Based on Lucas cell technique, the portable device named “PRASSI” (SILENA mod. 5S) was used to realise discrete radon measurements in water samples. The calibration factor is (64.766.9) cpmQBq21 and, with the present set-up for measuring radon in water, the detection and the determination limits are, respectively, 0.007 and 0.020 Bq. Thus, the device is well adapted for determining low radon content in karstic waters. Several discrete radon samplings were performed nearly every week in the three main springs of the Bastareny karstic system (Catalonian

Pyrenees, N-E Spain). Thus, it was possible to estimate the reproducibility of the methodology as well as to characterise every spring of the aquifer in relation to

their hydrogeological features. With the portable radon monitor, some useful measurements were achieved to estimate radon loss due to degassing process over short distances, with important consequences for radon studies in karstic waters. Lastly, it was possible to perform intensive sampling during a flood and the results point out the great variability of radon levels, closely related to the flow rate trend. That permitted also an intercomparison with other radon probes installed previously in every spring, for continuous radon measurements

Baixeras, C.

Corbin, J.C.

Freixes, A.

Gourcy, L.

Manolopoulou, M.

Meus, P.

Morin, J.-P.

Ramoneda, J.

Seidel, J.-L.

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IL NUOVO CIMENTO Maggio-Agosto 1999VOL. 22 C, N. 3-4Radon measurement in karstic waters with Lucas cell technique (*)(**)J.-P. MORIN (1), C. BAIXERAS (1), L. GOURCY (2), J. RAMONEDA (2), A. FREIXES (2)M. MANOLOPOULOU (3), P. MEUS (4), J.-L. SEIDEL (5), J. C. CORBIN (6)(1) Grup de Física de les Radiacions, UAB - Bellaterra, Spain(2) Junta d’Aigües - Barcelona, Spain(3) Physics Department - Thessaloniki, Greece(4) CNRS, Laboratoire Souterrain - Moulis, France(5) CNRS, Géofluides-Bassins-Eau - Montpellier, France(6) CNR, Istituto di Geocronologia e Geochimica Isotopica - Pisa, Italy(ricevuto il 9 Giugno 1998; approvato il 18 Dicembre 1998)Summary. — Based on Lucas cell technique, the portable device named “PRASSI”(SILENA mod. 5S) was used to realise discrete radon measurements in watersamples. The calibration factor is (64.766.9) cpm QBq21 and, with the present set-upfor measuring radon in water, the detection and the determination limits are,respectively, 0.007 and 0.020 Bq. Thus, the device is well adapted for determining lowradon content in karstic waters. Several discrete radon samplings were performednearly every week in the three main springs of the Bastareny karstic system (Catalo-nian Pyrenees, N-E Spain). Thus, it was possible to estimate the reproducibility ofthe methodology as well as to characterise every spring of the aquifer in relation totheir hydrogeological features. With the portable radon monitor, some usefulmeasurements were achieved to estimate radon loss due to degassing process overshort distances, with important consequences for radon studies in karstic waters.Lastly, it was possible to perform intensive sampling during a flood and the resultspoint out the great variability of radon levels, closely related to the flow rate trend.That permitted also an intercomparison with other radon probes installed previouslyin every spring, for continuous radon measurements.PACS 91.30.Px – Phenomena related to earthquake prediction.PACS 92.40.Kf – Groundwater.PACS 29.40 – Radiation detectors.PACS 01.30.Cc – Conference proceedings.1. – IntroductionAmong the numerous applications of radon in earth sciences [1], one is beingdeveloped especially in the field of hydrogeology. A part of this research is dedicated to(*) Paper presented at the “Fourth International Conference on Rare Gas Geochemistry”,Rome, October 8-10, 1997.(**) The authors of this paper have agreed to not receive the proofs for correction.G Società Italiana di Fisica 597J.-P. MORIN, C. BAIXERAS, L. GOURCY, ETC.598radioprotection or earthquake predictions, that implies an intensive measurementcampaign of groundwater radon contents. Some authors use hydrogeologicalconceptual models for explaining possible high radon levels in dwellings or incaves [2, 3]. Another research direction is the study of radon as a potential tracer forhydrogeological purposes [4-6]. Recent studies favour continuous radon measurementin the aquifer outlets, that permits a reliable comparison of the radon trend withphysical parameters as flow rate, temperature or conductivity [7, 8]. Accuratemeasurements of radon concentration can be performed thanks to techniques knownsince long but more available after recent technological progress; they are principallyliquid scintillation, ionisation chamber and Lucas cell techniques [9]. This paperpresents an application of this last technique to the study of radon levels in the whole ofthe springs of a Pyrenean karstic system (Catalonia-NE Spain).2. – The methodologyWater samples were collected in the degassing units directly from the outlet of thesprings and whenever it was possible in water without any important turbulence. Thedegassing units consisted from 250 ml borosillicate bottles supplied with a two-outlethead with two stopcocks and a sintered distributor with porosity 1 (100–160 mm). Justprior to the sampling the unit was evacuated with a hand pump down to about17–209 Hg. The vacuum was enough to pull the water in the degassing unit through atube, even if the sampling point was somewhat lower than the unit itself. A three-wayvalve inserted between the stopcock and the tube permitted to exhaust the air of thetube and to pull the water up to the connection with the stopcock using the handpump.The samples were generally analyzed in the laboratory with the portable radonmonitor PRASSI, which is a commercially available monitor manufactured by theItalian Company SILENA (1). This monitor is suitable for radon gas continuous or grabsampling measurements with the scintillation cell technique. It consists basically of a1.83 l cell coated with zinc sulfide activated with silver [ZnS(Ag)] coupled to a low–gain-drift photomultiplier. The sampled air is pre-filtered before reaching themeasurement chamber and the sampling flow rate is electronically regulated tocompensate for filter clog-up. A computation algorithm allows compensating the countscoming from radon daughters plate-out.The measurement is performed in a closed loop circuit (fig. 1), which includes thescintillation cell, the pump, the degassing unit and a dehumidification column ofdrierite (CaSO4 ). Just prior to the measurement the circuit is washed out from the airwith pure nitrogen. The flow rate during the degassing process was 3 l Q min21 ,electronically regulated, with duration of 15 min to ensure the total mixing in thecircuit. The measurements were carried out after 3 h, to allow equilibrium betweenradon and its daughters.Calibration of the above procedure was performed with a calibrated radium-226solution provided by Amersham. For this purpose, numerous samples were preparedwith known radium concentrations in the range (0.5–50) Bq Qm23 . These values reflectthe usual radon concentrations found in karstic waters. The calibration factor obtained(1) SILENA, Società per l’Elettronica Avanzata SpA, Via Firenze, 3, I-20063 s/N., Italy.RADON MEASUREMENT IN KARSTIC WATERS WITH LUCAS CELL TECHNIQUE 599Fig. 1. – Water measurement set-up.for radon concentration measurements is (64.766.9) cpm QBq21 . With the presentset-up for measuring radon in water, the detection and the determination limits are,respectively, 0.007 and 0.020 Bq with a mean background count rate equal to (1.1760.08) cpm [10]. The value of the calibration factor and its associated uncertainty pointout that reliable measurements can be achieved in the case of waters with radon levelsas low as 0.5 kBq Qm23 .The samples were generally carried to the laboratory for analysis. In order to testthe radon loss during the transportation procedure duplicate samples were collectedand analyzed in the field and in the laboratory, several times. No statisticallysignificant loss of radon could be found during these measurements. Generally, all thesamples (4 to 6) are analyzed in less than three days.3. – Field measurementsThe Bastareny karstic system is located in the Cadí-Moixeró Natural Park, in theSouthern Catalonian Pyrenean unit, N-E Spain. Three permanent springs drain thisaquifer: Adou del Bastareny, Bullidor de Sant Esteve and Violí springs. A permanentsmall spring, Tortera, is located in the upper part of the catchment area and is relatedto the epikarst.The three main springs were equipped with devices for recording, in continuousflow rates, temperature and conductivity. Automatic radon probes based onsemi-conductor technique (Clipperton II probes) were also installed for continuousmeasurement of the gas in every spring.3.1. Normal sampling at Bastareny springs. – Radon measurements with PRASSIwere performed nearly every week in the three main springs of the Bastareny system.Some measurements were also realised in Tortera spring.Nine complete samplings were performed at each spring. When it was possible, intwo of the springs, one pair of samples was collected at the same time to check theJ.-P. MORIN, C. BAIXERAS, L. GOURCY, ETC.600TABLE I. – Radon concentrations measured with PRASSI at Adou system springs and reproducibilitytest.Spring Mean Rn Standard Reproducibility( kBq/m3 ) deviation( kBq/m3 ) meandifference%standarsdeviation%maximumdifference%minimumdifference%no. pairssamplesno. pairsE15%ViolíBullidorAdouTortera7.226.024.2621.940.850.450.518.5210.16.411.58.68.89.0—5.725.823.6—14.00.11.0—2.786136513reproducibility of the measurement. The mean radon concentration and the standarddeviation as well as the mean difference, its standard deviation, the maximum andminimum difference and the number of sampling pairs are presented in table I. Thedifference between the results for a given pair of samples ranges between 1.0% and14.0%, except for three pairs, two in Violí and one in Bullidor springs, due probably towater turbulence effects. In this case, differences between 20% and 26% have beenobserved.All the samples were collected during rather stable hydrological conditions(winter-spring 1997). The radon concentration in every spring appeared to be ratherconstant and it was possible to detect a significant difference of radon content betweenthe three springs (see table I). The Adou spring, the main one of the karstic system,differs clearly from the two others by lower radon contents. Recent studies confirmthat this spring is connected to the upper part of the saturated zone of the aquifer,probably rather karstified and is influenced by water supplies from the infiltrationzone [11]. So, there must be important exchanges between the water phase and the airphase, at least at the vicinity of the outlet, that favours radon degassing. The higherradon content found in the two other springs can reflect longer residence time of thewater in the aquifer, associated with lower flow rates than in Adou spring. Moreover,Bullidor spring has a clear relation with the saturated zone of the aquifer [11], thatlimits a lot radon loss by degassing process.The whole of these data is in the range of the radon concentrations given in theliterature for karstic waters [3, 12, 13]. Tortera spring was sampled three times and showedclearly much higher radon content. That is probably due not only to the relatively highRa-226 content (about 50 Bq Qkg21 ) in the soils at the immediate vicinity of the spring,but also to the diffusive characteristics of the water flows supplying the spring.Because of technical problems of installation, a Clipperton probe was settled in thestream flowing from Adou spring, 100 m downstream from it. Thus, it was necessary toestimate the difference in radon concentration between the two points. With PRASSIdevice, it was possible to detect an important radon loss due to degassing process overthis short distance. During stable hydrological conditions, this loss remains ratherconstant; after 9 samplings, it equals (5463)%.At Violi spring, all the electronic devices are set in a deposit located at about 12 mfrom the real spring. The two points are connected by a small gallery through whichthe water can flow with more or less turbulence, leading to radon losses. These losseshave been estimated with PRASSI protable monitor and are the range (8–28)%.RADON MEASUREMENT IN KARSTIC WATERS WITH LUCAS CELL TECHNIQUE 601Fig. 2. – a) Comparison between radon concentration and hydrological parameters during theflood at Adou spring (June 1997). b) Comparison between radon concentration and hydrologicalparameters during the flood at Violi spring (June 1997).As mentioned in the case of Adou spring, the consequences of these observations onthe interpretation of radon measurement in karstic waters could be very importantsince in the final part of the karstic network, the flow can be also very turbulent, withan important contact between the air phase and the liquid phase.3.2. The flood at Bastareny system. – At the beginning of June 1997 a flood wasstudied at the springs of Bastareny system. Radon samples were taken every two hoursJ.-P. MORIN, C. BAIXERAS, L. GOURCY, ETC.602Fig. 3. – Comparison between the radon concentration measured with PRASSI and withClipperton probes during the flood at Violí spring (June 1997).the first day and less frequently when the flow rates began to decrease. The PRASSIand the associated equipment could be easily carried to the field, thus, it was possible tostart immediately the sample analysis. Figure 2 shows the comparison between theradon trend and the hydrological parameters. The radon concentration is mainlyrelated to the flow rate. In both springs, it increases nearly simultaneously with thedischarge. Then, the radon concentration falls off faster than the flow in agreementwith other observations [8].In Violí spring, the radon level rises immediately with the flow rate, by a factor 2according to the basic level. Afterwards, the concentration decreases very quickly incomparison with the flow discharge, then more slowly until the basic level over aboutfour days. Waters coming from the infiltration zone must leach, at least at thebeginning of the flood, the radon-enriched soils. Then, concentration decreasesbecause of a temporary impoverishment of the gas in the infiltration zone which hasprobably a small extent in the case of Violí spring [11].The radon trend in Adou spring is not so clear. However, it seems that the radoncontent begins also to increase immediately as a consequence of a piston effect (fig. 2a),but for reaching a maximum only 15% greater than the basic level. It is probable thatradon comes mainly from the materials laid down in the karstified parts of the aquifer,by erosion or by dissolution. Then, in Adou spring, a dilution effect takes place for twodays approximately and the concentration is 30% lower than the basic level. It must benoted also that the turbulences in the spring during the flood were very important andthe radon losses by degassing effects must be important. Laslty, three days after themaximum flow rate, the radon content goes back to the basic level.The PRASSI device can be also a very useful tool to intercompare with other typesof radon probes. During the flood, intensive sampling with PRASSI makes possiblesome comparison of the results with that from the Clipperton probes, which recordradon levels every hour (fig. 3). In Violí spring, one of the probe does not seem torespond to the important radon input. In return, the other probe records a radoncontent rise of about 45%, but more delayed in time according to the PRASSI results;the radon decrease is also slower and more regular.In Adou spring, the radon trend recorded by the Clipperton probe remains more orless constant. But the probe is located 100 m downstream from the spring and it isprobable that the degassing phenomena hide the real radon variations at the spring.RADON MEASUREMENT IN KARSTIC WATERS WITH LUCAS CELL TECHNIQUE 6034. – ConclusionThe PRASSI device which combines a portable and autonomous radon monitor witha high-precision scintillation cell technique gives a very reliable way to perform radonmeasurements in karstic waters, generally poor in this gas. Thus, it was possible todetermine precisely the basic radon level in every spring of the studied karstic system,that constitutes a good basis for making assumptions about radon origins, incorroboration of the hydrological data. Though it is not possible until now to performcontinuous radon measurements with this methodology, the interest of this last onewas greatly confirmed during the flood study, thanks to an intensive radon samplingand sample analysis directly in the field. The results point out the great variability ofradon levels, closely related to the variability of flow rates. Systematic studies of floodsin karstic systems must give useful and precise information about radon origins andabout the waters which carry the gas towards the outlets. In this case, comparison ofthe radon trend not only with the flow rate but also with the conductivity and thetemperature is essential.The results obtained during the flood pointed out the significative differences of thedata trends from Clipperton probes and PRASSI monitor.Finally, this study points out the great importance of the radon degassingphenomena in water. Firstly, it can be a serious problem for measuring accuratelyradon concentrations in springs with a lot of turbulence. Secondly, the measurementsgive an idea of the radon loss which can occur inside the karst itself, in particular withinits more karstified parts where gas exchanges are very important.* * *This work was realised in the framework of the European project “Amultidisciplinary global approach of groundwater flows in karstic areas and itsconsequences for water resources and environment studies”, No. CHRX-CT94-0567.R E F E R E N C E S[1] MONNIN M. and SEIDEL J. L., Nucl. Instrum. Methods Phys. Res. A, 314 (1992) 316.[2] LEGRAND H. E, Groundwater, 25 (1987) 59.[3] SURBECK H. and MEDICI F., Int. Ass. Hydrol., Mem., 22 (1991).[4] ANDREWS J. N. and WOOD D. F., Inst. Min. Metall. Trans., Sect. B, 81 (1972) 198.[5] HOEHN E. and VON GUNTEN H. R., Water Resources Res., 25 (1989) 1795.[6] LAWRENCE E., POETER E. and WANTY R., J. Hydrol., 127 (1991) 367.[7] SURBECK H. and EISENLOHR L., Radon as a tracer in hydrogeology; a case study, in GasGeochemistry, supplement to Environ. Geochem. Health, 16 (1995) 91-100.[8] SURBECK H. and EISENLOHR L., Radon as a naturel tracer to study transport processes in akarst system, An example in the Swiss Jura, C. R. Acad. Sci. Paris, 321-IIa (1995) 761-767.[9] SURBECK H., Nucl. Tracks Radiat. Meas., 22 (1993) 463.[10] CURRIE L. A., Anal. Chem., 40 (1968) 586.[11] FREIXES A., GOURCY L. and RAMONEDA J., The Bastareny karstic system, final report of theEuropean project, A multidisciplinary global approach of groundwaters flows in karsticareas and its consequences for water resources and environment studies (CHRX - CT94 -0567), Recursos Hídrics Subterranis i Recerca, Junta d’Aigües de Catalunya (1997).[12] KOBAL I., VAUPOTIC J., MITIC D., KRISTAN J., ANCIK M., JERANCIC S. and SˇKOFLJANEC M.,Environ. Internat., 16 (1990) 141.[13] HUNYADI I., HAKL J., LE´NA´RT L., GE´CZY G. and CSIGE I., Nucl. Track Radiat. Meas., 15(1990) 321.

Radon measurement in karstic waters with Lucas cell technique

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Radon measurement in karstic waters with Lucas cell technique

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