Radon determination in ground water

Revista
Mexicana
de Física 38, Suplemento
1 (1992) 242-248
Radon determination in ground water
N.
SEGOVIA AND S. BULBULIAN
Instituto Nacional de Investigaciones Nucleares
Apartado postal 18-1027, 11801 México, D.F., México
Recibido el 15 de agosto de 1991; aceplado el 29 de abril de 1992
ABSTRACT.
In this paper a review of environmental radioactivity studies in groundwater from
a11uvial and volcanic aquifers in Mexico is presented. Thc water samples were taken from wells and
springs in San Luis Potosi, Mexico, and Michoacan states in Mexico. 222Rn was extracted from
lhe samples wilh pure loluene and radiaclivily measured wilh lhe Iiquid scinlillalion lechnique.
Results indicate a low radon content confirming that water samples from alluvial and basaltic
rocks have in general low radon levels.
RESUMEN. En este trabajo se presenta una revisión de los estudios sobre la radiactividad natural
realizados en algunas muestras de agua de pozos y manantiales ubicados en los Estados de San Luis
Potosí, México y Michoacán, México. Todas las muestras estudiadas provienen de acuíferos ya sea
aluviales o de origen volcánico. El radón (222Rn) de las muestras se extrajo con tolueno puro y se
midió por la técnica de centelleo líquido. Se encontró que estas muestras tenían un contenido muy
bajo de radón lo cual confirma que las muestras provenientes de acuíferos aluviales o volcánicos
basálticos contienen muy baja concentración de radón.
PAes: 89.60.+x
INTRODUCTION
The groundwater 222Rn source is the decay of 226Ra within the solid matrix of an aquifer
and exhaled from 226Ra dissolved in the water. 226Ra, with a half Jife of 1 600 years,
supplies a continuous source of 222Rn to the fluid in the pores of the matrix. I3eing
chemically inert, radon incorporates into ground water systems via molecular diffusion.
Ground water flow is then the dominant mechanism by which radon is transported;
the
Iimiting factor in the transport length being its half-Iife. In areas where surface waters
infi¡trate to aquifer, radon may be used as a tracer for qualitative investigatious.
The natural radiactivity of ground water varies markedly from site to site. In general,
granite areas and those where pegmatites are abundant, seem to favor e¡evated levels
of 222Rn and other natural radionuclides in ground water. On the other hand, 222Rn
concentration could be mueh lower in aquifers imbedded in alluvial deposits and basaltic
rocks [1,2,31 when the uranium content is low. l!owever, given a site, the actual situation
may tUfO out to be quite
different.
As a mattef
of fact, in the case of 222Rn al least,
the concentration
depends not only on that of its progenitor but also on the degree of
fracturation of the rocks and on the percolation time of the water. The rnaxilllum 222Rn
content is achieved when the velocity of the percolating water is low enough so that it can
pick-up as much 222Rn as possible, but not too slow so that 222!ln does not have euough
time to decay in large amounts [41.
RADONDETERMINATION
IN GROUNDWATER
243
FIGURE 1. Sampling Locations in San Luis Potosi State.
Studies on natural radioactivity in ground water were started in Mexico in San Luis
Potosi state followed by samplings from deep wells and springs in the states of Mexico and
Michoacán [5,6,71. In this paper sorne of the radon concentration levels found, as analysed
by the radon extraction in toluene method, are reviewed.
SITES DESCRIPTION
1. San Luis Potosi State
The water samples were collected from 4 wells in the Villa de Reyes valley and 3 wells
in the San Luis Potosi valley, both located in the State of San Luis Potosi, Mexico,
approximatelly
400 krn NE of Mexico City (Figure 1).
The Villa de Reyes valley is forrned by cernented sandstones, alluvial deposits and
volcanic tu!fs. A non confined aguHer is found forrned by clastics and rhyolites. Ground
water movernent is non-horizontal; downward movement occurs is the center of the valley
where rainfall and water from excess irrigation recharges the aguifer system.
On the other hand, the San Luis Potosi valley has a shallow alluvial aguifer and a deep
one separated by a clay and clayey sand body located in the middle of the valley [51.
2. Mcxico
and Michoacan
Sta/es
The sampling incllldes 46 wells drilled up to 200 m frorn the City of Toluca, and 6 springs
located at the sw part of the sta te.
244
N. SEGOVIAANDS. BULBULlAN
... .
. .•."-,.
..
-~=
,o
••.
FIGURE 2. Sampling Localions in lhe Cily of Toluca.
The aquifers of the studied zone in the state of Mexico are found mainly in basaltic
racks, particularly around the City of Toluca where igneous racks are found produced by
ancient eruptions from Nevado de Toluca volcano [5].
Water samples fram 2 spriugs belonging to lhe geothermal field of Los Azufres in the
state of Michoacan have al so been studied.
The geothermal field of Los Azufres belollgs to the Solfataras regio n located at the
Sierra de San Andres in the Central Part of the Mexican Neovolcanic bel!. The rocks
found in the are a are ba,alts. rhyo!ites, dacites and andesites. llydrothermal
springs are
found in this under exploilation geothermal field [7J.
The sampling lacations are indicalecl in Figures 2 and :l.
EXPERIMENTAL
The samples were analyzed for solubilized and nOHa-supporled
also studied for 234U/238U activity ratio.
222Rn. Sorne oflhem
were
Sampling
222Rn in natural water easily escapes from waler samples, lherefore, the sarnpling has
to be done \Vilh gleal careo The \Valer musl rUII steadily in lhe one liler \Vashed and
decontarninated
container
achieved after sampling
and wit!Jout
any air buhblcs,
in arder lo avoid degassing.
and pcrff'ct sealing
has to be
RADONDETERMINATION
IN GROUNDWATER
;
(;-e.' ,,-...
'''.
,
245
,-
'-/
,
,
,,
(4 .••.••.•.•
I
'" !
~--t
,
,,
{
FIGURE 3. Springs Sampled in Mexico and MidlOacan States.
Finally lhe sample musl he transporled
to the laboratory
conlenl w¡lhin 24 h from the lime of sampling.
Radon determination
lo be analysed
for 222Rn
in the water saml'les
Measuremenl
of radon and shorl Iived radon daughlers aclivily in water samples was
perfomed with a Packard No. 4530 scintillation deteclion system.
The toluene extraction melhod reporled by Noguchi and \Vakila [8] was slightly modified in order to measure very low 222Rn conlenl in the water samples. These authors
reported the radon extraction from water with a toluene base Iiquid scintillator, obtaining an exlraction efficiency of 80%. \Vilh lhe modificalion used in lhe presenl work, an
extraction efficiency of 97% was obtained when using pure loluene lo extracl radon from
lhe waler samples.
Aliquot.s of 100 mI sample and 10 mi of pure toluene were lransferred lo a separatory
funnel and vigorously shaken fol' 2 minules for radon separation. \Vhen lhe mixture reseltled in two phases, the organic phase was lransferred into a counting vial lo which 10 mi
of INSTAGEL scintillation solution was added. Samples were counted immediately after
radon separation
and again about fOUf hours after the first measurerncnt,
radioac.tive eqllilibrium with his daughters.
Corrections for decay of radon amI decay and growth of the daughter
samples \\'ere necessary.
upon rcaching
products
in the
246
N. SEGOVIAANDS. BULBULlAN
TABLE 1. Results obtained in San Luis Potosi State [5).
U
234U/238U
Temperaturc
Radon
(Bq/l)
(I'g/i)
SLP
901
507
918
2.18
1.31
1.42
0.56
0.47
0.49
1.72
0.96
1.02
36.5
36.2
36
VR
397
416
398
354
5.26
8.7
3.2
1.21
0.34
0.72
0.32
0.48
1.03
1.09
1.68
1.07
31
28.6
27.8
41.6
Well
('C)
Calibration of the collnter w•.., performed with a 226Ra-standard solution from NBS. Systematic countings of selected samples wcre made during several days in order to ascertain
that the decay of the extracted nllelide in tolllene corresponded to the 222Rn half-life.
Radon supported radium determination
226Ra support of 222Rn was determined in sorne samples from the same water ali'luots
from where original 222Rn present in the samples was extracted. Those ali'l"ots were sealed
and left for one month in order to reach equilibrium between 226Ra present in water amI
222Rn. Once the time elapsed, 222Rn extraction was performed from the samples in the
same way as specified in the previous paragraph.
Measurement
01234 Uf38 U activity ratio
Uranium was concentrated from twenty-liter water samples by co-precipitation
in b•..,ic
media. The precipitate was dissolved in 0.1 N HCl solution. Uranium was then purified
by the extraction-chromatographic
method using di-2-ethylhexylphosphoric
acid.
Thin foils of U compollnd were prepared for use in o-spectrometry.
Alpha-counting
was performed IItilizing a sllrface barrier detector collpled to a 1024 channel analyzer.
The time of measurement of the activity of the sources ranged from several hOllrs to two
days, depending on the intensity of the a-sources.
RESULTS
ANO OISCUSSION
Results of 222Rn, U and 234U j238 U activity ratio as obtained from water samples of seven
wells in San Luis Potosi and Villa de Reyes valleys are shown in Table 1. The water
temperatllres at sampling time are also indicated in this tableo
From these results we can scc that radan solubility in water dccreascs with incrcasillF;
temperature. No corrclation wa.sfound bclween radon and uranium conccntration Icvels
HADON DETERMINATION
IN GROUND WATER
247
"
"
,o
,
.
FIGURE 4.
Radon Concentration in Water Samples Crom the City oC Toluca [6).
in water. This lack of correlation has often been reported in aquifers since radon migration
iuto the pore fluid is higher than uranium. The values of the activity ratios obtained, very
c10se to equilibrium, together with the low uranium content seem to confirm a rapid water
transit from the recharge zone to the sampling site. Radon content for the samples from
Villa de Reyes is higher than that of San Luis Potosi by an average factor of 2.8, suggesting
a radon enrichment from a flow of more ancient subsurface water into the aquifer of Villa
de Reyes.
The distribution of 222Rn concentration as found in 46 wells from the City of Toluca
is shown in Figure 4. It can be observed that 58% of the sampled wells have less than
2..1 Bq/I; the highest value obtained being 11.3 Bq/l.
Those wells with the higher 222Rn concentration were found in regions where local rock
composition is mainly andesites and rhyolites.
222Rn supported by 226Ra dissolved in the water samples was below the detecting
threshold of the measuring equipment (0.009 Bq/I as reported by Olguin el al. [6]),
showing that radon input into the fluids of the aquifer is much higher than that of 226Ra .
.Radon content in the water from eight springs in the states of Mexico and Michoacan
is shown in Table 2. The spring water samples were taken in the same month (November)
as those of the wells of the City of Toluca.
CONCLUDING
In this papcr,
REMARKS
uranium
and radon rncasurements in sorne ground water samplcs werc
reviewed. As it is Iikely, since the embedding rocks of the studied aquifers are of basaltic
or sedimentary essence, low levels of radioactive nuclei have beeu found. In addition, the
samples were retreived at the eud of the rainy season when the aquifers were undergoing
248
N. SEGOVIA ANO S. BULBULlAN
TABLE JI. Radan concentrations
in spring water samples from ~lexico and Michoacan states [7].
Radon (Bq/I)
Spring
Mexico State
Las Tazas
Tonatico
Ix tapan de la Sal
Nevado de Toluca
Ixtapan del Oro I
Ixtapan del Oro Il
Michoacan State
San Alejo
Los Azufres
Temperature
0.491
1.518
2.362
0.375
1.037
0.117
17
33
32
16
31
25
1.335
0.25
41
82
(OC)
their latest recharging
phase. Accordingly,
the dilution effect was at its maximum,
which
also accounts for the presented findings. It" is worth mentioning
that the use of pure toluene
for radon extraction
allowed us to obtain a higher radon extraction
yield.
ACKNOWLEDGEMENTS
The authors are grateful to the technicians
of the Chemistry
and Nuclear Tracks Departments, ININ; to R. Noriega for linguistic revision. \Ve acknowledge
financial support
from
CONACYT, Mexico.
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1.
2.
3.
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