Journal of Asian Earth Sciences · 2020. 4. 2. · Radon emanation of heterogeneous basin deposits in Kathmandu Valley, Nepal Frédéric Giraulta,⇑, Ananta Prasad Gajurelb, Frédéric

Journal of Asian Earth Sciences 40 (2011) 595–610

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Journal of Asian Earth Sciences

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Radon emanation of heterogeneous basin deposits in Kathmandu Valley, Nepal

Frédéric Girault a,⇑, Ananta Prasad Gajurel b, Frédéric Perrier a, Bishal Nath Upreti b, Patrick Richon c,d

a Equipe de Géomagnétisme, Institut de Physique du Globe de Paris, UMR-7154 and Université Paris-Diderot, 1 rue Jussieu, F-75238 Paris Cedex 05, Franceb Department of Geology, Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepalc CEA, DAM, DIF, F-91297 Arpajon, Franced Equipe de Géologie des Systèmes Volcaniques, Institut de Physique du Globe de Paris, UMR-7154, 1 rue Jussieu, F-75238 Paris Cedex 05, France

a r t i c l e i n f o

Article history:Received 28 May 2010Received in revised form 15 October 2010Accepted 20 October 2010

Keywords:Radon emanationEffective radium concentrationKathmandu BasinStratigraphic studyRadon health hazard

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⇑ Corresponding author. Tel.: +33 (1) 83 95 74 76;E-mail address: [email protected] (F. Girault).

a b s t r a c t

Effective radium-226 concentration (ECRa) has been measured in soil samples from seven horizontal andvertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong tothe Thimi, Gokarna, and Tokha Formations, dated from 50 to 14 ky BP, and represent a diverse fluvio-deltaic sedimentary facies mainly consisting of gravelly to coarse sands, black, orange and brown clays.ECRa was measured in the laboratory by radon-222 emanation. The samples (n = 177) are placed in air-tight glass containers, from which, after an accumulation time varying from 3 to 18 days, the concentra-tion of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of3.8 days, is measured using scintillation flasks. The ECRa values from the seven different profiles of theterrace deposits vary from 0.4 to 43 Bq kg�1, with profile averages ranging from 12 ± 1 to 27 ± 2 Bq kg�1.The values have a remarkable consistency along a particular horizon of sediment layers, clearly demon-strating that these values can be used for long distance correlations of the sediment horizons. Widely sep-arated sediment profiles, representing similar stratigraphic positions, exhibit consistent ECRa values incorresponding stratigraphic sediment layers. ECRa measurements therefore appear particularly usefulfor lithologic and stratigraphic discriminations. For comparison, ECRa values of soils from different local-ities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg�1 in soils of Syabru–Bensi (Cen-tral Nepal), 23 ± 1 Bq kg�1 in red residual soils of the Bhattar-Trisuli Bazar terrace (North of Kathmandu),17.1 ± 0.3 Bq kg�1 in red residual soils of terrace of Kalikasthan (North of Trisuli Bazar) and 10 ± 1 Bq kg�1

in red residual soils of a site near Nagarkot (East of Kathmandu). The knowledge of ECRa values for thesevarious soils is important for modelling radon exhalation at the ground surface, in particular in the vicinityof active faults. Importantly, the study also reveals that, above numerous sediments of Kathmandu Valley,radon concentration in dwellings can potentially exceed the level of 300 Bq m�3 for residential areas; afact that should be seriously taken into account by the governmental and non-governmental agenciesas well as building authorities.

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1. Introduction

Radon-222 is a radioactive gas, with a half-life of3.8224 ± 0.0018 days (Collé, 1995), belonging to the uranium-238decay chain and a daughter product of radium-226, with a half-lifeof 1600 years. Radon has its source in rocks and in soils and itstransport to the atmosphere is governed by two main mechanisms:diffusion and advection by geological fluids (Tanner, 1964). Inhaledradon is, first of all, a health hazard which accounts for half of theannual effective dose of natural radioactivity for the average pop-ulation (Porstendörfer, 1994; Darby et al., 2004; Al-Zoughool andKrewski, 2009). It is therefore important to assess radon emissionfrom the ground, especially in highly populated areas.

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The ability of sediments or rocks to release radon-222 is linkedto the concentration of parent radium-226. However, not allradium atoms are able to release radon in the pore space, only afraction (defined by the emanation coefficient E). This parameter(E) depends on porosity, water saturation, and on the spatial repar-tition of radium in the mineral grains (Bossew, 2003; Adler andPerrier, 2009); it can vary from 1% to 40%. The radon source term,therefore, is the product of the radium concentration (CRa) and theemanation coefficient. This is called the effective radium concen-tration (ECRa) and is expressed in Bq kg�1 (Stoulos et al., 2004). Ingeneral, it is difficult to estimate a priori the value of E. Therefore,the knowledge of CRa is insufficient and the actual radon sourceterm, the product ECRa, must be determined in each case by labora-tory measurements (Stoulos et al., 2003).

Knowing the radon source term ECRa is not only important toassess radon health hazard and to produce regional potential maps

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596 F. Girault et al. / Journal of Asian Earth Sciences 40 (2011) 595–610

(Reimer, 1992; Clamp and Pritchard, 1998), but also to be able tomodel radon flux at the surface of the earth which might give someinteresting insights on geodynamic processes (Tanner, 1964).Many fundamental questions, however, remain unanswered.While it was noted long ago that ECRa is related to the underlyinggeological context (Botset and Weaver, 1932), it is not known towhich extent it can be used as a geological tool. If properlycalibrated, ECRa can also potentially be a discriminating index forstratigraphical and lithological studies (Åkerblom et al., 1984).

The ability of soils to produce radon depends on their origin(type of rock) and on the variety of sediment (Wiegand, 2001). Asmall number of studies have given ECRa values of soils, in particu-lar in the Himalayas. Indeed, ECRa of soil samples have been mea-sured, most often in India (Ramola and Choubey, 2003; Prasadet al., 2008): for example, Singh et al. (1999) have found a valueof 13 ± 1 Bq kg�1 for one soil sample from Bihar, whereas Singhet al. (2007) have found an average value of 3.2 ± 0.2 Bq kg�1 forsoil samples from Himachal Himalayas.

To evaluate the geological relevance of ECRa, it is appropriate tofocus on heterogeneous sediments (Miles and Ball, 1996). Such acontext is provided by the Kathmandu Valley sediments, Nepal.Indeed, this valley is characterized by several terraces with differ-ent types of sediments belonging to lacustrine and deltaic facies,with depositional age ranging from about 2 Ma to present. Mostlandscapes and sedimentary outcrops in the Kathmandu Basinhave been little modified by human activities, thus the outcrop

Fig. 1. Topographic map of the Kathmandu Valley obtained from Digital Model data wsimplified location map of Nepal indicating the location of Syabru–Bensi geothermal ar

scarps are easily accessible all year round. Furthermore, the geol-ogy of this basin is being documented with increasing precision(e.g. Sakai et al., 2008). Thus, the Kathmandu Basin is a naturalreference laboratory, particularly well suited for stratigraphicstudies.

In this paper, we report the first ECRa measurements obtainedfrom sedimentary successions of the Kathmandu Basin. We presentseven prototype vertical and horizontal profiles in the GokarnaTerrace, located in the northern part of the Kathmandu Valley. Inaddition, these data are compared with the results obtained fromsoil samples from other locations in Nepal.

2. Sampling sites

The Kathmandu Basin is located between Mahabharat Lekh (orRange) to the south and Shivapuri Lekh to the north, and is one ofthe largest among several intermontane basins in the Midland ofthe Lesser Himalayan Belt (Fig. 1). The basin-fill sediments areunconformably overlying the Lower Paleozoic rocks of Tethysaffinity (Phulchauki Group), and gneiss–granite of BhimphediGroup both belonging to the Kathmandu Complex (Stöcklin andBhattarai, 1981). The sediments of the Kathmandu Valley weredeposited in the Paleo-Kathmandu Lake that appeared about1 Ma before present (Sakai et al., 2006) and represent a fluvio-lacustrine facies succession reaching more than 600 m in depth

Syabru-Bensi

ith the position of the sites studied of this paper. Upper right inset represents aea.

F. Girault et al. / Journal of Asian Earth Sciences 40 (2011) 595–610 597

in its central part (Sakai et al., 2008). These sediments, dating from1 Ma to present, are grouped into different formations. From southto north, they are: the Lukundol, Kalimati, Gokarna, Thimi, Tokhaand Patan Formations; the latter five formations are characterizedby open-lacustrine to fluvio-deltaic facies (Shrestha et al., 1998;Sakai et al., 2002, 2008). In the northern areas of the valley, theGokarna, Thimi and Tokha Formations have produced the GokarnaTerrace surface whereas in the central area the Patan and ThimiFormations have given rise to the Patan and Thimi Terrace surfaces.All these terraces are predominantly composed of fluvio-deltaicsediments comprising mainly sands, silt and black clay (Dillet al., 2001). These formations are particularly interesting becauseof having a high variability in Paleo-Kathmandu Lake level changesmainly related to two overlapping tectonic effects: the uplift of theMahabharat Lekh and the subsidence of the Kathmandu Basin(Sakai, 2001; Gajurel, 2006).

The Gokarna Terrace, consisting of fluvio-lacustrine sediments,is formed by three major formations: the Gokarna Formation atthe base, the Thimi Formation at the middle and the Tokha Forma-tion at the top. The Gokarna Formation is uncomformably overlainby the Thimi and Tokha Formations. The Gokarna Terrace is morethan 50 m high with a range of altitude from 1335 to 1390 ma.s.l., including the deposits of the Gokarna Formation. The GokarnaFormation is dated at 50–34 ka, whereas the sediments of theThimi Formation and the Tokha Formation are dated at 34–24 kaand 20–14 ka, respectively (Sakai et al., 2008). This terrace ismainly constituted of sand, gravel and clayey sediments. Black claybeds of 1–5 m in thickness are common in the Gokarna Formationand some erosional surfaces are also encountered and associatedwith sudden lake-level decrease (Sakai et al., 2006). Part of thesediment successions may be related to an important climatechange of a colder and drier regime at the same period of the Euro-pean main Würm event (Yoshida and Igarashi, 1984). In our study,seven ECRa profiles called P1–P7, of the Gokarna Terrace, located to

Sangla K

Baniyatar

View from South East

View from South West

6

P3

P2

Baniyatar

Dhalmal Ridge

P6P5

Fig. 2. Simplified bird view of the studied Gokarna Terrace located in the northern pardisplayed with pictures: P1 near Jhuldol, P2 and P3 near Baniyatar, P4 near Dhalmal Vil

the south-west (P1–P6) and to the east (P7) of Tokha village, havebeen explored (Figs. 1 and 2). GPS locations of all these profiles arelisted in Table 1. More precisely, the profile P1 is located near Jhul-dol, P2 and P3 near Baniyatar, P4 near Dhalmal Village, P5 and P6near Dhalmal Ridge, and P7 near Painyatar (Fig. 2). Their relativelocation and their lithology are displayed in Fig. 2 and on the strati-graphic columns in Fig. 3, respectively.

The other investigated sites are located in the eastern part of theKathmandu Basin, near Nagarkot, and near Trisuli Bazar, to thenorth outside Kathmandu Valley (Fig. 1). The Nagarkot site ischaracterized in its upper part by lateritic soil, where one profile(P8) was sampled (Table 1, Fig. 1). The Bhattar-Trisuli Bazar siteis located on an approximately flat plateau of altitude 500 m, withan area of more than 14 km2

, bordered by the Trisuli River to thenorth and the Thadi Khola River to the south. This area is coveredby a characteristic dark red residual soil on top of often highlyweathered basement rocks. One profile (P9) was sampled in detail(Table 1, Fig. 1). To the east, another profile (P10) was also sampledin the black residual soils covering the basement of black-schisttype.

3. Experimental methods

The same sampling method is used at all locations. For soil sam-pling, first, the outer surface of the sample site is cleaned, and then,the sample is collected using a small shovel or a spoon cleanedafter each sampling. For the vertical high-sampling resolution inprofile P6, where the distance between two samples was 3 cmonly, a hollow tube was hammered horizontally into the soil. Thesample was then extracted from the tube with a piston.

ECRa is measured using a standard accumulation method byplacing the sample in a sealed container (Stoulos et al., 2003). Inour experiment, to be able to measure a significantly large amount

1360 m 1350 m

1340 m

Bishnumati Khola

holaTokha

Jhuldol

View from West

North

00 m P1

Jhuldol

View from WestP4

DhalmalDhalmalVillage

t of the Kathmandu Valley. The six profiles used for the measurement of ECRa arelage, and P5 and P6 near Dhalmal Ridge.

Table 1ECRa of soil samples from all profiles and separated samples of this study. A, B, C and D represent the Kalikasthan red soil, the red soil between Kalikasthan and Betrawati Bazarlocalities, the red soil of the Bhattar-Trisuli Bazar site, and the outcrop close to the Shivapuri Lekh, respectively. Mean corresponds to a simple average.

Location Profile orreference

Figurereference

Number ofsamples

ECRa (Bq kg�1) GPS location Date(s) ofsampling

Mean Rangemin–max

Altitude(m)

(N) (E)

Jhuldol P1 Fig. 4 43 12.4 ± 1.3 0.38–33.5 1309 27�45.3700 85�19.5570 01/2008Fig. 5 11/2008

01/2009Baniyatar P2 Fig. 6 24 12.7 ± 1.6 1.91–24.8 1340 27�45.2450 85�19.3780 08/2009

P3 Fig. 9 12 15.3 ± 2.4 7.08–31.2 1347 27�45.2420 85�19.3650 07/2009Dhalmal Village P4 Fig. 7 12 13.9 ± 3.3 2.79–31.8 1345 27�45.7740 85�19.6400 12/2009Dhalmal Ridge P5 Fig. 10 12 20.7 ± 3.8 2.79–38.1 1355 27�45.7370 85�19.5440 07/2009

P6 Fig. 11 24 26.7 ± 2.2 9.59–43.2 1360 27�45.7330 85�19.5430 08/2009Painyatar P7 Fig. 8 12 19.7 ± 3.2 6.99–37.9 1398 27�45.8650 85�17.9980 08/2010Nagarkot P8 Fig. 12 10 10.4 ± 0.9 6.28–16.0 1537 27�42.7350 85�28.4060 01/2009Bhattar-Trisuli Bazar P9 Fig. 13 12 17.4 ± 0.7 11.9–21.0 493 27�51.6040 85�07.9880 01/2009Tadipul Chhahare P10 6 23.2 ± 1.3 19.5–28.6 499 27�51.7590 85�08.1720 01/2010Betrawati A 1 17.1 ± 0.3 2017 27�59.3000 85�11.9730 01/2009

B 1 14.1 ± 0.4 705 27�58.7480 85�11.2360 01/2009South of Trisuli Bazar C 4 23.4 ± 0.5 22.1–24.3 578 27�53.5920 85�08.9240 09/2009

570 27�53.2660 85�09.0050

591 27�51.9080 85�07.6520

556 27�51.7590 85�07.4770

Chandeshvari D 4 8.54 ± 2.59 4.80–16.2 1391 27�46.6770 85�20.3880 08/20091391 27�46.6860 85�20.3840

Syabru–Bensi 102 9.24 ± 0.42 2.03–26.6 1410 28�09.7690 85�20.2520 2007–2010


of samples, we did not use elaborated accumulation chambers butsimple glass bottles, with volumes varying from 260 mL to 710 mL,closed by a pre-perforated natural rubber stopper. Air tightness ofeach bottle was checked before each accumulation experiment bydepressurization and subsequent pressure monitoring. Backgroundradon emanation by the glass was found to be negligible in accu-mulation experiments without soil sample. Sample mass in thebottles was about 100–200 g maximum so that the free air volumein the bottle was sufficient to avoid back diffusion effects (Chaoet al., 1997; Tuccimei et al., 2006).

Radon-222 concentration is measured at various times usingscintillation Lucas flasks (Lucas, 1957) with 125 mL volume, previ-ously evacuated to pressure smaller than 60 hPa. Dilution effectsduring sampling are taken into account by measuring initial and fi-nal pressures in the sampling flask. This correction varies from +3%to +30%, and introduces an experimental error varying from ±0.5%to ±5%. Counting rate of the scintillation flasks is determined in aCALEN™ photomultiplier (Algade, France). Counting rates variedfrom a few dozen to several hundred counts in 3 min, leading tostatistical errors ranging from 20% to smaller than 3%, respectively.

For each sample, measurements were repeated 3 or 4 timeswith different accumulation times, ranging from 3 to 18 days. Aweighted average was then calculated to obtain the final value ofECRa for each sample. Relative errors included, in quadrature, thecounting statistical error and the error from the dilution correction.For our samples, relative errors vary from less than 6% to 30%,respectively for high and low values of ECRa. For ECRa values smallerthan 1 Bq kg�1, the total error is always higher than 10%. Therefore,this technique is more accurate to determine high ECRa, typicallygreater than 1 Bq kg�1. When comparing with other results, anabsolute error of 5% due to the calibration coefficient of the flasksis to be added in quadrature.

The measurements were all performed at room temperature. Inprinciple, the emanation coefficient E depends on temperature, butthis effect is small (Iskandar et al., 2004) and can be neglected inour case. More problematic is the sensitivity of E on water content.The parameter E is indeed known to be significantly reduced forwater volumetric saturation lower than 15% (Adler and Perrier,2009). In our case, it is important to estimate ECRa in laboratoryconditions representative of field conditions. Therefore, the sam-ples were not submitted to any treatment, in particular no

desiccation. Note that thermal treatments in an oven could alsomodify the chemical properties of the considered soil, and there-fore they have to be avoided.

Nevertheless, to estimate the possible uncertainty associatedwith variable and unknown water content, for two sets of soils,we repeated the measurements after the soils were left drying ina room without heater or ventilation. The mass loss varied from0.9% to 14% after 2 months. The ECRa was reduced on average by6.9 ± 1.4%. Most of this difference, however, could be attributedto the mean temperature change in the laboratory. Thus, weconsider that possible effects due to the water content are smalleror of the same magnitude as the quoted experimental errors. Noneof the conclusions of the present study, therefore, are affected bythe water content of the samples.

4. The Gokarna Terrace results

In this section, each profile will be presented in more detailstogether with the obtained ECRa results. Locations are displayedin Fig. 2, and the stratigraphic position in Fig. 3. Results are sum-marized in Tables 1 and 2.

4.1. The Jhuldol profile P1

An initial study of ECRa determination from soil samples wascarried out at profile P1. This profile is located near the Jhuldolvillage (Fig. 2) and belongs to the Thimi Formation (Fig. 3). Thissite, forming a terrace scarp, contains heterogeneous sedimentscomposed of sand to black clay layers (Figs. 3a and 4a). A first sur-vey in January 2008 at the site had shown that it might be a prom-ising site for an extensive study of ECRa. Indeed, the first four soilsamples (BC1, BM2, S3 and BC4) had heterogeneous values of ECRa

representing at least three orders of magnitude, from 0.52 ± 0.05 to20.0 ± 0.4 Bq kg�1 (Figs. 4 and 5, in green). In the subsequent moredetailed surveys, two horizontal and two vertical profiles for ECRa

were carried out (Fig. 4a). Black clay and massive black mud weresampled with horizontal profiles BCh and BMh, respectively, in or-der to obtain ECRa characteristic values for the two horizons. Inaddition, two vertical profiles P1a and P1b, shown in Fig. 5, werestudied. Each of the nine samples from each vertical profile had

Thimi Formation

Gokarna Formation

Tokha Formation

Clay Sand Gravel

Clay Sand

Clay Sand

Sandy soil Orange clay

Black clay Brown sandy clay Ochre sand White Sand

Fine sand

Coarse sand

Gravely sand

Boulders

2 m

1335 m

1367 m

1341 m

P5P6

P2,3P4,7

P1

LEGEND

a

Fig. 3. Simplified stratigraphic column of the studied Gokarna and Thimi Terraces in the northern part of the Kathmandu valley (a) and schematic stratigraphic south-northcross-section through the northern part of the Kathmandu valley showing the position of studied profiles (b) (modified after Sakai et al. (2008)). This last figure is insertedwith the permission of the Journal of Nepal Geological Society.


its counterpart sample from the same layer in the other verticalprofile.

Results from the two horizontal profiles are shown as a functionof position in Fig. 4b. For each horizontal profile, the obtained dataare similar for the same layer. The weighted average values are25.0 ± 0.4 Bq kg�1 and 10.6 ± 0.1 Bq kg�1 for BCh and BMh profiles,

respectively. While the values show a standard variation smallerthan 16% for profile BMh, a small but significant increase of ECRa

is noted from left to right for profile BCh.The measurements in vertical profiles P1a and P1b are pre-

sented in Fig. 5b. These profiles show, first, that different layerscan have significantly different values of ECRa. For example, the

Gokarna Fm

Dhamarsthali Fm

Basement of granite-gneiss

Thimi FmPatan Fm

Kalimati Fm

P5 & P6

P2,3 P4,7

P1

Tokha Fm

Thimi Terrace

Gokarna Terrace

Patan Terrace

Kalimati Fm

b

Fig. 3 (continued)


averaged values of ECRa are 0.56 ± 0.06 Bq kg�1 and 21 ± 2 Bq kg�1

for sand and for black clay, respectively. Similar types of material,by contrast, have similar values of ECRa. For instance, the two layersof sand, or likewise the two thick layers of black clay, below andabove the massive black mud, have similar values of ECRa. Second,the technique is here particularly reproducible. The two profiles,indeed, have remarkably similar values of ECRa although they wereseparated by around 10 m. At this site, the overall average ECRa

value of the 43 samples is 12 ± 1 Bq kg�1.

4.2. The Baniyatar, Dhalmal Village and Painyatar vertical profiles P2,P4 and P7

The second studied site of the Gokarna Terrace is referenced asprofile P2 and is located near Baniyatar village (Fig. 2). This site ischaracterized by two layers of sand located at base and top of threeclay layers (Figs. 3a and 6b). The clay layers have different colourand thickness. From the bottom to the top of this profile, threesamples of sand, two samples from each layer of clay and threesamples of sand again at the top were sampled. An examinationof two similar profiles spaced at a distance of about 30 cm was per-formed. As for the profiles P1, each sample has its counterpart inthe other profile. The vertical profile P4, located near the DhalmalVillage, was also sampled (Fig. 2). The lithological succession of P4is similar to the profiles P2a and P2b (Figs. 3a and 7), but the P4profile is situated about 1 km away from P2a and P2b profilesand, differences, particularly in thickness of the clay layers, areencountered. In particular, the black clay layer becomes thickertoward north. Moreover, another profile, the profile P7, locatednear Painyatar village (Fig. 2), was also sampled. The lithologicalsuccession is also similar to the profiles P2a and P2b (Figs. 3aand 8), but the profile P7 is situated about 2.5 km to the east fromP2a and P2b profiles. The thickness of the black clay is approxi-mately the same as in P2a and P2b profiles, but the orange andbrown clay layers have different thickness.

Results of ECRa of the two profiles P2a and P2b are displayed inFig. 6c as a function of vertical position. Like previously observedwith the P1 profile, the ECRa of clay layers are significantly higherthan in sand layers: 2.8 ± 0.4 Bq kg�1 and 22 ± 1 Bq kg�1 for coarsesand and black clay, respectively. Orange and brown clay layershave intermediate ECRa values. This ECRa profile is characterizedby a gradual increase in ECRa from sample 3 to 6, and then rapidlydecreases from sample 8 to 10. A similar spatial variation isobserved for the two profiles, which also confirms a good andsufficient reproducibility of the measurement technique.

In order to better appreciate the spatial reproducibility of thisECRa determination method, the profile P4 of Dhalmal Village isdisplayed in Fig. 7c. Similar results are obtained with, however,higher ECRa values for the black clay (26 ± 2 Bq kg�1) and the brownclay (31.5 ± 0.2 Bq kg�1), and a smaller value for the orange clay(11 ± 2 Bq kg�1). The latter fact could be the effect of mixing ofthe orange clay with sand. To compare this sedimentary successionat another scale, typically plurikilometric, the profile P7 of Painyataris displayed in Fig. 8c. Similar results are also obtained, for examplefor the orange clay (16 ± 1 Bq kg�1), but higher ECRa values arenoted for the black clay (31 ± 2 Bq kg�1) and the brown clay(36 ± 2 Bq kg�1). This technique does appear to be reproducible atsignificantly different scales – metric to kilometric.

4.3. The Baniyatar and Dhalmal Ridge horizontal profiles P3 and P5

The sampling site P3 is located near the Baniyatar village, justabove the last profile P2 (Fig. 2). This scarp is characterized by anorange-greenish clay layer overlain by a top soil layer (Fig. 9aand b). Two horizontal subprofiles of ECRa were performed: anupper profile P3a and a profile P3b about 1 m below P3a. All sam-ples of the subprofile P3a are taken from the top soil layer,whereas, because of the presence of dirt and vegetation, only thefour samples from 1 to 4 m of the subprofile P3b belong to the claylayer. The last two samples from 5 and 6 m belong to the top soil(Fig. 9). The sampling site P5 is located near Dhalmal Ridge(Fig. 2). This outcrop is characterized by three distinct layers: fromtop to bottom, a top soil layer, an orange-greenish clay layer, and ablack clay layer (Fig. 3). For each layer, a horizontal subprofile ofECRa with four soil samples was extracted (Fig. 10a and b): subpro-files P5a, P5b and P5c, respectively from top to bottom. No sampleswere taken from recent debris fall deposits.

Results of the sampling site P3 are displayed in Fig. 9c as a func-tion of the sample position. Through ECRa data, the layers are clearlydistinguishable: averaged ECRa values, indeed, are 26 ± 2 Bq kg�1

and 10 ± 1 Bq kg�1 for the orange-greenish clay layer and for thetop soil layer, respectively. Averaged ECRa value of all subprofiles atsite P3 is quite high and reaches up to 15 ± 2 Bq kg�1. However, somedifferences are observed among the samples belonging to the samelayer. These differences reach sometimes up to 21% in the orange-greenish clay layer and up to 34% in the top soil. A slight increaseof ECRa values is also noticed in the top soil layer, from position 5to 6 m in the two subprofiles.

Results of ECRa for each subprofile of the site P5 are shown inFig. 10c as a function of the sample position. The average of all sub-profiles at site P5 is relatively high: 21 ± 4 Bq kg�1. For each layer,

Table 2ECRa of soil samples listed according to type of lithology for all studied profiles and separated soil samples of this study. A, B, C and Drepresent the Kalikasthan red soil, the red soil between Kalikasthan and Betrawati Bazar localities, the red soil of the Bhattar-Trisuli Bazarsite, and the outcrop close to the Shivapuri Lekh, respectively. Mean corresponds to a simple average and STD range is the 1r standarddeviation range of variation. The associated statistical values are also graphically displayed in Fig. 15.

Type of sediment Profile or reference Number of samples ECRa (Bq kg�1)

Mean Rangemin–max

STD range

Sand 28 5.12 ± 0.63 0.38–13.3 1.76–8.47White sand P1 5 0.56 ± 0.06 0.38–0.73Coarse sand P2 4 2.76 ± 0.40 1.91–3.82Coarse sand P4 1 5.49 ± 0.27Medium sand P4 1 4.94 ± 0.27Medium to fine sand P7 3 7.42 ± 1.07 5.80–9.45Fine sand P2 4 5.49 ± 0.57 3.82–6.30Fine sand P4 1 9.79 ± 0.41Ochre sand P4 3 3.68 ± 0.58 2.79–4.77Fine to very fine sand P7 1 13.3 ± 0.4Sandy mud P2 4 7.79 ± 0.55 6.25–8.60Gravely sand P6 1 9.59 ± 0.38

Clay 76 24.5 ± 1.0 7.35–43.2 16.0–33.1Mixing Clay and Silt P7 2 18.6 ± 1.2 16.8–20.3

Orange clay 13 14.5 ± 1.0 7.35–20.5 10.9–18.1Orange clay P2 4 17.1 ± 1.7 12.5–20.5Orange clay P4 2 10.9 ± 2.4 7.35–14.2Orange clay P6 5 12.6 ± 0.6 11.2–14.5Orange clay P7 2 15.9 ± 1.3 14.0–17.8

Orange-greenish clay 8 26.7 ± 1.2 22.4–31.2 23.4–30.0Orange-greenish clay P3 4 25.8 ± 1.9 22.4–31.2Orange-greenish clay P5 4 27.5 ± 1.5 23.3–30.1

Brown clay 15 28.7 ± 1.6 16.3–37.9 22.4–35.0Brown clay P2 4 21.1 ± 1.8 16.3–24.8Brown clay P4 2 31.5 ± 0.2 31.2–31.8Brown clay P6 5 32.0 ± 1.0 29.7–35.1Mixing Brown clay and Gravely sand P6 2 23.0 ± 1.2 21.4–24.7Brown clay P7 2 35.5 ± 1.7 33.1–37.9

Black clay 38 26.2 ± 1.4 7.42–43.2 17.3–35.0Black clay P1 15 20.8 ± 1.9 8.53–33.5Black clay P2 4 21.8 ± 1.1 19.8–24.8Black clay P4 2 25.8 ± 2.1 22.9–28.7Black clay P5 4 31.0 ± 3.4 23.3–38.1Black clay P6 11 32.9 ± 2.7 22.0–43.2

Black clay hard member P6 5 23.9 ± 1.1 22.0–27.7Black clay weathered member P6 6 40.3 ± 1.0 36.3–43.2

Black clay P7 2 30.5 ± 2.3 27.3–33.8

Black mud 21 10.2 ± 0.7 4.45–14.3 7.19–13.2Massive black mud P1 17 10.5 ± 0.6 4.40–13.4Laminated black mud P1 4 8.95 ± 2.45 4.62–14.3

Debris of slope failure P1 2 3.03 ± 0.54 2.26–3.79

Top soil 14 10.1 ± 1.8 2.79–23.6 3.51–16.7Top soil P3 8 9.98 ± 0.74 7.08–13.4Top soil P5 4 3.59 ± 0.42 2.79–4.72Top soil P10 2 23.5 ± 0.1 23.3–23.6

Red residual soil 19 14.2 ± 1.4 6.28–24.3 8.32–20.1Red residual soil (Nagarkot) P8 10 10.4 ± 0.9 6.28–16.0Red-ochre residual soil (horizon A) P9 2 16.9 ± 0.1 16.8–17.0Red residual soil (Betrawati) A 1 17.1 ± 0.3Red residual soil (Betrawati) B 1 14.1 ± 0.4Red residual soil (Bhattar-Trisuli Bazar) C 4 23.4 ± 0.5 22.1–24.3Red residual soil (Chandeshvari) D 1 7.31 ± 1.33

Basement: black schist 5 31.2 ± 8.3 19.5–63.6Black residual soil P10 4 23.1 ± 2.0 19.5–28.6Black schist near P10 1 63.6 ± 0.5

Basement: mica-schist 11 16.4 ± 1.3 6.29–21.0Horizon B P9 2 20.3 ± 0.5 19.5–21.0Horizon C (saprolite) P9 8 16.7 ± 1.0 11.9–19.9Unweathered mica-schist near P9 1 6.29 ± 0.21

Basement: granite 3 8.95 ± 3.61 4.80–16.2Highly weathered granite (arenite) D 1 4.80 ± 0.30Slightly weathered granite D 1 5.89 ± 0.27Unweathered granite D 1 16.2 ± 0.4


Profile P1a Profile P1b

BCh

BMh

BC1

BM2 S3

BC4

East West a

b Jhuldhol

0

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BChBMh

BC1

BM2

Fig. 4. Location of the horizontal (BCh and BMh) and vertical (P1a and P1b) profilesnear Jhuldol used for the measurement of ECRa of soil samples belonging to theGokarna Terrace (a) and data obtained from horizontal (BCh and BMh) profiles (b).Green symbols indicate the location of the soil samples of the initial survey.


the obtained values of ECRa are 3.6 ± 0.4 Bq kg�1, 28 ± 2 Bq kg�1 and

31 ± 3 Bq kg�1, respectively, for top soil, orange-greenish clay andblack clay. The differences within each layer, for example for thetop soil, reach up to 32%.

4.4. The Dhalmal Ridge high resolution profile P6

In the Tokha Formation near the top of the Gokarna Terrace, in asmall vertical profile just opposite the outcrop of profile P5, ECRa

measurement of the soil (profile P6) was performed (Fig. 2). Thissite is characterized by a succession of three layers from the bot-tom to the top: an orange clay layer, a black clay layer, and a brownclay layer (Fig. 11). The black clay and the orange clay layers con-tain diatomaceous clay. The total length of profile P6 is 70 cm, witha 3 cm distance between two consecutive samples. At this level ofspatial resolution, we could observe differences within the blackclay layer. The black clay member at its lower part from wheresamples from 6 to 10 were collected was hard and intact, whereasthe upper part containing the sample from 11 to 16 showed a morealtered surface with some centimetre-scale fractures (Fig. 11a).

Fig. 11c shows ECRa values from profile P6 as a function of thesampling position. First, this profile appears extremely smooth,which demonstrates a striking spatial coherence of ECRa betweennearby samples, and consequently a good representation of ECRa

values at this spatial scale. The ECRa values range from around 10to 43 Bq kg�1, with an overall average of 27 ± 2 Bq kg�1, which isparticularly high in this profile. Second, the previously describedlayers are perfectly recognizable in the ECRa profile. Four groupsare obviously represented, respectively, by the data of the samples

from 1 to 5, 6 to 10, 11 to 16, and 17 to 21. These four groupscorrespond to the orange clay, the hard member of black clay,the altered member of black clay, and the brown clay layers,respectively (Fig. 11). The averaged ECRa values of these layers,from the bottom to the top of the profile, are: 13 ± 1 Bq kg�1,24 ± 1 Bq kg�1, 40 ± 1 Bq kg�1, and 32 ± 1 Bq kg�1. These valuesare particularly high, especially for black and brown clays. Theuppermost layer shows some debris flow deposits consisting ofgravely sand from samples 22 to 24 (Fig. 11a and b). Note thatthe sample 23 is entirely composed of gravely sand, whereas thesamples 22 and 24 are largely composed of brown clay with minoramount of gravely sand. This lithological variability, thus, might besufficient to explain the large difference in their ECRa values.

5. Complementary results from other soil profiles

5.1. The Nagarkot soil profile P8

The profile P8 is located in the north-eastern part of the Kath-mandu Valley, near Nagarkot (Fig. 1). The Nagarkot residual soildeposit is mainly characterized by 2-m-thick top red soil layer(Fig. 12a). Two horizontal profiles, P8a in the upper part of thelayer and P8b in the lower part of the residual soil, were studiedfor ECRa measurement. Underlying bedrock consists of alteredphyllite from the Tistung Formation (Stöcklin and Bhattarai, 1981).

Sampling points and the obtained values of ECRa in soil samplesare represented in Fig. 12a and b, respectively. ECRa values rangefrom around 6 to 16 Bq kg�1, and the overall average is10 ± 1 Bq kg�1. The lateral variation, with a standard deviation of29 ± 3%, is larger than noted with the previous profiles. The twoP8 profiles, however, appear remarkably similar, indicating thatthe relative variation in ECRa is probably due to a significant lateralvariation in soil properties, rather than a heterogeneous noise inthe ECRa value. Again, the spatial coherence of the ECRa values ap-pears striking at the plurimetric scale.

5.2. The Bhattar-Trisuli Bazar soil profile P9

The ECRa profile P9 is located at the southern edge of theBhattar-Trisuli Bazar lateritic soil zone, just below Pipaltar village,in the vicinity of the bridge on the Thadi Khola River (Fig. 1 andTable 1). The samples were collected from the residual soil derivedfrom the mica-schist (Fig. 1). From bottom to top, the soil profilecan be distinguished into: a weathered basement rock comprisedof mica-schist belonging to the Tawa Khola Formation (Stöcklinand Bhattarai, 1981), overlain by highly weathered basement rockrepresenting the saprolite layer or horizon C, a 120 cm thick redsoil layer named as horizon B and a red top soil representing thehorizon A at the top (Fig. 13a). Two separate vertical profiles(P9a and P9b), with a spacing of 50 cm, were analysed for ECRa val-ues (Fig. 13a).

The results of ECRa values are displayed in Fig. 13b as a functionof position. The two profiles show a similar trend: lower ECRa val-ues for the weathered basement and higher ECRa values for the topred soil and the highly weathered basement. The obtained ECRa val-ues range from around 12 to 21 Bq kg�1 and the average value forthe two profiles P9 is 17 ± 1 Bq kg�1.

5.3. Further measurements with red soils and associated soils

Four further samples were taken in the Bhattar-Trisuli Bazarlateritic soil zone. They are noted by the letter C in Fig. 1. The ECRa

results for these soils, although they are separated by several kilo-metres, are very similar: the values vary from 22.1 to 24.3 Bq kg�1

100cm

60cm

45cm

20cm

45cm

80cm

Coarse sand Fine sand Orange clay

Black clay Brown clay Sandy mud

3

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1

2

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Clay Sand1340 m b 0

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11 12

Profile P2b

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30cm

Profile P2a

a c

Baniyatar

NorthSouth

Fig. 6. Profiles P2a and P2b near Baniyatar used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented, from left to right, a detailedpicture with sample levels of the two vertical profiles of ECRa (a), a simplified stratigraphic log of the outcrop (b), and the data obtained from these two profiles (c). Note: Thephotograph does not have the same scale as in figures (b) and (c).

BCh

BC1BC2

LBM1 LBM2

S1 BM1

BM3 S2 S3

BC3 BC4

BC5

BMh BM2

b

Jhuldhol

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0.1 1 10 100Effective Radium concentration (Bq kg-1)

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Profile P1aProfile P1bBMh (average in black dot)BCh (average in black dot)First survey (01/2008)

a Clay Sand

1309 m

BC5BC4

BC3

S2 S3BM3BMh (12 samples) BM2BM1

S1LMB2LMB1

BC2BC1BCh (9 samples)

Black clay

Laminated black mud

White sand

Occhre sand

Massive black mud Debris of slope failure

a

Fig. 5. Profiles P1a and P1b near Jhuldol used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented, from left to right, a simplifiedstratigraphic log of the outcrop (a), the data obtained from the two vertical profiles, with each sample taken from the same layer, in logarithmic scale for ECRa (b), and adetailed picture of the profile P1b showing the different nature of deposits with sample levels (c). Here and after, on the photograph, small flags indicate sampling locations.Note: The photograph does not have the same scale as in figures (a) and (b).


0

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West Eest

15cm

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Ochre sand Orange clay and sand

Black clay

Brown clay

Coarse sand Medium sand Fine sand

1 2

3

4 5

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7

8 9

10

11

12

Dhalmal Villagea c Profile P4

1345 mClay Sand

b Profile P4

Fig. 7. Profile P4 near Dhalmal Village used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented, from left to right, a detailedpicture with sample levels of the vertical profile of ECRa (a), a simplified stratigraphic log of the outcrop (b), and the data obtained from this profile (c). Note: The three figureshave the same scale.


(Table 2), with an average of 23.4 ± 0.5 Bq kg�1 and a standarddeviation of 1.1 Bq kg�1.

Additional red residual soil samples were also taken north ofTrisuli Bazar: near Kalikasthan (A) and one site located betweenKalikasthan and Betrawati localities (B). The ECRa values are17.1 ± 0.3 Bq kg�1 and 14.1 ± 0.4 Bq kg�1 (Table 2), respectively,significantly different compared with the red soils from theBhattar-Trisuli Bazar plateau.

Another soil profile has been carried out east from the Bhattar-Trisuli Bazar lateritic soil zone (P10 in Fig. 1). This profile can bedescribed by a succession of black residual soils overlain by ochretop soil. These soils cover a basement constituted of black schist.The basement was characterized by a higher ECRa value(64 ± 1 Bq kg�1) than the black residual soils (23 ± 2 Bq kg�1) andthe top soil (23.5 ± 0.1 Bq kg�1), which appeared similar (Table 2).

5.4. Other reference samples

Several samples were also collected north of Tokha (D in Fig. 1)near the Chandeshvari village, the closest zone near the GokarnaTerrace sampling sites with outcropping Shivapuri Lekh granite–gneiss bedrock. One reddish residual soil sample was characterizedby an ECRa value of 7.3 ± 1.3 Bq kg�1 while granite samples hadcontrasting values: from 4.8 ± 0.3 Bq kg�1 for a highly weatheredsample to 16.2 ± 0.4 Bq kg�1 for an unweathered sample (Table 2).

Finally, ECRa was also measured with 102 soil samples from asite in a widely different context: the Syabru–Bensi geothermal

zone of central Nepal (Fig. 1, Table 1). For these samples, ECRa

was found to be rather homogeneous, with values varying from2.0 to 27 Bq kg�1 and an overall average of 9.2 ± 0.4 Bq kg�1.

6. Discussion

6.1. Characterization of ECRa values in the deposits of KathmanduValley

The range of values obtained from the Kathmandu deposits,which extend from 0.38 ± 0.06 Bq kg�1 to 43.2 ± 1.4 Bq kg�1, is par-ticularly impressive. Concerning the lower ECRa values, however,they do not diverge dramatically from other soil sample valuesfrom the Himalayas (Singh et al., 1999; Ramola and Choubey,2003; Singh et al., 2007; Prasad et al., 2008). The distribution ofECRa results from the sediment samples of the Gokarna Terrace,displayed in Fig. 14, shows three dominant modes, with a signifi-cant tail towards the higher values. The overall average, 17 ±1 Bq kg�1, is, in this case, a rather poor representation of the typicalvalues. For comparison, the distribution obtained from soils of Sya-bru–Bensi, is also shown in Fig. 14. This distribution is single mode,approximately log-normal, with an average of 9.2 ± 0.4 Bq kg�1.This is a more commonly observed situation. Indeed, the typicalvalue of ECRa for a soil is about 5–10 Bq kg�1 (Nazaroff, 1992; Leh-mann et al., 2000).

The various clusters of values in the distribution of the GokarnaTerrace sediments, shown in Fig. 14 with the number of samples,

1398 m Clay Silt Sand

12

11

10987

65 4

3

21 b

Fine to very fine sand Clay and silt Orange clay

Black clay

Brown clay Medium to fine sand with lenses of coarse sand

128cm

14cm

16cm

4cm

30cm

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Effective Radium Concentration (Bq kg-1)

Sam

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(cm

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Profile P7 Painyatar

a

cProfile P7

Fig. 8. Profile P7 near Painyatar used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented, from left to right, a detailed picturewith sample levels of the vertical profile of ECRa (a), a simplified stratigraphic log of the outcrop (b), and the data obtained from this profile (c). Note: The photograph does nothave the same scale as in figures (b) and (c).


summarized in Table 2, and displayed graphically in Fig. 15, reflectthe various lithologies in the basin deposits. For all studied sandlayers, the ECRa values remain low, mostly between 1.8 and8.5 Bq kg�1 (STD range), with an average for all sand samples of5.1 ± 0.6 Bq kg�1. Other values are significantly different, particu-larly for clay layers, where ECRa reaches up to 43 Bq kg�1. This max-imum value is about a factor of 2 larger than the value observed forthe red residual soils of the Bhattar-Trisuli Bazar plateau, and thusoffers an exceptional reference point. Moreover, interestingly,among the various clay layers of the Kathmandu deposits, the ECRa

average value appears to be specific to particular lithologies:orange clay (15 ± 1 Bq kg�1), orange-greenish clay (27 ± 1 Bq kg�1),brown clay (29 ± 2 Bq kg�1), black clay (26 ± 1 Bq kg�1) or blackmud (10 ± 1 Bq kg�1). ECRa values of all black clay samples, exceptfor the Jhuldol profile P1, have high values comprising between 20and 43 Bq kg�1. The non red top soils from P3 and P5 profiles andthe red soil from P8 and P9 profiles, together with A, B, C and Dsites, have different ECRa values, typically higher on average byabout a factor of 2 (1.8 ± 0.3) for red soils, compared with nonred top soils. This fact could be explained by preferential sorptionof radium onto amorphous ferric oxyhydroxides which occurs to agreater extent in red residual soils than in others (Ames et al.,1983c; Flexser et al., 1993).

The second striking fact, observed with the Kathmandu Valleydeposits and with the red soils of Bhattar-Trisuli Bazar plateau, isthe homogeneity of the ECRa values within a given stratigraphichorizon. This is demonstrated by the horizontal profiles BCh andBMh (Fig. 4), P3 (Fig. 9), P5 (Fig. 10), but, also, by the correspon-dence between points at the same stratigraphic level in parallel

vertical profiles, such as P1 (Fig. 5), P2 (Fig. 6), P9 (Fig. 13). Indeed,the overall difference between the two corresponding ECRa values,for P1, P2 and P9 profiles, is 27.3 ± 6.8%. This horizontal homogene-ity of ECRa, however, is less clear with the Nagarkot or the Bhattar-Trisuli Bazar data, which are not terrace deposits. This probablyreflects the fact that the stratigraphic homogeneity of theKathmandu deposits is maintained particularly well because oftheir fluvio-lacustrine nature. Such deposits therefore, by contrastwith other types of soils, are particularly well suited to studystratigraphic variations of ECRa.

To discriminate all the various sediment layers, this study indi-cates that the determination of the ECRa of at least 3 or 4 samplesfor each layer could be sufficient. It appears, thus, that ECRa mea-surements are a particularly effective method, easy to implement,to distinguish and characterize different layers of a sedimentarysequence. In addition, for layers with a slight difference in litholog-ical composition, for example the black clay members of thehigh-sampling resolution profile P6 (Fig. 11), ECRa values are signif-icantly different and can potentially discriminate two internalmembers. Note, however, that stratigraphic discrimination cannotrely on few measurements. The statistical significance of a givenprofile, as well as the characteristic value for each particular layer,will have to be assessed carefully in each case.

6.2. The ECRa values in residual soils versus bedrock properties

The radon source term is a complex combination of several ef-fects: the distribution of radium atoms within the rocks and thetransport properties of the granular assembly, which contributes

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Debris of slope failure

b Subprofile P3b

Subprofile P3a

c

Baniyatar

1m

1m 1m

South North

a Debris of slope failed materials

Top soil

Fig. 9. Horizontal subprofiles P3a and P3b near Baniyatar used for the measure-ment of ECRa from soil samples belonging to the Gokarna Terrace. Are represented,from top to bottom, a detailed picture of the two horizontal subprofiles of ECRa withlocation of sample spots (a), a simplified stratigraphic interpretation of the outcrop(b), and the data obtained from these two subprofiles (c). Note: The figures (a) and(b) do not have the same scale as in (c) but each consecutive two samples have 1 mdistance in the field.


to the emanation coefficient E. In a first approximation, since weconsider here only crushed samples, the main differences betweensamples should originate from a difference in the distribution ofradium atoms (Nazaroff, 1992; Flexser et al., 1993). The radiumatoms can be a primary component of minerals, mainly zirconand sphene for uranium (Flexser et al., 1993), for example, result-ing from primary uranium decay, or can be a secondary elementresulting from mineral alteration, then leaching and transport, ofradium or its parent radioactive elements, by geological fluids. Inthe former case, radium will be distributed throughout the grains,while it will be concentrated at the grain surface in the latter, espe-cially by sorption for example onto micaceous minerals (Ameset al., 1983b). Transport of secondary radium phases can also befollowed by metamorphic evolution. The radium content of a rockor soil, thus, can be the result of a complex distribution signifi-cantly disturbed by geologic/geomorphic history (Wiegand,2001); in general, it remains poorly understood so far.

It is therefore interesting to discuss whether our results canbring some insight in this matter. The obtained results can be usedto address a possible relationship between the ECRa of the sedi-ments and the local bedrock (Gundersen and Schumann, 1996).In particular, we can investigate whether alteration processes, in

general, have a systematic effect on the value of ECRa. The data ofprofile P9 (Fig. 13) in Bhattar-Trisuli Bazar soil zone provide an ini-tial answer (Table 2), with an indication for an increase of ECRa

from the unweathered mica-schist, slightly weathered mica-schist,highly weathered mica-schist and soil horizon which has the max-imum value. The samples from Chandeshvari (D), however, do notconfirm this trend: in this case, the unweathered granite–gneissfrom Shivapuri Lekh shows the maximum ECRa value. The sameobservation is made for the profile P10, characterized by a maxi-mum ECRa value for the black schist basement compared to theECRa values of the black residual soil and the top soil (Table 2).The relationship between soil radon potential and local geology,thus, while probably a characteristic of an erosion pattern, doesnot exhibit a systematic evolution. This fact, on which questionshave been raised for decades (Botset and Weaver, 1932), providesserious limitations in radon risk assessment based on regionalgeology only. The particular nature of the rocks, and in particularits clay content, might play an important role in the distributionof radium between the evolved material and that eliminated byerosion.

Similarly, the lower value of ECRa observed for the Syabru–Bensisoils is also difficult to relate to the local rocks, dominated byGreater Himalayan Crystalline rocks, gneiss and quartzites, butwhich also contains sedimentary sequences from the Lesser Hima-layan Sequences. It is also difficult, at this stage, to explain theanomalously high values of ECRa from the Dhalmal Ridge black clay,but it is known that radium can be redeposited by sorption ontoclays (Ames et al., 1983a). It might be due to an anomalously highuranium concentration in the original rock, or an alteration processleading to significant concentration of radium by leaching andredeposition. It is also known that radium is preferentially associ-ated with the organic matter, which is here the main constituent ofclays, and this affinity is stronger than for Fe-oxides (Greemanet al., 1999). This could explain that the ECRa values of the Fe-richred residual soils are lower than those of the black and brownclays. More work will be needed to clarify this point. However,some additional data, already available, might provide some inter-esting clues.

6.3. The ECRa of the Baniyatar deposits versus other availableparameters

The outcrop of ECRa profile P2 near Baniyatar belongs to the Go-karna Formation, which happens to have also been studied usinggeochemical analyses by Gurung et al. (2007). While the samplesused for the geochemical analyses by these authors came fromother outcrops, some belong to the same layers that have beenstudied in this paper. Two major oxides, Fe2O3 and TiO2, are posi-tively linearly correlated with ECRa: for example, Fe2O3 concentra-tion of 1.2 wt% and 7.2 wt%, and TiO2 concentration of 0.2 wt% and0.9 wt%, are obtained respectively for ECRa values of 5.4 Bq kg�1

and 21 Bq kg�1. Concerning trace elements, Zn, Cu, Ni, Th, Cr, V,Y, Nb, Zr, Sc and As (6 and 7 mg kg�1

, respectively for the sametwo ECRa values), they are also positively correlated with ECRa,whereas Pb is anticorrelated (44 and 31 mg kg�1, respectively forthe same two ECRa values). These results are interesting, particu-larly for the relationship between Zr and ECRa. Zr (82 and216 mg kg�1, respectively for the same two ECRa values) is indeedknown to be encountered in zircon minerals, which concentrateU, which could partly explain the correlation between Zr and Ra.Another interesting point is the relationship between As and ECRa,which could be explained by the alteration of granites and gneis-ses, which may concentrate As in sediments. However, to betterunderstand such relationships, we need further investigations,especially to confirm whether ECRa could be a relevant proxy of As.

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30cm

Top soilOrange-greenish clay Black clay Debris of slope failure

Subprofile P5a

Subprofile P5b

Subprofile P5c

30cm

South North

a b Debris of slope failed materials

c

Dhalmal Ridge

Fig. 10. Horizontal subprofiles P5a, P5b and P5c near Dhalmal Ridge used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented,from left to right, a detailed picture of the three horizontal subprofiles of ECRa with location of sample spots (a), a simplified stratigraphic interpretation of the outcrop (b), andthe data obtained from these three subprofiles (c). Note: The figures (a) and (b) do not have the same scale as in (c) but each consecutive two samples have 30 cm distance inthe field.

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Black clay Profile P6

Brown clay

Gravely sand

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16 17

21 22

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a

b c Profile P6

Dhalmal Ridge1360 m

Fig. 11. High resolution vertical profile P6 near Dhalmal Ridge used for the measurement of ECRa from soil samples belonging to the Gokarna Terrace. Are represented, fromleft to right, a detailed picture of the vertical profile of ECRa of high resolution with location of sample spots (a), a simplified stratigraphic log of the outcrop (b), and the dataobtained from this profile (c).


7. Consequences in terms of potential health hazard

The results presented above indicate that ECRa values can reachsignificant levels in the Kathmandu valley, and, therefore, poten-tially can produce important radon fluxes to the atmosphere, con-sequently, significant exposure of the population. For our purpose,here, we shall consider that a significant risk arises when concen-tration reaches a threshold value of 300 Bq m�3. The main conclu-sions actually do not depend on the choice of this threshold. Thus,

we need to estimate the ECRa values able to exceed this thresholdconcentration. For this, a simple two-phase model of radon diffu-sion from the soil, and stationary regime in a dwelling, can be used.We shall thus consider a basic house with a single room havingbeaten soil as a floor with a surface area S (m2) and a volume V(m3). This actually still corresponds to numerous dwellings in thearea, and presents probably a relevant estimate of the exposure.To facilitate estimations of radon derived from underlying materi-als, radon gas produced by the building materials of the house is

A1

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170cmProfile P8a

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ive

Rad

ium

con

cent

ratio

n (B

q kg

-1)

Profile P8a (A1 to A5)Profile P8b (A6 to A10)

Fig. 12. Horizontal profiles P8a and P8b near Nagarkot used for the measurement ofECRa from soil samples belonging to the Nagarkot residual soil deposits. A detailedpicture of the outcrop with location of soil samples (a) and the ECRa data obtainedfrom these two profiles (b) are displayed.

Fig. 14. Distribution of the entire data set of ECRa, obtained in this study, from soilsamples of the Gokarna Terrace. The distribution obtained with soil samples of theSyabru–Bensi geothermal area (Langtang region, Nepal) is also plotted forcomparison.


neglected. The equilibrium concentration of radon in such a roomcan be written as (Richon et al., 2005):

CRn ¼SV

UkV þ k

; ð1Þ

where k is the radon decay constant (s�1), kV the ventilation rate(s�1) and U the radon flux (Bq m�2 s�1). We take S/V = 0.5, a valuefrom which it is difficult to depart significantly. The ventilation rateis more difficult to estimate and, in particular, can be highly variableand time dependent. Houses in the considered area tend to havehigh ventilation rates and open windows and doors even in the win-ter time. However, to obtain a conservative estimate, it is reason-able to assume a standard value of 1.5 � 10�4 s�1 (Nazaroff,1992). The value of the radon flux able to generate the thresholdvalue of radon concentration is then 300 � (kV + k) � V/S � 90� 10�3 Bq m�2 s�1. This is the estimate of the radon flux from the

50 cm

120 cm

120 cm

120 cm

A1

A2

A3

A4

A5

A6

A7

A8

A9

A10

A11

A12

Profile P9a Profile P9b

Horizon A

Horizon B

Horizon C Saprolite

a Bedrock

(mica-schist)

0

50

100

150

200

250

300

350

1

Sam

ple

dist

ance

(cm

)

Fig. 13. Vertical profiles P9a and P9b near Bhattar-Trisuli Bazar used for the measuremenpicture of the outcrop with location of soil samples (a) and the ECRa data obtained from

soil that is deemed tolerable in our context. Note that the meancontinental radon flux is 22 � 10�3 Bq m�2 s�1 (Nazaroff, 1992).

It is then necessary to constrain the conditions able to generatesuch a radon flux from the ground given the measured value ofECRa. For this purpose, we shall consider a homogeneous soil ofporosity e, water content Sw, air filled porosity ea = e(1 � Sw) andwater filled porosity ew = eSw. The radon flux can then be expressedas (Lehmann et al., 2000):

U ¼ffiffiffiffiffiffiffiffikDS

p qECRa

ea þ ewjw; ð2Þ

where DS is the soil radon diffusion coefficient (m2 s�1), q the den-sity of the soil (kg m�3), and jw the partition ratio of radon betweenwater and air. The parameter jw depends only on temperature,

North South

b

Bhattar-Trisuli Bazar

0 12 14 16 18 20 22


Profile P9a(A1 to A6)Profile P9b(A7 to A12)

t of ECRa from soil samples belonging to the Trisuli residual soil deposits. A detailedthese two profiles (b) are presented.

a

b

Fig. 15. Graphical presentation of the ECRa measurement results from differentstudied lithologies of Kathmandu deposits (a) and from the various studied claylayers of the Gokarna Terrace (b). For each considered group of measured samples,the vertical white line represents the ECRa average with the horizontal white linecorresponding to its uncertainty, the black filled region is the 1r standard deviationrange of variation of ECRa values, and the grey filled region corresponds to globalmin–max range of ECRa values. The ECRa values and the associated statistical valuesused here are summarized in Table 2.

Fig. 16. Calculated surface radon flux from the ground as a function of totalporosity of the soil (a) and water content of the soil (b). Two different homogeneousvalues of ECRa of the underlying soil are used for each graph. The horizontal dottedline corresponds to the calculated surface radon flux threshold below which it isassumed the absence of radon health hazard inside a traditional Nepalese housebuilt potentially onto this underlying soil.


varying from 0.4 at 5 �C to 0.2 at 30 �C. Here, we take an average va-lue of 0.3. To describe the variation of DS with porosity and watercontent, we use the following empirical parameterization (Rogersand Nielson, 1991):

DS ¼ea

ea þ ewjwD0ee�6Swe�6S14e

w ; ð3Þ

where D0 is the diffusion coefficient of radon in air(D0 = 1.2 � 10�5 m2 s�1). The radon flux is shown in Fig. 16 as afunction of e and Sw for two representative values of ECRa. Duringthe winter, when the water content is close to 30%, the radon fluxis higher than during the summer, when the water content is closeto 80%. For ECRa = 50 Bq kg�1, the threshold value of90 � 10�3 Bq m�2 s�1 is exceeded over the whole range of porosity,except at the highest values of water content. For ECRa = 5 Bq kg�1,the threshold value of radon flux is only reached at low water con-tent. For average conditions of e = 20% and Sw = 50%, the thresholdvalue of the radon flux is reached when ECRa = 7.7 Bq kg�1, whichcan be considered as the maximum acceptable ECRa value in termsof health hazard in our conditions. In our case, we note that 77% ofour samples exceed this threshold value. Thus, the range of valuesobtained in this study suggests a definite health hazard in unpro-tected dwellings on most sediments of the Kathmandu valley, andproper attention should be given to the exposure of population.

8. Conclusion

In this paper, experimental results are, for the first time, pre-sented on the radon emanation potential of soils from Nepal. Froma methodological standpoint, this study has shown that determina-tion of ECRa in the laboratory from soils and sediments of the Kath-mandu Valley is a promising tool in characterizing sedimentarydeposits. Furthermore, present data (in association with otherchemical, lithological and stratigraphical characteristics) are ableto facilitate the establishment of a long distance correlation ofthe sediments in the basin. The potential of the Kathmandu Valleyhas been confirmed as a natural laboratory for pilot stratigraphicstudies.

Our ECRa results corroborated the observation that radiumatoms are strongly adsorbed by soils and are enriched in the fineclay fraction (Greeman et al., 1999). In addition, radium atomshave a particular affinity for organic matter and for clay, but inlower importance for Fe-oxides. We have also noted lower ECRa

values in average for red residual soils than for clays. However,the observations made by Greeman et al. (1999), that A horizons(especially in organic-rich A horizons) are enriched in radium orthat radium content increases with depth appear in the light ofour measurements to depend on the studied sedimentary site.The diverse clays studied in this paper are composed primarily oforganic matter, micaceous minerals, but also fine minerals frac-tions, which have a high specific surface area to adsorb radiumatoms onto their grain surfaces. Consequently, in clays, radium isalso concentrated in the finest fraction of the minerals (Breitneret al., 2008). Moreover, leaching plays a major role, associated withon one hand the exchangeable-organic fraction (Greeman et al.,1999) and, on the other hand, with the more leachable species ow-


ing to the radium recoil effect (Breitner et al., 2008), the main ra-don source remaining the easily removable adsorbed radiumatoms.

Finally, ECRa measurements are an essential asset; whenattempting to constrain the sources of radon in the soil; to developa better understanding of the physical transport of radon gas in thesurrounding environment, especially via the diffusion process; andto assess radon hazard and risk in highly populated areas. Moredata would be considerably useful to make radon hazard and riskmaps in urban as well as rural areas of Kathmandu Valley andother populated places in the Himalayas. Such hazard and riskmaps obviously are most useful to policymakers and governmentagencies for awareness and radon risk reduction programs.

Acknowledgements

The authors thank the Department of Mines and Geology, Kath-mandu, for its continuous support over the years. Khem Puri, SaranSilwal and Buddha Lama are thanked for their assistance duringsampling. The original version of the manuscript was greatly im-proved thanks to the careful work and numerous positive sugges-tions of Tadeusz Przylibski and one anonymous reviewer. This isIPGP contribution number 3101.

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Journal of Asian Earth Sciences · 2020. 4. 2. · Radon emanation of heterogeneous basin deposits in Kathmandu Valley, Nepal Frédéric Giraulta,⇑, Ananta Prasad Gajurelb, Frédéric