United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-92/119 December 1992
*& EPA Project Summary
Modeling Radon Entry Into
Florida Houses with Concrete
Slabs and Concrete-Block Stem
Walls, Florida Radon Research
Program
K.L. Revzan, W.J. Fisk, and R.G. Sextro
A finite-difference numerical model
was used to examine the influence of
soil, fill, and construction characteris-
tics on the convective entry of radon
and soil gas into slab-on-grade houses.
Such houses, built with a perimeter,
hollow-core concrete block stem wall
and an above-grade floor slab resting
on fill, are typical of a portion of the
Florida housing stock. When the build-
ing is depressurized with respect to
ambient pressure, radon-bearing soil
air flows through various combinations
of soil, fill, and blockwall components,
entering the house through perimeter
slab-stem wall gaps, or interior cracks,
or other openings in the floor slab. At a
constant building depressurization, the
model predicts the steady-state pres-
sure, flow, and radon concentration
fields for a soil block 10 m deep and
extending 10m beyond the 7-m-radius
slab. From the concentration and pres-
sure fields, radon and soil gas entry
rates are then estimated for each entry
location. Under base case conditions,
approximately 93% of the soil gas en-
try is through the exterior section of
the stem wall, 5% through the interior
section of the stem wall, 2% through
an interior slab opening, and less than
1% through gaps assumed to exist be-
tween the stem wall and footing or the
stem wall and floor slab. In contrast,
57% of the radon entry rate occurs
through the interior section of the stem
wall, 22% through the interior slab
opening, 20% through the exterior sec-
tion of the stem wall, and less than
0.5% through the gaps. Changes in fill
permeability have significant effects on
radon entry, while changes in blockwall
permeability are largely offset by in-
creased flow and entry through struc-
tural gaps.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of a research project that is fully
documented as a separate report with
the same title (see Project Report or-
dering information in the back).
Introduction
The role of convective flow of soil gas
in transporting radon into buildings is
widely acknowledged; however, the fac-
tors that affect radon entry can be com-
plex. These flows depend on the driving
pressure, the type and location of the
openings connecting the building interior
with the surrounding soil environment, and
characteristics of the soil medium. The
nature of these openings is strongly influ-
enced by both the type of building sub-
structure and the specific construction de-
tails. The driving pressure, which is the
pressure difference between the surface
of the soil surrounding the building and
the building interior, is caused by the stack
effect (due to temperature differences be-
tween the inside of the building and the
outdoors), wind loading on the building
shell, and the operation of heating and/or
air conditioning systems.
Several detailed numerical models of
radon transport through soil and entry into
buildings have been developed to investi-
gate factors influencing soil gas and ra-
don migration, including characteristics of
Printed on Recycled Paper
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the building and the surrounding soil. In
the present study, a two-dimensional,
steady-state finite-difference numerical
model, utilizing cylindrical symmetry, has
been assembled, with boundary conditions
appropriate for one form of the slab-on-
grade construction used in Florida hous-
ing. The model has been used to explore
the influence of soil and building param-
eters on soil gas and radon entry. Implica-
tions for possible methods of limiting soil
gas and radon entry are also discussed.
Mode! Description and
Approach
Model Overview
The model used in this study is based
on a finite-difference numerical code in
whteh the soil is assumed to be isother-
mal and the relationship between gas flow
and driving pressure is assumed to be
linear (Darcy's law). In the present model,
the Cartesian coordinates are transformed
into a cylindrical coordinate system. This,
in effect, reduces the model to two dimen-
sions for computing purposes. Since many
of the structural elements of interest are
at the perimeter of the house or can be
chosen to have cylindrical symmetry, there
Is little loss of generality in using cylindri-
cal coordinates. This approach permits in-
creased resolution and/or more rapid con-
vergence with only modest loss in realism
In moving from a fully three-dimensional
treatment. In this parametric analysis, the
benefit of greater speed outweighs the
slight loss in accuracy compared with a
fully three-dimensional configuration.
Boundaries for the soil block have been
chosen to be 10 m from the footing in
both the radial (r) and vertical (z) direc-
tions, as Indicated in Figure 1. The bottom
surface of the slab and the outer surfaces
of the footing are assumed to be no-flow
boundaries. The model accounts for ra-
don transport by both convective flow and
diffusion.
A static pressure difference is applied
between the surface of the soil exterior to
the building and the floor slab (top) sur-
face, the mouth of the interior slab gap
and the opening between the slab edge
and the outer element of the stem wall
(subsequently referred to as the slab edge
opening), as illustrated in Figures 1 and 2.
Generally, the slab edge opening is as-
sumed to be sufficiently large so there is
no pressure drop associated with flow
through this opening. Thus, the static pres-
sure difference is effectively between the
Inner surfaces of the stem wall, the mouth
of any of the gaps, and the exterior soil
surface. Two cases have been modeled
.7m
Soil Surface
Slab
Opening
10m
10m
Figure 1. Vert/ca/ cross-section of the region modeled showing the dimensions of the soil block and
the location of the slab gap for the base case. Greater detail for the stem wall is presented
in Figure 2.
where this general picture is altered. In
the first case, the stem wall is assumed to
be filled with impermeable concrete, so
that the only gap is between the top of the
interior element of the stem wall and the
bottom of the floor slab. In the second
case, the size of the slab edge opening is
reduced so that pressure drop does occur
across it, reducing the pressure difference
between the exterior soil surface and the
stem wall interior. In all cases, soil air and
radon entering the stem wall interior also
pass through the slab edge opening into
the house. Soil gas and radon can also
enter the house through the interior floor
slab gap.
The model computes the pressure field
throughout the soil and fill region by solv-
ing the Laplace equation. Soil gas trans-
port is then calculated from Darcy's law,
which assumes a linear relation between
applied pressure and fluid velocity. The
mass balance equation describing radon
migration, including radon generation, ra-
dioactive decay, and both convective and
diffusive radon transport, is solved to de-
termine the radon concentration field. The
model then yields soil gas and radon en-
try rates at each entry point.
Building Substructure and Soil
Geometry
Many houses built in Florida are con-
structed with a slab-on-grade substruc-
ture, of which there are several variants.
For this work, the model has been set up
to simulate an above-grade concrete slab
floor which rests on a perimeter hollow-
core concrete block stem wall. The slab
edge rests on a chair block, which is the
top course of blocks in the stem wall.
There is an opening between the edge of
the floor slab and the outer section of the
stem wall, as noted earlier. The floor is
also supported by fill material placed within
the boundaries of the stem wall and el-
evated above the natural grade. A vertical
section of the substructure is shown in
Figure 1. As indicated in Figure 2, where
the floor and stem wall are shown in
greater detail, gaps are assumed to exist
between both the inner and outer ele-
ments of the stem wall and the footing,
and between the inner portion of the stem
wall and the bottom of the floor slab. The
gap dimensions are chosen as an input
parameter. The effect of eliminating the
gaps at the bottom of the stem wall on
soil gas and radon entry has also been'
examined.
The inner and outer elements of the
concrete blocks that comprise the stem
wall are assumed to be permeable to air
flow; this permeability is another input pa-
rameter for the model. These wall ele-
ments are modeled as vertically homoge-
neous; i.e., no provision is made for differ-
ences due to mortar joints between the
blocks. To simplify the model, the block
webs—sections of the block that connect
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Concrete
Block Wall
House'V
Side V
Gaps V
Figure 2. Detail of the stem wall, showing the fixed dimensions for the wall height, dimensions of the
footing, and fill depth and location. The size of the gaps at the top and bottom of the stem
wall is exaggerated in this diagram. In the base case, their widths are 3 mm. The floor slab
thickness is 10 cm.
the inner and outer wall elements—have
not been included. Generally, the interior
of the block is open and flow through the
webs themselves should not significantly
affect the results. Where the stem wall is
filled with concrete, these webs are also
not present in the model, and thus no flow
path is provided. The concrete footing and
floor slab are assumed to be impermeable
to gas flow. An interior gap in the slab
floor is included in the model, with radial
location and gap width as model inputs.
The length of this gap is defined by the
radial location.
As shown in Figure 2, the fill below the
slab and on top of the footing is defined
as a separate region to enable us to
specify fill properties that may differ from
those of the natural soil. The two param-
eters of greatest interest here are air per-
meability and the radium content of the
soil or fill.
Base Configuration
A set of parameters have been chosen
to constitute a base case for the model-
ing. These have been selected based on
reviews of the available data on Florida
housing and on soil and fill properties. To
evaluate the effects of varying several of
the soil and/or building substructure fea-
tures on soil gas and radon entry, a range
was established for the variation of each
parameter. The base case values and
ranges are summarized in Table 1.
In the base case, the model uses an
effective radon diffusion coefficient of 2 x
10 -6 m2 s-1 for the soil and fill and an
'infinite depth' radon concentration, C , of
37 kBq nr3 which is equivalent to soil or
fill with a radium concentration of 46.5 Bq
kg-1 and an emanation coefficient and po-
rosity of 0.2 and 0.4, respectively. The
pressure difference between the top of
the slab and the top of the soil outside the
building was chosen to be -2.4 Pa.
The parametric investigation was car-
ried out using two approaches. First, each
parameter was van'ed individually, with the
remaining parameters held fixed at their
respective base case values. Second, in
some cases more than one parameter
was varied at the same time to explore
more fully the effects of the parameters of
interest. In these cases,
1) the soil permeability was varied for
high (10 -9 m2) and low (10 -15 m2) fill
permeabilities;
2) the soil and fill permeabilities were
varied independently when the slab
gap was the only soil gas entry path;
3) the soil, fill, and stem wall perme-
abilities were varied independently
when the core of the concrete blocks
making up the stem wall was filled
with impermeable concrete;
4) the stem wall permeability was var-
ied when gaps between both the bot-
tom of the stem wall and the concrete
wall footing were completely closed;
and
5) the size of the slab edge opening
was varied.
Results and Discussion
In the base case, the predicted soil gas
and radon entry rates due to convective
flow are 5.1 x 10 ^ m3 s-1 and 1.6 Bq s~\
respectively. The distribution of soil gas
and radon flows through the various entry
points shown in Figure 2 is summarized in
Table 2. The model simulations predict
that 93% of the total soil gas enters through
the exterior side of the stem wall, while
about 6% proceeds through the interior
surface of the stem wall. Most of the gas
flow is through the sides of the stem wall,
rather than through the 3 mm wide gaps
at the top and bottom of the stem wall.
Only 1.6% of the total soil gas is predicted
to enter at the interior slab gap, which in
the base case is located at 3 m radius.
This corresponds to a crack length of 18.8
m. These relative entry rates are consis-
tent with the path length of the flow lines—
and therefore the resistance to flow—con-
necting the exterior soil surface and the
specific entry point.
The distribution of the radon entry rates
associated with this air flow is different,
with almost 59% predicted to occur through
the interior side of the stem wall, 21%
through the exterior side of the stem wall,
and 20% through the interior slab crack.
The predicted radon concentrations at
each entry point, shown in Table 2, indi-
cate that, although the largest fraction of
gas flow occurs through the exterior side
of the stem wall, the radon concentration
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Tabla 1. Base Case Value and Range for Model Parameters
Parameter Base Case Value
Range
Soft air permeability (nf):
a, total soil block
b. soil layer 0-1 m deep'
c. soil layer 0.5-1.5 m deep*
Fill a!r permeability (m2):
a. aHfill
b. exterior to stem wall
Stem wall air permeability (m*):
a. both vertical wall elements
b. inner wall element
c. outer wall element
Slab opening:
a. width
b. radial distance (m)
Radium content (relative to base case):
a. fill
b. soil below 0.6 m depth*
c. soil below 4.6m depth *
Water table (m):
a. depth betow surface
10-"
10-"
10-"
4x10-"
4x10-"
10-'°
10-'°
3mm
3
10
10 -a- 10-s
10-'*-10-°
W'z-10-a
10 "'s- 10-°
10'1S-10-S
10-1S. to-9
1 mm -10 cm
0-7
0.1-10
0.1-10
0.1-10
0.5 -10
* Depth with respect to grade level
Tabto 2. Base Case Soil Gas and Radon Entry at Various Entry Points
Entry Location
Fraction of
Soil Gas Entry*
(percent)
Radon
Concentration
(percent of Cm)
Fraction of
Radon Entryf
(percent)
Interior side of the stem wall:
a. top gap 0.06
b. bottom gap 0.06
c. side of wall 5.0
d. bottom of wall 0.23
e. top of wall 0.24
Exterior side of the stem wall:
a. bottom gap 0.98
b. side of wall 88.
c. bottom of wall 4.
Slab opening 1-6
88
87
88
87
88
5
3
5
98
0.65
0.65
53.
2.5
2.6
0.67
18.
2.7
20.
Total base case soil gas entry = 5.1 x
Total base case radon entry ~ 1.6 Bq s-'
in the adjacent soil is low due to diffusion
to the atmosphere and to dilution by the
atmospheric air entering the soil through
a short flow path. In contrast, the radon
concentrations are much higher in the fill
materials located adjacent to the interior
side of the stem wall and below the inte-
rior of the slab.
In comparison with the convective ra-
don entry rate, the diffusive entry rate,
based on a radon diffusion coefficient for
concrete of 5 x 10 •" m2 s'1 and a concrete
porosity of 0.2, is 0.5 Bq s'1. Thus, for a
single-story house with a volume of 500
m3 and an average air exchange rate of
0.5 rr1, the total indoor radon concentra-
tion would be 31 Bq nr3 for this base case
soil and substructure.
Results of selected model runs in which
the effects of different parameters are
evaluated are shown in Table 3 and in
Figure 3. The effects of changes in per-
meability of the soil were extensively in-
vestigated, both alone and in conjunction
with variations in other parameters or as-
sumptions. Changes in soil permeability
alone had a somewhat modest effect on
radon entry in the base case, since flows
at the higher soil permeabilities are then
limited by the fill permeability. The role of
the fill in determining flows is demonstrated
by comparing the predicted radon entry
rates when the fill permeability is chosen
to be either high (10 "9 m2) or low (10 -15
m2). For high fill permeability, radon entry
is limited by the permeability of the under-
lying soil. When both are high, the in-
creased radon entry rate is significant,
almost 30 times the base case. On the
other hand, if the fill has a low permeabil-
ity, total radon entry is quite low and is
essentially unaffected by changes in soil
permeability.
Another effect that arises when soil per-
meability is varied is the change in the
importance of the various radon entry lo-
cations. As soil permeabilities are reduced
below that of the base case, radon entry
through the exterior of the stem wall
changes only slightly as soil permeabilities
range from 10 -12 to 10 -9 m2. However,
entry through the interior side of the stem
wall is reduced as soil permeability is re-
duced below the base case, and increases
as soil permeability increases. Radon en-
try at the interior slab gap behaves simi-
larly, although it does not increase as
much with increasing soil permeability.
Thus at the low end of the range of soil
permeabilities modeled here, radon entry
through the exterior side of the stem wall
is the largest single component; as soil
permeability increases, the relative impor-
tance of this entry pathway decreases. At
the high end of the soil permeability range,
approximately 88% of the radon enters
through the interior side of the stem wall,
almost 10% through the interior slab gap,
and about 2% through the exterior side of
the stem wall.
If the soil is layered, the effects on
radon entry of variations in the permeabil-
ity of the layer depend on the location of
the layer. Two layered soil cases were
modeled in which the permeability of the
soil layer was varied while those of the fill
and the remaining soil were held fixed at
the base case values. In the first case,
the soil layer began at grade level (in
direct contact with the fill material) and
extended 1 m deep. In the second case,
the soil layer began at 0.5 m below grade
(which is the depth of the bottom of the
footing) and extended to 1.5 m below
grade. As shown by the results in Table 3,
when the. layer is in contact with the fill
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Table 3. Effects of Selected Parameters on Radon Entry
Parameter
Radon Entry (Bq s •')
Soil air permeability (m *):
a. all other parameters = base case
b. till permeability = 10 -9 m *
c. fill permeability = 10 ~'5 m z
d. filled stem wall
e. soil layer 0 to 1 m deep'
f. soil layer 0.5 to 1.5 m deep'
g. slab opening only
Fill air permeability (rrf):
a. all other parameters = base case
b. filled stem wall
c. slab opening only
Radium content (relative to base case):
a. fill
b. soil below 0.6 m"
c. soil below 4.6 m*
Width of slab edge opening (cm):
a. all other parameters = base case
10-'*
0.4
0.63
5.X 10"
0.16
0.64
0.74
0.14
10 -(S
5.3x10-*
6.x 10"
3. x 10 -5
0.1
1.3
0.75
1.5
0.1
0.88
10-"
1.6
2.1
5.X 10"
1.2
1.6
1.6
0.55
10-"
5.x 10*
4.8x10*
3.x 10*
1
1.6
1.6
1.6
0.2
1.5
10-™
6.6
13
5.x 10"
3.8
4.2
2.5
1.1
10-"
1.1
0.8
0.2
5
2.6
5.1
1.7
1
1.6
10-*
13
47
5.x 10"
5.1
7.9
3.3
1.2
10-*
2.1
1.6
1.6
10
4
9.3
2.0
5
1.6
" Depth with respect to grade level
(assuming the fill has the base case per-
meability), the layer has a larger effect on
radon entry than when the soil layer is
deeper.
Interestingly, filling the stem wall inte-
rior with impermeable concrete has only a
modest effect on total radon entry. In this
case, a gap is assumed to exist between
the top of the concrete-filled stem wall
and the bottom of the floor slab. As shown
in Table 3 and Figure 3, total radon entry
still increases with increasing soil perme-
ability, though for a given permeability the
radon entry rate is lower than in the base
case. One can also see that the effects
on radon entry of changing the fill perme-
ability when the stem wall is filled with
concrete are also modest. These results
can, in general, be explained by the fact
that the pressure field distribution in the
adjacent fill is altered when the stem wall
interior is impermeable. The larger pres-
sure gradient at the remaining entry point,
which compensates somewhat for the re-
duced number of entry points, results in a
higher soil gas and radon entry rate.
Similarly, changing the permeability of
the stem wall itself has very little effect on
total radon entry, as can be seen from
Figure 3. Again, this is due to compensat-
ing effects. As long as the wall permeabil-
ity is greater than that of the adjacent fill,
flow through the wall is the most impor-
tant. As the wall permeability decreases
below that of the fill, the gaps between
the wall and the footing and between the
wall and the floor slab become increas-
ingly important flow pathways as the pres-
sure field is altered due to the changing
wall permeability.
The effect of the size of the slab edge
opening on the radon entry rate was ex-
amined parametrically. In the initial prob-
lem definition, this opening was assumed
to be sufficiently large so that no pressure
drop occurred at this point—effectively ap-
plying the full -2.4 Pa static pressure dif-
ference between the exterior soil surface
and the inner surfaces of the stem wall. In
actual construction practice this opening
may in fact be much smaller, in effect
reducing the driving force for convective
flow into the stem wall interior. Holding all
the soil and wall parameters at their base
case values, the effect of closing this open-
ing to 1 mm reduced the total radon entry
by about 40%. Radon entry via this open-
ing drops by about a factor of 4 in this
case, but the predicted entry via the inte-
rior slab gap increases by almost a factor
of 2, compensating somewhat for the re-
duction at the stem wall. This increased
entry rate at the interior slab gap arises
because the pressure gradient in the fill
region near the stem wall is reduced, thus
more of the air flow through the soil is
directed toward this interior opening.
In addition to the flow of soil gas into
the stem wall, via the wall material itself or
through the gaps indicated in Figure 2, air
flows through that portion of the exterior
wall that is above grade. In fact, in the
base case, this flow is 6.3 x 10 -3 m3 sr\
which is about 12 times the total predicted
soil gas flow from the soil into the house
(neither this entering outdoor air nor infil-
tration has been included as a radon
source). In order to investigate the effects
of changing the flow balance between the
inner and outer stem wall elements the
permeability of the above-grade portion of
the exterior stem wall element was in-
creased to 10 -9 m2 and fixed the perme-
ability of the remainder of the wall at 10 -12
m2 (as might be achieved with a wall coat-
ing or sealant). With the slab edge open-
ing reduced to 1 mm, the radon entry rate
through the stem wall is reduced dramati-
cally to 0.01 Bq s-1 from 0.9 Bq s-1 in the
base case. Total radon entry predicted for
the entire substructure is not reduced as
much, to about 37% of the base case
rate, because radon entry through the in-
terior slab gap increases in response to
the changes in the pressure field distribu-
tion, as described earlier.
The effect of water table depth on the
predicted radon entry rate was found to
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I 1 I I I I 11"[ 1 I I I I lll| 1 1 I I Mil
23 Variable Soil
g Variable Soil (Filled Stem Wall)
3 Variable Backfill
_ _e Variable Backfill (Filled Stem Wall)
Variable Stem Wall
10"
Permeability (m2)
Range of radon entry rates produced by variations in the soil, fill, and stem wall permeabilities for open and concrete-filled stem walls. Also shown
are entry rates when the stem wall/footing gap is eliminated. In each case, all other parameters have the base case value.
be small. For a water table (modeled as a
change in the position of the no-flow
boundary at the bottom of the soil block)
depth between 2.5 and 10.5 m below
grade, the radon entry rate was essen-
tially unchanged. At depths less than 2.5
m, the entry rate reduction was small; at
0.5 m, the radon entry rate was predicted
to be 0.88 Bq s-1.
Finally, the effect on predicted radon
entry of changes in the radium content of
the soil and fill was examined. First, it
should be noted that, if the radium con-
tent (and thereby the soil gas radon con-
centration) was increased uniformly in both
the soil and fill, the radon entry rate would
increase proportionately (except for minor
reductions due to the slight increase in
diffusive losses from the soil surface). If
the fill radium content is changed from the
base case, the radon entry rate does not
change proportionately, as can be seen
from Table 3. Larger changes in radon
entry can occur if the radium content of
the soil below 1.5 m were to increase, as
might be the case where a high radium
soil layer was close to the surface. The
effects of similar changes in radium con-
tent of soil below 5.5 m are diminished,
reflecting the fact that any additional ra-
don from the enhanced radium content is
transported through the soil by means of
diffusion into the soil and fill region where
convective transport into the structure be-
comes important.
Conclusions
Application of finite-difference models,
incorporating key features of the soil, fill,
and substructure, has provided additional
insight into transport of soil gas and radon
through the soil and into a building. The
model results have also shown that
changes in the characteristics of various
entry locations or pathways can impact
radon migration and entry at other loca-
tions, leading to compensating effects. As
one example of this, a reduction in the
permeability of the stem wall elements
reduces flow through the wall materials,
but soil gas and radon entry increases
through the gaps at the top and bottom of
the stem wall in response to the changes
in the pressure field in the adjacent fill.
Thus the total radon entry rate is not sig-
nificantly affected. Similarly, a reduction in
the size of the opening at the slab edge to
1 mm or less is necessary to effect any
significant reduction in the total radon en-
try rate. If the interior opening in the floor
slab is eliminated (but the stem wall entry
is unchanged), the total radon entry rate
is reduced by only 10% over the base
case rate. If, on the other hand, all entry
points at the stem wall are eliminated (as
might be accomplished by use of a solid,
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one-piece wall and floor slab) the total
radon entry rate is reduced by 66% (as-
suming that the floor-slab gap is present).
Changes in the air permeability of the
soil and fill can have the most significant
effect on radon entry. Increased soil per-
meability (above the 10 -11 m2 value as-
sumed in the base case) will increase
total radon entry; if accompanied by an
increase in fill permeability, the increase
in radon entry rate is more dramatic. On
the other hand, if the fill permeability alone
is reduced below the base case value (4 x
10 -11 m2), radon entry is reduced substan-
tially. At very low fill permeabilities, con-
vective flow of radon from the soil is es-
sentially negligible, and is largely invariant
with regard to changes in other param-
eters. Even at a more modest fill perme-
ability of 10 -12 m2, total radon entry is
reduced by 80% from the base case rate.
Note that these results assume that the fill
material maintains its integrity; i.e., no
cracks or gaps develop in the fill or in
those regions of the fill penetrated by util-
ity pipes or conduit.
Changes in radium content of the fill
have some effect on total radon entry,
though the more significant effects occur
for fill radium contents more than 3 times
the base case. Changes in the soil radium
concentration can have a more important
effect, depending on the depth of the ra-
dium-bearing layer. Where the radium con-
tent of the soil below 1.5 m is a factor of 5
times that of the base case, radon entry
increases by more than 3 times the base
case value, while a 10-fold increase in
radium provides a radon entry rate that is
6 times greater than in the base case. For
a radium-rich soil layer below 5.5 m, the
changes are less pronounced, with only a
25% increase in radon entry arising from
a 10-fold increase in the radium content.
•U.S. Government Printing Office: 1992 — 750-071/60157
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K.L Revzan, W.J. Fisk and R.G. Sextro are with Lawrence Berkeley Laboratory,
Berkeley, CA 94720
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Modeling Radon Entry Into Florida Houses with
Concrete Slabs and Concrete-Block Stem Walls, Florida Radon Research
Program," (Order No. PB92-201128; Cost: $26.00; 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 and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-92/119
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