United States
Environmental Protection
Agency
Research and Development
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
EP/V600/SR-94/218
Project Summary
Soil Radon Potential Mapping of
Twelve Counties in North-Central
Florida
Kirk K. Nielson, Rodger B. Holt, and Vern C. Rogers
This report describes the approach
methods, and detailed data used to pre-
pare soil radon potential maps of twelve
counties in North-Central Florida. The
maps were developed under the Florida
Radon Research Program to provide a
scientific basis for implementing
radon-protective building construction
standards in areas of elevated risk and
avoiding unnecessary regulations in
areas of low radon risk.
Calculated soil radon potentials re-
flect geographic variations by model-
ing the potentials as the rate of radon
entry into a reference house that Is
successively modeled on the soils in
each radon map region. Individual soil
profile properties are defined by hori-
zon from county soil survey data Ra-
don source properties are defined from
aeroradiometric data and from soil ra-
dium and radon emanation measure-
ments. Calculated rates of radon entry
into the reference house are grouped
into tiers for display on radon potential
maps. Comparison of the calculated
radon entry rates with indoor radon
data yields a geometric standard de-
viation of 2.1 and indicates that the
reference house is consistent with the
aggregate properties of the Florida
houses in the comparison.
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 the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
Information at back).
Introduction
Soil radon potential maps are being de-
veloped for Alachua, Citrus, Clay, Duval
Flagler, Lake, Levy, Marion, Nassau'
Putnam, St. Johns, and Volusia counties
in Florida. They are designed to show
from soil and geological features the ar-
eas that have different levels of radon
potential. The maps are being developed
under the Florida Radon Research Pro-
gram to provide a scientific basis for imple-
menting radon-protective building construc-
tion standards where they are needed and
to avoid the cost of unnecessary imple-
mentation where they are not needed.
Soil radon potentials are defined for
mapping purposes as the calculated
annual-average rate of radon entry from
soils into a reference house. They are
calculated to represent geographic radon
sourcia distributions, minimizing the influ-
ences of house and occupant variations
temporal variations, and political and insti-
tutional boundaries (city, county, etc.). The
mapping approach consists of
Definition of radon map polygons
(geographic areas on a radon map)
from existing soil and geologic
maps.
Definition of the soil profiles asso-
ciated with each radon map poly-
gon and their associated radon gen-
eration and transport properties.
Calculation of numeric soil radon
potentials for individual soil profiles
and an area-weighted average to
represent each radon map polygon.
a.
b.
c.
Printed on Recycled Paper
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d Grouping map units with similar ra-
don potentials and plotting the ra-
don map polygons by color-coded
radon potential tiers.
Radon map polygons ^redefined by
tho digital intersection of STATSGO soil
map units with digitized surface-geology
map units. The intersections to def.ne the
polygons were performed with a geo-
maphic information system under Arclnfo
format at the University d Florida GeoPlan
Center. The STATSGO soil maps defined
29 units in Alachua County, 13 in Citrus
County, 14 in Clay and Duval counties, 13
in Flagler County, 16 in Lake County. 21
in Levy County, 20 in Marion County, 11
in Nassau County, 20 in Putnam County,
18 in St. Johns County, and 21 in Volus.a
County. The map units occurred in mul-
tiple polygon areas in each county, rang-
ing from about 40 to 120 polygons per
county. When intersected by geology poly-
gons, the number of map polygons was
approximately doubled.
Soil profiles in each polygon were de-
fined from county soil sunwdate comi-
piled at the University of Florida Soil and
Water Science Department for each hori-
zon in each of several profiles. The data
included horizon depth and thickness, den-
sity, textural analyses and classifications,
water drainage characteristics, high water
table depths and durations, and related
physical properties.
Radon Entry Modeling
Soil radon potentials were defined to
eliminate house and occupant variables
by using a hypothetical "reference house
with invariant properties, including indoor
pressure, ventilation rate, slab and foun-
dation design and attributes, and other
reference conditions. The reference house
was modeled as if it were located on each
of the soil profiles that made up the land
areas of each map polygon to reflect the
average differences and variabilities in ra-
don potential between and within the map
polygons. The annual-average rates of ra-
don entry into the reference house were
modeled using complete, multiphase ra-
don generation and transport equations.
They characterize indoor radon entry by
both diffusion (concentration-driven) and
advection (with pressure-driven air flow).
The soil radon entry modeling for the
reference house utilized detailed soil pro-
file data defined by county soil survey
analyses and surface geology data. The
detailed soil profiles analyzed with the ref-
erence house for each location included
individual radon source and transport prop-
erties of each soil horizon. The reference
housa was defined with the approximate
characteristics of Florida slab-on-grade
single-family dwellings. It consisted of an
8 6 x 16.5m rectangular structure with a
perimeter shrinkage crack between the
floating floor slab and the stem walls. The
indoor pressure, house ventilation rate.
floor slab properties, and other house char-
acteristics were based on typical values
measured in Florida houses. The radon
entry modeling represented soil moisture
profiles under the reference house by an
annualized distribution that was defined
from the reported high water table depths
and durations.
Since the map calculations utilized a
fixed set of house and foundation charac-
teristics and considered only vertical varia-
tions in radon source and transport prop-
ertiesMhe 2-dimensionaURAdon-Emana--
tion and TRansport into Dwellings
(RAETRAD) model was used to develop
a more efficient, 1-dimensional algorithm
that gave equivalent results for the refer-
ence house. The specialized radon poten-
tial cartography algorithm was named
RnMAP and was shown to approximate
the reference-house RAETRAD analyses
within about 5%. Most of this difference
resulted from the finite-difference math-
ematics in RAETRAD compared to ana-
lytical radon calculations in RnMAP. The
1 -dimensional radon generation and trans-
port calculations in RnMAP define the
sub-slab radon concentration, from which
radon entry rates are computed with em-
pirical functions for radon diffusion through
the intact floor slab and for diffusive and
advective transport through floor cracks.
The empirical functions, fitted to the
sub-slab soil properties, also define the
coupling of the soil region to the
reference-house slab properties based on
corresponding RAETRAD calculations.
Radon Source and Transport
Parameters ,
Radon potentials of each map polygon
were calculated from the radon source
and transport properties of the soil pro-
files comprising the polygon region. Ra-
don source properties were estimated from
National Uranium Resource Evaluation
(NURE) aeroradiometric data for shallow
horizons (0 to 2 or 2.5m). and from geo-
logical classifications of the soils for deep
horizons (to 5m depth). The NURE data
were averaged [to obtain a geometric
mean and geometric standard deviation
(GSD)1 for each polygon from data in an
flight-line segments within the polygon, and
were converted from equivalent uranium
concentrations to corresponding radium
concentrations for the model calcutalions.
Polygons not intersected by NURE flight
lines were represented by the geometric
mean and GSD of all NURE data for their
geological classification in the county. The
NURE flight-line data were partitioned digi-
tally into map polygon segments with the
same geographic information system used
to define the map polygons.
Radon amanation coefficients for each
of the NURE-based radium concentrations
were defined from a measured trend of
increasing emanation with radium concen-
tration. The trend had the form
E » 0 15Ra + 0.20 for radium levels below
23 pCi g-1 and remained constant at
0 50-0.55 for higher radium levels. The
trend was based on emanation measure-
ments from over 200 samples from the
twelve counties. Most of the samples were
from University of Florida soil-survey ar,
chives and corresponded to the soils used
to develop the STATSGO soil maps The
remainder were from U.S. Geological Sur-
vey borings. To attain adequate precision
at low radium concentrations, the radon
emanation measurements utilized a new
effluent technique that is described and
validated in the full report.
Deep-soil radium concentrations were
defined from surface geology classifica-
tions and radium measurements in a larger
qroup of over 600 soil samples. These
included the samples used for emanation
measurements plus additional samples
from the same sources.
Radon transport properties were esti-
mated from soil profile physical data com-
piled for each STATSGO soil map unit.
The radon transport properties (radon dif-
fusion coefficients and air permeabilities)
were calculated from empirical correlations
with soil horizon water contents, porosi-
ties, and particle sizes. Soil horizon water
contents were calculated from their height
above the water table using soil water
drainage data. Steady-state water balance
calculations indicated that for water tabes -
in the 5m or shallower range, sub-slab
soil water contents were well-approximated
by the soil drainage-curve moisture at a
matric potential that was equal to the dis-
tance above the water table. Water drain-
aqe data compiled for each soil horizon in
each soil profile therefore were directly
interpolated from the horizon-to-water-tab e
distance to estimate soil moistures Field
measurements of near-surface soil water
matric potentials at 46 locations in Central
Florida confirmed the range of matric po-
tentials being used.
Calculation of Soil Radon
Potentials
Soil radon potentials were computed Dy
mathematically modeling the reference
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house as if it were located on each soil
profile of each of the radon map poly-
gons. The RnMAP calculations used the
specific radon source and radon transport
properties of the horizons in each soil
profile. The radon potentials were calcu-
lated as the rate of radon entry into the
reference house in annual units (mCi y1)
to emphasize the long-term average na-
ture of the radon potential estimates.
Radon potentials were calculated for
each of several soil profiles in each poly-
gon, at each of two or three seasonal
water table depths. They then were aver-
aged seasonally to obtain annual-average
radon potentials, which in turn were aver-
aged over the different profiles to repre-
sent each polygon. Radon potentials also
were calculated for both low- and
high-radium geology, and the applicable
geologic classification was used afterward
to select the appropriate values to repre-
sent each polygon.
Separate radon potentials were calcu-
lated for the estimated median of the dis-
tribution in each polygon and also for ra-
don potentials corresponding to the 75,
90, and 95% confidence limits. The confi-
dence limits were based on the geometric
means and GSDs of radium computed
from the NURE data distributions in each
polygon and also on the varied properties
of the different soil profiles that comprised
the polygon.
Production and Interpretation
of the Radon Maps
The resulting radon potentials were par-
titioned into seven tiers of similar numeri-
cal values for display on the radon poten-
tial maps. The tiers corresponded to the
<0.4, 0.4-1, 1-2, 2-3, 3-6, 6-12. and >12
mCi y1 levels of radon potential. This set
of tiers provided suitable ranges for using
a uniform tier scale on all of the radon
potential maps. Map polygons were col-
ored according to the appropriate tier clas-
sification for intuitive visual interpretation.
However the numerical values of the ra-
don potentials for each map polygon are
presented in the report for more quantita-
tive map interpretations. A radon potential
of 3 mCi y1 corresponds to approximately
3.9 pCi L1 in the reference house.
Separate maps were plotted for the me-
dian, 75, 90, and 95% confidence limits of
radon potentials to give a better perspec-
tive of radon potentials in a given polygon
(region). Regions with low potentials on
both the median and higher-confidence-limit
maps exhibit reasonable assurance of hav-
ing minimal indoor radon risk. Regions with
high radon potentials on the median and
higher-confidence-limit maps conversely
have a relatively high probability of el-
evated indoor radon levels. Regions with
low median radon potentials but high pd-
tentials for higher confidence limits are
heterogeneous (low median; high GSD)
arid may have generally low radon poten-
tials but occasional to frequent anomalies
with high radon potential. Special consid-
erations may be needed to define
radon-protective building needs in these
areas.
The calculated soil radon potentials ware
compared with 804 indoor radon mea-
surements in the twelve counties from the
state-wide land-based radon survey. The
comparison was consistent with the
reference-house indoor radon accumula-
tion rate of 1.3 pCi L1 per mCi y1 of soil
radon potential and with an ambient out-
door radon concentration of approximately
0.3 pCi L-'. The GSD between measured
indoor radon levels and those predicted
from the maps was 2.08, which is the
approximate level of precision associated
with the calculated soil radon potentials.
The total variation among measured in-
door radon levels was partitioned to esti-
mate a house variability of GSD=3.7, com-
pared to soil variability on the order of
GSD=2.2 to 2.4. Uncertainties are much
higher in predicting an indoor radon level
for a particular house than for predicting
the median level in the reference house
for a given polygon.
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K Wfe/son, R Holt, and V. Rogers are with Rogers and Associates Engineering
' Corp., Salt Lake City, UT 84110-0330.
changa) 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 tor Environmental Research Information
Cincinnati, OH 45268
Official Business
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PERMIT No. G-35
EPA/600/SR-94/218
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