Radiation Measurements 43 (2008) S364–S368www.elsevier.com/locate/radmeas

Seasonal variation measurements of radon levelsin caves using SSNTD method

G. Espinosaa,∗, J.I. Golzarria, R.B. Gammageb, L. Sajo-Bohusc,J. Viccon-Paled, M. Signoret-Poillond

aInstituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, D.F., MexicobOak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6480, USA

cDepartamento de Física, Universidad Simón Bolívar, VenezueladEl Hombre y su Ambiente, UAM-Xochimilco, México D.F., Mexico

Abstract

The results of radon concentration measurements inside of the Gabriel caves of Mexico, during three consecutive two-month periods coveringalmost three seasons, are reported in the present work. The radio-ecological importance of this site is related to the radon and its concentration-dynamic behavior in the cave. Further interest in radiation safety motivated this initiative since routine biological field work is done, withpeople spending long periods of time there. CR-39 passive nuclear track detector was chosen for this survey. Radon concentration levelsdecrease during the rainy season and show different values depending on the ventilation and geometeorological structure. Measured valuesrange between 956 and 4931 Bq m−3, an indication that radon doses may exceed the allowed values for workers. This project is part of a largerstudy of indoor radon alpha emitters in Mexican caves.© 2008 Elsevier Ltd. All rights reserved.

Keywords: Radon in caves; CR-39; Caves; Radon exposure

1. Introduction

Radon-induced human health hazard has been the objectiveof several studies, with high levels having been reported incaves. A large number of surveys have put in evidence thehealth impact of radon and the estimated risk level both to oc-casional cave visitors, employees, and speleologists. Mexicois a very rich country in caves and underground rivers, manyof them known for centuries. Visitors who often frequent thecaves may be divided in two main groups, those who do it asan amateur sport and those involved in scientific projects. Bothgroups, however, are exposed to possible radiological risk dur-ing their stay inside the cave, since prolonged exposure to highradon concentration has been linked to lung cancer and tumorgrowth. Elevated concentrations of radon (222Rn) have beenrecorded and experimental data confirm that long-term users of

∗ Corresponding author. Tel.: +52 55 5622 5051; fax: +52 55 5622 5050.E-mail address: espinosa@fisica.unam.mx (G. Espinosa).

1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2008.04.033

limestone and karstic deep caves are at risk for estimated an-nual dose values up to 120 mSv (ICRP-65, 1993; Sajo-Bohuset al., 1997). Mexican caves are visited by biologists, archeol-ogists, and speleologists, who spend many more hours inside acave than the average tourist or occasional visitors; therefore,some level of health related radiation risk is expected. To es-timate the consequences of prolonged exposure to high radonconcentration, a survey is necessary so that the radiological riskmay be established from the obtained data base including anapproximate value for expected lung cancer among other healthrelated hazards. To address the question of potential radon dosefrom radon concentration measured inside each specific cave,the radon concentration was measured employing CR-39 pas-sive nuclear track detectors.

The main goals of this work included the evaluation of a newcave system as a part of a larger survey of the radon levels insideof the Mexican caves (Borau et al., 1993; Espinosa et al., 1997).The data base obtained will be utilized to produce a MexicanAtlas of radon in caves, as a reference for the mentioned visitinggroups.

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2. Methods

2.1. Cave site description

The Gabriel cave was selected for this study and is lo-cated in “Cerro Mojarra” (18◦27′25′′N, 96◦40′34′′W) at an al-titude of 110 m asl, in the “Acatlan” county (Oaxaca, Mexico)(Mejía-Ortíz et al., 2003); on the geological formation called“Sierra Madre del Sur” with karstic origin dated from the creas-tic superior period. The climate in this region is classified astype Aw2(w) (i’)g, for its tropical, warm sub-humid, with highrain falls in summer, and very low rain in winter (less than5% of the total annual rain); with an average temperature of25.2 ◦C, with a maximum in summer of 30.5 ◦C and a minimumof 17.2 ◦C in winter time. The cave entrance is approximately20 m wide and 30 m height, with an almost horizontal develop-ment (Fig. 1), it is 1500 m long having corridors with 5–10 mheight, and domes with 25 m wide and 50 m long. The floor iscovered of mud and mire, and this layer increases at the end of

Fig. 1. Gabriel cave scheme with the sample locations in Oaxaca State,Mexico.

the cave. At the 250 m from the cave entrance a chimney withapproximately 30 m height is located; through this chimneythe air flows in or out depending on the season of the yearand meteorological conditions above the cave surface. Alongthe cave between 500 and 700 m, there is a very rich zone instalactites and stalagmites denoting a relatively high humidity.From 1000 to 1500 m the surface floor of the cave is full of mudand at 1500 m an underground river exists, with a depth of 1.2 min the first 200 m. After this point the cave is unexplored (MejíaOrtíz et al., 1997; Cruz-Hernández et al., 2002; Mejía-Ortízet al., 2003). At the rainy season, a small brook runs constantlyfrom the end of the cave to the entrance, and disappears in thedry season. Probably, this can be explained by the closeness toa hydroelectric dam.

2.2. Radon concentration measuring technique

Radon measurements inside caves have often been reportedto be technically and physically complicated, due to the diffi-culties to reach the locations and the local conditions such ashumidity, temperature, perhaps the presence of dangerous envi-ronment, and the lack of electricity. The applications of nucleartrack methodology (NTM) did show their advantages, and inregard to other alternative methods, often is preferred for thelow cost, massive measurements, and few limitations imposeddue to the surrounding physical environment.

During the past decades several methods to evaluate indoor222Rn have been studied (Borau et al., 1993; Choubey et al.,2005; Sajo-Bohus et al., 1997); and in our case the passiveclose-end cup measuring device within the NTM was selectedmainly due to its high detection efficiency for alpha particlesand the experience accumulated during the past decades. De-tectors were positioned in several sections and at different dis-tances from the entrance of the cave so that experimental resultscould show a seasonal variation with a relatively low statisticaluncertainty. The general description of a close-end cup device,with CR-39 Lantrack�500 �m thickness as material detector,can be found elsewhere (Espinosa and Gammage, 1993); hereit is sufficient to mention that it is set in such a way that radongas and radon progeny concentration in the air of the cave envi-ronment can be discriminated and the intrinsic interference dueto humidity, air flow and others that may intervene are reducedto negligible level.

The exposed detectors were chemically etched and counted atthe Instituto de Fisica, UNAM (IFUNAM), using 6.25M-KOHsolution and kept at 60±1 ◦C in a thermo-regulated water bath,following a very well-established protocol described in detail(Espinosa and Gammage, 1993). The automatic track densityanalyzer complete with a DIAS system (Espinosa et al., 1996;Gammage and Espinosa, 1997) allowed us to determinethe radon concentration with a one sigma uncertainty bet-ter than ±7%. The detectors are calibrated every year at theORNL-USA following the established protocols (Espinosa andGammage, 1993), and the verification and calibration are madeat the IFUNAM chamber every time that a new set of detectorsheets is used.

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2.3. Detectors locations

Starting at the entrance of the Gabriel cave, the sampling lo-cations were placed at every 50 m, along a walking path 700 mlong. The number 01 corresponds to the first location near theentrance and the number 15 is deep in the cave. Detectors werealways placed in the same position inside the cave; each lo-cation had a serial number and was marked with a flag at apoint between 1.5 and 1.8 m height depending on the accessi-bility to the wall of the cave. The cave is an irregular tunnelcharacterized by arch of vault, rooms with different volumes aswas mentioned before, and, in particular, at the 250 m shows achimney through which air may flow to reach open space, lo-cated between the points marked 05 and 06. The effect of theradon transport due to the existence of this air flow is reflectedin this study.

2.4. Exposure periods

From the fall of 2004 (October–December) to the win-ter 2004–spring 2005 (December–February, February–April)measurements of indoor 222Rn concentration in Mexicancaves were carried out in collaboration with the “Man and hisEnvironment” group of the Universidad Autónoma Metropoli-tana (Mexico, DF). The Gabriel cave was selected due to itsimportance from the biological research point of view, since agreat variety of species, as anthropodes, sweet water fish, andcrustacean exist.

The integration time for each set of detectors was 60 dayscomprised the first period of 2 months between October andDecember. Due to the severe rainy season the floor of the cavewas partially inaccessible and detectors could be collected onlyup to the location No. 11. The second measurement period,with no rain, from December to February was made underimproved conditions; the water level diminished substantially

0

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15LOCATION

Rad

on L

evel

s (B

q m

-3)

Oct-DecDec-FebFeb-Apr

Fig. 2. Gabriel cave indoor radon levels.

which allowed access to all the locations and high humidityand drips from the ceiling could be observed. The third runof measurements occurred during dry season from February toApril, and complete access to the detection sites was availablesince no more water was there on the floor nor could condensedwater was on walls of the cave be found. After each period of60 days, the detectors were replaced with new ones in order tocomplete a period of 6 months of measurements.

3. Results

The results are shown in Fig. 2. As it can be observed, eachlocation presents different indoor radon concentration levelsduring the three measurement periods. The changing internaland external environmental cave conditions clearly explain thevariation. (Gammage et al., 1992).

Table 1 shows the indoor radon levels and statistical analysisof the data, including the averages from the first 11 locationsand for the total of 15 locations. Results evidence that deeperin the cave higher concentration may be found; this shows alsothe existence of air flow impedance. From the data obtained,the average value and standard deviation of indoor radon levelsfor the locations along the cave for October–December (11 lo-cations) was 1.73 and 0.76 kBq m−3, for December–February(14 locations) was 1.87 and 0.76 kBq m−3; and for February–April (15 locations) was 3.23 and 1.19 kBq m−3. The lowestaverage level was found in the period of December–February,with the lowest level of 1.10 kBq m−3 at location 05, and thehighest level was found in the February–April measurementperiod, at the end of the cave, location 14, with 4.93 kBq m−3.The radon concentration levels in the entrance of the cave arelow because of the air flow exchange; the minimum concentra-tion value of 0.96 kBq m−3 was measured in the second period(December–February). The correlation factor R between the2nd and 1st period was 0.94; for the 3rd and 1st was 0.71, and

G. Espinosa et al. / Radiation Measurements 43 (2008) S364–S368 S367

Table 1Statistical analysis of the obtained data (kBq m−3)

Location A B C Average Coefficient variat Ratios (%)(October–December) (December–February) (February–April) of variation (%)

B/A C/A B/C

1 0.98 0.96 2.62 1.52 ± 0.95 63 97 266 372 1.60 1.25 2.28 1.71 ± 0.52 31 78 143 553 1.26 1.23 2.10 1.53 ± 0.50 32 98 167 584 1.29 1.21 1.89 1.46 ± 0.38 26 94 147 645 1.16 1.10 1.55 1.27 ± 0.26 19 96 134 716 1.49 1.42 2.08 1.66 ± 0.36 22 95 139 687 1.42 1.22 2.29 1.64 ± 0.57 35 86 161 538 2.22 2.14 4.16 2.84 ± 1.14 40 96 187 529 1.39 2.02 4.68 2.69 ± 1.75 65 145 336 43

10 3.06 2.68 4.21 3.31 ± 0.80 24 88 138 6411 3.21 2.96 4.52 3.56 ± 0.84 24 92 141 6512 ND 2.33 2.91 2.62 ± 0.41 16 ND ND 8013 ND 2.62 3.82 3.22 ± 0.85 26 ND ND 6914 ND 3.11 4.93 4.02 ± 1.29 32 ND ND 6315 ND ND 4.43 4.43 ND ND ND NDAverages1–11 1.73 ± 0.21 1.65 ± 0.14 2.94 ± 0.34 2.11 ± 0.721–15 1.73 ± 0.21 1.87 ± 0.76 3.23 ± 1.19 2.28 ± 0.93

Table 2Comparative values of indoor radon concentration in other caves already reported

Cave site Cave name Min Max Average Reference(kBq m−3) (kBq m−3) (kBq m−3)

Oaxaca, México Cueva Gabriel 1.10 4.93 2.28 This paperGuerrero, México Zacatecolotla 1.02 1.28 1.16 Borau et al. (1993)Guerrero, México Pozas Azules 1.01 1.29 1.14 Borau et al. (1993)Guerrero, México San Jerónimo 1.28 1.80 1.53 Borau et al. (1993)Zonguldak, Turquia Gokgol cave 0.02 4.48 1.92 Aytekin et al. (2006)Zonguldak, Turquia Cehennemagzi cave 0.30 0.88 0.66 Aytekin et al. (2006)Arizona, USA Cavernas Kartchner ND ND 3.33 Buecher (1999)Garhwal Lesser Himalaya, India Pindal River 4.70 12.20 6.97 Choubey et al. (2005)Al-Somman Plateau, Saudi Arabia Abu-Sakheel-1 0.05 0.20 0.10 Al-Mustafa et al. (2005)Al-Somman Plateau, Saudi Arabia Abu-Sakheel-1 0.07 0.19 0.11 Al-Mustafa et al. (2005)Al-Somman Plateau, Saudi Arabia Bin-Gazi 0.05 0.13 0.07 Al-Mustafa et al. (2005)Al-Somman Plateau, Saudi Arabia Al-Ferry 0.05 0.11 0.08 Al-Mustafa et al. (2005)Al-Somman Plateau, Saudi Arabia Abu-Wrken 0.07 0.62 0.45 Al-Mustafa et al. (2005)Venezuela Quebrada Amarilla 0.10 80 1.00 Sajo-Bohus et al. (1997)Venezuela El Indio 1.40 3.00 ND Sajo-Bohus et al. (1997)Venezuela El Mirador 0.30 1.20 ND Sajo-Bohus et al. (1997)

Table 3Values of doses and risks of the averages of each measured period

1.74 kBq m−3 1.88 kBq m−3 3.23 kBq m−3

Dose rate (�Sv/h) 5.45 5.88 10.15Annual dose rate (mSv y−1) 2.18 (0.44 WLM y−1) 2.35 (0.47 WLM y−1) 4.06 (0.81 WLM y−1)Cumulative dose (mSv) 2.18 (0.44 WLM) 2.35 (0.47WLM) 4.06 (0.81 WLM)Excess lifetime cancer risk 0.012% (1:8106) 0.013% (1:7505) 0.023% (1:4352)

between the 2nd and 3rd period was 0.88. Table 2 shows com-parison values for indoor radon concentration in the Gabrielcave and values already reported in the literature.

The effective dose and therefore the derived risk are asso-ciated mainly with the inhalation of the short-lived polonium

(218Po and 214Po) alpha emitter (radon progeny). A detailedanalysis of the radiation dose to the cave workers or other per-sonnel should contemplate detailed information on the aerosolother than the degree of disequilibrium between radon and itsprogeny for each site and for each season. We expect that the

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aerosol progeny particle size bimodal distribution will have di-ameter values around 1 and 170 nm with an average unattachedfraction of 15% (cave environments have low concentration ofsuspended particulate matter—spm) (Husar, 1976). Values re-ported in the literature for the equilibrium factor are between0.36 and 0.52 and suggest that an average value of 0.4 could bean accepted one to estimate radon progeny exposure from radonconcentration measurements (UNSCEAR, 2000). Thus, assum-ing 2 kBq m−3 as the average indoor cave radon concentrationfor the first period from October to December and employingWISE Uranium Project calculator, it is possible to perform radi-ation dose calculations for individuals exposed and dose rate forknown activity concentration or annual exposure of radon; val-ues used in this calculation are from ICRP-65 (1993) (for furtherdetails see website http://www.wise-uranium.org/rdcrnh.html).Values for the radiation dose rate, dose, and health risk for anindividual exposed to radon and its decay products are given inTable 3 assuming the time scale reported in the literature andaccepted in the field of 400 h per year of occupancy.

4. Discussion and conclusions

Last year, Mexican diplomacy experienced a most regretfulincident at the international level related with to the radon con-centration in caves (the politicians believe that the speleologistswere inspecting the cave searching for uranium), pointing outthe importance of understanding and evaluating radon levels incaves.

In general terms, the indoor radon concentration in theGabriel cave is relatively high. However, at present, the re-search groups and speleologists have detailed knowledge onthe radon levels inside caves where they are working; there-fore taking into account possible health hazards, the scheduledwork program can be modified to reduce as much as possiblethe risk related to radon presence.

Dose calculation given here suggests that on the average,the occasional visitors and amateur enthusiasts are exposed toa dose rate of 2.40 �Sv/h that corresponds to an indoor caveradon concentration of 0.96 kBq m−3 and after 4 h of exposure(average visiting time), the dose is approximately 9.6 �Sv. Inthe case of other groups, perhaps the personnel exposure timeis longer, such that radon concentration may change requiringaverage values to establish the corresponding additional dose.

Measurement results show that meteorological conditionsinfluence and characterize the indoor cave radon concentration;approaching the raining season concentrations changes up to336%. An index factor on the dynamical behavior of cave radongas is derived from the ratio values given in Table 1. Then fora given site in the cave (e.g. point 9) the index factor(assuminga reference value 1 for B/C) for the three ratios B/C, B/A andC/A are 1, 2, and 7, respectively. Higher the index value lessadvisable would be the presence of visitors. Due to cave in-accessibility this study was restricted to the mentioned period,

and there is no plan to extend the measurements on the nextfuture.

Acknowledgments

The authors wish to thank Jocele Wild and Eugenio Ley Koofor their useful comments. This work was partially supportedby Oak Ridge National Laboratory, managed by UT-BattelleCorp. for the U.S. Department of Energy under contract numberDE-AC05-00-OR2-2725 and PAPIIT-DGAPA-UNAM project1N107707.

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