United States
Environmental Protection
Agency
National Kisk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/161
February 1996
& EPA Project Summary
Site-Specific Characterization
of Soil Radon Potentials
Kirk K. Nielson, Rodger B. Holt, and Vern C. Rogers
The Florida Department of Commu-
nity Affairs is developing construction
standards for incorporating radon-re-
sistant building features in areas of el-
evated soil radon potential. Although
statewide maps have been developed
to show the regions where the features
are required, there is also a need for
simple methods to assess the radon
potential of specific building sites. The
report gives results of the development
and evaluation of a mathematical basis
for using simple site measurements to
estimate soil radon potential. The ap-
proach utilizes a lumped-parameter
model of radon generation and entry.
Site-specific soil radon potential is de-
fined as the rate of radon entry into a
reference house, consistent with previ-
ous definitions used for the statewide
radon maps. The model shows that, in
the simplest case, soil radon potential
is reduced to a simple function of two
measurable parameters: the soil sur-
face radon flux and the soil moisture
(as a fraction of saturation). The flux
gives the radon generation rate of the
soil profile, and the moisture is a sur-
rogate for radon transport parameters,
including air permeability and radon
diffusion coefficient.
Field tests of soil radon flux and
moisture measurements were con-
ducted at 26 house sites in Polk County,
Florida, to evaluate their utility in pre-
dicting site-specific radon potentials.
Radon fluxes also were measured from
bare concrete surfaces, where they
were accessible, to better estimate non-
advective radon entry rates. Gamma-
ray intensity also was measured in the
yards, but it failed to correlate well with
the radon fluxes. The measured soil
radon fluxes and moistures showed lo-
calized trends that compared well with
mapped radon potentials in some
cases, but not in others. For the 26
sites, the radon potentials estimated
from site-specific measurements aver-
aged twice the potentials from the gen-
eralized radon maps. A large geomet-
ric standard deviation (GSD = 4.7) was
associated with individual sites.
The site-specific estimates also were
compared with prior indoor radon mea-
surements. When the reference-house
ventilation rate was attributed to the
houses, the calculated/measured radon
ratios averaged 1.06 ± 0.72. Slightly
greater bias but improved precision
(0.87 ± 0.56) was obtained using con-
crete-surface radon flux measurements
in addition to the site radon potential
measurements. The empirical measure-
ments suggest that the precision of
site-specific evaluations is marginal,
leaving an uncertainty of about a fac-
tor of 2 in site-specific estimates. Al-
though potentially useful for some ap-
plications, the site-specific measure-
ments studied here do not greatly im-
prove the radon potential estimates
over the regional estimates already
available from radon maps.
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
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fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Radon (222Rn) gas generated by natu-
rally occurring radium (226Ra) in soils can
enter buildings through their foundations.
With elevated entry rates and inadequate
ventilation, radon can accumulate indoors
to levels that pose significant risks of lung
cancer with chronic exposure. The Florida
Department of Community Affairs (DCA)
and the EPA have jointly developed radon-
resistant building standards to help re-
duce radon entry from soils. The stan-
dards address improved understructure
sealing, altered air pressures, and other
engineered features developed under the
DCA's Florida Radon Research Program
(FRRP). Statewide radon potential maps
also have been developed to identify re-
gions where the radon-resistant features
are needed. This report examines the fea-
sibility of estimating soil radon potentials
from simple measurements to help deter-
mine more accurately the radon protec-
tion needed at a specific building site.
Soil radon potential is defined using the
same reference house as was used previ-
ously for the maps. A lumped-parameter
model simplifies the theoretical basis, and
indicates minimum parameters and surro-
gates for characterizing the site radon po-
tential. Field tests of the methods include
measurements of selected parameters and
surrogates at 26 house sites that were
already being studied under the FRRP.
Soil radon potentials estimated from the
field measurements were compared with
mapped regional estimates, and were also
compared with data from measured radon
levels in the houses at the sites.
Previous Estimates of Radon
Potential
Radon indices and simple models have
been proposed previously for estimating
soil radon potential. These have depended
variously on house ventilation rates, ema-
nating soil radium concentrations, soil air
permeability, soil radon concentrations, soil
porosity, soil radon migration distance, soil
water permeability, and soil equivalent ura-
nium concentration. A more detailed ap-
proach numerically analyzes advective ra-
don transport into houses. This approach
utilizes soil air permeability, soil radon gen-
eration rate (radium concentration, den-
sity, and radon emanation coefficient),
foundation crack geometry, and house air
pressures. It defines radon potential as
the rate of radon entry into a house in
picocuries per second. A review of site
measurement methods shows the need
for detailed radon source and transport
measurements, including soil density, par-
ticle size, texture classification, moisture,
permeability, diffusion coefficient, radon
emanation coefficient, radium concentra-
tion, and radon concentration profiles.
These properties are used in a radon
source potential index that depends also
on site drainage conditions, site ground-
water conditions, and site climatology.
A more detailed modeling approach
characterizes radon entry from house and
soil parameters, including radon move-
ment by both advection and diffusion. Us-
ing the RAETRAD model, this approach
uses detailed soil radium distributions; ra-
don emanation fractions; and soil density,
moisture, permeability, and diffusion coef-
ficients with house air pressure and crack
distributions. Soil radon potential is de-
fined on an annual basis (in millicuries per
year) to emphasize the long-term average
nature of equivalent steady-state radon
entry rates and exposures.
Reference House
The site-specific soil radon potential is
defined as the annual rate of radon entry
from soils into a hypothetical reference
house that is defined to represent Florida
slab-on-grade houses. The reference
house provides a constant, typical inter-
face between the indoor exposure volume
and the varied soil conditions that control
radon potential. Although house and soil
parameters cannot be completely sepa-
rated for modeling radon entry, the use of
a reference house avoids the large differ-
ences in radon potential that would other-
wise result from differences in house de-
sign, construction, ventilation, and occu-
pancy.
The present reference house corre-
sponds to the house defined previously
for radon potential mapping. The house is
an 8.6 x 16.5 (28 x 54 ft), slab-on-grade
single-family dwelling. Its volume is based
on that of a median U.S. family dwelling,
and is similar to that of typical Florida
houses. Its area is estimated from its vol-
ume using a nominal 2.4-m (8-ft) ceiling
height. Its ventilation rate is about half the
normal median U.S. house ventilation rate,
based on measurements in Florida houses.
A perimeter floor crack approximates a
floating-slab shrinkage crack to permit ad-
vective radon entry from pressure-driven
air flow. The stem wall and footing pen-
etrate 61 cm (2 ft) into the natural terrain,
and enclose an additional 30 cm (1 ft) of
above-grade fill soil beneath the slab. The
indoor pressure is -2.4 Pa, typical of pres-
sures from thermal and wind-induced pres-
sures in U.S. houses, and also typical of
the average pressures measured in 70
Florida houses under passive conditions.
Concrete slab air permeabilities, radon dif-
fusion coefficients, and other properties
are estimated from data measured on
Florida floor slabs.
Soils beneath the reference house are
modeled as uniform, isotropic soils with a
bulk density of 1.6 g cm"3. A 30-cm layer
of sandy fill soil is located beneath the
slab, below which the site-specific soil is
represented by its textural class and its
associated water content at a matric po-
tential of -30 kPa. From these properties,
the soil air permeability and radon diffu-
sion coefficient are calculated from em-
pirical relationships.
Lumped-Parameter Model
The mathematical definition of site-spe-
cific radon potential utilizes a lumped-pa-
rameter model, which is based in turn on
the detailed RAdon Emanation and
TRAnsport into Dwellings (RAETRAD)
model. The lumped-parameter model was
developed primarily from RAETRAD sen-
sitivity analyses, which identified the most
significant house and soil parameters. The
analyses suggested simplified approxima-
tions to express average indoor radon lev-
els as a function of radon source strength
and house radon resistance and ventila-
tion parameters. Radon source strength
was defined in terms of the sub-slab ra-
don concentration. House radon resistance
was defined from floor openings, pressure
driving forces, and slab diffusivity. The
lumped-parameter model uses a simpli-
fied relation between indoor radon and
the radon entry rate:
Cnet=Cin-Cout =3.6Q/(XhVh) (1)
where Cnet = net indoor radon concen-
tration from sub-slab
sources (pCi L~1)
Cin = total indoor radon con-
centration (pCi L1)
Cout = outdoor background radon
concentration (pCi L1)
3.6 = unit conversion (pCi L1 rr1
per pCi rrr3s"1)
Q = radon entry rate (pCi s"1)
Xh = rate of house ventilation
by outdoor air (rr1)
Vh = n Ah = interior house vol-
ume (m3)
h = mean height of the inte-
rior volume of the house
(m)
Ah = house area (m2).
The radon entry rate in equation (1) is
defined in the lumped-parameter model
for the reference house as:
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Q = AhCsub[fc(vdc-vacAP)
+ Vs|ab+VSc] (2)
where Csub = sub-slab radon concen-
tration (pCi L1)
f, = area of floor openings
as a fraction of total
floor area (dimension-
less)
vdc = equivalent velocity of
radon diffusion through
floor openings, depen-
dent on the radon dif-
fusion coefficient of the
soil (0.0143 mm s'1)
vac = equivalent velocity of
radon advection through
floor openings, depen-
dent on the air per-
meability of the soil
(mm s'1 Pa'1) = exp(-
3-0.045e6S)
S = soil water saturation
fraction (dimensionless)
AP = indoor air pressure (Pa)
vsiab = equivalent velocity of
radon diffusion through
the slab, dependent on
the radon diffusion
coefficient of the slab
(mm s-1) = 2.9x10-7
exp(11.4W)
W = water/cement ratio of
the slab concrete (dimen-
sionless)
vsc = radon entry velocity ad-
justment for house size
and crack location (mm
s-1) = 3.5x10-=(xcrk/xh)
+ 4.6x10-5/xh
xcrk = location of dominant
floor crack opening from
house perimeter (m)
xh = house width (m).
Only the parameters Csub and vac in
equation (2) are site-dependent; therefore
reference-house values were substituted
for all of the others, leading to a simplified
relationship for defining the site-specific
soil radon potential:
Qss =1.68Csub [0.019
+ exp(-3-0.045 i
(3)
where Qss = site-specific soil radon po-
tential (pCiV1).
Although the indoor radon concentra-
tion for a reference house can be directly
estimated from Qss in equation (1), indoor
radon concentrations for specific houses
are better estimated by using as many
defining parameters in the lumped-param-
eter model as are known. Using the defi-
nitions associated with equation (2) to de-
fine the radon entry rate for equation (1),
indoor radon concentrations can poten-
tially be better estimated from house-spe-
cific data.
Surrogate Estimates of Model
Parameters
Many model parameters are difficult to
measure directly, and therefore are sel-
dom quantified. However, most can be
estimated from related parameters that
are directly measurable. The site-specific
value for the soil water saturation fraction
(S) can be readily estimated from mea-
sured soil moisture contents as:
S = 0.01 Mv/e = 0.01 pMw/e (4)
where M =
P =
M =
soil moisture (volume per-
cent)
total soil porosity (dimen-
sionless) = 1 - p/pg
soil bulk dry density (g
cnr3)
soil specific gravity (nomi-
nally 2.7 g cnr3)
soil moisture (dry weight
percent).
Using the reference-house slab param-
eters and assuming that the radon-gener-
ating soil profile is deep (unconstrained
by a water table or bedrock), Csub can be
approximated as:
Csub = (90 + 5,900JS)
/(1.13 + 35 ,/DJ
where J =
D =
(5)
radon flux at the soil sur-
face (pCi rrr2 s'1)
radon diffusion coefficient
of the soil pore space (cm2
s-1).
Field Tests
Sensitivity analyses with the lumped-
parameter model have demonstrated a
relatively strong dependence of radon po-
tential on Csub,Xh, W, AP, S, h, and f., and
a smaller dependence on vdc, xh, xcrk. Field
measurements therefore were directed at
quantifying the important parameters.
Where the parameters could not be ad-
equately measured, default values typical
of the reference house were used.
Site-specific field measurements were
conducted during March 17-22, 1993, to
evaluate the sensitivity, precision, and util-
ity of selected parameters for estimating
site-specific radon potential. The measure-
ments were conducted in the yards of 26
houses in Polk County, FL, for which in-
door radon data were already available.
The protocol at each site included mea-
surements on all four sides of the house
of soil moisture, gamma-ray activity, and
radon flux. In addition, radon flux was
measured from a bare concrete surface
where suitable locations were accessible.
The site protocol concentrated on rapid,
inexpensive measurements that could
most directly estimate the site-specific ra-
don potential, Qss. As indicated by equa-
tion (3), the sub-slab radon concentration
(Csub) and the soil water saturation fraction
(S) were of primary interest. The soil mois-
ture, Mv, was measured using a time-do-
main reflectometer probe, which charac-
terized the top 30 cm of soil. The value of
S was calculated from equation (4), as-
suming a soil porosity of e =0.407.
Although prior FRRP measurements of
Csub were planned, these data were gen-
erally unavailable, and consisted of only
single measurements in a few cases.
Therefore, the soil surface radon flux mea-
surements at each site became the pri-
mary estimator of Csub, using equation (5)
as the basis. The flux measurements were
made using the small-canister method,
which gives equivalent results to EPA
Method 115. The radon fluxes were
sampled over a 24-hour period, after which
the charcoal canisters were retrieved,
sealed, and submitted for laboratory as-
say of radon activity. The value of Ds
required for calculating Csub was estimated
from the same porosity and moisture us-
ing the predictive correlation:
Ds = D0 e exp(-6 e S - 6S
14e,
(6)
where Do = diffusion coefficient for ra-
don in air (0.11 cm2 s'1).
The radon flux measurements on bare
concrete slab surfaces, when accessible,
estimated more directly the radon entry
through the intact portions of the concrete
slabs. The flux measurements directly es-
timate v for use in equation (2) as:
v slab-Jslab/Csub (7)
radon
crete slab surface (pCi nr2
where Jslab = radon flux from the con-
The small charcoal canisters were found
to have marginal sensitivity for the lower
fluxes from concrete surfaces. Therefore,
an alternative method used approximately
230 cm3 of granular activated carbon
spread over a paper napkin mounted in a
30-cm diameter wooden compression
frame and covered with a polyethylene
sheet. The frame was sealed to the con-
crete surface with rope caulk. After a 24-
hour deployment, the charcoal was re-
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trieved and sealed into metal cans for
assay of radon. The method was cali-
brated against the small-canister samplers
using a thin-sample radon source. The
large-area method measured fluxes equal
to those from small canisters, but with 13
times greater sensitivity.
The gamma ray measurements were
intended for possible correlation with the
soil radon flux measurements, or as po-
tential surrogates for surface soil radium
concentration. The measurements were
made 1 m above the soil surface using a
5 x 5-cm sodium iodide scintillation probe.
Test Results and Analysis
The site-specific measurements were
first analyzed for simple empirical correla-
tions with the measured soil radon fluxes.
The measurements also were used to pre-
dict site-specific radon potentials, which in
turn were compared with the FRRP indoor
radon concentration data. The gamma ray
measurements showed greater differences
among the different house sites than the
radon flux measurements, which in turn
showed greater differences than the mois-
ture measurements. All of the variations
among house sites were significant at the
p<0.01 level in analyses of variance.
Since radon flux expectedly varies with
radium in uniform soils, a linear flux vs.
gamma relationship was sought by least-
squares linear regressions. The regres-
sion on individual gamma intensity mea-
surements (y in uR h'1) gave a correlation
coefficient of only r = 0.26 for the fitted
line Js = -0.2 + 0.146 y. A regression corre-
sponding to Js = 0.23 y062 was obtained
from log-transformed data. The regressions
were strongly affected by some low flux
points at high gamma intensity that were
associated with wet soils. The high uncer-
tainty in the flux vs. gamma relationship
limits the usefulness of gamma intensity
as a surrogate for soil radon flux.
A similar regression of soil radon flux
on soil moisture gave similar scatter. This
regression on individual moisture measure-
ments had a correlation coefficient of r =
0.26 for the fitted line Js = 6.9 - 0.268Mv.
Log-transformed data gave the line Js =
28 M;118. Low fluxes were associated with
high soil moisture levels.
Measured ra_don fluxes also were re-
gressed on yV Ds-tanh(xsV A/DS), where
X is the radon decay constant (2.1x10'6
s'1) and xs is the soil thickness dominat-
ing the flux (i.e., above the water table).
Since y is a surrogate for radium concen-
tration, this lumped parameter is the theo-
retical surrogate for radon flux, Jsur, for
uniform soil. The measured radon fluxes
were regressed on Jsur, which utilized ra-
don diffusion coefficients from equation
(6) and measured moistures. The regres-
sion had an improved correlation coeffi-
cient of r = 0.55 for the fitted line Js = -
0.85 + 1.51 Jsur. Log-transformed data gave
the least-squares fitted line Js = 0.6 Jsur.
Although this relation better predicts ra-
don flux, it still exhibits considerable un-
certainty, which limits its potential use in
predicting radon flux.
Estimation of Site Radon
Potentials
Site-specific soil radon potentials were
estimated using equation (3). Measured
soil moistures were used to determine S
from equation (4), and measured radon
fluxes were used to determine Csub from
equation (5). The calculated site-specific
soil radon potentials were averaged by
neighborhood groupings for area-based
comparisons with the mapped soil radon
potentials. Figure 1 illustrates the result-
ing geometric means and geometric stan-
dard deviations, and gives side-by-side
comparisons with the median mapped ra-
don potentials. For illustration purposes,
average error bars also were applied to
the three houses not associated with other
neighborhood groupings. As illustrated, the
Qss values were higher than the mapped
radon potentials (Q ) in six of the ten
comparisons. In statistical analyses of the
seven comparisons involving multiple
houses, the Qss values averaged 0.8 stan-
dard deviations higher than the Qm? val-
ues. This average bias is not significant
(p<0.41). The geometric mean of all 27
ratios of Qss/Qmap was 2.02, with a geo-
metric standard deviation of 4.7 (GSD of
the mean is 1.35). The positive bias in Qss
may result in part from radon flux sam-
pling near the houses, which can increase
102
1 10°
cc
"5
tn
io-1
Ir
D
Neighborhood
Figure 1. Comparison of site-specific and mapped soil radon potential distributions for seven neighborhood groupings (A-G) and three isolated houses.
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the flux compared to an open-field sample
that is away from the house foundation.
Comparison of Soil Radon
Potentials with Indoor Radon
Data
Indoor radon concentrations were esti-
mated from the soil radon potential mea-
surements for comparison with the FRRP
indoor radon data. The indoor radon con-
centrations were estimated from equation
(1) using the reference-house ventilation
rate (Xh = 0.25 h"1) with individual esti-
mates of house volumes. The ratios of
calculated/measured indoor radon concen-
trations averaged 0.94 ± 0.65 for the 13
slab-in-stem-wall houses and 1.18 ± 0.80
for the 13 monolithic-slab houses. The
overall average ratio for all 26 houses
was 1.06 ± 0.72. Despite the relatively
large scatter, this comparison demon-
strates close average agreement of calcu-
lated and measured radon concentrations.
A separate comparison also utilized the
measured radon fluxes from concrete sur-
faces, Jslab. Using equation (7), the fluxes
defined vs|ab, which was used in equation
(2) with reference-house values for the
other parameters. The ratios of calculated/
measured radon concentrations averaged
0.97 ± 0.64 for the 10 slab-in-stem-wall
houses, and 0.69 ± 0.35 for the five mono-
lithic-slab houses where concrete surface
fluxes were measured. The overall aver-
age ratio for all 15 houses was 0.87 ±
0.56, compared to 1.18 ± 0.89 for the
corresponding houses by the previous ap-
proach. The lower variation from using
the concrete fluxes shows that they im-
prove the estimate of radon transport
through slabs over the generic assump-
tion of equation (2). This approach has
slightly larger biases, but significantly im-
proves the precision over the previous
approach.
Discussion and Conclusions
The analyses in this report demonstrate
a theoretical basis for measuring site-spe-
cific radon potentials from a variety of
potential surrogate parameters. Although
radon flux and soil moisture were the pri-
mary parameters evaluated, others (e.g.,
soil radium, soil radon, soil air permeabil-
ity, and soil density) may also provide
useful results. The flux and moisture mea-
surements were chosen for their simplicity
and low cost. However, their marginal pre-
cision leaves an uncertainty of about a
factor of 2. This uncertainty can compro-
mise the basic purpose of site-specific
measurements, which is to reliably deter-
mine the potential for elevated radon at a
particular building site.
The need for better precision and accu-
racy may require other measurements. For
example, soil radium profiles measured
from borehole samples support more de-
tailed model analyses that determine ra-
don potentials with greater precision and
accuracy. Although more costly, radium
profile measurements have demonstrated
model and measurement agreement within
10-20%.
If site-specific measurements are con-
sidered for construction decisions, the plan-
ning should consider measurement costs,
radon control costs, measurement uncer-
tainty, and radon control uncertainty.
Ample design safety margins should allow
for measurement and control uncertain-
ties. The safety margins also give benefits
of reduced health risk from lower radon
levels, thereby serving a greater purpose
than simply assuring attainment of a < 4
pCi L1 indoor radon goal.
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K. Me/son, R. Holt, and V. Rogers are with Rogers and Associates Engineering
Corp., Salt Lake City, UT 84110-0330
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Site-Specific Characterization of Soil Radon Poten-
tials, "(Order No. PB96-140553; Cost $17.50, 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
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POSTAGE & FEES PAID
EPA
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EPA/600/SR-95/161
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