prediction of indoor radon based on soil radon and soil permeability

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Prediction of indoor radon based onsoil radon and soil permeabilityDouglas G. Mose a & George W. Mushrush aa Department of Chemistry , George Mason University , Fairfax,Virginia, 22030, U.S.APublished online: 15 Dec 2008.

To cite this article: Douglas G. Mose & George W. Mushrush (1999) Prediction of indoor radonbased on soil radon and soil permeability, Journal of Environmental Science and Health,Part A: Toxic/Hazardous Substances and Environmental Engineering, 34:6, 1253-1266, DOI:10.1080/10934529909376894

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J. ENVIRON. SCI. HEALTH, A34(6), 1253-1266 (1999)

PREDICTION OF INDOOR RADON BASED ON SOIL RADON AND SOILPERMEABILITY

Key Words: Indoor radon, soil radon, soil permeability

Douglas G. Mose and George W. Mushrush

Department of Chemistry, George Mason UniversityFairfax, Virginia 22030, U.S.A

ABSTRACT

Evidence indicates that for homes within a geographically small area, indoor

radon can be predicted using soil radon and soil permeability. In northern Virginia, an

area of temperate climate and rolling topography, soil measurements taken adjacent to

and under 150 homes were compared to their indoor radon. Indoor radon above 5

picoCuries/Liter (pCi/L) occurred in homes with soil radon in excess of 2000 pCi/L, but

it also occurred with soil radon as low as 1000 pCi/L if the permeability exceeded 0.5

inches/hour.

INTRODUCTION

National interest in the relationship between the development of lung cancer and

the concentration of radon in typical homes has greatly increased the amount of available

1253

Copyright © 1999 by Marcel Dekker, Inc. www.dekker.com

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1254 MOSE AND MUSHRUSH

indoor radon data (Kerr 1988; Alter and Oswald 1987; Cohen and Gromicko 1988; Nero

et al., 1986). This interest has develop because an estimated 8 to 25 percent of all current

lung cancer deaths are thought due in part to past exposure to inhalation of airborne

radon (Puskin and Yang 1988). This concern has intensified since the discovery that

inhaled radon passes through the lungs to be dissolved in body fluids and tissues (Pohl

Ruling 1967; Lykken and Ong 1989; Henshaw et al., 1990) and consequently may initiate

soft tissue cancers.

Attempts to identify areas with an unusually high number of homes that contain

elevated indoor radon concentrations is a very popular activity. Recent state-wide

compilations reveal that about 20% of the homes in the United States have lowest livable

level indoor radon levels in excess of 4 pCi/L, through the state to state variations are

significant (White and others 1992; Alexander et al., 1994; Marcinowski et al., 1994).

Health officials would like to concentrate their rather limited funds on these

areas. Radon testing and home repair companies would like to concentrate their efforts

in a cost effective fashion. The compilation of radon measurements from testing

companies have been found to be useful only in areas where abundant indoor radon

measurements are already available (Mose et al., 1988). Radon potential maps for areas

with relatively few indoor radon measurements must rely on other measurable quantities,

such as the physical and chemical properties of the material underlying the homes (Mose

et al., 1990a).

Several methods have been examined, most of which involve bedrock

composition, soil radon, and soil permeability. Unfortunately, as demonstrated by several

studies such as that by Levesque et al., (1997), local variations in indoor radon are not

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PREDICTION OF INDOOR RADON 1255

well associated with geological maps or with home construction, other than the well-

known differences in lowest-livable-level radon concentrations between homes with

basements and homes without basements. This article will show that a combination of

soil radon and soil permeability is probably the best indicator for detecting potential

radon problems on any particular homesite.

MATERIAL AND METHODS

Indoor Radon and Soil Measurements

The indoor radon measurements in this study were from homes in Fairfax

Country in northern Virginia and the immediately adjacent Montgomery County in

southern Maryland. Very different geological materials underlay the area, Figure 1. The

Coastal Plain Province consists of poorly cemented sand and clay strata that were

deposited during the opening of the modern Atlantic Ocean, which began about 130

million years ago. The Culpeper Basin is in ancient rift valley that formed between about

190 and 170 million years ago and now contains basaltic rock and sandy sedimentary

rocks. The Piedmont Province is composed of crystalline rock that formed a great depth

when the Appalachian Mountains were created between about 600 and 300 million years

ago and were exposed by slow uplift and erosion.

Homeowners in this study were provided with a series of alpha track indoor

radon monitors (type SF indoor radon monitor, Tech/Ops Landauer Corporation). Each

alpha track monitor was left in place for three months. At the 90 percent confidence

level, these alpha track monitors carry a 25 percent uncertainty (Mose et al., 1990b).

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Gull Coastal Plain Province| 1 Piedmont Province• I Culpeper Basin

0 10 km

0 if mi

FIGURE 1Geological provinces in Montgomery County, Maryland and Fairfax County, Virginia.

Three month exposure intervals were selected so as to as to examine seasonal

variations. The winter exposure interval was November, December and January, the

spring exposure was February, March, and April, the summer exposure interval was May,

June, and July, and the fall exposure was August, September, and October. Although

not all homeowners started their year of radon measurement in the same season, each

home was provided with a sequence of four alpha track indoor radon monitors to

measure all four seasons.

To approximate the soil permeability at each home site, the homeowners who

obtained a soil radon monitor were provided with directions for making a simple

percolation (i.e., infiltration) test. The homeowners were asked to bury the soil radon

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monitors in a 2 ft hole, between 5 and 20 ft from the home; when the monitor were later

exhumed for analysis, the homeowner made the percolation test. This test consisted of

filling the hole with water, letting this water, soak into the soil, and then pouring 1 gal

of water into this presoaked hole. The homeowner was asked to note the time required

for the water to soak into the soil and the diameter of the hole (most were about 6 in.).

The soil permeability measurement was therefore a measure of rate of water flowing

through the surface of a hole against which the 1 gal of water was poured and was

recorded as inches per hour.

RESULTS AND DISCUSSION

A compilation of basement radon data shows that the winter season tends to be

the season of highest indoor radon (Table 1). Winter measurements tend to be taken

during the time when the home is most often closed and therefore least affected by the

low radon outside air.

A detailed compilation of the measurements shows that basement measurements

in the winter tend to be about 50 percent higher than first floor measurements, in terms

of the average radon measurements, in terms of the average

radon median indoor radon, and the percentage of homes over 4 pCi/L (Table 2). The

radon characteristics of this study group are believed to be similar to that of the entire

Fairfax County and Montgomery County population, in that about 35 percent of the

community have basements annual (i.e., year long average) indoor radon above 4 pCi/L,

and about 20 percent have first floor annual radon above 4 pCi/L. In the study area a

relatively large portion of the approximately 1 million people who live in the study area

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TABLE 1Percentage of Homes that Recorded their Highest Seasonal

Indoor Radon Concentrations for Each Season*

CountvNumberOfHomes

Fairfax, VA 689

Montgomery, MD 202

Fairfax ^Montgomery 891

Season of highest indoor radon concentrationWinter

41

32

39

Spring

18

21

19

Summer

12

12

12

Fall

29

30

30

* This compilation is for a set of homes that successfully completed four seasons ofradon measurements and is expressed in percent of homes.

TABLE 2Compilation of Seasonal Indoor Radon Measurements*

Season and CountvNumber

ofHomesBasement Radon MeasurementsWinterFairfax CountyMontgomery CountySpringFairfax CountyMontgomery CountySummerFairfax CountyMontgomery CountyFallFairfax County 898Montgomery County

844293

829242

927323

4.2307

Indoor RadonAverage Median 4

% ofHomes (oCi/LllOoCi/L

Dver, 20 oCi/L

("basement" has one or more soil-facing walls)

4.4pCi/L 3.2pCi/L4.9 3.1

4.1 3.04.7 3.1

3.3 2.53.6 2.7

3.2 354.4 3.2

First-Floor Indoor Radon Measurements ("first floor" hasWinterFairfax County 180Montgomery CountySpringFairfax County 132Montgomery CountySummerFairfax County 133Montgomery CountyFallFairfax County 131Montgomery County

3.135

2.833

2.844

3.141

2.2 214.4 3.6

1.9 185.1 2.2

1.8 152.3 1.7

2.3 203.3 2.5

38%40

3336

2427

436

6%11

59

24

19

1%2

12

02

1

no soil-facing walls)

335

323

117

030

010

011

00

00

3

4

0

0

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reside in their home for less than five years, and the prevalence on indoor radon has

affected the home sales and home loan markets.

As shown in Tables 1 and 2, about 75 percent of the study participants occupy

homes in Fairfax County, Virginia. Since most of the indoor and soil radon measurements

are from Fairfax County, the following discussions will emphasize „Fairfax County.

Although fewer data are available for the Montgomery County, Maryland the

observations that will be made for Fairfax County would also generally apply to the

homes in Montgomery County.

To explore the possible correlation a between indoor radon and soil radon, alpha

track monitors were deployed to measure indoor as well as soil radon. Soil radon

measurements (type SM soil radon monitor, Tech/Ops Landauer Corporation) and soil

permeability measurements were made at 150 of the study homes for which we have

indoor radon measurements.

Virtually all the available Comparison between soil radon and indoor radon using data

from alpha track monitors and exposure intervals of several months are very rare

(Brookins 1986, 1988). Virtually all of the available studies were conducted with soil

probes that measure soil gas radon (and estimate permeability) over only a few minutes

(Tanner 1988; Reimerand Gundersen, 1989; Varley and Flowers 1998). Unfortunately,

the radon emanation depends on several factors other than the radon concentration of the

soil and its permeability. Most of these others factors are related to variations in

weather, which change the amount of soil moisture, soil temperature, and atmospheric

confining pressure (Rose et al., 1988; Luetzelschwab et al., 1989). Since measurements

using soil probes are only momentary estimates of the radon emanation of a soil, the

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study reported in this article utilized only alpha track monitors to estimate the radon

content of the soil, and the exposure intervals were one to three months.

It is intuitively obvious that soil permeability also should be examined. Early

studies have shown that radon and other soil gases flow more rapidly as the sorting and

grain size increase (Tanner, 1964). There are factors other than permeability which affect

the flow of radon into homes, but are not very important within a small geographic area

like northern Virginia and southern Maryland (approximately 1000 square miles). These

factors include variations in weather (particularly precipitation),

topography and vegetation. Consequently, apparent correlations between soil radon and

indoor radon are probably most applicable to each study area (Morris and Fraley, 1994;

Gundersen and Schumann, 1996). Virginia and Maryland have temperate climates, with

year-round rainfall averaging one modest rainfall per week, and gently rolling hills that

are almost completely vegetated.

Indoor Radon and Site Specific Measurements

The median ratio between the soil radon and the indoor radon for the subset of

homes with both indoor and soil radon measurements was bout 150:1, but the individual

soil to indoor ratios ranged from less than 10 to more than 2000. Similar trends were

noted when comparing indoor radon and soil radon for particular seasons. Presumably

this variation in soil radon to indoor ratios is due in large part to variations in soil

permeability, since gas can flow more rapidly through sandy soil, as compared to clay-

rich soil. The variation in soil to indoor radon ratios is, in fact, so severe that soil radon

measurements alone can not reasonably be used to identify homesites that might have a

potential indoor radon problem. Similarly, soil permeability alone can not serve to

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identify which home sites might have a radon problem, since permeability and

radioactivity are both quite variable.

The assumption that there ought to be a tendency for indoor radon to increase

if the soil radon and the soil permeability both increase caused us to hypothesize that

both parameters could be combined into a simple to understand chart that could be used

by home builders and homeowners. Following a lengthy comparison between soil radon,

soil permeability, and indoor radon, the chart shown in Figure 2 was constructed. The

predictability of indoor radon concentration based on soil radon and soil permability

measurements using this chart is in excess of 70 percent (Table 3).

Several additional observations proved to be instructive. An additional 10% of

the indoor radon measurements were within 1 pCi/L of being "correct", and so they were

in the group identified as "almost correct". However, these data points were randomly

distributed, in that they were not all from homes with similar soil radon or similar

permability. Similarly, the "predicted too low" and the "predicted too high" data were

randomly distributed, though in this group of incorrect predictions, homesites were more

often "predicted too low" than "predicted too high."

The usefulness of a radon potential chart for a particular homesite, such as the

chart shown in Figure 2, needs to be qualified. The comparison of soil data and indoor

radon data was developed from homes only in the northern Virginia and southern

Maryland study area. In .other areas, one might expect different soil to indoor

relationships. In a colder climate, where homes are more insulated and windows less

often opened, one would expect generally higher indoor radon values for particular soil

radon values. The opposite would be true for a warmer climate. One might also predict

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SOIL

RADON

LESS THAN .5 .5 TO 1.0 1.0 TO 5 MORE THAN 5SOIL PERMEABILITY IN INCHES/HOUR

LOW

HIGH

(0-6

(16*

pCI/L)

pCI/L)

PREDICTED RADON

3 MEDIUM (6-16 pCI/U

FIGURE 2Indoor radon prediction chart, developed using soil radon and soil permeabilitymeasurements.

TABLE 3Summary of Measurements Used to Develop the Radon Prediction Chart

Field of indoorradon predictionLess than 5 pCi/L5 to 15 pCi/LMore than 15 pCi/L

Numberof homes

139111

Correct7073100

Predictions^0/^Almost Correct(within 1 pCi/LÏ

9189

Too low2090

Too hieh100

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that in a wetter climate, where more frequent rainfall more often the soil and prevents

vertical escape of soil radon to the atmosphere, the indoor radon values would be

greater.

In this study, homeowners were allowed to test their soil radon and soil

permeability at locations they selected, though they were encouraged to test near their

home. Most tested their homesite soil at a distance of 5-20 feet of their basement. Some

tested within 5 feet of the basement. A few were able to test under the basement floor.

As shown in Table 4, the best prediction of indoor radon came from the homes tested

below the basement floor, and the most incorrect predictions came from the homes tested

at distances of 5-20 feet of the basement. Evidently, variations in soil radon and/or soil

permeability, possibly related to the excavations conducted when the home was

constructed, are important when selecting measurement sites. Similarly, testing

conducted on property before homes are constructed may prove to not be useful in terms

of predicting indoor radon.

CONCLUSION

Indoor radon measurements are now available for many populated areas in the

United States, particularly in the states within the Appalachian Mountain system.

Homeowner interest allowed us to gather serval thousand indoor radon measurements

for home in northern Virginia and southern Maryland. These data, together with

measurements of soil radon and soil permeability, have been examined to determine ways

in which to predict indoor radon concentrations.

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TABLE 4Summary of Measurements Used to Develop the Radon Prediction Chart

Location of Soil Predicted Number Correct Almost IncorrectRadon Monitor Indoor Radon of Homes Prediction Correct Prediction

Below Floor

Within 5 Feet

ofBasement

5-20 Feet

ofBasement

Less Than 55-15 pCi/L

More Than 15

Less Than 5

5-15 pCi/L

More Than 15

Less Than 5

5-15 pCi/L

More Than 15

74

0

31

2

0

85

2

1

63

0

24

1

0

58

0

1

01

0

2

1

0

12

0

0

10

0

5

0

0

15

0

0

Studies of indoor radon, soil radon, and soil permeability at individual home sites

show that neither soil radon alone nor soil permeability alone can adequately predict

indoor radon. A combination of soil radon and soil permeability is quite effective using

commercially available and homeowner placed soil radon monitor and simple water

infiltration test. Since this conclusion was based on homeowner workmanship, which is

of variable quality, and is based on only one soil radon and permeability test, we suspect

that multiple measurements by a trained technologist would yield still high indoor radon

predictability. Simple measurements are important to develop if a national reduction of

indoor radon is to ever be a reality. The predictive methods that involve estimations soil

radon and soil permeability are most useful to homebuilders who wish to construct the

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home so as to anticipate problems and to homeowners who wish to estimate the cost of

remediating a home with a radon problem.

These predictive methods are also useful to determine if a home with presently

low indoor radon might change as the home settles. If a homeowner can obtain a

simultaneous measurement of soil radon and soil permeability, correct predictions about

indoor radon can be made in the majority of homes.

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prediction of indoor radon based on soil radon and soil permeability

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