Radon content in groundwaters drawn from the metamorphic basement, eastern São Paulo State, Brazil

Radiation Measurements 42 (2007) 1703 – 1714 www.elsevier.com/locate/radmeas

Radon content in groundwaters drawn from the metamorphic basement, eastern São Paulo State, Brazil Fábio de Oliveira Lucas, Fernando Brenha Ribeiro ∗ Departamento de Geofísica do Instituto de Astronomia, Geofísica e Ciências Atmosféricas da Universidade de São Paulo, Rua do Matão, 1226, Butantã, 05508-090 São Paulo, Brazil Received 5 February 2007; received in revised form 15 May 2007; accepted 22 June 2007

Abstract The 222 Rn activity concentration was measured in groundwaters drawn from three wells drilled in different rocks of the metamorphic basement exposed at eastern São Paulo State, southeastern Brazil. The first well cuts a K-feldspar bearing granite, the second well cuts a granite–gneiss and the third well cuts a gneiss. The groundwater samples were collected between July 2005 and August 2006 with a time interval of about one month between sampling campaigns. The year-long mean activity concentrations were (76 ± 7) Bq/dm3 for the first well, (26 ± 3) Bq/dm3 for the second well and (39 ± 4) Bq/dm3 for the third one. The activity concentrations show a time-dependent variability that is interpreted as a consequence of rainfall seasonal variations. © 2007 Elsevier Ltd. All rights reserved. Keywords: Radon; Groundwater; Natural radioactivity

1. Introduction Groundwater drawn from granitic and metamorphic rocks frequently presents relatively high radioactivity, mostly due to the presence of uranium, radium and radon isotopes. In particular, the radon isotope 222 Rn seems to have particular significance since it is normally much enriched in groundwater in relation to other uranium series isotopes and is in large disequilibrium with its 226 Ra mother isotope. Furthermore, radon is one of the main agents of radioactivity transfer from crust uppermost layers to lower atmosphere. Some authors (e.g. Cothern et al., 1986) identify radon isotopes released by groundwaters as an important contribution to the natural radiation background. There are many reports on the presence of radon in groundwater drawn from granitic and metamorphic terrains (e.g. Asikainen and Kahlos, 1979, 1980; Dillon et al., 1991; Zhuo et al., 2001). Radon content in groundwaters in these terrains is quite variable and in part reflects the aquifer rock type. For instance, Zhuo et al. (2001) report, for the Fujian ∗ Corresponding author. Tel.: +55 11 3091 4755; fax: +55 11 3091 5034.

E-mail address: [email protected] (F.B. Ribeiro). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.06.005

Province, eastern China, mean radon concentrations between 108 and 270 Bq/dm3 for groundwaters drawn from fractures in different types of granite and between 72 and 128 Bq/dm3 for groundwaters drawn from fractures in different metamorphic rocks. Radon concentrations may show a large spread in groundwaters drawn from a single hydrogeologic unit. For instance, for a hydrogeologic unit of the Georgia Piedmont (USA), composed mainly of amphibolites associated with muscovite and biotite schists, biotite gneisses and quartzites, Dillon et al. (1991) reported radon concentrations varying between 5.9 and 5700 Bq/dm3 . At the eastern part of São Paulo State, southeastern Brazil, a metamorphic terrain outcrops with a large number of waterbearing fractures that composes a set of fractured aquifers. Groundwater is drawn from these aquifers through a large number of relatively shallow drilled wells, mostly less than 200 m deep. The radioactivity, and in particular the radon activity concentration, of the groundwaters drawn from this metamorphic terrain has not yet been investigated in detail. Oliveira et al. (2001) presented a general survey of the radioactivity in drinking water supplies of the São Paulo State, including groundwater from some of these aquifers. These authors, however, do not

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Fig. 1. General location of the study region. The large shadowed area in the lower insertion corresponds to the Morungaba batholith outcrop. (Composed with modified figures from Vlach, 1993.)

identify the rock type in their data presentation. In the whole São Paulo State data set, including sedimentary areas, 222 Rn activity concentrations vary from 0.40 to 315 Bq/dm3 . This paper reports the results of a year-long observation of the 222 Rn activity concentration variations in three wells drilled in a limited region of the metamorphic terrain at eastern São Paulo State, between 22◦ 40 S and 23◦ 03 S and between 46◦ 40 W and 47◦ 00 W (Fig. 1). 2. The geological setting The Ribeira folded belt with Pan-African-Brasiliano cycle age outcrops along the southeastern Brazilian coast. It is mainly composed of metamorphic rocks of variable grade, ranging from meta-sediments and meta-volcanic rocks to granulites and

a large number of late to post-orogenic granitic bodies. These rocks are densely fractured and the fractures represent, in many places, important sources of groundwater, which are exploited for domestic and economic purposes. The Ribeira belt rocks are characterized by variable radioactivity, with natural uranium concentrations varying between 0.5 and 18 g/g (6.1–221 Bq/kg of 238 U) and thorium concentrations varying between 3.0 and 100 g/g (12–410 Bq/kg of 232 Th). The potassium element concentrations vary in range between 0.01% and 4.8% of the rock mass (3.3.1590 Bq/kg of 40 K) (Tassinari and Barretto, 1991; Pascholati et al., 1997; Andrade, 1998; Vlach, 1998). In the region defined in Fig. 1, the metamorphic basement is composed of gneissic–migmatitic rocks intruded by the granitic plutons that form the large Morungaba batholith.

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

The 226 Ra activity concentration in groundwater drawn from fractures at the Morungaba Batholith was recently reported by Lucas and Ribeiro (2006). The report corresponds to a year-long observation in two wells drilled in the granites at the southern part of the batholith. The mean observed 226 Ra activity concentrations in these wells were about 45 and 52 mBq/dm3 and no systematic time dependence of the 226 Ra activity concentrations was clearly identified.

3. Well location and lithologic description The first well is located downtown Itatiba (7453.68 km North and 310.40 km East, UTM coordinates, central meridian 45◦ ). With 155 m total depth, it cuts 10 m of clay–sand soil, 21 m of pink-whitish weathered granite and ends in grayish porphyroid granite with K-feldspar porphyries. In the following, this well will be referred as well 1. The second well (well 2) is located at the outskirt of the city of Itatiba and has UTM coordinates 7450.13 km North and 309.07 km East. With 52 m total depth; it was drilled in the metamorphic basement rock, cutting 14.3 m of soil, 4.70 m of weathered rock, with quartz, feldspar and abundant mica and ending in gray granite–gneiss. The third well (well 3), with UTM coordinates 7475.42 km North and 302.34 km East, is located in the rural area between the locality of Souzas and the city of Pedreira, at the sheared northwest contact between the metamorphic basement and the granitic batholith. It has 54 m total depth and cuts 17 m of soil, 7 m of weathered rock and 30 m of quartzose and micaceous melanocratic gneiss. Four fractures were identified at depths of 27, 29, 30 and 42 m.

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4. Water sampling Water samples were collected in special bottles designed to avoid contact between samples and the atmospheric air (Fig. 2). The sampling bottle has a volume of about 3 dm3 . Its lid has one inlet, controlled by a gas tighten tap, which drives water to the bottle bottom, and one outlet, also controlled by a gas tighten tap, which drains the bottle when it is completely filled with water. The lid has also a plug-in gas connection that allows a striping gas to be bubbled in water through a porous glass plug near the bottle bottom. When opened to the air, the plug-in connection also drains the bottle when it is completely filled. The water samples are collected at the well mouth after having discarded a volume of at least 15 min of flow through the sampling drain. At least three independent sample aliquots are collected at each well in a sampling campaign. A sampling bottle, with the internal volume clean and free of oil and fat, is connected to the well drain through a plastic hose. Keeping the inlet, the outlet and the plug-in connection open, water is allowed into the sampling bottle. Water completely fills the bottle and drains through the outlet and the plug-in connection, removing all the air. After 5 min of continuous flushing, the plug-in connection is closed and at least more 50 dm3 of water is forced to flow through the sampling bottle volume. Special care is taken to ensure that only minimal volume of air bubbles, if any, remain in the plastic hose and in the sampling bottle during the entire flushing period. After the flushing period, the two taps are closed sealing a 3 dm3 water sample with a radon activity that should be representative of the groundwater at the sampling site. The sampling time is recorded and the bottles are taken to the laboratory for radon activity measurement.

Fig. 2. Water sampling bottle.

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a Poisson probability distribution. A 2obs variable was defined as (Knoll, 1988)

5. Radon stripping from solution and activity measurement procedure The activity of the 222 Rn isolated in the sampling bottles was measured by the emanation technique (Lucas et al., 1990), using the radon extraction from solution and concentration system described by Lucas and Ribeiro (2006). A brief description of the measurement procedure is given below. The sampling bottle is connected by its tap-controlled outlet to the extraction system. With the tap open, argon is bubbled in the 222 Rn-containing water through the sampling bottle plug-in gas connection (Fig. 2). The argon flow removes 222 Rn from the solution and crosses, in sequence, two glass columns. The first column, empty and cooled at dry ice and acetone mixture temperature (−76 ◦ C), retains most of the water vapor. The second column contains silica gel, which retains traces of vapor eventually still present, and ascarite (sodium hydroxide on silica), which retains all carbon dioxide. The argon flow then crosses an activated charcoal column, cooled with dry ice and acetone bath, where argon and a fraction of the 222 Rn stripped from the solution are retained. The charcoal column is then heated to 350 ◦ C and a fraction of the degassed argon–radon mixture is transferred to a scintillation detector (Lucas cell type detector). The detector is then left at rest for at least 4 h, which is the time interval necessary for 222 Rn to reach secular radioactive equilibrium with its short-lived daughters: 218 Po, 214 Pb, 214 Bi and 214 Po. After this time interval, the detector is coupled to a photomultiplier tube and associated electronics for alpha decay counting. 6. Background counting and detection limits To estimate the total background that affected the 222 Rn activity measurements, three distilled water samples were closed in sampling bottles, left at rest for at least four months and then submitted to the radon extraction and activity measurement procedure. The background counts of three clean scintillation detectors and associated electronics were integrated in 24 consecutive, 15 min long, time intervals. For each counting data set, it was verified if its 24 elements could be considered as extractions of

2obs = (N − 1)

s2 , x¯

(1)

where x¯ and s 2 are the sample mean and variance of the data set. N is the data number, 24, in this case. The statistical test consists in seeking to reject the hypothesis that the population mean and variance, estimated by x¯ and s 2 , are equal, as in a Poisson distribution, considering a previously established significance level. The decision of rejecting or not the null hypothesis is made by applying the usual 2 test on the variable 2obs (e.g. Bevington, 1969). In these tests, a significance level of 0.05 was adopted. The null hypothesis could not be rejected for the three data sets. Since the detector background together with the detection system background counts can be considered as belonging to Poisson distributions and since the mean values are about 20, which allows to approximate a Poisson distribution to a normal distribution, a critical detection level and a limit of qualitative detection can be established following Curie (1968). Table 1 presents the mean background counts integrated in 15 min long time intervals and the calculated critical level (Lc ) and limit of qualitative detection (LD ). In establishing Lc and LD , a type I statistical error () of 0.05 and a type II statistical error () of 0.05 were adopted, respectively. The three distilled water samples were then radon purged and the resulting gas mixture transferred to the tested scintillation detectors. The alpha counts of each distilled water sample were also integrated in 24 consecutive, 15 min long, time intervals. Application of the described 2 test led to the conclusion that these counts can also be considered as values extracted from Poisson distributions. Table 1 also presents the mean water sample counts integrated in 15 min long time intervals. Comparison with the calculated Lc and LD shows that a liquid signal could not be detected in any of the distilled water samples. As a consequence of this test, it was decided to adopt the counting rates observed in the clean scintillation detectors prior to radon activity measurements as the background counting rate.

Table 1 Comparison between the sterile sample counts with the scintillation detector backgrounds Sampling bottle volume (±0.028) (dm3 )

Detector mean background countsa

Critical levelb (Lc ) (counts)a

Limit of qualitative detectionb (LD ) (counts)a

Sterile sample mean counts

Observation

3.040 3.020 2.980

20.4 20.4 20.7

10.5 10.5 10.6

23.7 23.7 23.9

33.7 31.6 30.8

< LD < LD < Lc

All listed background and sterile sample counts correspond to the mean value of 24 consecutive alpha activity observations integrated in 15 min long time intervals. Critical levels (Lc ) and limits of qualitative detection were calculated following Curie (1968). a Counts in 15 min time intervals. b Counts above the background mean value.

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714 Table 2 Calibration of the dissolved

222 Rn

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activity measuring procedure

Aliquot

Bottle volume (±0.028) (dm3 )

226 Ra

activity inside the bottle (Bq)

222 Rn build-up time (min)

Calculated 222 Rn activity inside the bottle (Bq)

222 Rn decay rate inside the detector (d.p.m.)

Estimated conversion factor () (Bq/d.p.m.)

1 2 3 4 5

2.940 3.040 3.020 3.020 3.020

40.0 ± 2.0 41.3 ± 2.1 41.1 ± 2.1 41.1 ± 2.1 41.1 ± 2.1

25 640 27 327 31 796 28 700 30 195

38.4 ± 1.9 40.0 ± 2.0 40.3 ± 2.1 40.0 ± 2.0 40.2 ± 2.1

1015 ± 13 1016 ± 21 919 ± 13 1140 ± 19 931 ± 11

0.0378 ± 0.0019 0.0394 ± 0.0021 0.0439 ± 0.0024 0.0351 ± 0.0018 0.0442 ± 0.0024

The individual estimates of the conversion factor were used to define a mean conversion factor of (0.040 ± 0.002) Bq/d.p.m.

7. Calibration of the dissolved radon activity measuring procedure 0.5 mol/dm3

Twenty kilograms of a HCl solution were contaminated with radium to produce a radium solution with (13.60 ± 0.68) Bq/dm3 activity concentration. The radium activity concentration was measured twice in three independent 0.5 dm3 aliquots of the solution by the emanation method, following Lucas and Ribeiro (2006). The indicated uncertainty corresponds to one standard error of the mean. About 15 dm3 of the radium solution has the dissolved radon purged by flushing it, for about 1 h, with an atmospheric air flow sufficiently large to stir the whole solution volume. The total solution volume was then divided and closed completely filling five sampling bottles and left at rest for radon buildup. Table 2 presents the bottle volumes, the total radium activity inside the bottles, the radon build-up time and the calculated radon activities inside the bottles. These activities were calculated assuming that there was no radon dissolved at the bottles’ closing time. The dissolved 222 Rn was purged from the solution aliquots and the activities inside the scintillation detectors measured. A conversion factor () between the 222 Rn activity inside the detector and the 222 Rn activity in the solution can be defined as

of time in the form A = A0 e−Rn t ,

where Rn is the 222 Rn decay constant, t is the time after radon purging and A0 corresponds to three times the initial 222 Rn counting rate inside the detector. The total alpha counting rate is integrated during 6 h divided in consecutive, 15 min long (t) , time counting intervals, to give a set of 24 consecutive Gi gross countings. The net Ai consecutive counting rate set, defined by Ai =

Gi − B, t

i=1

2 Ai −2 e Rn ti i=1 2 Ai

A0 =  24

8. Groundwater aliquots data processing The alpha counting begins 4 h after radon has been purged from groundwater aliquots and transferred to the scintillation detectors. At this time, 222 Rn is in secular radioactive equilibrium with its short-lived, alpha-emitting daughters and the total alpha counting rate inside the detector (A), which is three times the counting rate due to radon alone, varies as a function

(5)

with an unbiased variance estimate given by   24 1 1  (Ai − A0 e−Rn ti )2 2   A0 =  2 . 2  23  Ai 24 e−Rn ti i=1

activity in solution (Bq)  = 222 . Rn decay rate inside the scintillation detector(d.p.m.)

Table 2 presents the 222 Rn decay rate measured in the detectors and the five () estimates. The mean conversion factor is (0.040 ± 0.002) Bq/d.p.m., where the indicated uncertainty corresponds to one standard error of the mean.

(4)

where B is the mean scintillation detector background counting rate, is then least squares fitted to Eq. (3) to estimate A0 in the form 24 Ai e−Rn ti

222 Rn

(2)

(3)

i=1

Ai

(6) Assuming that the gross 15 min long observations are extractions of Poisson distributions characterized by the mean value estimated by Gi , the variances 2 Ai are estimated by 2 Ai =

Gi (t)2

+ 2 B,

(7)

where 2 B is the variance of the background counting rate. The goodness of this fitting process is evaluated by applying the 2 test with a significance level  (Bevington, 1969) to the variable defined as 1  (Ai − A0 e−Rn ti )2 . 23 2 Ai 24

2obs =

(8)

i=1

The initial radon counting rate is successively transformed in the initial radon decay rate inside the detector, using the individual scintillation detectors calibration factors, and in the

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water aliquot radon activity at the purging time, using the conversion factor estimated for the extraction line. Each calculated activity is then divided by the corresponding sampling bottle volume to give the activity concentration of the aliquot. The three groundwater aliquots collected in a particular well give three values of radon activity concentration inside the sampling bottles at the time of radon purging Coj (j = 1, 2, 3). Since the determinations occur at different times tj after sampling, which can last for a few days, it is necessary to correct for the radon decay and to estimate the radon activity concentration (CG ) in the groundwater aliquot at the sampling time. After groundwater sampling, the radon activity concentration C(t) as a function of time inside the sampling bottles is given by C(t) = CG e−Rn t + CRa (1 − e−Rn t ),

(9)

where CRa is the radium activity concentration contained in the groundwater aliquot. In general, CG is expected to be much larger than CRa , since 222 Rn is normally significantly enriched in relation to 226 Ra in groundwaters. Eq. (9) can be re-written in the form C(t) = (CG − CRa ) e−Rn t + CRa ,

(10)

where C(t) is expressed as a linear function of the variable z = e−Rn t . CG is estimated by fitting by least squares the observation pair (Coj , z) to Eq. (10). a = CRa

(11)

being the intercept of the fitted linear function and b = (CG − CRa )

(12)

its slope, the estimated radon activity concentration in the groundwater aliquot at the sampling time is given by CG = b + a

(13)

with an estimated variance given by 2 CG = 2 a + 2 b + 22 ab,

(14)

where 2 a and 2 b are the variances of the intercept and slope estimates, respectively, and 2 ab is the covariance between them. 9. Results and discussion Fig. 3 presents the temperature at the three sampling well mouths as a function of time, beginning on July 1st, 2005. In well 1, the temperatures varied between 22.9 and 23.9 ◦ C through the whole sampling period, whereas in well 2 the temperatures varied between 20.6 and 22.7 ◦ C. In well 3, the temperatures varied in range between 22.3 and 23.5 ◦ C. The differences of the mean temperature between the wells and the temperature variations in each well are small and do not reflect in the variations in the radon activity concentrations. Table 3 presents the analytical data obtained at well 1. The table presents, for each sample aliquot, the aliquot volume, the beginning alpha counting time after sampling in minutes, the 2obs for the counting data fitting to Eq. (3), the total activity and the activity concentration. The table also presents the activity concentration in the groundwater estimated from the

Fig. 3. Water temperature at well mouth as a function of time between July 2005 and July 2006.

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

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Table 3 Analytical data obtained with well 1 water Sampling Sample campaign aliquot

Aliquot volume (dm3 )

Time after water sampling (min)

2 for the alpha counting fitting

Aliquot total activity (Bq)

Aliquot activity concentration (Bq/dm3 )

Groundwater activity concentration (Bq/dm3 )

2 for the activity concentration fitting

1

1 2 3 4 5 6 7 8

2.980 ± 0.028 2.940 ± 0.028 3.020 ± 0.028 3.020 ± 0.028 3.060 ± 0.028 3.040 ± 0.028 3.040 ± 0.028 3.000 ± 0.028

337 3115 4670 7471 8917 11 724 13 160 14 737

23.5 25.4 21.4 33.1 35.7 16.7 20.3 27.9

282 ± 15 187.6 ± 9.8 145.5 ± 7.7 128.3 ± 6.7 111.0 ± 7.5 69.9 ± 3.6 58.8 ± 3.6 50.6 ± 3.3

94.7 ± 5.1 63.8 ± 3.4 48.2 ± 2.6 42.5 ± 2.2 36.3 ± 2.4 23.0 ± 1.2 19.3 ± 1.2 16.9 ± 1.1

95 ± 10

11.586

2

1 2 3

3.020 ± 0.028 3.040 ± 0.028 3.020 ± 0.028

376 1933 6270

36.0 28.5 19.4

221 ± 11 169.9 ± 8.7 95.5 ± 5.8

73.0 ± 3.8 55.9 ± 2.9 31.7 ± 1.9

74.7 ± 2.5

0.623

3

1 2 3

3.020 ± 0.028 3.000 ± 0.028 3.020 ± 0.036

4657 1740 3249

22.3 15.2 28.8

136.0 ± 8.2 200 ± 10 168.8 ± 8.9

45.1 ± 2.8 66.7 ± 3.5 55.9 ± 3.0

84.5 ± 2.4

0.134

4

1 2 3

3.020 ± 0.028 3.020 ± 0.028 2.940 ± 0.028

12 992 10 404 8703

21.4 37.1 23.7

41.9 ± 2.2 52.7 ± 2.7 64.0 ± 3.3

13.88 ± 0.74 17.44 ± 0.92 21.8 ± 1.1

58.2 ± 4.2

0.374

5

1 2 3

2.940 ± 0.028 3.020 ± 0.028 3.020 ± 0.028

392 8975 10 513

35.8 37.1 35.5

204 ± 10 70.7 ± 3.7 59.4 ± 3.1

69.4 ± 3.6 23.4 ± 1.2 19.7 ± 1.1

72.6 ± 1.7

0.204

6

1 2 3

3.020 ± 0.028 2.940 ± 0.028 3.020 ± 0.028

2837 4172 7247

22.1 22.2 31.4

173.8 ± 9.2 146.9 ± 7.5 106.4 ± 5.8

57.6 ± 3.1 50.0 ± 2.6 35.3 ± 1.9

80.45 ± 0.77

0.018

7

1 2 3

3.020 ± 0.028 2.940 ± 0.028 3.040 ± 0.028

21 664 18941 20 337

30.5 21.1 18.8

15.03 ± 0.79 23.6 ± 1.2 18.7 ± 1.1

4.98 ± 0.27 8.03 ± 0.42 6.16 ± 0.38

110.3 ± 6.7

0.154

8

1 2 3

2.980 ± 0.028 3.040 ± 0.028 3.020 ± 0.028

318 12 800 11 735

37.0 33.4 28.2

276 ± 17 57.6 ± 3.0 63.2 ± 3.2

92.5 ± 5.7 18.9 ± 1.0 20.9 ± 1.1

95.9 ± 3.1

0.272

9

1 2 3

3.040 ± 0.028 2.940 ± 0.028 3.020 ± 0.028

8591 9214 7200

28.4 31.8 33.8

35.2 ± 1.8 30.6 ± 1.6 41.4 ± 2.3

11.59 ± 0.60 10.41 ± 0.54 13.72 ± 0.76

35.5 ± 2.2

0.107

10

1 2 3

3.020 ± 0.028 3.040 ± 0.028 3.020 ± 0.028

375 1736 6050

23.4 29.6 24.9

149.7 ± 7.8 119.0 ± 7.2 70.4 ± 3.8

49.6 ± 2.6 39.1 ± 2.4 23.3 ± 1.3

50.8 ± 1.8

0.624

11

1 2 3

3.020 ± 0.028 3.020 ± 0.028 3.020 ± 0.028

2915 4232 7406

27.5 24.4 34.2

170.6 ± 8.7 140.1 ± 7.6 105.6 ± 6.4

56.5 ± 2.9 46.4 ± 2.6 35.0 ± 2.1

76.4 ± 4.2

0.575

sample aliquot activity concentrations through Eqs. (10) and (13) and the 2obs for the fitting involved in this estimate. In all cases, 24 alpha counting data points were fitted to Eq. (3) to estimate the A0 value for each sampling campaign. Considering the significance level of 0.05, most of the estimated 2obs should fall in the interval given by 11.7 2obs 38.1.

(15)

Since the fitting process was repeated 38 times, the expected number of observed chi-square values out of this interval is 2.

In the case of the first well data, the number of estimated 2obs out of this interval (none) did not differ much from the expected one. With the exception of the first sampling campaign, three activity concentrations were fitted to Eq. (10) to estimate the slope and the intercept defined by Eqs. (11) and (13). In these cases, the estimated 2obs were expected to fall between 0.001 and 5.02, considering a significance level of 0.05. Since the fitting process was repeated 10 times, estimated 2obs were expected between none and 1. Again none 2obs was observed out of the range. In the case of the first sampling campaign (eight activity concentration data point), the 2obs also fell in the acceptable

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F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

Fig. 4. Radon (222 Rn) activity concentration in the groundwater tapped by well 1 (downtown Itatiba city) as a function of the time between July 2005 and July 2006.

2 value range for the corresponding number of degrees of freedom (6). Fig. 4 shows the 222 Rn activity concentration of the groundwater drawn from this well as a function of time, beginning on July 1st, 2005. The horizontal continuous line represents the mean radon activity concentration of (76 ± 7) Bq/dm3 , the indicated uncertainty corresponding to one standard error of the mean. No low degree polynomial, including a constant equal to the mean value and no periodic function with small number of harmonics fit well to the individual values. Since the individual activity concentration uncertainties were established with some care and seem not to be underestimated, the activity concentrations seem to present a high variability about the observed mean. A noticeable local maximum activity concentration of (110.3 ± 6.7) Bq/dm3 occurred in the middle of March 2006, separating two distinct local minima in the middle of November 2005 and the beginning of June 2006. Table 4 presents the analytical data obtained at well 2. With one exception, all estimated 2obs for the alpha counting fitting to Eq. (3) fell in the range defined in (15). Since the fitting process was repeated 30 times, the expected number of observed chisquare values out of this interval lays between 1 and 2. For Eq. (10) slope and the intercept estimates, none of the 2obs values fell out of the interval between 0.001 and 5.02. Fig. 5 shows the 222 Rn activity concentration of the groundwater drawn from this well as a function of time, beginning on July 1st, 2005. The horizontal continuous line represents the mean radon activity concentration of (26 ± 3) Bq/dm3 (one standard error of the mean). A broad and less evident local maximum of about 33 Bq/dm3 occurred between the beginning

of February 2006 and the beginning of June 2006, separating a clear local minimum at the beginning of January and a less evident local minimum at the beginning of July of the same year. Table 5 presents the analytical data obtained at well 3. In this case, three estimated 2obs for the alpha counting fitting to Eq. (3) fell out of the range defined in (15). Again, the expected observed chi-square values out of this interval lays between 1 and 2. For Eq. (10) slope and the intercept estimates, one of the 2obs values fell out of the interval between 0.001 and 5.02. Fig. 6 shows the 222 Rn activity concentration of the groundwater drawn from this well as a function of time, beginning on July 1st, 2005. The horizontal continuous line represents the mean radon activity concentration of (39 ± 4) Bq/dm3 (one standard error of the mean). A noticeable local maximum activity concentration of (59 ± 11) Bq/dm3 occurred at middle March 2006, separating two distinct local minima in the beginning of December 2005 and in the middle of June 2006. The different mean values observed in each well seem to reflect the different rock types they were drilled in. The groundwater drawn from the granitic body (well 1) presents a distinctly higher mean activity concentration than the groundwaters drawn from metamorphic rocks (wells 2 and 3). It is worthwhile to point out that the radioactivity of the rocks cut by the wells was not measured. Fig. 7 presents the total precipitation in each month, from April 2005 to July 2006, observed in two stations, about 5 km apart and located at the city of Campinas. The two stations are about 30 km from the Itatiba wells and about 15 km from

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

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Table 4 Analytical data obtained with well 2 water Sampling Sample campaign aliquot

Aliquot volume (dm3 )

Time after water sampling (min)

2 for the alpha Aliquot total counting fitting activity (Bq)

Aliquot activity concentration (Bq/dm3 )

Groundwater activity concentration (Bq/dm3 )

2 for the activity concentration fitting

1

1 2 3

3.020 ± 0.028 2.980 ± 0.028 3.020 ± 0.036

5960 9141 7437

21.2 27.3 21.4

44.5 ± 2.3 33.9 ± 2.1 37.9 ± 2.0

14.74 ± 0.77 11.38 ± 0.71 12.54 ± 0.66

25.9 ± 2.7

0.318

2

1 2 3

3.020 ± 0.028 2.940 ± 0.028 3.020 ± 0.036

7229 6093 8928

32.7 31.4 23.3

30.2 ± 1.6 32.9 ± 1.7 28.2 ± 1.7

9.98 ± 0.52 11.2 ± 0.59 9.32 ± 0.57

18.1 ± 2.1

0.336

3

1 2 3

3.040 ± 0.028 3.040 ± 0.028 3.020 ± 0.028

1503 2823 7498

29.6 33.7 19.1

56.9 ± 3.4 51.8 ± 2.8 21.9 ± 1.5

18.7 ± 1.1 17.4 ± 0.94 9.25 ± 0.50

23.2 ± 1.1

0.755

4

1 2 3

3.040 ± 0.028 3.020 ± 0.028 3.040 ± 0.028

4358 5860 7631

32.2 37.2 8.7

14.85 ± 0.84 14.60 ± 0.87 11.21 ± 0.58

5.21 ± 0.28 4.83 ± 0.29 3.69 ± 0.19

8.8 ± 1.1

1.392

5

1 2 3

3.020 ± 0.028 2.940 ± 0.028 2.980 ± 0.028

1568 3056 4457

33.9 21.9 34.8

84.0 ± 4.3 61.2 ± 3.7 56.2 ± 2.9

27.8 ± 1.5 20.8 ± 1.3 18.85 ± 0.98

33.1 ± 3.2

1.625

6

1 2 3

3.020 ± 0.028 3.000 ± 0.028 2.980 ± 0.028

11 682 10 420 15 969

30.4 35.5 21.1

23.3 ± 1.3 24.8 ± 1.3 12.79 ± 0.67

7.71 ± 0.42 8.28 ± 0.44 4.29 ± 0.23

31.5 ± 3.0

1.277

7

1 2 3

3.000 ± 0.028 3.020 ± 0.028 3.040 ± 0.028

9031 7712 10 590

23.5 20.9 23.0

32.8 ± 2.0 35.9 ± 2.0 24.4 ± 1.3

10.93 ± 0.67 11.88 ± 0.65 8.02 ± 0.42

34.1 ± 5.9

1.694

8

1 2 3

2.940 ± 0.028 3.000 ± 0.028 3.000 ± 0.028

4474 5979 7517

22.1 20.9 21.4

56.8 ± 3.4 47.8 ± 2.5 41.5 ± 2.3

19.3 ± 1.2 15.93 ± 0.85 13.74 ± 0.76

32.1 ± 1.4

0.105

9

1 2 3

3.040 ± 0.028 3.020 ± 0.028 3.000 ± 0.028

3258 5037 7558

16.7 16.4 21.8

28.7 ± 1.6 26.0 ± 1.4 20.1 ± 1.1

9.44 ± 0.53 8.61 ± 0.47 6.70 ± 0.37

13.1 ± 1.0

0.716

10

1 2 3

3.020 ± 0.028 3.040 ± 0.028 3.020 ± 0.028

335 1298 5905

29.8 30.9 27.5

106.2 ± 5.5 101.2 ± 5.3 61.8 ± 3.7

35.2 ± 1.3 33.3 ± 1.8 20.5 ± 1.2

36.92 ± 0.86

0.472

well 3. The stations, code D4-044 and D4-047, are run by the São Paulo State Water and Electric Energy Department. In this figure, the black bar corresponds to the month precipitation at D4-044 and the hatched bar corresponds to the month precipitation at D4-047. The gray bar represents the observations mean. This figure shows a heavier precipitation between October 2005 and March 2006, with a total of about 1000 mm. The maximum precipitation occurs in January. The precipitation minima occur in August 2005 and in June 2006. The 222 Rn activity concentration variation observed at wells 1 and 3 partially reproduces this pattern, with the maximum activity concentration shifted about two months ahead of the precipitation maximum. Also, the activity concentration in these two wells partially reproduces the precipitation variation observed between April and June 2005. In the case of well 2, this relation is less clear.

During the rain period, radon is trapped by the water percolating downward in rock fractures and fissures. The groundwater progressively enriched in radon migrates through the rock and eventually is collected in a well. During the dry season, downward water percolation diminishes lowering the radon transfer from rock to groundwater. Since 222 Rn is a short-lived isotope, the concentration excess decays. If this interpretation is correct, at least in general terms, the time lag between the maximum precipitation at surface and maximum radon activity in groundwater may represent an estimate of the time interval necessary for the fractured aquifer respond to changes in precipitation at the ground surface. 10. Conclusions The 222 Rn activity concentration was monitored during a year-long observation in the groundwater drawn from three

1712

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

Fig. 5. Radon (222 Rn) activity concentration in the groundwater tapped by well 2 (outskirt of the city of Itatiba) as a function of the time between July 2005 and July 2006. Table 5 Analytical data obtained with well 3 water Sampling Sample Aliquot Time after water 2 for the alpha Aliquot total Aliquot activity Groundwater activity 2 for the activity campaign aliquot volume (dm3 ) sampling (min) counting fitting activity (Bq) concentration (Bq/dm3 ) concentration (Bq/dm3 ) concentration fitting 1

1 2 3

3.013 ± 0.036 3.013 ± 0.036 3.013 ± 0.036

1407 3039 4265

34.7 33.1 37.1

117.2 ± 7.1 100.3 ± 5.5 88.4 ± 4.6

38.9 ± 2.4 33.3 ± 1.9 29.4 ± 1.6

45.11 ± 0.40

0.010

2

1 2 3

3.020 ± 0.028 3.020 ± 0.028 3.060 ± 0.028

3358 8983 1943

20.9 21.2 23.8

92.2 ± 4.7 49.1 ± 2.6 126.4 ± 6.8

30.5 ± 1.6 16.26 ± 0.87 41.3 ± 2.3

49.1 ± 4.2

2.897

3

1 2 3

3.000 ± 0.028 8837 2.980 ± 0.028 10 198 3.020 ± 0.036 11 540

38.4 32.4 32.2

43.9 ± 2.3 35.4 ± 1.9 31.6 ± 1.6

14.63 ± 0.78 11.87 ± 0.55 10.47 ± 0.55

43.3 ± 4.5

0.375

4

1 2 3

3.020 ± 0.028 7558 3.040 ± 0.028 8987 3.020 ± 0.028 10 429

41.6 33.1 44.0

23.8 ± 1.3 20.1 ± 1.2 16.10 ± 0.85

7.88 ± 0.43 6.61 ± 0.40 5.33 ± 0.29

21, 38 ± 0.73

0.058

5

1 2 3

3.020 ± 0.028 3.040 ± 0.028 2.940 ± 0.028

1698 2929 4314

14.8 36.1 26.1

67.5 ± 3.5 62.0 ± 3.8 50.6 ± 2.8

22.3 ± 1.2 20.4 ± 1.3 17.21 ± 0.95

26.9 ± 1.0

0.204

6

1 2 3

3.020 ± 0.028 8851 2.980 ± 0.028 10 080 3.040 ± 0.028 6036

24.5 30.8 15.6

49.1 ± 2.5 37.8 ± 2.1 67.2 ± 4.1

16.25 ± 0.85 12.67 ± 0.70 22.1 ± 1.4

49.9 ± 6.5

1.290

7

1 2 3

3.040 ± 0.028 14 683 3.040 ± 0.028 20 299 3.020 ± 0.028 21 733

23.1 23.8 16.0

27.3 ± 1.4 10.91 ± 0.56 10.41 ± 0.63

8.97 ± 0.47 3.59 ± 0.19 3.45 ± 0.21

59 ± 11

5.336

8

1 2 3

3.040 ± 0.028 1462 3.000 ± 0.028 10 383 3.040 ± 0.028 11 800

25.5 12.8 28.1

104.3 ± 5.4 31.5 ± 1.7 24.6 ± 1.3

34.3 ± 1.8 10.49 ± 0.56 8.08 ± 0.42

41.8 ± 1.6

0.507

9

1 2 3

3.040 ± 0.028 3.040 ± 0.028 3.040 ± 0.028

1808 2876 4184

30.4 30.0 21.3

81.0 ± 4.2 67.3 ± 3.4 63.4 ± 3.3

26.6 ± 1.2 22.1 ± 1.1 20.9 ± 1.1

31.0 ± 3.4

1.481

10

1 2 3

3.040 ± 0.028 3.040 ± 0.028 2.980 ± 0.028

419 4219 6280

21.9 19.7 27.4

78.8 ± 4.2 53.5 ± 2.7 38.7 ± 2.0

25.9 ± 1.4 17.60 ± 0.90 12.98 ± 0.68

26.9 ± 3.0

3.437

11

1 2 3

3.000 ± 0.028 3.040 ± 0.028 3.060 ± 0.028

1757 3000 4690

26.1 30.5 25.5

81.8 ± 4.2 67.6 ± 3.5 53.8 ± 2.9

27.3 ± 1.4 22.1 ± 1.2 17.58 ± 0.96

34.72 ± 0.89

0.133

F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

1713

Fig. 6. Radon (222 Rn) activity concentration in the groundwater tapped by well 3 (rural area between Souzas and Pedreira) as a function of time between July 2005 and July 2006.

Fig. 7. Total precipitation between April 2005 and July 2006 observed in two stations, 5 km apart, in the city of Campinas. The two stations are about 30 km from the Itatiba wells and 15 km from well 3. Black bar corresponds to data collected at station D4-044 and the hatched bar corresponds to station D4-047. The gray bar represents the observations mean.

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F. de Oliveira Lucas, F.B. Ribeiro / Radiation Measurements 42 (2007) 1703 – 1714

wells, drilled in different rocks of the exposed metamorphic basement at eastern São Paulo State. The mean activity concentrations in part reflect the rock type crossed by each well, with the higher value being observed in groundwater drawn from granite. Comparing with the 222 Rn activity concentrations in groundwater drawn from metamorphic terrains elsewhere, the three Ribeira Belt sampled well groundwaters have relatively low radon contents. The activity concentrations show a time-dependent variability that can be interpreted as induced by rainfall fluctuations during the year. Acknowledgments The work described in this paper was financially supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo—FAPESP and by the Conselho Nacional do Desenvolvimento Científico e Tecnológico—CNPq. The authors are indebted to “Covabra Supermercados” administration, in particular to Mr. Rodnei Barros, for permission to access well 1, to “Dover do Brasil Ltda” administration, in particular to Mr. João Almeida, for permission to access well 2 and to Mr. Pierre Alexis Fonteyne for the permission to access well 3. F.O.L. wishes to thank the Coordenadoria de Aperfeiçoamento do Ensino Superior—CAPES for a graduate fellowship. F.B.R. research work is partially supported by CNPq, proc. 300599/2005-6. References Andrade, T.C.Q., 1998. Taxa de produção de calor radiogênico no embasamento da Bacia do Paraná. Ph.D. Thesis, Instituto Astronômico e Geofísico da Universidade de São Paulo, São Paulo, Brazil. Asikainen, M., Kahlos, H., 1979. Anomalously high concentrations of uranium, radium and radon in water from drilled wells in the Helsinki region. Geochimica et Cosmochimica Acta 43, 406–409.

Asikainen, M., Kahlos, H., 1980. Natural radioactivity of drinking water in Finland. Health Phys. 39, 77–83. Bevington, P.R., 1969. Data Reduction and Error Analysis for the Physical Sciences. McGraw-Hill, New York. Cothern, C.R., Lappenbusch, W.L., Michel, J., 1986. Drinking water contribution to natural background. Health Phys. 50, 33–39. Curie, L.A., 1968. Limits for qualitative detection and quantitative determination: application to radiochemistry. Anal. Chem. 40, 586–593. Dillon, M.E., Carter, G.L., Arora, R., Kahn, B., 1991. Radon concentrations in ground water of the Georgia Piedmont. Health Phys. 60, 229–236. Knoll, G.F., 1988. Radiation Detection and Measurement. second ed.. Wiley, New York. Lucas, F.O., Ribeiro, F.B., 2006. Radium content in ground water from a granitic batholith of the metamorphic basement, eastern São Paulo State, Brazil. Appl. Radiat. Isot. 64, 735–749. Lucas, H., Markun, F., Boulenger, R., 1990, Methods for measuring radium isotopes: emanometry. In: The Environmental Behavior of Radium, vol. 1. International Atomic Energy Agency Technical Reports Series no. 310, Vienna, pp. 149–172. Oliveira, J., Mazzilli, B.P., Sampa, M.H.O., Bambalas, E., 2001. Natural radionuclides in drinking water supplies of São Paulo State, Brazil, and consequent population doses. J. Environ. Radioact. 53, 99–109. Pascholati, E.M., Amaral, G., Hiodo, F.Y., Okuno, E., Yoshimura, E.M., Yukihara, E.G., 1997. Survey of environmental gamma radiation around Itu. In: Fourth Meeting on Nuclear Applications, vol. 1. Poços de Caldas, MG, pp. 381–385. Tassinari, C.C.G., Barretto, P.M.C., 1991. Uranium in granitoids: recognition criteria of uranium provinces in Brazil. In: New Developments in Uranium Exploration, Resources, Production and Demand. Proceedings of a Technical Committee Meeting Jointly Organized by the International Atomic Energy Agency and the Nuclear Energy Agency of the OECD, Vienna, 26–29 August 1991, pp. 13–22. Vlach, S.R.F., 1993. Geologia e petrologia dos granitóides de Morungaba, SP. Ph.D. Thesis, Instituto de Geociências da Universidade de São Paulo, São Paulo, Brazil. Vlach, S.R.F., 1998. Teores de U, Th e K dos granitóides de Morungaba, SP, e implicações para a produção de calor radiogênico e para radiação ambiental. In: 40◦ Congresso Brasileiro de Geologia, vol. 395. Belo Horizonte, MG. Zhuo, W., Iida, T., Yang, X., 2001. Occurrence of 222 Rn, 226 Ra, 228 Ra and U in groundwater in Fujian Province, China. J. Environ. Radioact. 53, 111–120.