United States
Environmental Protection
Agency
National Kisk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-96/010
March 1996
& EPA Project Summary
Test Cell Studies of Radon Entry
A. D. Williamson, C. S. Fowler, and S. E. McDonough
Although slab edge detail and fill
composition are logical factors to con-
sider in radon resistant construction,
little research has documented their ef-
fects on radon entry into slab-on-grade
structures. This study was conducted
to contrast the effectiveness of slab-in-
stem wall (SSW) with floating slab (FS)
construction practices, to measure ra-
don transport and entry for model test-
ing, to develop protocols relevant to
depressurized radon measurements,
and to determine the effect of high ra-
dium fill soil on indoor radon concen-
trations. The effects of the slab edge
details were investigated in two test
cells built on 6- x 6-m slabs placed
over 8-pCi/g radium soil. The high ra-
dium fill study was conducted on two
3- x 3-m poured concrete foundations
in which different depths of fill soils
could be placed and a movable build-
ing put on top. The native soil con-
tained about 0.2 pCi/g radium. The fill
soils ranged from 0.2 to 33 pCi/g ra-
dium. The indoor radon concentrations
in the FS cell were 3.5 times higher
than those in the SSW cell. These re-
sults agreed with predictions by a ra-
don entry and transport (RAETRAD)
model. Whole building stresses and
slab area and crack length radon entry
were measured, and they yielded com-
parable results. Experiments in the fill
study suggest that the amount of ema-
nating soil radium is a good predictor
for radon entry into a structure, indi-
cating that elevated radium fill soil can
contribute significantly to indoor radon
concentrations.
This Project Summary was developed
by the National Risk Management Re-
search Laboratory's Air Pollution Pre-
vention and Control Division, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The purpose of the Florida Radon Re-
search Program (FRRP) was the develop-
ment of building codes and standards for
radon resistant buildings and construction
standards for mitigation of radon in exist-
ing buildings. A working draft of proposed
building standards was developed based
on fundamental studies. Emphases shifted
to field evaluation or validation of specific
areas of the proposed standards. These
demonstration studies centered primarily
on two research components: intense,
long-range, controlled studies in a few re-
search structures; and less comprehen-
sive, shorter duration, less controlled mea-
surements in many houses enrolled in new
house evaluation studies. The study re-
ported here is part of the research house
component.
One objective of these research house
studies was to validate the effectiveness
of "barrier" construction features, specifi-
cally contrasting a floating slab (FS) with
a perimeter crack with a slab-in-stem wall
(SSW) with no perimeter crack. Another
objective was to measure the radon trans-
port and entry required for model testing
and validation. The primary model being
tested was Rogers & Associates Engi-
neering (RAE) Corporation's radon entry
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and transport (RAETRAD) code. The third
objective was the development of trans-
ferable protocols relevant to depressur-
ized radon measurements. The main pro-
tocol under investigation in this study was
the "stress" test, although several differ-
ent applications of it were included: whole
building, slab area, crack lengths, and
short- versus long-term. A final objective
of this work was to determine the effect of
high radium fill soil placed over a low
radium native soil on indoor radon con-
centrations.
In December 1990, the Florida Depart-
ment of Community Affairs (DCA) con-
structed two test cells on the site of the
Florida Institute of Phosphate Research
(FIPR) in Bartow, FL. These 6- x 6-m
structures were built over a spoil bank of
sand tailings reclaimed from phosphate
mining operations. The soil had moder-
ately low permeability and moderate soil
radium levels. The soil gas radon concen-
trations measured were 3200-3500 pCi/L
at 0.9-m depths prior to construction with
the soil relatively dry and 5000-12,000 pCi/
L at 1.2-m depths after construction when
the soil was fairly wet. One of the cells
was built with an FS with a significant
edge crack, and the second was con-
structed on an SSW foundation.
In January 1992, plans and construc-
tion of a movable building were begun.
Two 3- x 3-m foundations were con-
structed. The site had never been mined,
and the native soil was sand with less
then 1 pCi/g soil radium. The site was
underlain by relatively impermeable clay
that prevented soil gas radon produced
from deeper and potentially higher radium
sources from making its way into the sur-
face sand. Both of these 0.6 m deep
poured concrete foundations were built in
and surrounded by this low radon poten-
tial sand. In one foundation (F1) an addi-
tional 0.3 m of native soil was placed,
allowing for 0.3 m of fill soil. In the second
foundation (F2) 0.6 m of fill soil could be
added. The movable building had a pair
of fixed floor cracks 1.2 m long and 6 mm
wide, 0.3 m from the side walls of the
building, and a 75-mm diameter hole in
the center of the floor.
Procedure
Sites and Structures
FIPR Cells-
Test cell 1 was built with a floating slab,
and test cell 2 had an SSW construction.
No polyethylene vapor barrier was placed
under either of the 100 mm slabs poured
in these two cells. The cells' superstruc-
tures were wood frames with stucco over
plywood cladding. A 25-mm layer of spray
urethane foam was applied inside the ply-
wood surface, in the walls and under the
roof, providing both thermal insulation and
a considerable degree of air tightness.
Each cell had only a single steel door with
a foam core opening into the unit with
positive magnetic seals on all four edges.
Neither cell was built with any other fixed
or planned opening. The bottom of the
slabs was just above the outside grade.
Each test cell was later equipped with a
wall-mounted ductless split heat pump for
space conditioning. All of the penetrations
were sealed with spray urethane and/or
caulk and were leak-tested to ensure that
the sealing was adequate and complete.
Leak tests were performed by depressur-
izing the whole structure, and using chemi-
cal smoke inside the structure to detect
infiltration sources.
IMC Movable Cell—
A "pad" of relatively uniform sandy soil
was mounded in a field that had never
been mined. Two 3- x 3-m foundations
were dug in this pad about 3 m apart.
Concrete stem walls, 0.6 m high and 150
mm thick, were poured in each founda-
tion. Polyvinyl chloride (PVC) access holes
and notches for running the sampling lines
were placed prior to the pouring. The mov-
able cell itself was constructed of 1.2-m
panels made of 0.8-mm aluminum skin
bonded to both sides of a 75-mm thick
extruded polystyrene (EPS) core. The cell's
only major opening was a 3068 insulated
steel door with magnetic weatherstripping.
The cell was equipped with a wall-mounted
ductless split heat pump. The refrigerant
lines and electrical wires fed through a
single opening in the back wall which was
sealed around the feed-through. The floor
of this movable cell was constructed of
panels 75 mm thick, but the EPS core
was only 64 mm thick. Sheets of 11-mm
plywood were placed on top of the EPS,
and the 0.8 mm aluminum skin was
bonded to the EPS on the bottom and to
the plywood on the top. In the floor, 0.3 m
from each of the two sides, was placed a
1.2 m long fixed floor fault 6 mm wide. In
the center of the floor was a 75-mm diam-
eter hole with a 100- x 75-mm closet flange
placed on top and bottom. The two 1.2-m
edge faults were to simulate perimeter
cracks, and the center hole was to repre-
sent a plumbing penetration.
Measurements
The critical measurements in the stud-
ies were those determining radon concen-
trations and differential pressures.
Radon Measurements—
Most of the indoor radon was measured
in the FIPR test cells with continuous ra-
don monitors (CRMs). Generally, for in-
door radon concentration measurements,
the CRMs were set up to count for hour
intervals and store those counts for later
processing. In the IMC movable cell, the
indoor radon concentrations were also
measured with CRMs, but the counts were
fed directly into the data acquisition sys-
tem (DAS) hardware and into a 286 com-
puter for storage. Another form of critical
radon concentration that was measured
was grab sampling. A filtered sample of
air was drawn into and sealed in a flask or
cell that had a zinc sulfide phosphor coat-
ing on its interior surface. The flasks were
generally counted about 4 hr after filling to
allow the short-lived radon decay prod-
ucts to reach equilibrium with the radon.
Correction factors were applied to the
counting results to compensate for decay
during the time between collection and
counting. In both continuous monitoring
and grab sampling, calibration coefficients
were determined for the instruments and
flasks used by exposure to known con-
centrations at the National Air and Radia-
tion Environmental Laboratory (NAREL)
in Montgomery, AL. Examples of circum-
stances in which grab sampling was the
measurement of choice include soil gas
sampling, sub-slab sampling, and mea-
surements made during the "stress" tests
in which the test cells were depressurized
at different differential pressures for given
time intervals, and the effect on indoor
radon concentrations were monitored.
Differential Pressures and Other
Measurements—
The third type of critical measurement
that was made was that of differential
pressures. These measurements were
usually made using electronic digital mi-
cromanometers. Examples of differential
pressure measurements taken during the
course of these studies include indoor/
outdoor pressures, sub-slab/outdoor pres-
sures, soil/outdoor pressures, stem wall
block core hole/outdoor pressures, and
pressure drops across an orifice to deter-
mine flows when a depressurizing/pres-
surizing fan was active. A Campbell Sci-
entific, Inc., (CSI) 21X data logger was
installed in one of the FIPR test cells for
most of the study period. At first it was
used primarily to monitor indoor and out-
door temperatures and some of the differ-
ential pressures on a continuous, rather
than an episodic, basis as with the
micromanometers. Later in the project, af-
ter all of the components of the weather
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station were installed, the data logger also
stored information on wind speed and di-
rection, barometric pressure, solar flux den-
sity, the outdoor air temperature and rela-
tive humidity, and rainfall. The data from
the CSI 21X were retrieved on a schedule
varying from once a day to once every
two weeks. In the movable cell, a more
powerful DAS was set up. It monitored
the pressure drop across the orifice and
various pressure differentials, up to 16
thermocouples, three CRMs, and other
inputs. Besides receiving, reducing, and
storing the data, the DAS also controlled
the zeroing functions of the pressure trans-
ducers, the switching of the multiplexed
transducers and CRMs, and the flushing
of the scintillation cells on the quasi-con-
tinuous CRMs. Between the two founda-
tions, a weather station was set up that
contained a wind speed and direction in-
dicator, a barometer, a rain gage, an out-
door air temperature and relative humidity
sensor, and a solar flux density monitor.
Signals from these devices fed into the
CSI 21X data logger located inside a
weatherproof box at the base of the
weather station. Also stored in the 21X
were soil moisture matric potential data
collected from 10 locations.
Several other types of measurements
were made at the test cells associated
with specific tests or experiments. One of
these is the soil permeability, including
measurement of soil gas radon concen-
tration at the deepest penetration. After
each test cell was leak-tested with chemi-
cal smoke and all the infiltration sites were
sealed as well as possible with urethane
foam or caulk, air infiltration was deter-
mined using a modified "blower door" ap-
proach. A computer program modified from
the actual blower door code was used to
estimate the equivalent leakage area (ELA)
of each test cell. Later in the project, when
sufficient sources and detectors could be
obtained, the Brookhaven National Labo-
ratory Air Infiltration Measurement Sys-
tem (BNL/AIMS) was used to monitor infil-
tration integrated over longer time peri-
ods. Air leakage was also measured in
the movable cell, generally after each
move. The BNL system was not used in
the movable cell. When the soil
permeabilities were being measured, an
auger core near each test cell was ex-
tracted for analyses by other laboratories
both for additional documentation of the
site for this study and also in support of
other related projects in the FRRP.
Samples were analyzed for soil moisture
percent, soil radium concentration, and
radon emanation percentage in accor-
dance with the FRRP Standard Protocols.
Model Predictions
Information from the RAETRAD model
generic predictions was useful in the plan-
ning and design phases of the FIPR
project. At that time a variety of "reason-
able" parameters representing some of
the ranges that may be expected were
used as hypothetical starting points. Later,
as actual data were collected, the models
were refined to reflect the conditions of
the test cells and their environment more
realistically. Similar iterations were made
with other input parameters. The model
could deal with only one fixed crack. When
the slabs produced unplanned settling
cracks where they could not be easily
modeled, new estimates of the slab per-
meability had to be used to approximate
the "effective" permeability of the intact
and cracked slab. Soil and slab param-
eters such as densities, permeabilities,
saturation fractions, and porosities, were
similarly first approximated and later cor-
rected if necessary as more data were
collected. Models of radon entry were run
for smaller footprint structures to ensure
that we could go to the 3- x 3-m founda-
tions of the movable cell without undue
loss in generality. The model predictions
proved similar to those of larger struc-
tures. Most of the data required as input
for the movable cell runs were well char-
acterized before the runs were needed;
so the iterative process described above
for the FIPR cells was not required.
Radon Entry Theory and
Applications
The total amount of radon in a given
volume is VC, where V is the volume of
the space and C is the radon concentra-
tion. V is constant for the structure, and C
generally varies with time. This quantity of
radon changes as radon enters and leaves
the volume. It is assumed that the radon
enters at some fixed entry rate, I, for any
given pressure applied in the building.
Radon exits by way of the exfiltration of
the structure air, represented by QC, where
Q is the exfiltration rate and is assumed
to be constant at a given building condi-
tion of closure and applied pressure. The
net rate of change in the radon quantity is
= V— = - QC c\\
dt \ '
This is the governing differential equation
describing the change in radon. The as-
sumed initial condition for solving this
equation will be that the concentration at
time zero will be zero: C(0) = 0. Other
initial conditions can be accommodated
fairly easily. A solution that satisfies the
governing equation and initial condition is
C(t) = Cs(1-e-
(2)
where Cs is the steady-state (long time
equilibrium) radon concentration, and X is
the ratio of the exfiltration rate to the vol-
ume
Q
V
(3)
The radon entry rate, I, (assumed to be
constant for a given structure condition)
must equal the equilibrium radon
exfiltration rate after a sufficiently long time
for the indoor concentration to stabilize, or
I = QCs
(4)
Study Design
This project was implemented in phases
such that similar study objectives were
addressed in slightly to very different en-
vironments. The initial activity in the FIPR
test cells was performed prior to any
planned compromise of the slab integrity.
Necessary modifications such as sealing
and installation of ports, openings, and
sampling tubes were accomplished be-
fore altering the slabs. A full battery of
experiments was conducted, with as much
instrumentation as possible in place. After
the operation in the cells with the slabs
relatively intact was accomplished, three
100-mm slab cores were removed from
each cell. Sections of 100-mm PVC pipes
were caulked into these core holes so
that they could be capped off or opened
with some degree of control. Additional
10-mm holes were drilled through the slabs
along other radials, and stainless steel
tubes were placed to depths of 0.3-1.2 m
and sealed so that differential pressure or
sub-slab radon measurements could be
checked in other directions. Many experi-
ments and measurements similar to those
made with the intact slabs were repeated.
Finally, after the tests and experiments
were completed, samples were taken to
characterize the sub-slab soil. These in-
cluded soil permeability and soil gas ra-
don samples and soil cores from approxi-
mately 0 to 1 m deep. At the movable cell
site, similar characterizations of the site
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and structure to those made at the FIPR
site were made of each of the fill soil
configurations being tested.
Stress Tests
One of the objectives of this work was
to develop transferable protocols relevant
to depressurized radon measurements.
The specific protocols relevant to the other
objectives of this project dealing with ra-
don entry and barrier effectiveness are
the whole structure and zonal "stress
tests." These were developed for both
short- and long-term measurements. The
radon entry rates were approximated by
the change in concentration at constant
flow over the time period. These radon
entry rates were expressed as a function
of cell depressurization where appropri-
ate. Usually as the levels reached a pla-
teau, the entry rate would become pro-
portional to the exhaust flow rate.
Results and Discussion
Characterization and Other
Measurements
Test Cell Air Infiltration
Measurements—
The cells were depressurized with
an exhaust fan, and the resulting pres-
sures and flows were measured simi-
lar to blower door tests. Because of
the low flows in these tight cells and
the resulting larger impact of wind ef-
fects, the precision of the calculated
leakage areas and infiltration could
not be tightly controlled. The ELA leak-
age area calculated for test cell 1
varied from 580 to 1290 mm2, while
that of cell 2 ranged from 390 to 840
mm2. It was thought that the perim-
eter crack in the FS of cell 1 was the
source of some, if not all, of the addi-
tional leakage found there. From the
pressure-flow data, one could also
calculate the infiltration in air changes
per hour (ACH). Applying these cal-
culations to the test cell 1 measure-
ments indicated 0.02-0.06 ACH at 4
Pa in cell 1 and 0.02-0.04 ACH at 4
Pa in cell 2. At 50 Pa depressuriza-
tion, the results were 0.17-0.25 and
0.11-0.20 ACH in cells 1 and 2, re-
spectively. From the analyses of the
BNL data collected in two weeks in
July, the infiltration at 20 Pa and 4.7
L/s flow for both cells was about
0.2 ACH. When the cells were oper-
ated under no mechanically induced
depressurization and with all known
openings closed tightly, the infiltration
was 0.028 ACH in cell 1 and 0.024 in
cell 2. Also, placing other tracers at
the base of the foptings in both cells
allowed for the estimation of the frac-
tion of their emissions that were fpund
in the test cells under both conditions.
In cell 1, 45-57% of the emissions
from the buried tracers were detected
when the cell was depressurized at 20
Pa, while 2.7-3.9% was detected indoors
when the cell was not mechanically de-
pressurized. For cell 2, the corresponding
numbers were 23-32% and 1.0-2.5%.
Soil Radium, Radon Emanation,
Moisture, and Other
Characteristics—
During construction of the test cells,
GEOMET gathered samples of the soils
and sent them to UF for analyses. Later
Southern sent some deeper samples to
UF and two entire cores to RAE for some-
what similar analyses. In both test cells
the fill soils were slightly lower in soil
radium content, but higher in emanation
percentage. During late June 1991, tensi-
ometers were placed in the soil around
the test cells to measure the matric poten-
tial of the soil moisture. Generally the soil
matric potential increased with depth and
varied fairly little over the time the mea-
surements were made. Since there were
regular and frequent rains, the 0.3-m depth
generally had higher moisture concentra-
tion. In late July, after the slabs had been
penetrated, three tensiometers were
placed through the slab and into the sub-
slab soil of each test cell. Generally, the
soil matric potentials there decreased with
depth and proximity to the edge of the
slab. Over a relatively short time period,
the moisture was effectively constant un-
der the slabs, and those from the outdoor
tensiometers showed little change from
the previous month.
Stress Tests and Related
Radon Entry Calculations
Radon Flux and Stress Test
Measurements—
The 0.382 m3 "zonal plenum" was used
to measure the buildup of radon over a
0.8 m2 area of intact exposed slab in both
cells. A CRM was placed underthe sealed
plenum, and the radon in that space was
measured over time. A form of Eq. (2)
seemed to describe the observed behav-
ior fairly well. The zonal plenum was also
used to conduct a depressurized stress
on 0.8 m2 of slab area in both test cells.
The plenum was placed at -20 Pa and
about 0.5 L/min regulated flow through a
bleed valve. The radon entry rate for cells
1 and 2 seemed to be about 2.5 and 2.4
pCi/min, respectively. The second kind of
area plenum used was a crack plenum. It
was used to measure the radon concen-
trations pulled through the perimeter crack
around the floating slab in test cell 1. It
was found that -1 Pa at 0.2 L/min flow
was a stable, reproducible setting. When
the measured radon concentrations were
fit to entry Eq. (4), the radon entry rate
was found to be about 11.1 pCi/s, or 24.3
pCi/s per meter of perimeter crack. With
no depressurization, the radon entry rate
appeared to be between 0.6 and 2.2 pCi/s
or about 1.3 to 4.6 pCi/s per meter of
crack depending on the portion of the
crack measured. This crack plenum was
also placed over 0.45 m lengths of the
unplanned settling cracks that occurred
in the slabs of both test cells. The radon
entry rates in cell 1 estimated by three
applications of Eq. (2) were 2.3 pCi/s for
-40 Pa, 0.22 L/min flow, 2.2 pCi/s when
the flow was increased to 0.26 L/min with
bleed air, and 0.14 pCi/s for the "passive"
case with no imposed depressurization.
In cell 2 the radon entry rate was calcu-
lated to be about 0.3 pCi/s with no de-
pressurization and 10.9 pCi/s at -40 Pa
and about 0.2 L/min flow.
Whole Building Radon
Measurements and Long Term
Stresses—
Measurements of the indoor concentra-
tions in the test cells were started May 31,
1991. Radon entry rates were calculated
by fitting Eq. (2) for several controlled
conditions for each cell. These conditions
included "passive" modes, in which no
depressurization was applied to the cell,
and a number of states during which a
variety of depressurizations were applied
at various flow rates of make up air. Cell 1
maintained higher indoor radon concen-
trations and entry rates than cell 2 when
they were operated in the same condition.
Radon entry rates for both cells were also
calculated over a time when significant
rainfall, high winds, and a drop in baro-
metric pressure made the cells behave
differently from their controlled conditions.
The sub-slab pressures were elevated at
this time, perhaps spiked by the drop in
barometric pressure, the pumping action
of the rain, and/or some contribution from
the increased wind. As the cells "relaxed,"
they returned to more normal entry rates,
but only after several days. The cells were
also operated at various depressurizations
and flows, with fixed openings at various
placements in the slab. The radon entry
rates of the two cells were much closer to
each other with a fixed hole open. Radon
entry rates were calculated in cell 1 for
several months after cell 2 was being used
for another project.
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Pressure Field Extension
Measurements and Model
Comparisons
After the slabs had been penetrated on
July 10, the test cells were depressurized
with one hole open in the cap of the
center core hole. The pressure field ex-
tension was then measured at each of the
slab tubes and at the soil tubes placed
outside the structure earlier. The
RAETRAD model was run to predict the
pressures and radon concentrations ex-
pected. It was found that, because of the
extensive cracking and some detected sub-
sidence under the slab in cell 1, adjust-
ments had to be made in some of the
parameters for the model to fit the mea-
sured values. The primary adjustments
were to make the slab and the stem wall
more "porous" to the applied pressures.
With such adjustments made, the model
was made to agree fairly well with the
values observed during the pressure field
extension measurements. The model pre-
dictions are in good agreement under and
within 0.3 m of the slab, but the agree-
ment is not as good 1.8 m from the slab.
Some of this discrepancy may relate to
the fact that the model predicts for a cylin-
drical approximation rather than for a
square structure. The RAETRAD model
also predicts soil gas radon concentra-
tions. Some of these were taken as grab
samples over the course of the study. The
samples were taken over a wide range of
time, conditions, and states of operation,
leading to some fairly wide ranges of val-
ues. The model seems to predict values
relatively close to those ranges.
Studies of Radon Entry from
"Hot" Fill Using Movable Test
Cell
In March 1992, the experimental stud-
ies of the effects of "hot" fill soil began on
the second host site using the movable
test structure described earlier. The first
series of tests were performed on the
foundation designed for 0.3 m depth of fill,
but used the native soil for a control soil.
A week of depressurized operation was
performed, followed by a week of passive
recovery and a second week of depres-
surized operation. As expected from the
low soil radium level, the soil gas and
indoor radon levels were low. The indoor
radon concentrations averaged little more
than 1 pCi/L. Some trends are clear in
this data set which are observed in the
higher-radium fill soils. During the period
of 20 Pa depressurization, a slow drop in
sub-slab radon concentrations is seen
which may be interpreted as depletion of
soil gas radon under the structure. Under
passive operation, the sub-slab radon ap-
pears to recover somewhat and approach
previous levels. Similarly, a trend toward
higher indoor radon is noted. A layer of
0.6 m of moderate radium fill was placed
in F2 on May 15. This fill consisted of
sand tailings with a measured radium level
of 4.12pCi/g, and emanation fraction of
14%. The cell was moved onto this foun-
dation on May 22 and left in passive op-
eration through June 4. The cell was de-
pressurized approximately 7.5 Pa with ap-
proximately 2.5 L/s average exhaust flow
rate, then left passive. The pattern seen
in the earlier period was demonstrated
again, but with a greater signal/noise ra-
tio. The sub-slab radon built up to a steady
state value after a week of passive opera-
tion, then dropped by one-third during the
4-day depressurization, and began to re-
cover during the final 3 days of the experi-
mental cycle. The indoor radon quickly
reached a steady state value of about 3
pCi/L under depressurization, then climbed
over a several day period when mechani-
cal exhaust ventilation was removed. Be-
tween June 12 and October 15, the cell
was mounted over foundation F1, which
was filled with scrap concentrate to repre-
sent a probable upper limit to the range of
radioactive fill soil likely to be encoun-
tered. This material contained 32.9 pCi/g
radium with an emanation fraction of 11%.
The indoor radon in the cell was allowed
to equilibrate under passive conditions;
then it was depressurized at 10 Pa for 2
days, then allowed to remain under pas-
sive conditions. The ratio of the indoor
radon to the sub-slab radon concentration
was used to compare the data. During
periods where the sub-slab radon is chang-
ing slowly relative to the indoor ventilation
rate, this ratio gives a measure of relative
soil gas entry. During the periods of pas-
sive operation, the ratio approaches com-
parable steady-state values even though
the indoor and/or the sub-slab concentra-
tions may not have attained comparable
degrees of steady state.
On October 16, the cell was moved to
foundation F2 for the final fill condition.
This fill consisted of a layered "cold over
hot" soil. The lower 0.3-m layer at the
bottom of the foundation consisted of the
same scrap concentrate hot fill used in
the previous test series. This was covered
with a 0.3 m layer of the base soil from
the site. The cell was left passive until
October 20, when it was depressurized to
12 Pa with an exhaust flow rate of 4.7 L/s.
After October 23, a series of experiments
were conducted with the center and edge
openings alternately opened and closed.
This range of conditions allowed radon
entry and depletion to be measured be-
neath the slab for different locations of
opening.
Radon Entry and Slab Edge
Details
There were 10 periods of time when
both the FIPR test cells were operated at
no applied depressurization. The indoor
radon concentrations were recorded and
fitted to the infiltration equation, and equi-
librium radon concentrations and radon
entry rates were calculated. In every in-
stance the indoor radon concentrations in
cell 1 greatly exceeded those measured
in cell 2. In fact, the equilibrium concen-
trations ranged from being 2.2 to 5.4 times
higher and the radon entry rates from 2.3
to 5.0 times higher in the FS cell than in
the SSW cell. While sub-slab radon con-
centrations indicate that the source poten-
tial was as much as 40 to 50% higher
under cell 1 than under cell 2 because of
the lateral variability of the site, this does
not account for the three to fourfold in-
crease in indoor concentrations. When the
cells were depressurized at various lev-
els, the ratio of the steady state concen-
trations and entry rates reduced to less
than the factor of 3 to 4. The use of the
box plenum over areas of an intact slab in
both passive and depressurized modes
indicated that 1 to 2 pCi/s radon entry
came through the intact concrete in either
test cell. The crack plenum was similarly
used over the perimeter crack in cell 1.
Radon entry was calculated at a rate of
30 to 600 pCi/s, depending on the degree
of depressurization. This plenum was also
used on the settling/shrinkage cracks in
both cells, and rates of 4-60 pCi/s and 8-
300 pCi/s were estimated to enter cells 1
and 2, respectively. The perimeter crack
of the FS seemed to be the primary con-
tributor to the cell's radon entry. With prac-
tically no depressurization, there would be
as much as 35 pCi/s radon entry in cell 1
and about 10 pCi/s in cell 2. The ob-
served passive state usually resulted in
radon entry rates from 54 to 200 pCi/s in
cell 1 and from 15 to 53 pCi/s in cell 2.
When the various plenum measurements
under depressurized conditions were ex-
trapolated, estimated cell 1 radon entry
rates of as much as 660 pCi/s were
reached. Whole cell depressurization of
10 to 20 Pa produced calculated entry
rates of 540 pCi/s. The cumulative ple-
num depressurizations in cell 2 extrapo-
lated cell entry rates to the 300 pCi/s
range, while experimental observations
produced entry rates from 135 to 340
pCi/s when the cell was depressurized
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from 10 to 20 Pa. These observations in-
dicate that the various plenum measure-
ments seem to have potential for estimat-
ing the whole building entry rates.
Long-term Trends in Radon
Entry
As experiments were conducted over
time in the FIPR cells, the radon concen-
trations seemed to increase for the same
cell under the same conditions over time.
The slabs were still in the process of ag-
ing and settling. This possibility was sub-
stantiated by the slow appearance of set-
tling/shrinkage cracks 5 to 6 months after
the slabs were placed. The slabs did not
have vapor barriers exacerbating the ef-
fects of the cracks. In addition to these
naturally occurring faults, the work that
was done in the later phases also com-
promised these slabs. Each of these al-
terations was sealed, caulked, and re-
paired as well as possible, but the result-
ing barriers may not have been as sub-
stantial as the initial 100 mm of concrete.
The two parameters that tended to indi-
cate most strongly the changes over time
were the calculated equilibrium radon con-
centrations and radon entry rates for the
cells with no applied depressurization. Lin-
ear regression calculations indicate that
the equilibrium concentrations and entry
rates were increasing significantly over the
time periods of observation. The equilib-
rium radon concentrations and radon en-
try rates were also calculated and recorded
for the experiments in which the cells were
depressurized. For cell 1, the equilibrium
concentration for the nominal 2.4 Pa de-
pressurization was averaged to be 131 ±33
pCi/L, while the average entry rate for
these passive runs was 104±47 pCi/s. For
cell 2, the corresponding concentration was
33±15 pCi/L, and the entry rate was 31±13
pCi/s. Both the equilibrium concentration
and the entry rate increased with depres-
surization. The radon entry rate is ex-
pected to be linear with applied depres-
surization and seemed to be quite signifi-
cant with slopes of 30±2 and 11±2 pCi/s/
Pa for cells 1 and 2, respectively. The
consistent increase in the equilibrium con-
centrations is expected in these tight struc-
tures. Since the contribution of leakage
air through the superstructure was mini-
mal, the dilution of the indoor radon was
primarily limited by the flow control valve,
effectively reducing the leakage area as
the applied pressure was increased. The
ratio of the cell 1 (5.1±0.5 pCi/L/Pa)to cell
2 (1.1±0.1 pCi/L/Pa) slopes of the equilib-
rium concentration regression lines was
4.6, and that of the radon entry rates was
2.7. These ratios are reasonably close to
the factor of three to four observed in
relative radon concentrations between the
two cells. As stated above, we attribute
this difference primarily to the perimeter
crack.
Hot Fill Effects on Radon Entry
During the course of the hot fill phase
of the study in 1992, radon entry rates
from four combinations of fill were studied
under passive and mechanically depres-
surized conditions. These experimental
results were compared with predictions of
radon entry using the RAETRAD model
and figures of merit based on soil proper-
ties. The model provides satisfactory fits
to the experimental data and predictions
of the effect of further variation of key
parameters. First, the measured radon
entry at 10 Pa depressurization was com-
pared with predicted entry rates for each
fill studied. The experimental data were
about 30% higher than the RAETRAD pre-
dictions for the fills studied. However, not
all the depressurization experiments were
carried to steady-state with respect to
depletion of the sub-slab radon as as-
sumed by RAETRAD. Had steady-state
been achieved experimentally, the experi-
mental numbers would be lower, and in
closer agreement with the model. The ex-
perimental and model results were com-
pared with a basic figure of merit: the total
emanating radium under the slab. This
number is calculated by multiplying the
measured soil radium times the emana-
tion fraction times the calculated mass of
the soil layer in the 0.9-m layer immedi-
ately under the footprint of the slab within
the foundation walls. A strong correlation
with the experimental and modeled entry
was noted.
Further parametric modeling was stud-
ied using RAETRAD for each of the four
experimental soil configurations as well
as a few hypothetical extensions of these
conditions. The first effect studied was the
presence of sub-slab void spaces. Indica-
tions of void spaces under the slab were
seen not only in the movable cell but also
under the FIPR cells and in past diagnos-
tic studies in existing houses. In order to
test the effects of void spaces, RAETRAD
calculations were repeated with no void
spaces and with a "worst case" void 9 mm
thick extending under the entire slab. The
presence of this gap increased the radon
entry by 10-20% over the pressures stud-
ied. Increases of the same magnitude were
noted for other fill soils and geometries.
The second parameter studied was the
position of the penetrations or openings
under the slab. Most of our experiments
were performed with both center and edge
penetrations open. The few studies per-
formed with one location closed indicated
little difference in radon entry between the
center only, edge only, or both open con-
ditions. While the experiments were not
conclusive, these results would be ex-
pected if a significant sub-slab void exists
and if the primary resistance to soil gas
flow is the soil itself rather than either
penetration. Investigation indicated that for
each geometry the ratio of radon entry
between the two channels varies only
weakly with pressure above roughly 1-2
Pa depressurization. As one might antici-
pate, entry through the side channels is
greater under pressure-driven flow condi-
tions, while the proportion entering the
center increases in the presence of the
sub-slab gap and/or at lower pressures.
Since both of these conditions are com-
mon, if not typical, we expect central pen-
etrations to be as significant a factor as
edge penetrations in general slab-on-grade
housing stock.
We would anticipate that the different
fill geometries studied would have differ-
ent response to applied depressurization.
In order to test these assumptions, calcu-
lations were performed over a pressure
range of 0.2-20 Pa, extending over an
order of magnitude below and above typi-
cal depressurization levels. Calculations
were performed for the four fill cases stud-
ied: The "base" case (low radium soil), the
0.6-m layer of 4.12 pCi/g "warm" fill, the
0.3-m layer of 32.9 pCi/g "hot" fill, and the
0.3-m layer of hot fill covered by 0.3 m of
base soil. In addition, calculations were
performed on a fifth case for comparison.
This case, 0.6 m "hot," was identical to
the 0.6 m "warm" case except for substi-
tution of the "hot" fill. For consistency, the
"gap" geometry was used for all cases.
Inspection of this comparison showed a
similar trend for all fill cases. At the lowest
pressures, the radon entry approaches a
constant value characteristic of diffusive
transport without convection. At higher
pressures, the entry rate increases with
roughly a power-law dependence, then
appears to approach an upper asymptotic
limit which we would assume to be deter-
mined by the limit on the production rate
of the radon source. The pressure depen-
dencies of all cases are similar, but not
identical. The radon entry rate of each fill
case was normalized to its rate for a 0.2-
Pa depressurization. The differences
among the cases can be rationalized as
follows: where the radium-bearing soil ex-
tends farther from the slab entry point.
greater depressurization is required to col-
lect an equivalent fraction of the available
radon and is likewise less subject to deple-
tion at higher pressures.
To extend the conclusions of the test
cell studies to new residential construe-
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tion, RAETRAD calculations were ex-
tended to the elliptical geometry of the
"reference" house used in other modeling
studies for the FRRP. The ratio of the
yard area to house area modeled is 6.25:1.
We tested the dependence on crack posi-
tion for a base scenario consisting of 0.6 m
of elevated, sandy fill over sandy base
soil. The radon entering the house through
the slab crack increases almost linearly
with crack position to a maximum of 3.7
and 4.3 m from the center; for larger radii,
radon entry decreases slightly until the
perimeter is reached. The total radon en-
try also includes roughly 23 pCi/s attribut-
able to direct transport through the slab;
the magnitude of this portion is essentially
independent of crack position. It was found
that the radon entry rate per unit crack
length varied by less than 3.5% for cracks
in the annular region 0.9 m or more from
both the center and edge of the slab. The
soil gas flow varies by 4.4% over this
range, increasing monotonically with the
radius. These results indicate that model
conclusions using perimeter cracks should
be applicable to interior cracks as well.
Calculations at 0.9- and 4.9-m radius
for cracks of 2.5- and 10-mm width were
also made. The calculated soil gas and
radon entry values were essentially invari-
ant over this factor of four variation in
crack width, indicating that crack length
rather than area is the proper normalizing
factor for cracks of this size. In evaluating
the effect of a hot fill it is convenient to
postulate that the contribution of fill soil at
all depths within the slab footing is roughly
constant and that the total emanating ra-
dium within this layer will largely deter-
mine the indoor radon. A series of studies
was performed to investigate this hypoth-
esis. Assumed radium contents of 2, 4, 8,
and 16 pCi/g were used in the fill layer.
Emanation fractions of 50, 55, 50, and
50% were assumed, in accordance with
the findings of others for soils in a five-
county area of Central Florida. Data gen-
erated suggest that the indoor radon may
indeed scale with the total fill radium. For
each assumed fill radium content, the rate
of increase in indoor radon with fill thick-
ness falls off with increasing thickness,
suggesting that the deeper fill layers do
not contribute quite as much as the top
layer. The incremental added radon attrib-
utable to each new layer falls off at a rate
of 52%/m in the top 0.6 m.
To extend the predictions of this study
to a range of conditions, calculations were
made with a range of base and fill soil
types. For those studies, a 0.6-m fill layer
was assumed over a radium-free base
soil of the same soil class as the fill.
Three facts were revealed. First, the varia-
tion of indoor radon is completely corre-
lated with the emanating radium content
of the fill for each soil type. This must of
necessity be the case within the numeri-
cal precision of RAETRAD due to the struc-
ture of the equations used in the model.
Second, there is an intercept of roughly
0.06 pCi/L across the range of soil types.
Third, the slopes of the lines do not vary
by more than a factor of two across the
range of soil types used. More insight into
this result, which might at first seem sur-
prising, can be gathered from calculating
results from the 4 pCi/g soil radium cases
in various soils. The predicted total radon
entry for this available radium content var-
ies from 79 to 166 pCi/s across the soils
studied. The portion of radon which en-
ters through the crack shows a much stron-
ger relative relationship varying from 10 to
143 pCi/s. The remaining radon entry
route, direct transport across the slab, ac-
counts for most of the radon entry for the
less permeable soil types. This rate drops
off for coarser soils as more of the radon
immediately under the slab is able to dif-
fuse to lower depths or is drawn into the
building through the crack; both of these
factors decrease the concentration gradi-
ent that drives the diffusive transport.
When the calculations are extended to
non-uniform soils, the results are less eas-
ily visualized. In fact, the variability of the
results is partly explained by the variation
of direct entry through the slab. For fill
soils coarser than clay the diffusive trans-
port through the slab decreases as the
coarseness of the fill and/or the underly-
ing base soil increases. The same mecha-
nisms operate to decrease the sub-slab
radon available for transport through the
crack, but are generally overwhelmed by
the orders of magnitude increase in soil
gas flow as the coarseness of base and/
or fill soil is increased. Thus, for a given
base soil type, radon entry generally in-
creases as the coarseness of the fill is
increased. For a given fill soil type, the
radon entry generally increases strongly
as the base soil coarseness is increased
to that of the fill, then levels off and often
will decrease with further increases of base
coarseness. As one might predict, the
crack entry rates collapse into a tight trend
when radon entry is made a function of
soil gas entry. There is more scatter in the
total entry rate, but, for a given soil gas
entry rate, the radon entry rates are clus-
tered within about ±15% of one another.
While the soil gas entry rate is not gener-
ally measurable for a given house, a sur-
rogate parameterization is to define an
effective total permeability, Keff as a
weighted average of the fill and base
permeabilities. The effective permeability
reflects the resistance to flow that would
be experienced by a streamline passing
150 mm from the footing boundaries. A
comparison of the analysis with this pa-
rameter rather than soil gas entry indi-
cates that Keff does indeed serve as rea-
sonable surrogate for soil flow rate.
Conclusions and
Recommendations
One objective of this research was to
validate the effectiveness of "barrier" con-
struction features, specifically contrasting
the "floating" slab construction of test cell
1 with the slab-in-stem wall foundation of
test cell 2. The observed indoor radon
concentrations of test cell 1 generally were
about three times higher than those of cell
2 when the two cells were operated under
equivalent conditions. Calculated radon
entry rates for the structures tended to
show this same general relationship of a
threefold or greater entry rate with the
floating slab. Specific measurements were
made estimating the radon entry through
the perimeter crack of the floating slab,
and as the modelers predicted, most of
the radon entry appeared to come through
the perimeter crack.
Another objective was to make detailed
measurements relevant to radon transport
and entry for model testing and validation.
Many measurements were made and com-
pared with the predictions of the Rogers &
Associates' RAETRAD model. It was dis-
covered that several adjustments to as-
sumed parameters were necessary to
bring about good agreement. Some of
these were made because insufficient in-
formation was available initially about the
actual conditions. Others were required
because imperfections in the actual slabs
and fill preparations caused them not to
perform as the modeled slab and fill base
predicted. Overall, the modeled pressure
fields and sub-slab radon concentrations
were brought into good agreement with
the observed measurements. Both hori-
zontal and vertical variability in the soil
radium content input parameter was ob-
served and incorporated in the model. The
model was able to accommodate most of
the variability that was detected.
Another objective was to develop trans-
ferable protocols relevant to depressur-
ized radon measurements. A solution to
the basic differential equation accounting
for the mass balance of radon within the
structure was found to explain fairly well
the radon concentration changes observed
when the cells or zones of the cells were
subjected to depressurized stresses. Over-
all the modeling applications were very
useful and informative and showed good
potential for further use in this type of
research.
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The final objective was to demonstrate
and quantify the effect on indoor radon
from high radium fill soil. The data col-
lected from a number of experiments with
the movable cell placed over different thick-
nesses of fill soils with different concen-
trations of soil radium are presented with
an analysis of which parameters affect
the indoor radon concentrations most
strongly and what some target "safe" val-
ues might be. A framework for a possible
radiological standard for fill soil is pre-
sented.
A. Williamson, C. Fowler, and S. McDonough are with Southern Research Institute,
Birmingham, AL 55305.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Test Cell Studies of Radon Entry," (Order No. PB96-
153549; Cost: $27.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-96/010
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