Persistence of radon-222 flux during monsoon at a geothermal zone in Nepal

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Journal of Environmental Radioactivity 100 (2009) 955–964

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Journal of Environmental Radioactivity

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Persistence of radon-222 flux during monsoon at a geothermal zone in Nepal

Frederic Girault a,*, Bharat Prasad Koirala b, Frederic Perrier a, Patrick Richon c,d, Sudhir Rajaure b

a Equipe de Geomagnetisme, Institut de Physique du Globe de Paris, UMR-7154 and Universite Paris-Diderot, 4 place Jussieu, F-75005 Paris, Franceb National Seismic Centre, Department of Mines and Geology, Lainchaur, Kathmandu, Nepalc CEA, DAM, DIF, F-91297 Arpajon, Franced Equipe de Geologie des Systemes Volcaniques, Institut de Physique du Globe de Paris, UMR-7154, 4 place Jussieu, F-75005 Paris, France

a r t i c l e i n f o

Article history:Received 30 March 2009Received in revised form25 June 2009Accepted 7 July 2009Available online 18 August 2009

Keywords:Radon-222 fluxCO2 fluxPrecipitationMonsoonAccumulation chamber method

* Corresponding author. Tel.: þ33 1 44 27 24 11; faE-mail address: [email protected] (F. Girault).

0265-931X/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.jenvrad.2009.07.004

a b s t r a c t

The Syabru-Bensi hydrothermal zone, Langtang region (Nepal), is characterized by high radon-222 and CO2

discharge. Seasonal variations of gas fluxes were studied on a reference transect in a newly discovered gasdischarge zone. Radon-222 and CO2 fluxes were measured with the accumulation chamber technique,coupled with the scintillation flask method for radon. In the reference transect, fluxes reach exceptionalmean values, as high as 8700� 1500 g m�2 d�1 for CO2 and 3400� 100� 10�3 Bq m�2 s�1 for radon. Gasesfluxes were measured in September 2007 during the monsoon and during the dry winter season, inDecember 2007 to January 2008 and in December 2008 to January 2009. Contrary to expectations, radonand its carrier gas fluxes were similar during both seasons. The integrated flux along this transect wasapproximately the same for radon, with a small increase of 11� 4% during the wet season, whereas it wasreduced by 38� 5% during the monsoon for CO2. In order to account for the persistence of the high gasemissions during monsoon, watering experiments have been performed at selected radon measurementpoints. After watering, radon flux decreased within 5 min by a factor of 2–7 depending on the point.Subsequently, it returned to its original value, firstly, by an initial partial recovery within 3–4 h, followed bya slow relaxation, lasting around 10 h and possibly superimposed by diurnal variations. Monsoon, in thispart of the Himalayas, proceeds generally by brutal rainfall events separated by two- or three-day lapses.Thus, the recovery ability shown in the watering experiments accounts for the observed long-termpersistence of gas discharge. This persistence is an important asset for long-term monitoring, for example tostudy possible temporal variations associated with stress accumulation and release.

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1. Introduction

The measurement of gas fluxes at the ground surface isa promising and sensitive tool in order to obtain information onprocesses taking place in the earth crust, and to study its transportproperties. For example, carbon dioxide (CO2) or radon-222 fluxmeasurements have been extensively implemented near volcanoes(e.g., Chiodini et al., 2008) and in tectonically active zones (e.g.,Lewicki and Brantley, 2000). Monitoring of gas fluxes is also carriedout in the context of environmental applications or industrialprojects such as geological sequestration of carbon dioxide (Lewickiet al., 2007a). While gas discharges may provide valuable infor-mation on subsurface conditions or deep crustal processes, they arepotentially sensitive to meteorological effects such as atmosphericpressure variations (Clements and Wilkening, 1974; Neeper, 2002),

x: þ33 1 44 27 33 73.

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wind (Lewicki et al., 2007b) and rainfall (Megumi and Mamuro,1973; Viveiros et al., 2009).

High CO2 and radon-222 soil fluxes have been identified in Nepal(Perrier et al., 2009), during the dry winter season, at a geothermalzone located within 10 km from the nucleation zone of Himalayanearthquakes. Thus, this location may be an outstanding candidate tostudy a possible relationship between gas discharge and earth-quakes. However, as this region is affected by heavy precipitationduring the monsoon, one may raise the concern that, in suchconditions, meaningful long-term monitoring cannot be attempted.

The effect of precipitation on gas fluxes is difficult to predict.Radon flux has been observed to drop dramatically to backgroundvalues just after a strong rainfall or after successive rainfall events(Ferry et al., 2001a). After a heavier rainfall, the reduction seems tobe larger (Jha et al., 2000) and flux remains low for several days(Megumi and Mamuro, 1973). Other factors can influence radonflux, such as duration of precipitation and time between twosuccessive rainfalls (Lawrence et al., 2009). Relatedly, radon fluxesare observed to be higher in the absence of rain (Ferry et al., 2001b).Similarly, CO2 flux drops by a factor of about 10 after rainfall (Jensen

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F. Girault et al. / Journal of Environmental Radioactivity 100 (2009) 955–964956

et al., 1996; Lewicki et al., 2008) and can be affected by only smallchanges of soil water content (Rochette et al., 1991). The effect ofrainfall on CO2 flux appears also to depend on the particularmeasurement point and its specific exposure to weather conditions(Viveiros et al., 2009). The effect of rainfall, thus, has to be studiedexperimentally at every location, especially when no data areavailable, as in the Himalayas. Note that the climate of the Hima-layas, with dry winter and wet summer seasons, offers optimalconditions to study the general and specific properties of theresponse of gas fluxes to precipitation.

The Syabru-Bensi geothermal zone (Perrier et al., 2009) offersattractive opportunities to study fundamental and methodologicalissues related to gas flux measurements in the Himalayas. In thispaper, we present radon-222 and CO2 flux measurements repeatedalong a reference transect located in a newly discovered gas dischargezone. Measurements, initially performed at the end of 2007 summerseason, are compared with measurements obtained during twosubsequent dry winter seasons: December 2007 to January 2008(winter 2007) and December 2008 to January 2009 (winter 2008).The results are discussed in the light of watering experiments.

2. Area description and meteorological context

Numerous hot springs are located in the Main Central Thrust(MCT) zone. One of the most important, in terms of outflow rate(reaching up to 0.23� 0.02 L s�1) and alkalinity load (reaching25 mmol L�1), is located near the Trisuli River in Syabru-Bensi, inthe Langtang National Park, 60 km north of Kathmandu (Fig. 1).Large gas discharges, referred to as Gas Zones 1 and 2, were firstobserved from the soil in the terrace, 10–20 m above this hot spring(Perrier et al., 2009). This location is characterized by mean annualtemperatures of 17 �C, with daily averages ranging from 5 �C at theend of January to 27 �C in the beginning of June. Diurnal temper-ature variation is about 20 �C over the whole year.

In 2007, following the indication provided bya pervading smell ofhydrogen sulphide, another gas discharge zone (called Gas Zone 3)was found near the road, 110 m from the entrance of the village,450 m south-west of the main hot spring (Fig.1). While no hot springwas noticed nearby, a cold spring was located about 60 m below thisGas Zone 3, close to the river. Differently from the Gas Zones 1 and 2,located in a partly cultivated and inhabited area, this new Gas Zone 3

Fig. 1. Panoramic view toward North-East showing the location of the Syabru-Bensi hot sptransect. MCT and MFT correspond to Main Central Thrust and Main Frontal Thrust, respec

offers an easy access throughout the whole year. Therefore, it wasselected as the measurement site for the present study.

The Syabru-Bensi area is characterized by a dry winter season,with nearly no precipitation from mid-October to mid-February,and a wet summer season (monsoon), with 80% of the annualrainfall between June and September. Annual rainfall is about1000 mm in Timure and about 2000 mm in Dunche, respectively,located 10.5 km north and 7 km south of Syabru-Bensi (courtesy ofDepartment of Meteorology, Kathmandu). One important feature isthe uneven rate of rainfall during monsoon (Dahal and Hasegawa,2008). At this location, from 3 to 8% of annual precipitation iscommonly observed during a single day, with a daily maximumvalue lower than 150 mm. Both the yearly amounts and the dailymaxima, compared with other locations in the Himalayas, arerather moderate in this area, because it is protected by the topo-graphic high of the High Himalayas (rain shadow effect). In addi-tion, the monsoon is characterized by an intermittent behaviour,with large amounts of precipitation in a time span of a few hours,followed by several days of low or no precipitation. The actualtemporal pattern of precipitation plays an important role on thelocal soil degassing, as will be shown below.

3. Instruments and methods

3.1. Measurement of radon-222 flux

The flux of radon-222 from the soil was measured using theaccumulation chamber method (Ferry et al., 2001b). At a giventime, a container is installed on the ground; gas accumulates insideand radon activity concentration A(t) in the container increaseswith accumulation time t. Radon-222 flux VRn, expressed as10�3 Bq m�2 s�1, is calculated using the following expression fromRichon et al. (2005), which is valid for accumulation times smallerthan 2 h:

FRn ¼VS

dAdt; (1)

where V and S are respectively the volume (L) and open surface(m2) of the chamber (footprint), and dA/dt is the rate of increase ofradon activity concentration (A, in Bq m�3) in the chambercomputed after accumulation time t.

rings, Gas Zones 1 and 2 (Perrier et al., 2009), the new Gas Zone 3, and the referencetively.

F. Girault et al. / Journal of Environmental Radioactivity 100 (2009) 955–964 957

Air from the accumulation chamber is sampled using previouslyevacuated Lucas scintillation flasks (Lucas, 1957). Radon concen-tration is then inferred from the number of alpha-disintegrations inthese flasks counted by a photomultiplier (CALEN�, Algade,France), after a period of about 3.5 h. With this method, it wasnecessary to take into account a systematic uncertainty of about 5%,due to the absolute calibration of the Lucas flasks. In order to beable to install the accumulation chamber on the soil, 3–5 cm of soilwas removed and a roughly flat area was prepared matching thechamber footprint area. After installation of the chamber, airexchange with the outside atmosphere was reduced as much aspossible by putting plasters of wet soil on the contact between theouter chamber rim and the soil surface (Perrier et al., 2009).Accumulation chambers with 8.3-L volume and 0.092-m2 footprintarea were used in this work.

Examples of accumulation curves are shown in Fig. 2. Theincrease of radon concentration with time was remarkably linear upto accumulation times as large as 80 min for points of relativelylower flux (points B and C in Fig. 2). At points with high rates ofradon-222 increase (points A1, A2 and D), corresponding to excep-tional values of radon flux, saturation was observed for accumula-tion times longer than 15 min (Fig. 2). In practice, because of thenumber of available Lucas flasks, the accumulation curve cannot bemeasured at all points. The value of the flux was obtained from onesampling of radon concentration in the accumulation chamber, andsometimes two samplings of radon concentration at differentaccumulation times. In order to always remain in the linear part ofthe accumulation curve, sampling was performed at accumulationtimes smaller than 10 min for points with the highest rates ofincrease. The relative uncertainty was dominated by the statisticaluncertainty on the counting rate, which can be higher than 30% forthe small values of flux, but it was only around 5% for high fluxes.

3.2. Measurement of CO2 flux

The accumulation chamber method was used also for measuringsoil CO2 fluxes (Welles et al., 2001; Lewicki et al., 2005). The CO2

flux VCO2, expressed in g m�2 d�1, was obtained from the observedlinear increase of concentration dC/dt, expressed as molar fractionper day, using the following equation (Chiodini et al., 1998):

FCO2¼ PatmMCO2

RTVS

dCdt; (2)

Fig. 2. Examples of radon-222 concentration accumulation curves obtained withscintillation flasks.

where Patm is atmospheric pressure (Pa), MCO2 is CO2 molar mass(44 g mol�1), R is the perfect gas constant, T is the absolutetemperature (K), and V and S are, respectively, volume of theaccumulation chamber (m3) and area of soil surface (m2) enclosedby the chamber.

The CO2 concentration was measured using TESTO� infraredsensors (Testo AG, Germany). This sensor saturates at 10 000 ppmof CO2 in air. For the winter 2008 session, an additional sensor wasavailable (Airwatch� PM 1500, Geotechnical Instruments Ltd,England). This sensor saturates at 50 000 ppm of CO2 in air. Bothsensors were given with an absolute calibration uncertainty ofabout 5% and were intercompared in the laboratory. Several accu-mulation chambers, different from the chambers mentionedpreviously for radon, were used in this experiment: for the TESTO�detector the chambers had 8.5-L volume and 0.092-m2 footprintand, for the Airwatch� sensor, a 2-L volume and 0.037-m2 foot-print. The installation of the accumulation chamber was performedas described previously for radon.

Examples of accumulation curves are shown in Fig. 3a for theTESTO� sensor and in Fig. 3b for the Airwatch� sensor. Saturationat some points was reached in less than 30 s, in which case the mostreliable accumulation curves were obtained with the Airwatch�sensor (points F and G). The increase in slope, dC/dt, was estimatedfrom the accumulation curve. In some cases, the accumulationcurve was linear and the estimate was then straightforward. Inother cases, for example for points D and E (Fig. 3a), the initial slopewas less well defined and was calculated after examination to reacha satisfactory manual linear fit. In all cases, an error was estimated,for each flux measurement, by varying the number of data pointsused for the fit. For high fluxes, this relative uncertainty dominatedthe relative error (up to 30%), but it was smaller using the Air-watch� sensor (up to 20%), thus making the flux value obtainedfrom the latter more reliable. For example, for the point G (Fig. 3aand b), the CO2 flux value obtained from the slope of the TESTO�accumulation curve was 6780� 220 g m�2 d�1, and6670� 90 g m�2 d�1 with the Airwatch� accumulation curve. Foreach point, at least two independent measurements of the fluxwere performed. All available values were averaged to obtain thefinal result.

Note that CO2 flux measurements are more accurate for lowvalues, whereas radon flux measurements have a smaller statisticalerror for high values. Thus, it is particularly relevant to studysimultaneously these two gases: the two experimental methodscomplement each other.

4. The reference transect

In the present study, rather than mapping a large area, wefocused on repeating measurements in a selected number of loca-tions in Gas Zone 3, free of human activity and appropriate for long-term studies. In this Gas Zone 3, a reference transect was definedand is depicted in Fig. 4. This transect was not straight because ofcemented sections of the road, which had to be avoided for fluxmeasurement, and also large boulders, which had to be circum-vented. The transect crossed rocky slopes with small amounts of soiland zones of unstable debris flows, encountered mostly betweenpositions P10 (corresponding to x¼þ10 m) and P14, and betweenpositions P24 and P28 (Fig. 4). While the amount of vegetation wasalmost nil in the dry season (Fig. 4a), it remains minimal duringmonsoon (Fig. 4b), compared with the luxuriant development oftropical bushes and grass family plants in the rest of the valley. Thistransect had a total length of 660 m and extends along the road up tothe village gate (at position P-110). It included 66 points, withreference P0 (x¼ 0) located at 28�9.5550 N, 85�20.1080 E, and at analtitude of 1420 m a.s.l., 450 m south-west of the main Syabru-Bensi

0

2000

4000

6000

8000

10000

0 2 4 6 8 10

Accumulation time (min)

CO

2 co

ncen

tratio

n (p

pm

)

A B

C D1

E1 F1

G1 H

a

0

10000

20000

30000

40000

50000

0 2 4 6 8 10

Accumulation time (min)

CO

2 co

ncen

tratio

n (p

pm

)

D2 E2

G2 I

F2

b

Fig. 3. Examples of CO2 accumulation curves: (a) obtained using a TESTO� sensor, (b) obtained using an Airwatch� sensor.


hot spring. In the central zone, having significant gas fluxes, thespatial distance between two measurement points was 1 m whenpossible for CO2 flux measured, but for radon flux measurements inSeptember 2007, a sampling step of 2 m was used at some places. Inthe tails of the transect where gas flux was low, distances betweenpoints varied from 10 to 50 m. Each location of the reference tran-sect was measured with care, and for most of the locations, severalmeasurements were averaged.

5. Results

Large gas discharge was only identified in the central part of thetransect, from position P0 to position P33. This work, therefore,concentrates on the central part of the transect. The CO2 and radonflux values versus location of sampling points are shown in Fig. 4,separately for each measurement session. The average point value ofall flux data collected at each point is also shown. In a given season,the plotted flux value is, for most points, the average of at least twomeasurements. The eight points located outside the central partreflected the local background flux values, namely a radon flux of31�14�10�3 Bq m�2 s�1 and a CO2 flux of 20� 5.3 g m�2 d�1.These values are similar to the mean continental radon flux of22�10�3 Bq m�2 s�1 (Nazaroff, 1992) and the CO2 flux of6.25 g m�2 d�1 and 7.58 g m�2 d�1 reported for grassland (Meyers,2001) and tropical soil (La Scala et al., 2000), respectively.

In order to assess temporal variations, commonly observed forsoil gas measurements, some points were monitored a largenumber of times during a given session. For example, more thanfifty CO2 flux measurements were performed at points P18 and P23over a period of two weeks during winter 2007. Radon fluxmeasurements were also repeated up to seven times for points P26and P28 over a period of two days during winter 2008. At thesepoints, temporal fluctuations remained within from 29� 2% andfrom 37� 8% of the average value for radon and CO2, respectively. Atsome points, spike-like fluctuations were observed, in which casemore measurements were performed in order to obtain a morestable estimate of the average. Therefore, we assume that the spatialdistribution shown in Fig. 4 is not dramatically affected by temporalfluctuations with characteristic times from 10 min to one day.

The measured flux values, both for CO2 and radon, vary overmore than two orders of magnitude. Small flux values wereobserved at the northern edge of the transect (points P0–P3) and atpoint P22, which separated two distinct degassing sectors of thetransect. These smaller values were comparable to the averagebackground values measured outside the gas discharge zone,shown as the shaded bands in Fig. 4. Large fluxes were observedboth north and south of P22: a wide gas discharge zone from P7 to

P21, and a narrower section in the slope debris from P24 to P30.Exceptional flux peaks were observed in the two subsectionsdefined above, with highest mean exhalation values of3400�100�10�3 Bq m�2 s�1 for radon at point P30 in September2007 and of 8700�1500 g m�2 d�1 for CO2 at point P26 duringwinter 2008 session. Averaged flux values larger than400�10�3 Bq m�2 s�1 for radon and 1000 g m�2 d�1 for CO2 wereobserved over a distance larger than 20 m. Thus, the peak value wascomparable with the peak flux observed near the main hot spring(Perrier et al., 2009). Gas discharge occurred, however, over a largerdistance for the new transect.

Note that advection by CO2 or H2O is the most common mech-anism proposed to explain large radon fluxes which cannot besustained by diffusion in the absence of uranium ore (Etiope andMartinelli, 2002). In our case, dry CO2 flux is directly observed andthus offers a reasonable explanation. For this new zone, one canthus tentatively consider the conceptual model proposed for GasZones 1 and 2 (Perrier et al., 2009). In this model, CO2 is producedby metamorphic decarbonation at depth and is transported to thesurface expression of MCT by shallow meteoric water (Evans et al.,2008; Becker et al., 2008). Radon is added to the degassing CO2

before reaching surface. In this framework, large flux of CO2 induceslarge advective radon flux.

The study of possible seasonal influences on the time-sequenceof soil gas flux values shows that, contrary to expectation, noevident changes were found between the wet monsoon season andthe dry winter season. Looking at the data in more detail, somedifferences, however, can be pointed out. In the points with thelowest fluxes, such as P0 and P2, soil degassing was significantlylower during monsoon. The averages of selected low values arelisted in Table 1 for the various measurement sessions. For suchpoints, monsoon flux values were reduced by a factor of 3.4� 0.6for radon and by a factor of 1.7� 0.8 for CO2 compared with thewinter flux values. Small radon fluxes, thus, appeared particularlysensitive to rainfall, as already observed. A more contrasted situa-tion was noticed for large gas fluxes. The average value of radon flux(see Table 1) was 660� 23�10�3 Bq m�2 s�1 during monsoon,comparable with the average value of 580� 43�10�3 Bq m�2 s�1

observed during winter seasons. Note also that comparable resultswere observed in winter 2007 and winter 2008. For CO2, theaverage value was definitely smaller during monsoon, with a valueof 1050� 72 g m�2 d�1 compared with an average of1560�140 g m�2 d�1 during winter.

Alternatively, let us consider the integral of the flux over thewhole transect (Table 1). This quantity was calculated by summingeach flux value multiplied by the distance to the next flux value. Inour case, as the spacing was rather regular, it was not much

Fig. 4. Radon and CO2 fluxes in the central region of the reference transect (Gas Zone 3), located 450 m south-west of the Syabru-Bensi hot springs. Results are shown separately formonsoon and the two winter sessions. The shaded bands represent the background flux values estimated outside the central region. Error bars correspond to one standard deviation(�1s, 68% confidence level). Panoramic views of the central zone in December 2007 (a) and in September 2007 (b) are shown above the graphs.


Table 1Values of radon and CO2 fluxes from the reference transect in Syabru-Bensi, separately for each measuring session. The quoted average is the arithmetic mean. Points P0 and P2have been selected to analyze the lowest flux values. Points P8, P21 and P26 have been selected to analyze the highest flux values. Fitting parameters are defined in the text.

Monsoon Dry winter season

Average 2007 2008

Radon-222Number of measurements 27 118 54 64Flux (10�3 Bq m�2 s�1):

Range min–max 6.5–3371 36–4797 36–2431 48–4797Average 660� 23 580� 43 530� 22 720� 37Average of selected high points 1300� 330 1600� 300 1300� 120 1900� 520Average of selected low points 20� 1.8 68� 10 65� 21 71� 0.4

Flux integral (10�3 Bq m�1 s�1) 23 600� 500 21 200� 700 17 900� 800 24 500� 1300

CO2

Number of measurements 100 417 226 191Flux (g m�2 d�1):

Range min–max 11–14 544 24–13 770 24–13 770 31–11 718Average 1050� 72 1560� 140 1430� 110 1890� 110Average of selected high points 3700� 1400 5200� 1200 4100� 900 6200� 1600Average of selected low points 37� 14 64� 17 55� 19 72� 15

Flux integral (g m�1 d�1) 35 300� 2400 56 500� 2400 48 700� 3800 64 300� 3700

Radon–CO2 correlationCorrelation coefficient 0.82 0.94 0.88

Parameter valuesPower law:a 0.54 2.01 2.14a 0.99 0.76 0.77Determination coefficient R2 0.73 0.89 0.79Normalized coefficient c2 29.9 6.90 4.46Linear law:b 0.55 0.40 0.35b 4.0 30 40Determination coefficient R2 0.70 0.85 0.77Normalized coefficient c2 35.7 14.2 11.5


different from the average multiplied by the total length but,fundamentally, it represents the total flux emitted per meter ofwidth perpendicular to the transect and, thus, it is physically a moremeaningful quantity. For radon, the integral amounted to23 600� 500�10�3 Bq m�1 s�1 during monsoon, to be comparedwith 21200� 700�10�3 Bq m�1 s�1 during winter. Thus, the totalradon discharge was larger during monsoon by 11�4%. By contrast,the total CO2 discharge amounted to 35 300� 2400 g m�1 d�1

during monsoon and to 56 500� 2400 g m�1 d�1 during winter,thus it is reduced by 38� 5% during monsoon. This comparisonsuggests that, rather than a significant change in the flux regime,a significant seasonal change in the relationship between radonand CO2 was observed.

Fig. 5. Radon flux versus CO2 flux in the central zone of the reference transect in GasZone 3, separately for monsoon and the two winter sessions. The open black circlesrefer to the data collected in 2006–2007 in Gas Zones 1 and 2 near the main hot spring(Perrier et al., 2009). The lines refer to the power law fit (Eq. (3)) with the parametersvalues given in Table 1.

6. Seasonal variations of the radon-CO2 relationship

Radon fluxes are shown versus corresponding CO2 fluxes inFig. 5, separately for the three measurement sessions. Data collectedby Perrier et al. (2009) at Gas Zones 1 and 2 located near the mainhot spring are also shown as black circles. This figure shows that, asmentioned earlier, error bars get larger for radon flux at lower fluxvalues, but for CO2 flux at higher flux values. The data from the newreference transect agree with the previous data, but are confinedto a narrower zone. The new transect, thus, was characterized bya better-defined relationship between radon and CO2.

The correlation can be quantified in various manners. First, thelinear correlation coefficient between radon and CO2 was 0.82 forthe monsoon period, 0.94 for winter 2007 and 0.88 for winter 2008.Second, the correlation in Fig. 5 can be represented by a power lawfitting:

FRn ¼ aFaCO2

: (3)

The obtained values of parameters a and a are given in Table 1.The determination coefficients R2 and the normalized coefficientsc2 varied from 0.73 to 0.89 and from 4.46 to 29.9, respectively. Thus,the simple power law offered a satisfactory parameterization of theobserved correlation. The values of the parameters were similar inwinter 2007 and in winter 2008, but appeared significantlydifferent during monsoon. This suggests a significant seasonalchange of the correlation between radon and CO2. The exponenta was larger during monsoon than during winter. This is


compatible with the previous observation that, during monsoon,high radon flux was slightly enhanced whereas CO2 flux wasreduced. While this power law appears as a good representation ofthe data, it does not correspond to a physical model of soildegassing (e.g., Tanner, 1964). A more realistic physical represen-tation in this case is a linear relationship:

FRn ¼ bFCO2þ b: (4)

These parameter values are also given in Table 1. In this case, thedetermination coefficient R2 and the normalized coefficient c2 varyfrom 0.70 to 0.85 and from 11.5 to 35.7, respectively, indicating thatthe functional relationship is less well supported by the data. Theseasonal variation of parameters, however, remained true also inthis case. Thus, whatever the mathematical parameterizationchosen, we observed a seasonal change in the relationship betweenradon and CO2 fluxes.

The physical mechanism behind the seasonal influence on soildegassing can be found in the different water content in shallowestsoil layers induced by precipitation. In the following chapter,therefore, we will focus on the effect of a variable soil water contenton soil degassing.

7. Watering experiments

Simulating the effect of rainfall was not straightforward. In orderto study this effect, we poured water over the soil surface onlywhere it was covered by the accumulation chamber. An amount of1 L of water was poured on the soil surface in less than 10 s. Thisamount corresponded to an integrated rainfall of 11 mm, or, alter-natively, to an instantaneous amount of rain of about 376 mm h�1.The above-integrated amount was smaller than a large monsoonstorm, which can deliver up to 60 mm of rain in the Syabru-Bensiarea in less than 1 h. However, the instantaneous rate of the pouredwater was larger than a typical monsoon storm. The watering, thus,represented a reasonable average monsoon effect.

The effect of such watering was studied on radon and CO2 fluxesat more than twelve points. The effect of watering was found todepend on the point and also on time. In the following, we will firstconcentrate on the effect of watering on two points (P26 and P28)that were studied extensively for radon flux and where reasonablyreproducible results were obtained. Point P26 was characterized bya high mean radon flux (about 2800� 340�10�3 Bq m�2 s�1),whereas point P28 was characterized by a radon flux which wasparticular stable from 2007 to 2009, with a value of840� 85�10�3 Bq m�2 s�1 (see Fig. 4). The results of watering areshown in Fig. 6.

After measuring the initial flux before watering, the water waspoured and the accumulation chamber was only installed duringthe measurement of the flux. This measurement required about5 min for P26 and 12 min for P28. The measurement was repeatedevery 30 min at the beginning and up to 1–2 h after a few hours.Immediately after watering, the radon flux decreased by a factor of1.7� 0.3 for P26 (Fig. 6a and b) and up to a factor of 7.4� 2.5 for P28(Fig. 6c and d). The observed reduction was thus smaller for thehigher mean flux. Also, the data indicate that the second wateringexperiment (Fig. 6b and d), in both cases, had a smaller effect thanthe first experiment, indicating a possible memory effect, forexample due to an hysteresis effect in infiltration and drainage.

After the initial reduction, a recovery of the radon flux to pre-watering levels was observed and, after about 10 h, the initial valuewas somewhat recovered. In the plots of Fig. 6, two phases can bedistinguished: an initial fast partial recovery and a subsequent slowrecovery. The partial recovery took place within 1 h for both pointsand amounted to about 60� 3.4% of the initial drop for P26 (Fig. 6a)

and about 51�2.6% for P28 (Fig. 6c and d). The subsequent slowrecovery was completed within a day. This assertion was compli-cated by the fact that other temporal variations occurred during thesame time period. The intrinsic variability of the gas discharge wassignificant, 31% on average for P26 and 27% for P28, as seen inFig. 6b and d on January 17th.

Similar effects were observed with CO2 flux. Selected experi-ments using watering with 1 L or 3 L in this case, are displayed inFig. 7. After 3 L of watering, the flux is initially reduced by a factorfrom 3 to 145, with a fast recovery, followed by a slow recoveryresembling the experiments shown in Fig. 6 with radon flux.Measurements of CO2 flux allow a better estimate of instantaneousflux, whereas radon measurements intrinsically average over time.We tend to notice large positive flux spikes after watering, as inFig. 7b and d. Such observations are not systematic but illustrate thecomplexity of the response to watering, previously pointed out byother workers (Viveiros et al., 2009).

These experiments showed the order of magnitude and thetemporal structure of the flux response to rainfall. However, somelimitations and intrinsic difficulties have to be pointed out. Thistype of watering on a restricted area limited to the footprint of theflux chamber is probably able to capture well the effects due toa change of water content, including complex infiltration/drainage/drying effects which are difficult to model properly. These effectsmost likely affect gas flux in a similar manner in these experimentsand in real rainfall. However, there are also important differences.Our watering experiments may lead to diversion of the gas flow toside areas which have remained dry, which does not happen witha uniform rainfall. This bias was opposite to that observed withpermanent flux meters, which remain dry while the local soil iswetted by rainfall (Viveiros et al., 2009). This problem could be inprinciple avoided with automated flux meters in which the accu-mulation chamber is installed only at the time of flux measure-ments (Ferry et al., 2001b).

The watering experiments shown in Figs. 6 and 7 probablytended to overestimate the effect of rainfall because of diversioneffect mentioned above. Nevertheless, they showed that, in thepresence of advection, recovery does take place within 10 h. Thisinformation is helpful to analyze the observed seasonal variation ofthe gas fluxes.

8. Discussion and conclusion

In this paper, we have studied extensively a reference transect ofgas discharge located 450 m south-west of the Syabru-Bensihot springs. Very high values of the flux, reaching up to4797� 96�10�3 Bq m�2 s�1 for radon and 14 544� 2909 g m�2 d�1

for CO2, were observed, similar to the values observed 10–20 m awayfrom the main hot spring (Perrier et al., 2009). Such high values forradon so far have only been reported on uranium mining waste piles(Lawrence et al., 2009; Ferry et al., 2001a) whereas for CO2 fluxeshave been reported only on active volcanoes (e.g., Werner and Car-dellini, 2006). Our transect showed that gas discharge may not beassociated necessarily with a major hot water spring. This observa-tion is important in order to build a geochemical and geophysicalmodel of the geothermal system, and, first of all, suggests that furthergas discharges, possibly at larger distances, should be searched.

Secondly, we observed that, in this new gas zone, the correlationbetween radon and CO2 was rather well defined. More detailedmodeling is necessary in order to quantitatively assess the generationand transport of radon and to confirm the hypothesis that advectionby degassing CO2 is sufficient to account for the observed radon fluxover the whole transect. The change of the correlation between radonand CO2 during monsoon might provide important clues to better

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understand transport processes in the subsurface bedrock and in thesoil.

Our third result was the fact that the gas discharge along ourtransect was similar during monsoon and dry winter season. On theother hand, rainfall is known to reduce both radon and CO2 fluxesby a large factor. For example, reductions in the radon flux up to

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a factor of about 10 were observed in Australia on a waste uraniummine (Lawrence et al., 2009): on average, they observed an increasein radon exhalation by a factor of 2.8� 0.4 during the dry seasonthan during the wet season. Similarly, a reduction by a factor of 2–6in the CO2 flux was observed at the Azores Archipelago duringrainfall (Viveiros et al., 2009). Our watering experiments showed

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that gas fluxes were reduced after watering, by a mean factor of3.3�1.2 for radon. However, our experiments also showed thatrecovery after watering took place within 10 h. The mean intervalbetween large monsoon storms in Syabru-Bensi, as well as in mostof the High Himalayas, is a few days. Therefore gas fluxes haveenough time for transient recovery between monsoon storms, sothat the mean gas flux is not dramatically affected by rainfall.Consequently, no large seasonal cumulative effects result from therainfall events. The relatively fast recovery that we observed in ourwatering experiments is compatible with the fast recovery of CO2

flux observed in the Azores (Viveiros et al., 2009). This fast recoveryis probably a result of the advective nature of the gas discharge inboth cases. The situation is totally different in the case of uraniumwaste for radon flux (Lawrence et al., 2009), which is of diffusivenature, associated with a large radium content in the local soil. Toconclude, in the case of our experiment, monsoon does not appearto be able to cause a dramatic reduction of gas flux because ofa sufficiently fast recovery of the flux after rainfall.

While, on average, gas discharge persists during monsoon,larger differences could be observed at some points. Some pointsexhibited a complex response to watering experiments and alsoshowed less clear seasonal stability. Pulse-like discharges weresometimes observed in our reference transect, as also reported inthe Azores (Viveiros et al., 2009). We were able to observe thisbehaviour especially with the CO2 technique which allows real timeobservation of the concentration in the accumulation chamber.Pulsing is difficult to observe with radon flux because of both theaveraging nature of the sampling method and its poor temporalresolution. It is unlikely that pulsing was due to the advectivesource at depth. Rather, it can be due to sudden drops in atmo-spheric pressure. Water can also play an important role. In the nearsurface two-phase rock and soil, water can accumulate locally so asto temporally stop the gas transport until, with gas pressureincreasing, the air entry pressure of the water is exceeded anda spike-like discharge of gas is produced. Note that an increase oftemporal instabilities was measured during the recovery phaseafter watering, as observed, for example, in Fig. 6d. The effect ofpore water on gas discharge thus remains a subject of furtherinvestigations. Watering experiments definitely show some utilityin studying the effect of water, and the complex response of soil gasemissions to rainfall. Our first results indicate that, in particular, theestimate of the recovery time seems reasonable. However, furtherwork is needed to refine the understanding of the soil physicsaccessible through such experiments.

To conclude, our study showed that long-term monitoring of thegas discharge is feasible in the context of the geothermal systems ofthe Himalayas. Monsoon does not appear to be able to affect the gasdischarge dramatically. This is due to the dynamic pattern of monsoonand, also, to the presence of locally large fluxes of gas. In order to makemeaningful monitoring, a good knowledge of transient effects isnecessary. Such transient effects depend both on the particular pointand time. It is therefore important to estimate the main properties ofthe points before monitoring, and to implement monitoring witha proper strategy. One possibility, to be able to identify possibletemporal variation of geodynamic origin, is to include points withdifferent response to precipitation. More data on the seasonal andtransient variation of gas discharge are needed in order to evaluatethe relevance of long-term monitoring strategies, which would beuseful for the surveillance of volcanoes, active faults, as well as theleakage of stored CO2 or radioactive contaminants.

Acknowledgements

The authors thank the Department of Mines and Geology,Kathmandu, for long-term collaboration and enthusiastic support,

and Nabin Tamang, Gautam Tamang, Hiralal Tamang and Khem Purifor assistance during the experiments. This experiment could becarried out, thanks to a grant from the Fluids-Fault-Flux program ofCNRS. The original version of this paper was considerablyimproved, thanks to the extremely careful work of the editor andthree anonymous reviewers. This is IPGP contribution number2529.

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Persistence of radon-222 flux during monsoon at a geothermal zone in Nepal

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