United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

EPA Project Summary

Research and Development EPA/600/SR-96/010 March 1996

Although slab edge detail and fill

composition are logical factors to consider in radon resistant construction,

little research has documented their effects on radon entry into slab-on-grade

structures. This study was conducted

to contrast the effectiveness of slab-instem wall (SSW) with floating slab (FS)

construction practices, to measure radon transport and entry for model testing, to develop protocols relevant to

depressurized radon measurements,

and to determine the effect of high radium fill soil on indoor radon concentrations. 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 radium 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 building put on top. The native soil contained about 0.2 pCi/g radium. The fill

soils ranged from 0.2 to 33 pCi/g radium. The indoor radon concentrations

in the FS cell were 3.5 times higher

than those in the SSW cell. These results agreed with predictions by a radon entry and transport (RAETRAD)

model. Whole building stresses and

slab area and crack length radon entry

were measured, and they yielded comparable results. Experiments in the fill

study suggest that the amount of emanating soil radium is a good predictor

for radon entry into a structure, indicating that elevated radium fill soil can

contribute significantly to indoor radon

concentrations.

Test Cell Studies of Radon Entry

A. D. Williamson, C. S. Fowler, and S. E. McDonough

This Project Summary was developed

by the National Risk Management Research Laboratory’s Air Pollution Prevention 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 Research Program (FRRP) was the development of building codes and standards for

radon resistant buildings and construction

standards for mitigation of radon in existing 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 research structures; and less comprehensive, shorter duration, less controlled measurements in many houses enrolled in new

house evaluation studies. The study reported here is part of the research house

component.

One objective of these research house

studies was to validate the effectiveness

of “barrier” construction features, specifically 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 transport and entry required for model testing

and validation. The primary model being

tested was Rogers & Associates Engineering (RAE) Corporation’s radon entry
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and transport (RAETRAD) code. The third

objective was the development of transferable protocols relevant to depressurized radon measurements. The main protocol under investigation in this study was

the “stress” test, although several different 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 concentrations.

In December 1990, the Florida Department of Community Affairs (DCA) constructed 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 moderately low permeability and moderate soil

radium levels. The soil gas radon concentrations 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 constructed on an SSW foundation.

In January 1992, plans and construction of a movable building were begun.

Two 3- x 3-m foundations were constructed. 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 surface sand. Both of these 0.6 m deep

poured concrete foundations were built in

and surrounded by this low radon potential sand. In one foundation (F1) an additional 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’ superstructures were wood frames with stucco over

plywood cladding. A 25-mm layer of spray

urethane foam was applied inside the plywood 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 depressurizing the whole structure, and using chemical 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 foundation. Polyvinyl chloride (PVC) access holes

and notches for running the sampling lines

were placed prior to the pouring. The movable 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 diameter 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 represent a plumbing penetration.

Measurements

The critical measurements in the studies were those determining radon concentrations and differential pressures.

Radon Measurements—

Most of the indoor radon was measured

in the FIPR test cells with continuous radon monitors (CRMs). Generally, for indoor 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 system (DAS) hardware and into a 286 computer 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 coating on its interior surface. The flasks were

generally counted about 4 hr after filling to

allow the short-lived radon decay products 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 concentrations at the National Air and Radiation Environmental Laboratory (NAREL)

in Montgomery, AL. Examples of circumstances in which grab sampling was the

measurement of choice include soil gas

sampling, sub-slab sampling, and measurements 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 micromanometers. Examples of differential

pressure measurements taken during the

course of these studies include indoor/

outdoor pressures, sub-slab/outdoor pressures, soil/outdoor pressures, stem wall

block core hole/outdoor pressures, and

pressure drops across an orifice to determine flows when a depressurizing/pressurizing fan was active. A Campbell Scientific, 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 outdoor temperatures and some of the differential pressures on a continuous, rather

than an episodic, basis as with the

micromanometers. Later in the project, after all of the components of the weather
3

station were installed, the data logger also

stored information on wind speed and direction, barometric pressure, solar flux density, the outdoor air temperature and relative 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 transducers, the switching of the multiplexed

transducers and CRMs, and the flushing

of the scintillation cells on the quasi-continuous CRMs. Between the two foundations, a weather station was set up that

contained a wind speed and direction indicator, a barometer, a rain gage, an outdoor 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 concentration at the deepest penetration. After

each test cell was leak-tested with chemical smoke and all the infiltration sites were

sealed as well as possible with urethane

foam or caulk, air infiltration was determined using a modified “blower door” approach. 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 Laboratory Air Infiltration Measurement System (BNL/AIMS) was used to monitor infiltration integrated over longer time periods. 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 extracted 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 accordance with the FRRP Standard Protocols.

Model Predictions

Information from the RAETRAD model

generic predictions was useful in the planning and design phases of the FIPR

project. At that time a variety of “reasonable” 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 permeability had to be used to approximate

the “effective” permeability of the intact

and cracked slab. Soil and slab parameters such as densities, permeabilities,

saturation fractions, and porosities, were

similarly first approximated and later corrected 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 foundations of the movable cell without undue

loss in generality. The model predictions

proved similar to those of larger structures. Most of the data required as input

for the movable cell runs were well characterized 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 concentration. 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 condition of closure and applied pressure. The

net rate of change in the radon quantity is

d(VC)

dt = Vdcdt = I − QC (1)

This is the governing differential equation

describing the change in radon. The assumed 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

−λt

) (2)

where Cs is the steady-state (long time

equilibrium) radon concentration, and λ is

the ratio of the exfiltration rate to the volume

λ =

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 environments. 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 before 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 experiments 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 included soil permeability and soil gas radon samples and soil cores from approximately 0 to 1 m deep. At the movable cell

site, similar characterizations of the site
4

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 radon 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 appropriate. Usually as the levels reached a plateau, the entry rate would become proportional 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 pressures and flows were measured similar to blower door tests. Because of

the low flows in these tight cells and

the resulting larger impact of wind effects, the precision of the calculated

leakage areas and infiltration could

not be tightly controlled. The ELA leakage 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 perimeter crack in the FS of cell 1 was the

source of some, if not all, of the additional leakage found there. From the

pressure-flow data, one could also

calculate the infiltration in air changes

per hour (ACH). Applying these calculations to the test cell 1 measurements 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 depressurization, the results were 0.17-0.25 and

0.11-0.20 ACH in cells 1 and 2, respectively. 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 operated 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 footings in both cells

allowed for the estimation of the fraction of their emissions that were found

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 depressurized. 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 somewhat 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, tensiometers were placed in the soil around

the test cells to measure the matric potential of the soil moisture. Generally the soil

matric potential increased with depth and

varied fairly little over the time the measurements were made. Since there were

regular and frequent rains, the 0.3-m depth

generally had higher moisture concentration. In late July, after the slabs had been

penetrated, three tensiometers were

placed through the slab and into the subslab 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 under 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 under the sealed

plenum, and the radon in that space was

measured over time. A form of Eq. (2)

seemed to describe the observed behavior 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 concentrations 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 calculated to be about 0.3 pCi/s with no depressurization 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 concentrations 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 concentrations 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 barometric 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.
5

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 extension 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 expected. It was found that, because of the

extensive cracking and some detected subsidence under the slab in cell 1, adjustments had to be made in some of the

parameters for the model to fit the measured 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 predictions are in good agreement under and

within 0.3 m of the slab, but the agreement 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 cylindrical approximation rather than for a

square structure. The RAETRAD model

also predicts soil gas radon concentrations. 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 values. 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 studies 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 depressurized 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 appears 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 4.12 pCi/g, and emanation fraction of

14%. The cell was moved onto this foundation on May 22 and left in passive operation through June 4. The cell was depressurized approximately 7.5 Pa with approximately 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 ratio. The sub-slab radon built up to a steady

state value after a week of passive operation, then dropped by one-third during the

4-day depressurization, and began to recover during the final 3 days of the experimental 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 mechanical exhaust ventilation was removed. Between June 12 and October 15, the cell

was mounted over foundation F1, which

was filled with scrap concentrate to represent a probable upper limit to the range of

radioactive fill soil likely to be encountered. 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 passive 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 changing slowly relative to the indoor ventilation

rate, this ratio gives a measure of relative

soil gas entry. During the periods of passive operation, the ratio approaches comparable steady-state values even though

the indoor and/or the sub-slab concentrations 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 beneath 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 equilibrium radon concentrations and radon

entry rates were calculated. In every instance the indoor radon concentrations in

cell 1 greatly exceeded those measured

in cell 2. In fact, the equilibrium concentrations 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 concentrations indicate that the source potential 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 increase in indoor concentrations. When the

cells were depressurized at various levels, the ratio of the steady state concentrations 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 contributor to the cell’s radon entry. With practically 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 observed 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 extrapolated, 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 plenum depressurizations in cell 2 extrapolated 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
6

from 10 to 20 Pa. These observations indicate that the various plenum measurements seem to have potential for estimating the whole building entry rates.

Long-term Trends in Radon

Entry

As experiments were conducted over

time in the FIPR cells, the radon concentrations seemed to increase for the same

cell under the same conditions over time.

The slabs were still in the process of aging and settling. This possibility was substantiated by the slow appearance of settling/shrinkage cracks 5 to 6 months after

the slabs were placed. The slabs did not

have vapor barriers exacerbating the effects of the cracks. In addition to these

naturally occurring faults, the work that

was done in the later phases also compromised these slabs. Each of these alterations was sealed, caulked, and repaired as well as possible, but the resulting barriers may not have been as substantial as the initial 100 mm of concrete.

The two parameters that tended to indicate most strongly the changes over time

were the calculated equilibrium radon concentrations and radon entry rates for the

cells with no applied depressurization. Linear regression calculations indicate that

the equilibrium concentrations and entry

rates were increasing significantly over the

time periods of observation. The equilibrium radon concentrations and radon entry 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 depressurization 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 depressurization. The radon entry rate is expected to be linear with applied depressurization and seemed to be quite significant with slopes of 30±2 and 11±2 pCi/s/

Pa for cells 1 and 2, respectively. The

consistent increase in the equilibrium concentrations is expected in these tight structures. Since the contribution of leakage

air through the superstructure was minimal, 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 equilibrium 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 depressurized conditions. These experimental

results were compared with predictions of

radon entry using the RAETRAD model

and figures of merit based on soil properties. 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 compared with predicted entry rates for each

fill studied. The experimental data were

about 30% higher than the RAETRAD predictions 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 assumed by RAETRAD. Had steady-state

been achieved experimentally, the experimental numbers would be lower, and in

closer agreement with the model. The experimental and model results were compared 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 emanation fraction times the calculated mass of

the soil layer in the 0.9-m layer immediately 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 studied 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. Indications of void spaces under the slab were

seen not only in the movable cell but also

under the FIPR cells and in past diagnostic 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 studied. 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 performed with one location closed indicated

little difference in radon entry between the

center only, edge only, or both open conditions. While the experiments were not

conclusive, these results would be expected 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 anticipate, entry through the side channels is

greater under pressure-driven flow conditions, 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 common, if not typical, we expect central penetrations 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 different response to applied depressurization.

In order to test these assumptions, calculations were performed over a pressure

range of 0.2-20 Pa, extending over an

order of magnitude below and above typical depressurization levels. Calculations

were performed for the four fill cases studied: 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 substitution 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 determined by the limit on the production rate

of the radon source. The pressure dependencies 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 extends farther from the slab entry point.

greater depressurization is required to collect an equivalent fraction of the available

radon and is likewise less subject to depletion at higher pressures.

To extend the conclusions of the test

cell studies to new residential construc-
7

tion, RAETRAD calculations were extended 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 position 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 entry also includes roughly 23 pCi/s attributable 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 invariant 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 radium within this layer will largely determine the indoor radon. A series of studies

was performed to investigate this hypothesis. 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 fivecounty area of Central Florida. Data generated 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 thickness 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 attributable 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 variation of indoor radon is completely correlated with the emanating radium content

of the fill for each soil type. This must of

necessity be the case within the numerical precision of RAETRAD due to the structure 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 surprising, 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 varies from 79 to 166 pCi/s across the soils

studied. The portion of radon which enters through the crack shows a much stronger relative relationship varying from 10 to

143 pCi/s. The remaining radon entry

route, direct transport across the slab, accounts 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 diffuse to lower depths or is drawn into the

building through the crack; both of these

factors decrease the concentration gradient that drives the diffusive transport.

When the calculations are extended to

non-uniform soils, the results are less easily 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 transport through the slab decreases as the

coarseness of the fill and/or the underlying base soil increases. The same mechanisms 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 increases 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 clustered within about ±15% of one another.

While the soil gas entry rate is not generally measurable for a given house, a surrogate 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 parameter rather than soil gas entry indicates that Keff does indeed serve as reasonable surrogate for soil flow rate.

Conclusions and

Recommendations

One objective of this research was to

validate the effectiveness of “barrier” construction 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 compared with the predictions of the Rogers &

Associates’ RAETRAD model. It was discovered that several adjustments to assumed parameters were necessary to

bring about good agreement. Some of

these were made because insufficient information 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 horizontal and vertical variability in the soil

radium content input parameter was observed and incorporated in the model. The

model was able to accommodate most of

the variability that was detected.

Another objective was to develop transferable protocols relevant to depressurized 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. Overall the modeling applications were very

useful and informative and showed good

potential for further use in this type of

research.
8

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

EPA/600/SR-96/010

Official Business

Penalty for Private Use $300

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

The final objective was to demonstrate

and quantify the effect on indoor radon

from high radium fill soil. The data collected from a number of experiments with

the movable cell placed over different thicknesses of fill soils with different concentrations of soil radium are presented with

an analysis of which parameters affect

the indoor radon concentrations most

strongly and what some target “safe” values might be. A framework for a possible

radiological standard for fill soil is presented.