Radon and Its Decay Products - American Chemical Society

Chapter 2 Investigations of Soil as a Source of Indoor R a d o n 1

Downloaded by CORNELL UNIV on June 4, 2017 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch002

R. G. Sextro, B. A. Moed, W. W. Nazaroff, K. L. Revzan, and Α. V. Nero Indoor Environment Program, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 The predominant source of indoor radon in most single-family housing in the U.S. is the soil adjacent to the house substructure. We have examined factors influencing the production and transport of radon in soil and into buildings. A number of important param eters have been identified and their effect on radon production and migration assessed, including radium concentration, moisture content, air permeability, and grain size distribution of soils. The potential regional variations in parameters affecting radon have been evaluated by examining geographic data, including surface radium concentrations and general soil data. We have also investigated factors influencing radon migration into individual dwellings. Coupling between the building shell and the surrounding soil has been demonstrated experimentally, and pressure-field map ping and soil permeability measurements have been car ried out. Soil is now recognized as a significant, i f not predominant, source of radon in the indoor environment. This is especially true in those dwellings with elevated indoor radon concentra222 tions. (In this paper, radon refers to Rn, and radium to 226 220 Ra. Although some of the discussion applies to Rn as well, its presence in indoor air is usually limited by its short halflife of 55 s.) Several recent studies of radon entry into homes have been done, including investigations or discussions of the influence of pressure differentials across the building shell, and the role of soil as a source of indoor radon (Nazaroff, e_t al . , 1985a; Nazaroff and Doyle, 1985; Nero and Nazaroff, 1984; DSMA, 1983) . There are a number of reasons to investigate both 'Current address: Environmental Engineering Science, California Institute of Technology, Pasadena, CA 91125 0097-6156/87/0331-0010$06.00/0 © 1987 American Chemical Society

Hopke; Radon and Its Decay Products ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11

Soil as a Source of Indoor Radon

2. SEXTRO ET AL.


t h e c h a r a c t e r i s t i c s o f s o i l s and t h e i n t e r a c t i o n i n g s t r u c t u r e . The i n t r i n s i c p r o p e r t i e s of the w i t h t h e d e t a i l s o f t h e b u i l d i n g s t r u c t u r e and i t important determinants of the radon e n t r y r a t e ture. I n a d d i t i o n , b o t h t h e h o u s e and t h e s o i l changes i n ambient e n v i r o n m e n t a l c o n d i t i o n s .

with the buildsoil, combined s operation, are i n t o the struca r e a f f e c t e d by

T h i s p a p e r s u m m a r i z e s our r e c e n t i n v e s t i g a t i o n s o f s o i l as a source of radon. These i n v e s t i g a t i o n s have a t w o - f o l d purpose, and attack the problem from two distinct but complementary approaches. The f i r s t e f f o r t i s t o a s s e m b l e d a t a a t a g e o g r a p h i c a l s c a l e , a p p r o p r i a t e f o r understanding the general c h a r a c t e r i s tics of soils. This approach provides information on the a p p r o p r i a t e s c a l e s and r a n g e s o f i m p o r t a n t s o i l v a r i a b l e s , and how t h e s e m i g h t v a r y w i t h i n and among r e g i o n s . A n o t h e r important aspect of t h i s approach i s to d e r i v e v a l u e s of parameters that may be u s e d i n a p r e d i c t i v e t e c h n i q u e f o r i d e n t i f y i n g a r e a s where h i g h i n d o o r r a d o n l e v e l s a r e most l i k e l y ( N a z a r o f f a t «1 1986; Sextro, 1985). The s e c o n d a p p r o a c h i s t o i n v e s t i g a t e b o t h e x p e r i m e n t a l l y and t h e o r e t i c a l l y t h e i n f l u e n c e o f s o i l s a t a l o c a l i z e d l e v e l , on t h e s c a l e o f an i n d i v i d u a l h o u s e , f o r e x a m p l e . P r o d u c t i o n and t r a n s p o r t o f r a d o n i n s o i l s and m i g r a t i o n o f r a d o n i n t o h o u s e s d e p e n d upon l o c a l soil conditions, and r e s u l t s o f t h e s e s t u d i e s w i l l improve our basic understanding of the physical processes involved. T h i s i n t u r n w i l l a i d i n the s y s t e m a t i c development o f more e f f e c t i v e e n t r y i d e n t i f i c a t i o n and m i t i g a t i o n t e c h n i q u e s t o p r e v e n t or reduce radon e n t r y i n t o d w e l l i n g s .

Factors

I n f l u e n c i n g P r o d u c t i o n and

General

Desr.rlpi-M on.

tion

and

trated

migration

Table

and

As ciated

U U

mining

i s a mixture

the

matter. rocks

and

1

and

from

are

nominal

which

the

a

for

are

illusvarious

presented

i s p a r t of

slightly of

much e.gof

larger

Bq

the

kg

range

soil

usually, often

1

an

.

In

of

soil

1980).

water

reflects

and that

d e r i v e d by p h y s i -

ranges

are

(Wollenberg, f o r measured rocks,

asso-

than

(Powers eJi «1•

( d u n i t e ) t o 1 t o 1835 those

not

more

~40

m a t e r i a l s are

for rocks, excluding a l k a l i

t h e means o b s e r v e d

by

observed

(gneiss)

than

produc-

are

these

values,

value

found,

solid

rocks

rocks

broader

varies

content The

of

of radon,

m a t e r i a l s , a i r and,

radium

radon

buildings

ranges

Soils

content of surface s o i l s

milling,

activity.

metamorphic

The

typical

been

for ultrabasic

ranges

mean v a l u e s

a

of s o l i d The

chemical

kg"

igneous these

with

influence

into

In

i n a l l soil materials.

milling

c o n c e n t r a t i o n s has

organic

Bq

i s found

mining

magnitude,

1.

their

I, radium

near

20

and

that

and

immediate p a r e n t

or

areas

of

the

with of

cal

Figure

shown i n T a b l e

order radium

soils

i n some c a s e s

I . Ra-226,

U decay s e r i e s ,

Soil

parameters

through

schematically i n

parameters, in

The

M i g r a t i o n n f Radon

from

~

Bq k g "

0 1

1984). for soils,

i s consistent

soils.


to for

While the with

12

RADON AND ITS DECAY PRODUCTS

Basement


Indoor Rn

radioactive

decay:

m i g r a t i o n t o air; trapping

Ra-

in s o i l g r a i n s

Water trap

• R A D O N M I G R A T I O N IN S O I L

-•Rn-

diffusion:

f(H 0,

convection: geochemical

emanating

processes

f(H 0, 2

2

e)

f(k, V P )

fraction

d , ...)

F i g u r e 1. Schematic i l l u s t r a t i o n o f f a c t o r s i n f l u e n c i n g t h e p r o d u c t i o n and m i g r a t i o n o f r a d o n i n s o i l s a n d i n t o b u i l d i n g s . Geochemical p r o c e s s e s a f f e c t t h e radium c o n c e n t r a t i o n i n t h e soil. The e m a n a t i n g fraction i s principally dependent upon soil moisture (Σ^Ο) >1, If

N

Pe

In

first

then

second

N

40 Bq k g " ( F i g u r e 3 b ) . A s c a n be s e e n from t h e f i g u r e , the d i f f e r e n c e s i n t h e area radium concentration distributions are q u i t e l a r g e . L e s s t h a n 0.5% o f t h e r a d i u m c o n c e n t r a t i o n d a t a from t h e C h i c o Q u a d r a n g l e a r e above 40 Bq k g , and l e s s than 0.01% a r e a b o v e 60 Bq k g " . I n c o n t r a s t , a l m o s t 6 5 % o f t h e s u r f a c e r a d i u m c o n c e n t r a t i o n d a t a from t h e San J o s e Q u a d r a n g l e h a v e v a l u e s g r e a t e r t h a n 40 Bq k g " w i t h 0.5% above 200 Bq k g " and 0.05% g r e a t e r t h a n 300 Bq k g " . 1

1

1

1

1

1

I n a d d i t i o n t o t h e s e f r e q u e n c y d i s t r i b u t i o n s , maps o f v a r i o u s types have been produced to depict the spatial d i s t r i b u t i o n of the s u r f a c e radium c o n c e n t r a t i o n s . One example i s shown i n F i g u r e 4, w h i c h r e p r e s e n t s d a t a from t h e a e r i a l s u r v e y o f t h e Newark Quadrangle, c o v e r i n g p a r t o f e a s t e r n P e n n s y l v a n i a and n o r t h e r n New J e r s e y . T h e d a t a i n t h i s f i g u r e a r e p l o t t e d on an a p p r o x i mately s i x m i l e by s i x m i l e g r i d . Thus e a c h p o i n t r e p r e s e n t s an average o f t h e data c o l l e c t e d along s i x m i l e s o f t h e f l i g h t l i n e . The frequency d i s t r i b u t i o n o f s u r f a c e radium c o n c e n t r a t i o n s f o r t h i s q u a d r a n g l e a r e a h a s a GM o f 35 Bq k g " , a GSD o f 1.45 a n d an a r i t h m e t i c mean o f 37 Bq k g . From t h e s e d i s t r i b u t i o n parameters, one c a n e s t i m a t e that approximately 7% o f t h e s u r f a c e r a d i u m c o n c e n t r a t i o n d a t a a r e g r e a t e r t h a n 60 Bq k g and l e s s t h a n 0.2% o f t h e d a t a a r e above 100 Bq k g . 1

1

1

W h i l e t h e s e d a t a and t h e r e s u l t i n g maps a r e u s e f u l i n i l l u s t r a t i n g b o t h t h e t r e n d s and v a r i a b i l i t y i n s u r f a c e radium concent r a t i o n s , d e t a i l e d f i e l d examination o f s u r f a c e radium concentrat i o n s i n s e l e c t e d areas i s needed t o determine t h e r e l i a b i l i t y o f


18


10.0

1

1

I


NARR: All Quadrangles GM =24.9 GSD = 1.75 A M =28.5 ASD = 14.7

0.00

-U 40.00

80.00

120.00

160.00

200.00

Radium (Bq/kg soil)

F i g u r e 2. D i s t r i b u t i o n o f t h e s u r f a c e radium concentration d a t a from t h e N a t i o n a l A i r b o r n e R a d i o m e t r i c R e c o n n a i s s a n c e s u r v e y f o r 394 1° b y 2° q u a d r a n g l e s c o v e r i n g most o f t h e c o n t i g u ous 48 s t a t e s . The d i s t r i b u t i o n p a r a m e t e r s a r e c a l c u l a t e d from t h e d a t a and t h e l o g n o r m a l d i s t r i b u t i o n b a s e d on t h e g e o m e t r i c mearij and s t a n d a r d d e v i a t i o n from t h e d a t a i s shown as a s o l i d curve.


2. SEXTRO ET AL.

19

Soil as a Source of Indoor Radon π • 1 » NARR: Chico Quadrangle AM

= 19.1

ASD = 6.57 GM = 18.2


GSD = 1.35

40.00

40.00

80.00 120.00 Radium (Bq/kg soil)

160.00

200.00

80.00 120.00 Radium (Bq/kg soil)

160.00

200.00

F i g u r e 3. D i s t r i b u t i o n o f s u r f a c e radium c o n c e n t r a t i o n s f o r a quadrangle (a) w i t h a GM < 20 Bq k g " and f o r a q u a d r a n g l e (b) w i t h a GM > 40 Bq k g . The c u r v e s r e p r e s e n t lognormal distri b u t i o n s b a s e d on t h e d i s t r i b u t i o n p a r a m e t e r s c a l c u l a t e d from the data. 1

1


20


226

• = < 20 Bq/kg soil • =

20-40


Pennsylvania

76

W

75 30'W

Ra

· =

40-60

· =

ê 60

New Jersey

75

W

74 30'W

74

W

F i g u r e 4. The s p a t i a l d i s t r i b u t i o n o f i n f e r r e d s u r f a c e radium concentrations f o r t h e Newark Q u a d r a n g l e , covering eastern Pennsylvania and n o r t h e r n New J e r s e y . The s t a t e boundaries a r e shown as s o l i d l i n e s , w h i l e t h e c o u n t y b o u n d a r i e s a r e i n d i c a t e d by broken l i n e s . The s p a c i n g b e t w e e n f l i g h t l i n e s i s 6 m i l e s , and e a c h p o i n t r e p r e s e n t s d a t a a v e r a g e d a l o n g s i x m i l e s of each f l i g h t l i n e .


2.

SEXTRO ET AL.

21


the data. Such a f i e l d v a l i d a t i o n study has been conducted f o r c o m p a r i s o n w i t h NARR a e r i a l d a t a o b t a i n e d i n t h e Spokane, WA, a r e a . One m i l e segments o f two d i f f e r e n t f l i g h t l i n e s were i n v e s t i g a t e d ; e a c h t r a v e r s e d g r a n i t i c o u t c r o p s and t h e a e r i a l d a t a f o r t h o s e f l i g h t - l i n e segments showed c o n s i d e r a b l e v a r i a t i o n . I n gene r a l , a correspondence was o b s e r v e d b e t w e e n t h e r a d i u m c o n c e n t r a t i o n s b a s e d on t h e a e r i a l d a t a a n d t h e 1 n-gl-hu m e a s u r e m e n t s o f r a d i u m c o n t e n t a n d r a d o n f l u x from s o i l (Moed eJ: a l - 1984) .


f

As n o t e d i n T a b l e I , a v e r a g e s u r f a c e r a d i u m c o n c e n t r a t i o n s a p p e a r t o v a r y b y a b o u t a f a c t o r o f 20. T h i s can a l s o be seen f r o m t h e d i s t r i b u t i o n s from t h e NARR d a t a . S o i l p e r m e a b i l i t i e s , on t h e o t h e r hand, h a v e much l a r g e r v a r i a t i o n s , a n d t h u s , i n p r i n c i p l e , may h a v e a g r e a t e r i n f l u e n c e on t h e s p a t i a l v a r i a t i o n s i n average i n d o o r radon c o n c e n t r a t i o n s t h a t have been observed. As w i t h t h e case o f s u r f a c e radium c o n c e n t r a t i o n s , t h e s p a t i a l v a r i a b i l i t y o f a i r p e r m e a b i l i t i e s o f s o i l s i s an i m p o r t a n t e l e m e n t i n developing a predictive capability. I n f o r m a t i o n on s u r f a c e s o i l s i s a v a i l a b l e from a number o f s o u r c e s , i n c l u d i n g s u r f a c e s o i l maps c o m p i l e d b y t h e U.S. G e o l o g i c a l S u r v e y and t h e g e o l o g i c a l s u r v e y s o f v a r i o u s s t a t e s . At the p r e s e n t t i m e , t h e c o v e r a g e o f s u c h maps i s n o t c o m p l e t e , n o r h a s any s y s t e m a t i c d a t a on a i r p e r m e a b i l i t y o f s o i l s b e e n compiled. However r e p o r t s i s s u e d b y t h e S o i l C o n s e r v a t i o n S e r v i c e (SCS) o f t h e U.S. D e p a r t m e n t o f A g r i c u l t u r e c o n t a i n i n f o r m a t i o n on most s o i l s on a c o u n t y - b y - c o u n t y b a s i s . W h i l e no d i r e c t a i r p e r m e a b i l ity information i s contained i n these r e p o r t s , t h e d a t a and d e s c r i p t i v e m a t e r i a l c o n t a i n e d t h e r e may b e u s e f u l i n e s t i m a t i n g air permeabilities. We h a v e u s e d t h e s e i n d i c a t o r s t o e s t i m a t e p e r m e a b i l i t y v a l u e s f o r s u r f a c e s o i l s i n Spokane C o u n t y , WA. A g e n e r a l s o i l map o f t h i s c o u n t y i s shown i n F i g u r e 5. T h e s p a t i a l e x t e n t o f two s o i l associations, the Naff-Larkin-Freeman a s s o c i a t i o n ( a r e a 1) and the Garrison-Marble-Springdale a s s o c i a t i o n ( a r e a 2) , a r e s h a d e d in the figure, and d e t a i l e d e s t i m a t e s o f a i r p e r m e a b i l i t y f o r t h e s e two a s s o c i a t i o n h a v e b e e n made, a s d e s c r i b e d b r i e f l y b e l o w . More i n f o r m a t i o n i s g i v e n i n N a z a r o f f a t a T . (1986). r

The N a f f - L a r k i n - F r e e m a n a s s o c i a t i o n c o n t a i n s f i n e - t o mediumtextured s o i l s , moderately-well t o wel1-drained. The G a r r i s o n Marble-Springdale soils, on t h e o t h e r hand, a r e g r a v e l l y and s a n d y ( c o a r s e - t e x t u r e d ) a n d a r e " s o m e w h a t - e x c e s s i v e l y " t o "excess i v e l y " d r a i n e d (USDA, 1 9 6 8 ) . In a d d i t i o n t o t h e s o i l t e x t u r e s , water p e r m e a b i l i t i e s a r e d e s c r i b e d q u a l i t a t i v e l y f o r these s o i l s , l e a d i n g t o t h e e s t i m a t e d r a n g e o f i n t r i n s i c p e r m e a b i l i t i e s shown i n T a b l e I I I f o r t h e s e two s o i l a s s o c i a t i o n s . T h e r e a r e a number o f u n c e r t a i n t i e s a s s o c i a t e d w i t h e s t i m a t ing a i r permeabilities i n this way. The s o i l associations d e s c r i b e d b y t h e SCS on t h e c o u n t y maps g e n e r a l l y c o n s i s t o f one o r more m a j o r s o i l s e r i e s a n d may a l s o i n c l u d e one o r more m i n o r soil series. T h e f r a c t i o n o f t h e mapped a r e a o c c u p i e d b y t h e s e minor soil series, and t h e d e g r e e o f s i m i l a r i t y of physical




22

F i g u r e 5. G e n e r a l s o i l map from t h e S o i l C o n s e r v a t i o n Service for Spokane C o u n t y , WA. Two s o i l associations discussed i n the t e x t a r e shaded. The Spokane R i v e r i s i n d i c a t e d b y a dashed line and t h e a p p r o x i m a t e boundary f o r the c i t y of Spokane i s shown b y a h e a v y b o r d e r .


2.

SEXTRO ET AL.

23


p r o p e r t i e s b e t w e e n t h e m a j o r and m i n o r s o i l s will c o n t r o l the u n c e r t a i n t i e s a s s o c i a t e d with the d e r i v a t i o n of s o i l c h a r a c t e r i s t i c s for a given geographical area. I n t h e c a s e o f t h e two soil a s s o c i a t i o n s d i s c u s s e d h e r e , 77 t o 80% o f t h e r e s p e c t i v e mapped a r e a s a r e a c t u a l l y o c c u p i e d by t h e m a j o r s o i l s e r i e s .

Table

III.

P e r m e a b i l i t i e s of s e l e c t e d s o i l s

i n Spokane C o u n t y ,

Permeability


Soil

Naff-LarkinFreeman

1θ"

Garrison-MarbleSpringdale

10"

1 4

1 2

to

10"

1 2

to

ΙΟ"

1 0

-2

X

(0.3

P e t a l Ίed

S U f t - g p p r i f^r.

StndUs.

)

M e a s u r e d by (Range)

D e r i v e d from SCS Information

Association

(m

Experimental

10

WA

LBL

11

- 60)

X

10

11

measurements

of

p a r a m e t e r s a f f e c t i n g s o i l gas t r a n s p o r t and e n t r y i n t o h o u s e s i s very limited. One r e c e n t s t u d y o f two homes w i t h b a s e m e n t s h a s d e m o n s t r a t e d c o u p l i n g b e t w e e n t h e b u i l d i n g s h e l l and t h e s u r r o u n d i n g s o i l and m e a s u r e d c o n v e c t i v e f l o w t h r o u g h t h e s o i l (Nazaroff s i . a l . , 1985b) . U t i l i z i n g an a r r a y o f s o i l p r o b e s surrounding e a c h h o u s e and by d e p r e s s u r i z i n g t h e basement u s i n g a b l o w e r door, the resulting negative pressure field i n the soil was mapped. The e f f e c t o f t h e e n h a n c e d b a s e m e n t d e p r e s s u r i z a t i o n was s e e n a t d i s t a n c e s up t o 5 m from t h e basement w a l l f o r one h o u s e and up t o 3 m i n t h e o t h e r . S o i l gas f l o w s , m e a s u r e d utilizing SFg as a t r a c e r gas, were a l s o d e t e r m i n e d . I n one c a s e n e t m i g r a t i o n v e l o c i t i e s of g r e a t e r than 1 m h were o b s e r v e d w i t h a base ment d e p r e s s u r i z a t i o n o f 30 Pa a t an i n j e c t i o n p o i n t 1.5 m from t h e basement w a l l . 1

A similar set of experiments were c o n d u c t e d as part of d e t a i l e d r a d o n i n v e s t i g a t i o n s i n one h o u s e i n t h e Spokane (WA) area. The pressure-field i n the surrounding soil was mapped u s i n g t e c h n i q u e s d e s c r i b e d i n N a z a r o f f a t aT.. (1985b). S o i l per m e a b i l i t i e s were m e a s u r e d 1η-s! t u u t i l i z i n g an a i r p e r m e a m e t e r s i m i l a r t o t h a t d i s c u s s e d i n DSMA, (1983) . An a r r a y o f 30 s o i l p r o b e s was p l a c e d i n t h e s o i l s u r r o u n d i n g t h e h o u s e , as c a n be s e e n from F i g u r e 6. The p r o b e d e p t h s r a n g e d from 0.5 to 1 m d e e p ; t h e d e p t h s were u s u a l l y l i m i t e d b y t h e a b i l i t y t o p e n e t r a t e t h e s o i l w h i c h c o n t a i n e d a l a r g e number o f r o c k s o f v a r y i n g s i z e s s c a t t e r e d t h r o u g h o u t the s o i l column. The d e e p e s t p r o b e s , with s o i l d e p t h s b e t w e e n 0.8 and 1.0 m, are i n d i c a t e d i n the figure w i t h s q u a r e s , w h i l e p r o b e s w i t h d e p t h s b e t w e e n 0.5 and 0.7 m a r e


24


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7.6 1

7.2·£ ·7·8 •6.3

Basement Perimeter

U#7.0

4.6·

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Radon and Its Decay Products - American Chemical Society

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