United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-91/050 Sept. 1991
Project Summary
An Assessment of Soil-Gas
Measurement Technologies
Harry E. Rector
This report reviews the technologies
for measuring radon In soil gas. The
review addresses methodologies Involv-
ing In-srtu detection, sample extraction,
and surface flux, focusing on identifying
the range of options for measuring ra-
don in the soil. The following aspects of
each measurement approach are evalu-
ated:
• Measurement objectives—the spe-
cific parameter(s) that each technol-
ogy Is designed to measure (e.g.,
soil gas concentration, flux density,
permeability).
• Equipment needs—commercial
availability of systems and/or com-
ponents, and specifications for fab-
ricated components.
• Procedural information—docu-
mented elements of field and labo-
ratory methodology and quality as-
surance.
• Underlying assumptions—concep-
tual and mathematical models uti-
lized to convert analytical outcomes
to estimators of radon potential.
Basic technologies and field data are
examined from a generic perspective
(e.g., the common denominators of pas-
sive detectors, hollow sampling probes,
flux monitors) as well as specific con-
figurations developed by individual in-
vestigators (e.g., sample volume, depth)
to develop the basis for separating ana-
lytical uncertainties from sampling un-
certainties. Available technologies are
also reviewed in terms of theoretical and
practical utility as well as cost effective-
ness.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering Infor-
mation at back).
Introduction
Afairly wide rangeof methods forcharac-
terizing the radon potential of land areas
has evolved over the last decade through
research programs in this country and
abroad. This reviews published technolo-
gies that could support soil-based estima-
tors of radon potential. Basic technologies
concentrate on measuring (1) radon in soil
gas, (2) radon flux from the surface, or (3)
radium content. Approaches may also in-
clude attendant measures of soil character-
istics and otherfactors to support predefined
indexes of radon potential.
Fundamental Considerations
Soil and rock are the main source of
radon in buildings. Although broad spatial
trends of indoor radon are in rough propor-
tion to soil radium concentrations, the ema-
nation and subsequent migration in the soil
and ultimately into buildings is determined
by processes and characteristics at work in
the soil, in the building, and in the surround-
ing environment.
Quantitative estimates of radon potential
for soils are predicated on a volume of soil
in flow communication to a building, a sup-
ply of radon to the pore spaces of the soil,
and transport mechanisms to convey radon
into the building. The situation is compli-
cated by a number of factors. The soil
volume of interest is not defined by physical
boundaries; rather, the strength of the trans-
port mechanisms coupling the building to
the soil defines the basic limits of migration.
Radon emanation rates to the soil pores are
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controlled by the radium content of the soil
grains, and are further tempered by soil
moisture.
In most soils, the pore space contains
both air and water, providing the opportunity
for radon to partition between the air and
water phases. If the volume portion of the
pore space occupied by water is small,
radon emanation is directed primarily to the
gas phase. As the volume fraction of water
grows, however, it does so at the expense
of the gas phase. The gas phase vanishes
at complete saturation.
Radon delivered to the soil pores can
migrate through the ground by: (1) diffusion,
in which the radon moves with respect to the
pore fluid in order to equalize concentration
gradients; and (2) forced convection, in
which the pore fluid moves under the influ-
ence of external forces, carrying the radon
along with it. Diffusion can occur with or
without forced convection.
For soil systems exposed to the air, the
large concentration differences between the
soil pores and the overlying atmosphere
create a concentrat ion profile in the soil that
increases with depth. While the production,
migration, and exhalation of radon in undis-
turbed soils is well-approximated by diffu-
sion, the presence of a building dramatically
changesthe system. First, excavation, grad-
ing, and fill modify the soil environment.
Second, the building interrupts the commu-
nication between the soil and the atmo-
sphere. Third, operation of building sys-
tems and environmental influences on the
building create pressure differences that
supply the basis for forced convective trans-
port through cracks, joints, and service pen-
etrations connecting the building to the soil.
Pressure-driven transport of radon-bear-
ing soil gas into the building through cracks,
joints, and service penetrations is favored
over diffusion if the building is depressur-
ized. Pressure-driven flows dominate trans-
port in the soil at higher permeabilities,
while diffusion is probably the dominant
transport mechanism in the soil for situa-
tions of low permeabilities.
Measurement Technologies
While the basis for judging radon poten-
tial is still evolving, measurement strategies
have converged along basic themes ad-
dressing (1) radium content, (2) soil gas,
and (3) radon flux. Other types of measure-
ments have been developed to quantify
moisture, bulk density, permeability, poros-
ity, and other soil properties that relate to
the production and migration of radon in the
soil. Concerns have been raised about rep-
resentative sampling. While measurements
of radon potential based on invariant soil
properties could alleviate some of these
concerns, representative soil conditions
would still need to be defined for this ap-
proach and model relationships would still
be required to adjust measured values.
Measurement strategies for estimating
radon potential hinge on detecting the ra-
dioactivity in a known sample volume (or
mass) whose history has been controlled to
represent one or more processes germane
to the production and migration of radon in
the soil. Radium content is measured by
isolating a defined volume of soil to retain
the emanating fraction. At radioactive equi-
librium, the activity concentration of radon
and radon progeny is equated with the
radium concentration. Soil-gas measure-
ments, on the other hand, seek to isolate
radon in the pore spaces without affecting
emanation or transport. Flux-based mea-
surements rely on natural or induced trans-
port through the soil column to deliver the
radon to a sampling volume defined over a
specified area of the soil.
Basic approaches for measuring the ra-
dium content of soils involve sealing a soil
sample in a leak-proof container, storing the
sealed sample for a long enough period of
time to establish radioactive equilibrium,
and analyzing for radionuclides of interest
using gamma spectroscopy. Protocols fre-
quently accommodate concurrent analysis
of moisture content, laboratory estimates of
radon emanation, and other analyses by
subdividing field samples. Variations in pro-
cedure include repeated analyses to evalu-
ate the secular equilibrium between radium
and radon.
Basic technologies for measuring radon
concentrations in soil gas have evolved
along three complementary pathways: (1)
gas extraction from depth using holbwtubes,
(2) analysis of bulk soil samples, and (3) in
situ detection. The reconnaissance probe
for soil-gas extraction is a relatively simple
system consisting of a small-diameter (6- to
9-mm) thick-walled carbon steel tube that is
driven to sampling depth (75 cm, nominal)
using a slide hammer. While the reconnais-
sance probe is intended for collecting grab
samples of soil gas, it has been suggested
that the system can be used for determining
soil permeability. The permeameter probe
is further equipped for controlled flow ex-
traction to allow for estimates of soil perme-
ability from pressure/flow relationships as
well as radon concentration. The packer
probe is a more complex apparatus that
features inflatable packers to intercept sur-
face air.
Basic approaches for determining soil-
gas concentrations from bulk samples of
soil generally involve sealing the sample
under known conditions and measuring the
evolution of radon in the sample with time.
Three basic patterns can be recognized: (1)
emanation, a variation of the standard labo-
ratory test for radium that infers pore gas
radon from time-related changes in a sample
at controlled dryness, (2) prompt bismuth, a
second variation that monitors time evolu-
tion from field conditions, and (3) exhala-
tion, involving analysis of radon escaping
from the sample to a headspace.
Both the emanation and the prompt bis-
muth techniques monitor the ingrowth of
radon in the soil sample, producing data to
readily estimate undepleted soil gas con-
centrations. The exhalation technique, on
the other hand, is used primarily to deter-
mine the time rate of release of radon, and
requires additional information to estimate
undepleted soil gas concentrations.
In situ detection involves direct burial of
detectors to estimate radon concentrations
in the soil. The main avenue of development
entails forming a suitable detection volume
in the soil and detecting alpha activity from
radon diffusing into the cavity and subse-
quent decays of the short-lived progeny.
Two basic techniques are evident: passive
and active detection.
Passive in situ detection is probably the
most widely used. While the alpha track
detector is the system most closely identi-
fied with in situ passive measurements in
the soil, the basis can be extended to other
technologies. The second approach, involv-
ing an active detection system, presents an
opportunity to study short-term effects but
has not been used widely.
While buried alpha track detectors have
been used widely, generalized criteria with
regard to placement have not emerged. A
recent theoretical analysis indicates that
passive in situ detectors could significantly
underestimate soil gas concentrations at
high moisture levels because diffusive trans-
port is reduced as the pores fill with water.
For cavities of the approximate size for
alpha track detectors, however, significant
departures may not appear until water satu-
ration is fairly high (e.g., 80%).
Measurement systems for radonflux seek
to determine the net transfer from the soil to
the atmosphere. Basic approaches have
focused on capturing radon leaving the soil
using (1) closed accumulators, (2) flow-
through accumulators, and (3) adsorption.
Each of these approaches involves isolat-
ing an areaof soil and measuring the amount
of radon captured over a defined period of
time. A fourth method, induced flux, in-
volves applying controlled suction to the
surface of the soil. This technology has not
been applied to soil gas radon, but could
directly simulate flow coupling of a building
to the soil.
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The closed accumulation approach in-
volves direct accumulation of radon into a
volume defined by the soil surface and a
vessel whose open face is affixed to the soil.
The radon concentration in the accumulator
begins to increase as soon as the vessel is
emplaced because dispersion to the atmo-
sphere is eliminated. To more closely simu-
late natural conditions in the collection vol-
ume, flow-through accumulation can be used
to sweep radon out of the accumulator and
replace it with radon-free (or nearly so) air.
If radon concentrations in the accumulator
are maintained low enough to suppress
back diffusion, radon flux into the accumu-
lator is proportional to the radon content of
the exiting air stream. The basic method for
adsorption involves placing a charcoal can-
ister in contact with the surface for a period
that may range from a few hours to a few
days.
Technical Considerations
Currently, there are no hard and fast
criteria to provide an unambiguous refer-
ence for judging the performance of mea-
surement technologies for radon potential.
While there is little doubt that site-specific
measurements can be used to determine
the radon potential of land areas, interpreta-
tions are driven by empirical correlations
and theoretical considerations. A broad
consensus, however, highlights the impor-
tance of examining the abundance of radon
in the soil and its propensity to migrate into
buildings. Ideally, then, methods would pro-
vide information on the undepleted soil con-
centration, diffusion coefficient, and perme-
ability through various combinations of di-
rect measurements and model assump-
tions.
Each measurement approach reviewed
in this report can provide useful information
to evaluate radon potential. Technologies
geared to measuring (1) radium concentra-
tions in bulk soil samples or (2) soil gas
concentrations are readily applied to the
problem of estimating the undepleted radon
concentration in soil gas. Measurements of
unattenuated flux provide estimates of dif-
fusive transport which, in turn, could be
used to estimate soil-gas concentrations at
depth. The induced flux method, although
untested, may provide the means to directly
simulate radon entry for slab-on-grade and
crawl space construction. Laboratory mea-
surements of exhalation, on the other hand,
while not readily extrapolated to the soil
environment, may provide clues to the rela-
tive strength of radon sources through com-
parative tests.
Radium-based measurements have the
distinct advantage of being suited to testing
water-saturated soils. Soil-gas-based mea-
surements (extraction probes, in situ detec-
tion, flux), on the other hand, generally fail to
obtain samples from saturated soils be-
cause the gas volume is nearly zero. Rec-
ognition factors to avoid generally saturated
conditions can be built into protocols, as can
rules to invalidate samples from saturated
layers encountered at depth.
Material that is permanently saturated in
the native state but likely to reach varying
degrees of dry ness after construction, how-
ever, is best characterized using radium-
based measurements. These circumstances
are likely to occur with fill material and may
occur in areas with a shallow water table
that could recede as property development
alters drainage patterns.
Quality assurance is a vexing question
for soil-gas measurements. Although ana-
lytical proficiency can be deemed accept-
able, there is little information at hand to
evaluate system-level performance because
relatively few studies have explicitly com-
pared technologies. A number of studies
have included more than one soil measure-
ment technique, but additional analysis
would be required to formally compare
methods.
Limited comparisons to date provide a
fair degree of reassurance that the different
methods are comparable, but the test con-
ditions incompletely reflect current practice.
It would be useful to address intermethod
comparability by reanalyzing data bases
from completed multicomponent studies as
well as studies that are nearing completion.
Staged intercomparisons are also recom-
mended; it is generally known that such
intercomparisons have been conducted on
an informal basis, but the results have not
been published as yet.
Excepting the induced flux technique, the
measurement strategies summarized in this
report represent stable technologies sup-
ported by operational experience. The ba-
sis for assembling generalized protocols
exists and needs to be evaluated in detail to
develop method-specific protocols that can
be circulated for consensus review.
Practical Considerations
Practical decisions are likely to be guided
by two absolutes: (1) avoidance of clearly
inappropriate technologies, and (2) meet-
ing the schedule demands of the situation.
For the radium-based measurements, the
all-weather capability must be judged against
the lengthy time period necessary to achieve
radioactive equilibrium. Delays could be
shortened by taking more counts during the
ingrowth period to extrapolate data to equi-
librium levels. For soils with a low emana-
tion fraction, a number of days may need to
elapse to resolve the trend, but turnaround
time could, in concept, be reduced to a
matter of days. Further, initial count data
offer information to provide a rough esti-
mate without extended waits.
While the soil-gas extraction techniques
are not suited to testing under saturated
conditions, the simplicity of equipment and
field operations for the hand-driven probes
can deliver prompt results, making the re-
connaissance probe and the permeameter
probe likely candidates for widespread use.
The packer probe is a bit more complex and
requires an augered hole, but delivers data
in a short timeframe.
In situ detectors offer possibly the least
expensive approach. Emplacing detectors
at a satisfactory depth (1 m) and retrieving
them may present a problem. The main
disadvantages, however, could arise from
the need to sample for relatively long peri-
ods of time and from unreliable results in the
presence of high moisture levels.
As noted earlier, measurements of
unattenuated flux can be converted to esti-
mates of soil-gas radon at depth. This con-
version, however, is predicated on model
assumptions that may go unverified in the
field. Similarly, laboratory exhalation can-
not be readily extrapolated to quantitative
estimators of radon potential. The induced
flux technique may prove to be a useful test
apparatus for soils receiving slab on grade
or crawl space construction. At the present
time, however, it is an untested technology.
Conclusions and
Recommendations
Each available technique for measuring
radon in the soil provides some useful and
informative data, but the means to apply
these data are still evolving. How this evo-
lution will affect future strategies and proto-
cols for soil measurements remains to be
seen. In the end, the utility of soil-based
measurements is probably more sensitive
to the interpretive framework than to the
technologies employed to collect the data.
A firm and quantitative basis needs to be
established for formally comparing different
soil-measurement technologies. Achieving
this basis would most likely require repli-
cated testing of the various technologies
across a range of soil types and sampling
conditions. Allied to this, existing multicom-
ponent data bases should be analyzed to
place different measurement parameters
on a common basis. The virtues of mea-
surements directed toward invariant soil
properties warrant further investigation from
technical and practical standpoints, and the
current range of accepted practice for soil-
gas measurements needs to be assembled
and compiled in a manner suited to consen-
sus review.
•6U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40061
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Harry E. Rector is with GEOMET Technologies, Inc., Germantown, MD 20874.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "An Assessment of Soil-Gas Measurement Technolo-
gies, " (Order No. PB91- 219 568; Cost: $17.00, subject to change) will be available
only from:
National Technical Information Se/v/ce
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
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POSTAGE & FEES PAID
EPA
PERMIT No. G-35
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Penalty for Private Use $300
EPA/600/S8-91/050
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