QUALITY CONTROL 21
5.1 Calibration of System 21
5.2 Accuracy and Precision 21
5.2.1 Sampling Technique 21
5.2.2 Sample Concentration 21
5.2.3 Sample Size 21
5.2.4 Purging 21
5.2.5 Aeration 22
5.2.6 Counting Time 22
5.2.7 Temperature 22
5.2.8 Relative Humidity 22
5.2.9 Background Effects 22
5.3 Comparison of RAD H2O with Other Methods 23
5.4 Quality Assurance 23
Fig. 7 Method Comparison 24
6 CARE, MAINTENANCE, and TROUBLESHOOTING 25
6.1 Warning on Pump Direction 25
6.2 Warning on Tipping the Aeration Unit 25
6.3 Frit Maintenance 25
6.4 High Humidity 25
6.5 Foaming 26
Fig. 8 RAD H2O with Bypass Assembly 26
6.6 Technical Support 26
7 DEVIANT SETUPS 27
7.1 Passive DRYSTIK (ADS-1) 27
7.2 Large Drying Unit 27
7.3 Oversized Dome 27
7.4 Extended Cycle Time and Cycle Count 27
7.5 Active DRYSTIK (ADS-2, ADS-3) 28
7.6 Large Water Samples 28
REFERENCES 29
Table of Contents 4
1 GETTING STARTED
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1.1 Unpacking
Examine the RAD H2O case contents and verify that you have all the items shown below. If anything is missing,
Continued on next page."
Section 1 Getting Started 5
RAD H2O Carrying Case!
• Rugged Pelican brand case!
• Dust proof and crushproof!
• Sculpted foam inserts to hold components
250mL Glass Vials!
• 250mL glass vial (x6)!
• Septum cap (x6)
40mL Glass Vials!
• 40mL glass vial (x12)!
• Septum cap (x12)!
• Labels for 40mL glass vials
Continued on next page."
Section 1 Getting Started 6
Retort Stand!
• Small adjustable retort stand!
• Clamp for retort stand
RAD H2O Aerator Cap Kit!
• Aerator Cap (x2)!
• Tubing for 40mL and 250mL vials
Indoor Faucet Adaptor!
• Plastic adaptor!
• 20-inch vinyl tubing
Drying and Charcoal Tube Kit!
• Small drying tubes x 4!
• Tube of activated charcoal x 1
RAD H2O Glass Frit Kit!
• Glass frit (x3)!
• RAD7 inlet filter (x2)!
• Tygon Tubing spacers for 40mL and 250mL vials
1.2 General Safety Instructions
ere is nothing particularly hazardous to the user in the RAD H2O, but care should be taken to make sure that
water never enters the RAD7. e check valve attached to the aerator should never be removed, as it protects the
RAD7 in the event that the tube connections to the instrument are reversed. For more information on preventing
water from entering the RAD7, see Section 6.2, Warning on Tipping the Aeration Unit.
Section 1 Getting Started 7
RAD H2O Tubing Set!
• From RAD7 to aerator cap, with check valve (~24”)!
• From aerator cap to drying tube (3”)!
• From drying tube to RAD7 (~24”)!
• RAD H2O Bypass Assembly!
• Vacuum grease container (use grease when connecting !
tubing to the aerator cap to ensure tight seal)
1.3 Taking a Look
Fig. 1 Aerating a 250mL water sample
Fig. 2 Aeration in progress"
e setup consists of three components: the RAD7
with printer, the water vial with aerator cap, and the
tube of desiccant, which is connected to the aerator
cap and supported by a clamp on the retort stand.
e components are connected to one another using
the included tubing, as shown in Fig. 3 on the
following page.!
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During the ve minutes of aeration, the radon
concentration in the air loop will approach
equilibrium with the remaining radon in the water."
Section 1 Getting Started 8
Fig. 3 RAD H2O Schematic
When the RAD7 is set to Wat250 or Wat-40 protocol, it automatically calculates the radon concentration in a
250mL or 40mL water sample based on the radon concentration of the air entering the instrument. For this
calculation to be performed accurately, the components must be assembled exactly as shown.!
1.4 Running a Test
ese are brief, simple instructions, just to gain an
initial introduction to the technique. A more
thorough treatment follows later in the manual.!
1.4.1 Preparing the RAD7
Before making a measurement, the RAD7 must be
free of radon, and dry. To achieve this, it should be
purged for some time. It is convenient to use the
larger, laboratory drying unit during the initial
purging process, to save the small drying tubes for
the actual measurement.
Hook up the laboratory drying unit to the RAD7
inlet, with the inlet lter in place (see RAD7 manual).
Purge the unit with fresh dry air for ten minutes.
Aer 10 minutes of purging with dry air, push the
[Menu] button and push [ENTER] twice to go into
the status window, and push the right arrow button
twice to see the relative humidity. If it is not yet down
close to 6%, start purging some more. To conserve
desiccant, aer the rst ten minutes or so, you may
connect the RAD7 outlet to the inlet of the laboratory
drying unit, thus forming a closed loop. is will
continue to dry out the RAD7 but will not introduce
more fresh air.
If the RAD7 has not been used for some time, or if it
has been lewithout the small tubing bridge in place
between the air inlet and outlet, then it will take
longer to dry it out, perhaps as much as 30 minutes of
purging, or even more. Once it has thoroughly dried
out, however, just 15 minutes of purging between
measurements will generally be sufficient.!
Section 1 Getting Started 9
1.4.2 Collecting a Sample
Getting a good sample requires care and practice.
Sampling technique, or lack of it, is generally the
major source of error in measuring the radon content
of water. e water sampled must be a) representative
of the water being tested, and b) such that it has never
been in contact with air.
To satisfy (a), make sure that the sample has not been
through a charcoal lter, or been sitting for days in a
hot water tank. To test a well, choose a faucet at the
well, or outside the house, before the water enters any
treatment process. Run the water for an hour, to
make sure that the sample comes freshly from deep in
the well.
To satisfy (b), one of three techniques may be used.
e rst is to attach a tube to the faucet and ll the
vial using the tube. e second is to hold a bowl up to
the faucet so that water overowing from the bowl
prevents the water leaving the faucet from touching
air. e vial is then placed at the bottom of the bowl
and allowed to ll. e third method combines the
rst two, by having a tube attached to the faucet
feeding water to the interior of the vial at the bottom
of the bowl.
Using the third method, above, allow water to
overow freely from the bowl. Take a 250mL vial if
the radon concentration is probably less than 3,000
pCi/L, or 100,000 Bq/m3, or a 40mL vial if it is
probably more. Take samples in both sizes if you have
no idea of the concentration. Place the vial in the
bottom of the bowl, and put the tube end into the
vial. Let the water ow for a while, keeping the vial
full and ushing with fresh water. Cap the vial while
still under the water. Make sure there are no bubbles
in the vial. Tighten the cap.
Remove the vial from the bowl, dry it and
immediately apply a label stating the date, time and
source of the water.
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1.4.3 Setting Up the Equipment
Find the two pieces of Tygon tube (One tube is longer
than the other). In the instrument case, as originally
shipped, the shorter tube is in the 40mL vial
assembled on the aerator in the middle of the case.
With the glass vial removed, the end of the frit should
be 75mm or 3” from the bottom of the aerator cap.
Measure and adjust as necessary. e longer tube is in
the foam at the near le-hand corner of the case
(immediately to the right of the 6th 250mL vial). e
end of the glass frit should be 150mm or 4 7/16” from
the bottom of the aerator cap. Adjust it as necessary.
Pick the tube appropriate to the size of vial
containing the water sample: short for the 40mL vial
and long for the 250mL vial. Push one end onto the
aerator barb, on the side opposite the check valve.
Apply vacuum grease to the two hose barbs on the
top of the aerator cap. is causes a tight seal to be
formed between the tubing and the hose barbs.
Without sufficient vacuum grease, air leakage can
occur, resulting in a low radon in water reading.
With the 3” (7.6cm) of 1/4” ID vinyl tubing, connect
the output of the aerator (without a check valve) to a
small drying tube. Use vacuum grease to improve the
t of the tubing over the hose barbs on the aerator
cap. If one end of the drying tube is pink, that end
should face down, towards to the aerator outlet.
Connect the other end of the drying tube, with 1/8”
ID tubing, to an inlet lter mounted on the RAD7
inlet. e 1/4” to 1/8” adapter makes this connection
easy and secure. Connect the RAD7 outlet to the
check valve on the aerator. e Bypass Assembly may
be added as a precaution against foaming, as
discussed in Section 6.5.
With the system as connected so far, set the RAD7 to
purge for another few minutes. While it is purging,
clamp the small drying tube on the retort stand, thus
supporting it vertically.
Stop purging. On the RAD7, go to Setup Protocol
Wat-40, or Wat250, depending on which size of vial is
being used, and push [ENTER]. It is essential that the
correct protocol be entered here, because this
controls the pumping and counting cycle, and the
calculation according to the size of sample vial. Set
the Format to short. Place the printer on the RAD7.
Make sure the printer has paper. Switch on the
printer. Switch offthe RAD7, then switch it on again.
It will print its identity and a review of the setup.
While the RAD7 is printing the header, insert the
glass frit into the tygon tubing extending from the
cap. Remove the cap from the water sample and lower
the glass frit into the water. Some water will spill
during this procedure. Carefully watch the glass frit to
make sure it does not hit the bottom of the vial; adjust
the position of the tubing if necessary. Screw the
aerator cap onto the vial. e vial can be inserted in a
slot in the RAD H2O case to keep it secure. It must be
upright while aeration is in progress. See Fig. 3 and 4.!
Section 1 Getting Started 10
1.4.4 Starting the Test
Once the RAD7 has nished printing out the header,
go to [Test], [Start] and push [ENTER]. e
pump will run for ve minutes, aerating the sample
and delivering the radon to the RAD7. e system
will wait a further ve minutes. It will then start
counting. Aer ve minutes, it will print out a short-
form report. e same thing will happen again ve
minutes later, and for two more ve-minute periods
aer that. At the end of the run (30 minutes aer the
start), the RAD7 prints out a summary, showing the
average radon reading from the four cycles counted, a
bar chart of the four readings, and a cumulative
spectrum. e radon level is that of the water, and is
calculated automatically by the RAD7. All data,
except the spectrum, is also stored in memory, and
may be printed or downloaded to a PC at any time.
Fig. 4 Aerator assembly
(a) 75 mm for 40mL; 115 mm for 250mL
1.4.5 Finishing the Test
Unscrew the aerator cap, raise the glass frit out of the
water, and set the RAD7 to purge. is will blow
water out of the frit, and also introduce fresh air into
the tubing.
If no more tests are to be analyzed, the equipment
may now be replaced in the case. If there is another
sample for analysis, keep the RAD7 connected as
above, and purging, for at least two minutes. e
laboratory drying unit may then be substituted for
the small drying tube. Continue the purge for
another ten minutes. Check the relative humidity, as
above, and continue the purge until the relative
humidity indication in the instrument drops to 6% or
below. Aer six or seven minutes, the RAD7 air
outlet may be connected to the input of the drying
unit, to form a closed loop, to conserve desiccant.
When the relative humidity is down to 6% or less,
another test may be conducted. Repeat from 1.4.1
above.!
1.4.6 Interpreting the Results
e printout will look similar to the one shown in
Figure 5, on the next page.
ere are two grab sample advisory statements, four
ve-minute cycles and a test summary. e summary
shows the RAD7 run number, the date and time of
the measurement, the serial number of the
instrument, the number of cycles in the test, the
average value, standard deviation, highest and lowest
readings, a bar chart of the complete set of readings,
and a cumulative spectrum.
e radon content of the water, at the time of the
analysis, is the mean value shown in the printout.
is value takes into account the calibration of the
RAD7, the size of the sample vial and the total
volume of the closed air loop, as set up. It is
important that the setup be as specied above, using
the tubing and a small drying tube, as provided.
Deviations from the standard setup may cause errors
in the result.
e nal step is to correct the measured value for
decay of the radon in the water during the time
between taking the sample and analyzing it."
Section 1 Getting Started 11
Fig. 5 RAD H2O printout"
Section 1 Getting Started 12
2 BACKGROUND
2.1 About Radon-in-Water
Radon originates from the radioactive decay of
naturally occurring uranium and radium deposits.
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 affected by high radon. [Milvy,
EPA]
Radon was rst noticed in water supplies by J.J.
omson, a pioneer in the science of radioactivity, in
the rst decade of the 1900s. [Hess, Frame] At rst,
scientists and doctors believed radioactivity to have
benign, even curative, effects 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, aer a number of
serious accidents and fatalities. [Cauleld]
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 effect 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 officially
proposed a Maximum Contaminant Level (MCL) for
radon in public drinking water of 300 pCi/L. 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.
e risk of lung cancer due to inhaled radon decay
products has been well documented through the
study of underground mine workers. 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. 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. e 10,000:1 ratio, while not to be
considered a hard rule, has been veried 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 aer the water usage
tails off. [Henschel]
In most houses, waterborne radon is a secondary
source of indoor radon, far exceeded by soil gas
inltration. 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
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. is test ought to be done before
any attempt to mitigate soil gas inltration,
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. is is known as aeration.
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
xed ratio for a certain temperature. is ratio,
derivable from the Henry's Law constant for radon
dissolved in water, is known as the distribution
coefficient or partition coefficient.
For radon in water at 20 degrees C (68 F) the
distribution coefficient 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 coefficient increases,
rising to 0.51 at 0° C (32° F). At higher temperatures
the distribution coefficient decreases, dropping to
about 0.11 at 100° C (212° F). An empirical
expression for the distribution coefficient 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 signicant
concentrations of uranium and radium. Radon is
continually being created in the ground so that
groundwater oen has high radon content. By
contrast, open water contains very little dissolved
radium. 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 oen 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 H2O.!
2.5 Standard Methods for Radon in Water
Analysis
Several methods have been developed to measure
radon in water. ree of these are Gamma
Spectroscopy (GS), Lucas Cell (LC) and Liquid
Scintillation (LS).
Gamma spectroscopy seeks to detect the gamma rays
given offby 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.
e Lucas Cell method has been in use for decades
for laboratory analysis of radon-222 and radium-226
(via radon emanation). 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 lls. 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)]
e Liquid Scintillation method dates to the 1970s. A
liquid scintillation cocktail is added to the sample in a
25mL glass LS vial. e cocktail draws the radon out
of the water, so that when it decays the alpha particles
scintillate the cocktail. e method uses standard LS
counters, which are highly automated and can count
several hundred samples in sequence without
intervention. 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
[Prichard, Whittaker, Hahn (Method 913.0), Lowry,
Vitz, Kinner, Hess]
In comparison with the above, the RAD H2O offers a
method as accurate as LS but faster to the rst
reading, portable, even less labour intensive, and less
expensive. It also eliminates the need for noxious
chemicals.
!
2.6 Mitigation Strategies
Two main strategies have emerged for the removal of
radon from water. Both of these are applicable to
point-of-entry (POE) water treatment in residences
and small public water supplies.
Granular Activated Carbon (GAC) attempts to lter
the water by adsorbing radon on a charcoal bed that
holds onto the radon until the radon decays. GAC
systems can be effective and relatively inexpensive for
residential use, but can create new problems. As the
radon and its progeny decay in the GAC column,
they give offgamma radiation. e gamma radiation
may be a health concern to residents when the
inuent radon concentration is high, the GAC
column is poorly shielded for high energy radiation,
and the residents are likely to spend signicant
periods of time in the radiation eld. Over time, a
long lived decay product, lead-210, builds up in the
column, which may pose disposal problems in
systems with moderate to high radon concentrations
in the inuent. For that reason GAC is most oen
recommended for inuent concentrations of up to
around 5,000 pCi/L. GAC maintenance is simple and
inexpensive, and the GAC bed has an expected
service life of 5 to 10 years. [Henschel, Lowry, Rydell]
Aeration brings water into contact with a stream of
low radon air, which strips the radon from the water,
then exhausts the radon bearing air to the
atmosphere. Aeration systems offer effective removal
of radon without the buildup of gamma radiation or
waste material, but tend to be substantially more
expensive than GAC to install and maintain in a
residential setting. Aeration can be used over the
entire range of inuent concentrations, though very
high inuent concentration may require a multiple
stage system to reduce the effluent concentration to
acceptable levels. [Henschel, Lowry, NEEP]"
Section 2 Background 15
3 RAD H2O TECHNIQUE
3.1 The Closed Loop Concept
e RAD H2O method employs a closed loop
aeration scheme whereby the air volume and water
volume are constant and independent of the ow
rate. e air recirculates through the water and
continuously extracts the radon until a state of
equilibrium develops. e RAD H2O system reaches
this state of equilibrium within about 5 minutes, aer
which no more radon can be extracted from the
water.
e extraction efficiency, or percentage of radon
removed from the water to the air loop, is very high,
typically 99% for a 40mL sample and 94% for a 250
mL sample. e exact value of the extraction
efficiency depends somewhat on ambient
temperature, but it is almost always well above 90%.
Since the extraction efficiency is always high, we see
little or no temperature effect on the overall
measurement.!
3.2 Desiccant
e RAD H2O requires that the desiccant be used at
all times to dry the air stream before it enters the
RAD7. If the desiccant is not used properly, the
RAD7 may give incorrect radon concentrations, or
may become damaged due to condensation on
sensitive internal components.
For water sample analysis, always use the small
drying tubes supplied, as the system has been
calibrated with these tubes. Do not use the large
drying column as its much larger volume would
cause improper dilution of the radon.
Make it a habit to inspect the RAD7 humidity
reading to be sure the desiccant is and has been
effective through the entire measurement session. All
relative humidity readings during the measurement
should remain below 10%. In the worst case, at least
the rst two counting cycles should be below 10%. If
the relative humidity is higher than that, then the
RAD7 should be purged, see below. See the RAD7
Operator's Manual for more information on
maintaining the desiccant."
3.3 Purging the System
Aer performing a water or air measurement, the
RAD7's internal sample cell will continue to contain
the radon that was measured. If this radon is still
present when you start a new measurement, it will
erroneously inuence the next measurement. is is
of special concern when the radon concentration of
the last measurement was high relative to the next
measurement. To prepare for the next water
measurement, you must remove, as thoroughly as
possible, the radon from the RAD7 and its air
conducting accessories, including the aerator head,
tubes, and desiccant. is procedure is known as
"purging the system."
To purge the system, you must have a source of
radon-free (or relatively radon-free) air or inert gas.
For most occasions ambient air is good enough, but
see below. Put the RAD7 into a purge cycle with the
"Test Purge" command, and allow the RAD7 pump to
ush the clean air through the entire system for at
least 10 minutes. Aer measuring very high radon
concentrations, you should purge the system for at
least 20 minutes. A purge time of 30 minutes should
be long enough to remove almost all the radon aer
measuring a sample at 100,000 pCi/L.
Be sure to allow all the hoses and ttings to ush
thoroughly by keeping them attached during the
purge cycle for at least the rst ve minutes. Also be
sure that the drying tube does not deplete its
desiccant during the purge cycle. If the depleted
(pink) desiccant gets to within 1 inch of the drying
tube outlet, replace the tube with a fresh (blue)
drying tube. Aer the rst two or three minutes of
purging, you may replace the small drying tube with
the large laboratory drying unit, to conserve the small
drying tube desiccant, and continue purging the
system.
Be careful about the air you use to purge! Ambient air
may not be adequately free of radon to properly
prepare the system for a low level sample. e radon
present in the purge air will add unwanted
"background" to the next measurement. For example,
a purge air radon concentration of 4 pCi/L will give
about 4 x 25, or 100 pCi/L additional radon
concentration to the next water result (40mL water
sample). is is too much background to neglect
Section 3 RAD H2O Technique 16
when measuring samples below 1,000 pCi/L, but if
you are measuring only water samples above 1,000
pCi/L, you may consider this amount of error to be
acceptable. To reduce the error due to purge air radon
you may either subtract offthe background from
every measurement, or adopt strategies to reduce the
background to acceptable levels. In any case, for
levels below 1,000 pCi/L you should preferably use
250mL vials when ambient air of 4 pCi/L will give
only 20 pCi/L additional radon concentration to the
next water result.
e best way to determine the background is to
measure a "blank", a water sample containing no
radon. To get radon free water, purchase distilled
water from your local pharmacy, or ll a container
with tap water, and allow the container to stand
closed and undisturbed for 4 weeks or more. e 4
week period allows any radon present in the water to
decay away. Store your radon free water in a closed
air-tight container. Remember that the background
due to purge air radon will change when the air
radon concentration changes, so if you intend to
subtract background you should measure a blank
sample at every measurement session.
An alternative method to determine background is to
make a measurement of the air in sniffmode and
note the count rate in window A, aer 15 minutes.
From a previous printout of a water measurement,
with the format set to medium or long, you can see
the count rate in window A corresponding to the
water radon concentration measured. Typically, for a
250mL vial, 1,000 pCi/L in the water will generate
about 50 cpm in window A. A background count rate
of 0.5 cpm in window A (equivalent to about 2 pCi/L
in air) will then produce an error of 1% in the nal
reading.
e obvious way to reduce background is to purge
with very low radon air. Outdoor air rarely exceeds
0.5 pCi/L at several feet above the ground, so you can
probably get the water background to below 13 pCi/L
by simply using outdoor air to purge. To get even
lower radon air, ll a tank or balloon with outdoor air
and let it age for several weeks. If you are using
compressed air or inert gas, be very careful not to
allow the RAD7 to pressurize, as this may cause
internal damage to the pump or seals.
Another method to reduce background is to use
charcoal adsorption to clean the remaining radon
from the system following the purge. A small column
containing 15 grams of activated carbon can remove
up to 98% of the remaining radon from the RAD
H2O system when connected in a closed loop. is
will reduce the system's radon to truly negligible
levels for the most accurate low level radon in water
measurement. e charcoal lter works best if you
use it only aer a complete purge with low radon air,
which avoids overloading the lter with radon. If the
charcoal lter becomes badly contaminated with
radon it can give offsome of the radon and actually
increase the background aer a purge. Store the
charcoal lter with the end caps installed to allow the
lter to "self-clean" by waiting for adsorbed radon to
decay over several weeks time. Always keep the
charcoal dry by using it in conjunction with a drying
tube, since water vapor can adversely affect charcoal's
capacity to adsorb radon.
Even if you choose not to use fancy methods to
reduce the background, you should always purge the
system between samples. It is much better to purge
with ordinary room air than not to purge at all. In
any case, it is also necessary to purge to remove any
accumulated water vapor from the system, and bring
the relative humidity back down to close to 5%.!
3.4 Background and Residuals
Purge air is one among several causes for background
counts in the RAD H2O. e most signicant other
causes are radon daughters and traces of radon le
from previous measurements. e RAD7 has the
unusual ability to tell the difference between the
"new" radon daughters and the "old" radon daughters
lefrom previous tests. Even so, a very high radon
sample can cause daughter activity that can affect the
next measurement.
Aer the high radon sample has been purged from
the system, its decay products stay behind until they
decay away. e polonium-218 isotope decays with a
3 minute half-life. In the 30 minutes following the
purge, the polonium-218 decays to about a
thousandth of its original activity. at still leaves a
background of 100 pCi/L aer a 100,000 pCi/L
sample.
In addition to polonium-218, the RAD7 is sensitive
to polonium-214, which can give counts for several
hours aer the radon has been removed. e RAD7
uses alpha energy discrimination to reject
polonium-214 counts from a measurement, but a
very small percentage of the polonium-214 decays
slip past the discriminator. is can add background
to a measurement that follows a high radon sample.
Section 3 RAD H2O Technique 17
e solution to the problem of daughter activity is
time. Simply wait for the activity to decay away.
Check the background with a blank sample. If it is
still too high, keep waiting, and keep checking. e
length of time you will wait depends on just how
much radon your high radon sample had, and just
how much background you are willing to tolerate
before the next measurement. If you expect the next
sample to be high also, you may want to go ahead
with the next measurement right away, considering a
small amount of background acceptable.
In the case of extremely high radon samples, you may
develop a background that is more persistent than
daughter activity. at is possibly due to off-gassing
of residual radon that has absorbed into internal
surfaces. In particular, rubber and plastic parts can
absorb a small fraction of the radon that passes
through the system. A small fraction of a very large
amount can still be a signicant amount. e radon
may desorb from these materials over many hours. In
the worst case you may have to allow the system to sit
idle for a day or more for the absorbed radon to
nish leaking out of these materials, then purge the
system again to remove the radon. A radon
concentration high enough to cause a concern of this
kind is very rare in natural ground water, but is
possible in articial radon sources such as radium
crocks or "Revigators".
Sustained counting of very high radon concentrations
can lead to the buildup of long lived lead-210
contamination of the RAD7's alpha detector. is
possibility is described in the RAD7 Operator's
Manual. It suffices to say that the RAD7's ability to
distinguish alpha particles by energy makes it far less
susceptible to the build up of lead-210 related
background than other radon monitors."
Section 3 RAD H2O Technique 18
4 RESULTS
4.1 How Calculation Is Made
e RAD7 calculates the sample water concentration
by multiplying the air loop concentration by a xed
conversion coefficient that depends on the sample
size. is conversion coefficient has been derived
from the volume of the air loop, the volume of the
sample, and the equilibrium radon distribution
coefficient at room temperature. For the 40mL
sample volume the conversion coefficient is around
25. For the 250mL sample volume the conversion
coefficient is around 4.
e RAD7 does not presently make any correction
for the temperature of the water sample. In theory,
such correction would slightly improve the analytical
accuracy for the larger (250 mL) sample volume, but
would make little or no difference for the smaller
sample volume.!
4.2 Decay Correction
If you collect a sample and analyze it at a later time
(rather than immediately), the sample's radon
concentration will decline due to the radioactive
decay. You must correct the result for the sample's
decay from the time the sample was drawn to the
time the sample was counted. If the sample is
properly sealed and stored, and counted within 24
hours, then the decay corrected result should be
almost as accurate as that of a sample counted
immediately. Decay correction can be used for
samples counted up to 10 days aer sampling, though
analytical precision will decline as the sample gets
weaker and weaker.
e decay correction is a simple exponential function
with a time constant of 132.4 hours. (e mean life of
a radon-222 atom is 132.4 hours, which is the half-
life of 3.825 days multiplied by 24 hours per day
divided by the natural logarithm of 2.) e decay
correction factor (DCF) is given by the formula DCF
= exp(T/132.4), where T is the decay time in hours.
You will notice that decay times of under 3 hours
require very small corrections, so you can ordinarily
neglect the decay correction for samples counted
quickly.
To correct your result back to the sampling time,
multiply it by the decay correction factor (DCF) from
the chart, Figure 6 opposite.!
4.3 Dilution Correction
If you intend to count samples that have very high
radon concentrations, you may wish to dilute the
sample by a xed ratio, then correct the result back to
its undiluted concentration.
Example: You take a 4mL sample and dilute it with
36mL of distilled water in a 40mL sample vial.
Overall, this would be a 10:1 ratio of nal volume to
initial volume, so you must multiply the result by 10
to correct for the dilution. If the RAD H2O reports a
result of 9,500 pCi/L for the 10:1 diluted sample, then
the original concentration must have been 10 X
9,500, or 95,000 pCi/L. Great care must be taken in
this process to avoid loss of radon from the sample.
e syringe should be lled and relled several times
from under water that is a true sample, see method 2
in section 1. e 40mL vial should contain 36 mL of
radon-free water. 4mL of the undiluted sample
should be injected slowly at the bottom of the vial,
and the vial quickly capped. Any air bubble should be
very small."
Section 4 Results 19
!
Fig. 6 Decay Correction Factors"
Section 4 Results 20
Hours
DCF
Hours
DCF
Hours
DCF
Hours
DCF
Hours
DCF
0
1.000
1
1.008
2
1.015
3
1.023
4
1.031
5
1.038
6
1.046
7
1.054
8
1.062
9
1.070
10
1.078
11
1.087
12
1.095
13
1.103
14
1.112
15
1.120
16
1.128
17
1.137
18
1.146
19
1.154
20
1.163
21
1.172
22
1.181
23
1.190
24
1.199
25
1.208
26
1.217
27
1.226
28
1.236
29
1.245
30
1.254
31
1.264
32
1.273
33
1.283
34
1.293
35
1.303
36
1.312
37
1.322
38
1.332
39
1.343
40
1.353
41
1.363
42
1.373
43
1.384
44
1.394
45
1.405
46
1.415
47
1.426
48
1.437
49
1.448
50
1.459
51
1.470
52
1.481
53
1.492
54
1.504
55
1.515
56
1.526
57
1.538
58
1.550
59
1.561
60
1.573
61
1.585
62
1.597
63
1.609
64
1.622
65
1.634
66
1.646
67
1.659
68
1.671
69
1.684
70
1.697
71
1.710
72
1.723
73
1.736
74
1.749
75
1.762
76
1.775
77
1.789
78
1.802
79
1.816
80
1.830
81
1.844
82
1.858
83
1.872
84
1.886
85
1.900
86
1.915
87
1.929
88
1.944
89
1.959
90
1.973
91
1.988
92
2.003
93
2.019
94
2.034
95
2.049
96
2.065
97
2.081
98
2.096
99
2.112
100
2.128
101
2.144
102
2.161
103
2.177
104
2.194
105
2.210
106
2.227
107
2.244
108
2.261
109
2.278
110
2.295
111
2.313
112
2.330
113
2.348
114
2.366
115
2.384
116
2.402
117
2.420
118
2.438
119
2.457
120
2.475
121
2.494
122
2.513
123
2.532
124
2.551
125
2.571
126
2.590
127
2.610
128
2.629
129
2.649
130
2.669
131
2.690
132
2.710
133
2.731
134
2.751
135
2.772
136
2.793
137
2.814
138
2.836
139
2.857
140
2.879
141
2.901
142
2.923
143
2.945
144
2.967
145
2.990
146
3.012
147
3.035
148
3.058
149
3.081
150
3.105
151
3.128
152
3.152
153
3.176
154
3.200
155
3.224
156
3.249
157
3.273
158
3.298
159
3.323
160
3.348
161
3.374
162
3.399
163
3.425
164
3.451
165
3.477
166
3.504
167
3.530
168
3.557
169
3.584
170
3.611
171
3.638
172
3.666
173
3.694
174
3.722
175
3.750
176
3.778
177
3.807
178
3.836
179
3.865
180
3.894
181
3.924
182
3.954
183
3.984
184
4.014
185
4.044
186
4.075
187
4.106
188
4.137
189
4.168
190
4.200
191
4.232
192
4.264
193
4.296
194
4.329
195
4.361
196
4.395
197
4.428
198
4.461
199
4.495
200
4.529
201
4.564
202
4.598
203
4.633
204
4.668
205
4.704
206
4.739
207
4.775
208
4.811
209
4.848
210
4.885
211
4.922
212
4.959
213
4.997
214
5.035
215
5.073
216
5.111
217
5.150
218
5.189
219
5.228
220
5.268
221
5.308
222
5.348
223
5.389
224
5.429
225
5.471
226
5.512
227
5.554
228
5.596
229
5.638
230
5.681
231
5.724
232
5.768
233
5.811
234
5.855
235
5.900
236
5.945
237
5.990
238
6.035
239
6.081
Table of contents
