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Radon monitoring in soils and water
Periplmon
Nud. ~mc&ap,~i~,.M~., Vol.22,Nm I--4,pp.463--468,1993 mNvierScsem~LUl PrimodinOmm lldmin. 096g-IO/8/94$6.00+.00
RADON MONITORING IN SOILS AND WATER
H. $URSECK Federal Office of Publg Health, RachoacttvttySurvedlance Se~-tlon(SUeR), 3 oh. du Mus6e, CH-1700 F.nbo~g, Swntz~land
ABSTRACT The soll is known to be the most Inportant radon source but there ere only few ~ r c l a l l ¥ available dat~tors for soll gas radon. Even fewer instruments allow for the 8~mltanoou8 m m s u r e ~ m t of the gas penmabilit¥ of a soil, the most important factor daternlning underground rado~ transport. Dynanlc processes llke radon transport in the sol1 would best be studied by continuous nonltorlng but this is rarely done. The situation is hardly better for radon-ln-water mmasurements. Batch sampling and subsequent measurement in the laboratory is common practice but direct nmasurmmnt in the field or cm3tlnumm monitoring is the exception. This paper shows that proi~r soll gas smmpllng including permeability measurement and continuous radon-ln-water nmasuremmt is possible with simple equipment. Some appllcatlmm ere shown to encourage further work in this field.
KEYWORDS
Radon; soil; water; continuous monitoring; gas sampling; natural tracer.
INTRODUCTION Radon (if not otherwise stated this means aaaRn), a naturally occurrlng noble gas As present at cmmlderable concentrations An soils and groundwaters all over the world. This may lead to a health problem if radml enters a building through cracJu8 or openings in the foundations. But there is also a positive aspect o f this mmlpresent g a s ; it can be used as a natural tracer to study underground gas and water transport (Monin 1991).It's half life of 3.8 days sets an upper llait for the useful time range to about 10 days. Transport is a dynamic procts8. Therefore continuous monitoring of the radon concentration is preferred over grab mmpllng. Even in the case of a true steady state in a soil where grab saupllng l i l y be adequate, the simultaneous nmssuraMmt of ~ transport property of the soil like gel permeability greatly 8inpllfle8 interpretation o f the concentration data. The l a c k o f a g e n e r a l l y a c c e p t e d s a ~ l ~ a n d m e a s u r e m e n t p r o t o c o l makes i t difficult t o compare s o i l gas r e s u l t s from d i f f e r e n t surveys. ContAnuous monitoring is rarely done. The 81tuatlcm la hardly better for radon-lnwater measurements; ~ t l n u o u s ~ I t o r l n g is the exception.
463
464
H. S U R B E C K
With t h i s paper I would l i k e to show that proper s o i l gas sampling, including permeability measurement and continuous radon-in-water monitoring i8 possible with e v e n s simple equipment. Some applications ere shown to encourage further work in this field.
S~.NPLING
Sell Few can be added to the extensive work of Allan B.Tanner on sell gas sampling (Tanner 1991). The Imavy 88mpllng equipment he describes represents the state of the art but is too sophisticated for routine work. I have developed an easily portable apparatus (Fig. I) that fits most needs (Surbeck and Piller 1989). With minor modifications, mainly the addition of a 5 min delay line to avoid problems with the 22°Rn, it can be used for continuous monitoring too. Just putting a continuous monitor into the soil i8 less recommended for you loose any information On the permeability that may vary considerably , mainly with the sell moisture co, tent. Still less detectors.
and
hard to interpret information
will
give
buryed
passive
Water Bubbling air through water and subsequent measurement of the radon concentration in the gas phase is an easy way to determine radon in water (Fig. 2 and Teng 1986). This method can be used for direct field measurements on hatch samples as well as for continuous monitoring. I use 8 closed air circuit for both types of measurements. This slightly reduces the temporal resolution for the alr passes several times through the water but leads to better equilibration of the radon between the water and the air. Bubbling air through water with an electric pump takes a lot of energy, a bad thing for field applications. Therefore I use a water-Jet pump driven by the water to ha measured. A water head of I m and a water flow of 500 cm~/min is sufficient to bubble 100 cm 3 air per minute through a 10 cm water column.
MEASUREMENT In the soil gas or in the alr bubbled through water you always have high radon concentrations. A det~tion limit of ~ kBq/m s is sufficient. Except for karstic sources where a t u b a l resolution of ~ I h is desirable, integration over I h IS adequate. To determine w i t h l n a n h o u r 10 kBqla 3 i n the s o i l gas o r 3 B q l l i t e r i n t h e water by dogasslng w i t h a counting error of less than 5% requires a detector with a sensitivity of at least (0.05 counts/h)/(Bq/ma).
Hearl¥ all continuous radon ~ i t o r s photomultiplier, l m l s o d g r i d i o~ c h ~ b e r , sensitivity so you can c h o o ~ a ~ i n g
(Lutes c e l l coupled to a s o l i d s t a t e d e t e c t o r ) have t h i s to price, power cc¢~unpticm
(important for field work) and ease of data storage and readout. Varying humidity of the air to be m ~ with ~ t l n u o u s monitors may lead to erroneous results. Therefore I always use 8 desoclcator (silica gel) in the gas input of the monitor. Silica gel turned out to have a negligible radon retention.
MONITORING IN SOILS A N D W A T E R
RADON
H t DDRILLED HOLE IR. 7 CH
•
E'
465
'
F-I.-.
::
110I con
II
~ m lii:iiiii:]
_.~
I
<'-
FILTER I N
J) ~s
TO LUCNSCELL (180
m GB$ T
ENTRONCE
8 CVLINDRICeL i NoRse TUBE.
ccM)
HOLE9 PROTECTION I.D. 1 2 Met
GRID
Fie. t : Soil |as sampling equipment. To delermlne Ihe loll permeabilily one measures Iho lime II lakes Io pump 1000 ccm Mr II1 conslanl pressure inb Iho boreholo. The constant pressure is produced by the pump piston's weight.
Water in
~er-Jel pump
~ ~ Air in
Walor oul
)
• Air out er leuel
~
~l,~r
RIr and waler mixed : ~ " ¢ - .. ~~-" . - :~"
Bubbler Fig. 2 : ExPerlmentsl setup For conUnuous radon monitorin9 in wnter.
/
/ ue.el
oul
466
H. SURBECK
see
100
I
I
~, ~O -O ""
I
I
I
I I illlll
10":'-..
%..
. ,~ 10
"'"",.
i
I
a
I i l lllil
I
"'-%
I,IIIIII
I n
"'"',,. "-o%
"-.%
~o%
"'"~"
"'"""
%~.%
"'"- ....
50
SO11 p e r m o a b U i ~
.... ,
'- ' ' " i ' l ' " ' [ 100
500
f"
%%~
8 ...., ......
' '''"'l"i'i'l 10
!
""..,
~ ..... ~,
' i',','IJ,ii'l 5
I
""",..
"'...,... ..., ........
1
I
"",..
""'"'- •% % .
a
I
%'-°.%
1~,,.
1 1
i
%~°.% %%
s
I
' 1000
2000
[ 40 - 4 3 m 2 ]
Fig. 3 : Radon concsnblUon I n d gas pormeabilily in odJsclnl soils. 9mnplin9 silo : Wellonborg, Conlrld 9tdlzorland. 9mill)ling depth : 50 cm. Parlmelor Iliuen in Rgum Is mo "Radon AmlUablliW', doRnod as the producl oF radon-concenlralion I n d pormmadillly. Unl! Is B4km. o 9oil douelopod From Q u a r l o m ~ doposi|. • 9oil douelopod From t,;alanlllnion mad. • Trsmsllion zone, nMdnly soil douoliopod From lJIsdanginion mw'l.
7000 6000 S000 iO ,moo
3000
8 2000 1000 0 2-NOU-g8
3-NOU-90
4-NOU-gO
5-NOU-gO
Fig. 4 : Rn-concenbraUon vs. lime For lap waJar Fribourg, 8wllzerland. Xeasured with the appwalus in Rgure 2.
L
RADONMONITORINGIN SOILSAND WATER
•
20
40
GO
O0
l , , , I t , , l i o , l , , , l , , , l
200
•
28000
20
40
4457
£0
O0
100
i l l l l l l l l , , , l , , , l t , , l
26000
2,,)oo
~
22000
0
20000 o 18000 16000 600
~
400 ~
~
Flow
' ' ' L
30O 785 Condudldly
Condu 775
g 01
=I.
766
1 , , , i , , , i , ~ ~ i , , ~ i , , , i 765 • 20 40 GO eO 100 Time [ h ] Slart a* 3.Rpr-1992 0 : 0 0
' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 20 40 60 O0 Timo[ h ] Start sl 8-Jun-1992 0:00
Fig. 5 : Radon. Row and conducUuily w . Umo For me source " l - l u h o s " n o r a F d b o u r o . 9 w i l z e r l a n d . The IoR p w t oF Iho ROwe shows Iho roacUon to • h o a w rainfall •Fief a long dry p e r i o d . The right pant oF m e ROute shows the b o h ~ i o r oF the source aRoT modorido p r o d p l t o l l o n 8 d u M 9 • wet p e r i o d . The Rot# oF INs source r o u t s wllh • d o l q oF some daws to Ihe p m c i p l t o l i o n $ . For the radon conconlraUon 20000 counts I h corresponds to a b o u t 6 B4 ! liter. Radon leuol$ h e w been mossorod with lhe appmrato$ In Rguro 2 .
.LOO
468
H. SUP.BECK
If 22"Rn is present in the air to be measured (soil gas) it is cheaper to add a 5 mln delay line than to use instruments that can discriminate between 22°Rn and 222Rn.
APPLICATIONS Figure 3 clearly shows the advantage of having permeability data together with soil gas radon concentrations. Radon levels alone would not allow for the discrimination between the two soils. The temporal variations of the radon concentration in tap water shown in Fig. 4 is due to the difference in the radon level of the two sources feeding this public water system. The base demand during the night and late in the afternoon is supplied by the hlgh-radon source. Low radon water is added during the high-consumption periods. The left side of Fig. 5 shows the radon response of a source to a flood after a long dry period. The simultaneous increase of the radon level and the conductivity is probably due to the infiltration of soil pore water (Surbeck and Medici 1991).The right side of Fig. 5 shows that the radon signal contains information that could not be gained from conductivity or flow data alone. Whereas conductivity and flow change only slightly there is a large end fast change of the radon level. Radon is still far from being a routine tracer. But it's omnipresence and that it can be monitored with a simple equipment clearly justifies more work in this field.
ACKNOWLEDGEMENTS I would like to thank Georges Piller for his help and Laurent Eisenlohr have made available conductivity and flow data.
to
REFERENCES Monin, M.M. (1991). Radon in soil, air and in groundwater related to major geophysical events: a survey,In: Proc. of the 2nd Workshop on Radon Monltoring in Radioprotectlon, Environmental and/or Earth Sciences, Miramare, Trieste, November 25-December 6, 1991, in press. Surbeck, H. and Piller, G.(1989). A closer look at the natural radioactivity in soils. In : Proceedings of The 1988 Symposium on Radon and Radon Reduction Technology, Denver CO, October 17-21, 1988, Environmental Protection Agency Report EPA-600/9-89-006, Research Triangle Park, 1989 Surbeck, H. and Medici, F.(1991). Rn-222 transport from soil to karst caves by percolating water, In : Proc. of the 22nd Congress of the IAH, Lausanne, Switzerland, August 27 - September I, 1990, Int. Ass. of Hydrologists, Memoires, Vol.XXII, Part I, 1991. Tanner, A.B. (1991). Methods of characterization of ground for assessment of indoor radon potential at a site, In: Gundersen, L.C.S. and Wanty, R.B. (Eds.), Field studies of radon in rocks, soils, and water, U.S.G.S. Bulletin 1971, U.S.Gov.Printing Office, 1991 Teng, T.-L. and Sun, L.-F.(1986). Research on groundwater radon as a fluid phase precursor to earthquakes, J.Geophys.Res. 91/B12, 1986.
Nud. ~mc&ap,~i~,.M~., Vol.22,Nm I--4,pp.463--468,1993 mNvierScsem~LUl PrimodinOmm lldmin. 096g-IO/8/94$6.00+.00
RADON MONITORING IN SOILS AND WATER
H. $URSECK Federal Office of Publg Health, RachoacttvttySurvedlance Se~-tlon(SUeR), 3 oh. du Mus6e, CH-1700 F.nbo~g, Swntz~land
ABSTRACT The soll is known to be the most Inportant radon source but there ere only few ~ r c l a l l ¥ available dat~tors for soll gas radon. Even fewer instruments allow for the 8~mltanoou8 m m s u r e ~ m t of the gas penmabilit¥ of a soil, the most important factor daternlning underground rado~ transport. Dynanlc processes llke radon transport in the sol1 would best be studied by continuous nonltorlng but this is rarely done. The situation is hardly better for radon-ln-water mmasurements. Batch sampling and subsequent measurement in the laboratory is common practice but direct nmasurmmnt in the field or cm3tlnumm monitoring is the exception. This paper shows that proi~r soll gas smmpllng including permeability measurement and continuous radon-ln-water nmasuremmt is possible with simple equipment. Some appllcatlmm ere shown to encourage further work in this field.
KEYWORDS
Radon; soil; water; continuous monitoring; gas sampling; natural tracer.
INTRODUCTION Radon (if not otherwise stated this means aaaRn), a naturally occurrlng noble gas As present at cmmlderable concentrations An soils and groundwaters all over the world. This may lead to a health problem if radml enters a building through cracJu8 or openings in the foundations. But there is also a positive aspect o f this mmlpresent g a s ; it can be used as a natural tracer to study underground gas and water transport (Monin 1991).It's half life of 3.8 days sets an upper llait for the useful time range to about 10 days. Transport is a dynamic procts8. Therefore continuous monitoring of the radon concentration is preferred over grab mmpllng. Even in the case of a true steady state in a soil where grab saupllng l i l y be adequate, the simultaneous nmssuraMmt of ~ transport property of the soil like gel permeability greatly 8inpllfle8 interpretation o f the concentration data. The l a c k o f a g e n e r a l l y a c c e p t e d s a ~ l ~ a n d m e a s u r e m e n t p r o t o c o l makes i t difficult t o compare s o i l gas r e s u l t s from d i f f e r e n t surveys. ContAnuous monitoring is rarely done. The 81tuatlcm la hardly better for radon-lnwater measurements; ~ t l n u o u s ~ I t o r l n g is the exception.
463
464
H. S U R B E C K
With t h i s paper I would l i k e to show that proper s o i l gas sampling, including permeability measurement and continuous radon-in-water monitoring i8 possible with e v e n s simple equipment. Some applications ere shown to encourage further work in this field.
S~.NPLING
Sell Few can be added to the extensive work of Allan B.Tanner on sell gas sampling (Tanner 1991). The Imavy 88mpllng equipment he describes represents the state of the art but is too sophisticated for routine work. I have developed an easily portable apparatus (Fig. I) that fits most needs (Surbeck and Piller 1989). With minor modifications, mainly the addition of a 5 min delay line to avoid problems with the 22°Rn, it can be used for continuous monitoring too. Just putting a continuous monitor into the soil i8 less recommended for you loose any information On the permeability that may vary considerably , mainly with the sell moisture co, tent. Still less detectors.
and
hard to interpret information
will
give
buryed
passive
Water Bubbling air through water and subsequent measurement of the radon concentration in the gas phase is an easy way to determine radon in water (Fig. 2 and Teng 1986). This method can be used for direct field measurements on hatch samples as well as for continuous monitoring. I use 8 closed air circuit for both types of measurements. This slightly reduces the temporal resolution for the alr passes several times through the water but leads to better equilibration of the radon between the water and the air. Bubbling air through water with an electric pump takes a lot of energy, a bad thing for field applications. Therefore I use a water-Jet pump driven by the water to ha measured. A water head of I m and a water flow of 500 cm~/min is sufficient to bubble 100 cm 3 air per minute through a 10 cm water column.
MEASUREMENT In the soil gas or in the alr bubbled through water you always have high radon concentrations. A det~tion limit of ~ kBq/m s is sufficient. Except for karstic sources where a t u b a l resolution of ~ I h is desirable, integration over I h IS adequate. To determine w i t h l n a n h o u r 10 kBqla 3 i n the s o i l gas o r 3 B q l l i t e r i n t h e water by dogasslng w i t h a counting error of less than 5% requires a detector with a sensitivity of at least (0.05 counts/h)/(Bq/ma).
Hearl¥ all continuous radon ~ i t o r s photomultiplier, l m l s o d g r i d i o~ c h ~ b e r , sensitivity so you can c h o o ~ a ~ i n g
(Lutes c e l l coupled to a s o l i d s t a t e d e t e c t o r ) have t h i s to price, power cc¢~unpticm
(important for field work) and ease of data storage and readout. Varying humidity of the air to be m ~ with ~ t l n u o u s monitors may lead to erroneous results. Therefore I always use 8 desoclcator (silica gel) in the gas input of the monitor. Silica gel turned out to have a negligible radon retention.
MONITORING IN SOILS A N D W A T E R
RADON
H t DDRILLED HOLE IR. 7 CH
•
E'
465
'
F-I.-.
::
110I con
II
~ m lii:iiiii:]
_.~
I
<'-
FILTER I N
J) ~s
TO LUCNSCELL (180
m GB$ T
ENTRONCE
8 CVLINDRICeL i NoRse TUBE.
ccM)
HOLE9 PROTECTION I.D. 1 2 Met
GRID
Fie. t : Soil |as sampling equipment. To delermlne Ihe loll permeabilily one measures Iho lime II lakes Io pump 1000 ccm Mr II1 conslanl pressure inb Iho boreholo. The constant pressure is produced by the pump piston's weight.
Water in
~er-Jel pump
~ ~ Air in
Walor oul
)
• Air out er leuel
~
~l,~r
RIr and waler mixed : ~ " ¢ - .. ~~-" . - :~"
Bubbler Fig. 2 : ExPerlmentsl setup For conUnuous radon monitorin9 in wnter.
/
/ ue.el
oul
466
H. SURBECK
see
100
I
I
~, ~O -O ""
I
I
I
I I illlll
10":'-..
%..
. ,~ 10
"'"",.
i
I
a
I i l lllil
I
"'-%
I,IIIIII
I n
"'"',,. "-o%
"-.%
~o%
"'"~"
"'"""
%~.%
"'"- ....
50
SO11 p e r m o a b U i ~
.... ,
'- ' ' " i ' l ' " ' [ 100
500
f"
%%~
8 ...., ......
' '''"'l"i'i'l 10
!
""..,
~ ..... ~,
' i',','IJ,ii'l 5
I
""",..
"'...,... ..., ........
1
I
"",..
""'"'- •% % .
a
I
%'-°.%
1~,,.
1 1
i
%~°.% %%
s
I
' 1000
2000
[ 40 - 4 3 m 2 ]
Fig. 3 : Radon concsnblUon I n d gas pormeabilily in odJsclnl soils. 9mnplin9 silo : Wellonborg, Conlrld 9tdlzorland. 9mill)ling depth : 50 cm. Parlmelor Iliuen in Rgum Is mo "Radon AmlUablliW', doRnod as the producl oF radon-concenlralion I n d pormmadillly. Unl! Is B4km. o 9oil douelopod From Q u a r l o m ~ doposi|. • 9oil douelopod From t,;alanlllnion mad. • Trsmsllion zone, nMdnly soil douoliopod From lJIsdanginion mw'l.
7000 6000 S000 iO ,moo
3000
8 2000 1000 0 2-NOU-g8
3-NOU-90
4-NOU-gO
5-NOU-gO
Fig. 4 : Rn-concenbraUon vs. lime For lap waJar Fribourg, 8wllzerland. Xeasured with the appwalus in Rgure 2.
L
RADONMONITORINGIN SOILSAND WATER
•
20
40
GO
O0
l , , , I t , , l i o , l , , , l , , , l
200
•
28000
20
40
4457
£0
O0
100
i l l l l l l l l , , , l , , , l t , , l
26000
2,,)oo
~
22000
0
20000 o 18000 16000 600
~
400 ~
~
Flow
' ' ' L
30O 785 Condudldly
Condu 775
g 01
=I.
766
1 , , , i , , , i , ~ ~ i , , ~ i , , , i 765 • 20 40 GO eO 100 Time [ h ] Slart a* 3.Rpr-1992 0 : 0 0
' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 20 40 60 O0 Timo[ h ] Start sl 8-Jun-1992 0:00
Fig. 5 : Radon. Row and conducUuily w . Umo For me source " l - l u h o s " n o r a F d b o u r o . 9 w i l z e r l a n d . The IoR p w t oF Iho ROwe shows Iho roacUon to • h o a w rainfall •Fief a long dry p e r i o d . The right pant oF m e ROute shows the b o h ~ i o r oF the source aRoT modorido p r o d p l t o l l o n 8 d u M 9 • wet p e r i o d . The Rot# oF INs source r o u t s wllh • d o l q oF some daws to Ihe p m c i p l t o l i o n $ . For the radon conconlraUon 20000 counts I h corresponds to a b o u t 6 B4 ! liter. Radon leuol$ h e w been mossorod with lhe appmrato$ In Rguro 2 .
.LOO
468
H. SUP.BECK
If 22"Rn is present in the air to be measured (soil gas) it is cheaper to add a 5 mln delay line than to use instruments that can discriminate between 22°Rn and 222Rn.
APPLICATIONS Figure 3 clearly shows the advantage of having permeability data together with soil gas radon concentrations. Radon levels alone would not allow for the discrimination between the two soils. The temporal variations of the radon concentration in tap water shown in Fig. 4 is due to the difference in the radon level of the two sources feeding this public water system. The base demand during the night and late in the afternoon is supplied by the hlgh-radon source. Low radon water is added during the high-consumption periods. The left side of Fig. 5 shows the radon response of a source to a flood after a long dry period. The simultaneous increase of the radon level and the conductivity is probably due to the infiltration of soil pore water (Surbeck and Medici 1991).The right side of Fig. 5 shows that the radon signal contains information that could not be gained from conductivity or flow data alone. Whereas conductivity and flow change only slightly there is a large end fast change of the radon level. Radon is still far from being a routine tracer. But it's omnipresence and that it can be monitored with a simple equipment clearly justifies more work in this field.
ACKNOWLEDGEMENTS I would like to thank Georges Piller for his help and Laurent Eisenlohr have made available conductivity and flow data.
to
REFERENCES Monin, M.M. (1991). Radon in soil, air and in groundwater related to major geophysical events: a survey,In: Proc. of the 2nd Workshop on Radon Monltoring in Radioprotectlon, Environmental and/or Earth Sciences, Miramare, Trieste, November 25-December 6, 1991, in press. Surbeck, H. and Piller, G.(1989). A closer look at the natural radioactivity in soils. In : Proceedings of The 1988 Symposium on Radon and Radon Reduction Technology, Denver CO, October 17-21, 1988, Environmental Protection Agency Report EPA-600/9-89-006, Research Triangle Park, 1989 Surbeck, H. and Medici, F.(1991). Rn-222 transport from soil to karst caves by percolating water, In : Proc. of the 22nd Congress of the IAH, Lausanne, Switzerland, August 27 - September I, 1990, Int. Ass. of Hydrologists, Memoires, Vol.XXII, Part I, 1991. Tanner, A.B. (1991). Methods of characterization of ground for assessment of indoor radon potential at a site, In: Gundersen, L.C.S. and Wanty, R.B. (Eds.), Field studies of radon in rocks, soils, and water, U.S.G.S. Bulletin 1971, U.S.Gov.Printing Office, 1991 Teng, T.-L. and Sun, L.-F.(1986). Research on groundwater radon as a fluid phase precursor to earthquakes, J.Geophys.Res. 91/B12, 1986.