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Environmental Geology, (2005) 47:(6), 745-750 |
Original
Article
Radon variations in an active landslide zone
along the
V.M. Choubey1 
, S.K. Bartarya1 and
R.C. Ramola2
Received: 22 May 2004 Accepted: 13 October 2004
Abstract Radon measurements were made in the soil and spring/seepage water in and around an active landslide located along the Pindar river in the Chamoli District of Uttaranchal in Garhwal Lesser Himalaya, to understand the application of radon in geological disasters. The landslide is a compound slide i.e. a slump in the crown portion, and debris slide and fall in the lower part. The bedrock consists of gneisses and schists of the Saryu Formation of the Almora Group of Precambrian age. The presence of several small slump scars and debris slide/fall scars along the length of the slide indicates continuous downward movement. The radon concentrations in the present study are much lower in comparison to values reported from other regions. However, the present radon data show relative variation in the slide zone. The concentration of radon measured in landslide zones varies from 3.1 Bq/l to 18.4 Bq/l in spring water and from 2.3 kBq/m3 to 12.2 kBq/m3 in the soil gas of the debris. Along the section of the slide, the radon values in water and soil are slightly higher in the upper slopes i.e. toward the crown portion of the landslide as compared to the distal portion. The relatively low concentration of radon both in soil gas and water in the toe portion of the landslide may be due to the high porosity of the debris, which does not allow radon to accumulate in the soil and water, whereas, towards the crown portion, the high frequency of fractures increases the surface area due to particle size reduction, and the near absence of debris enhances the radon emanation in soil.
Keywords Radon - Landslides -
Introduction
Radon gas is produced by the decay of naturally occurring uranium found in
almost all rocks and soils. However, radon concentration varies with specific
site and geological material. Especially at faults and active geodynamic zones,
radon gas moves upward following the so-called 
channels
and then diffuses in groundwater and aerial zones (Hess
et al. 1985;
Ball et al. 1991;
Choubey et al. 1999,
2001).
This movement makes up abnormal fields of radon radiation in groundwater and
soil gas (Kemsi et al. 2001).
Determination of abnormal fields of radon radiation might enable assessment of
the existence of active geodynamic or active seismic zones in an area. The
enhanced radon concentration in subsoil in old mining areas may be indicative
of subsidence due to fractioning of the underground rocks (Kies et.al. 2004).
The profound impact of landslide-related slope instability is particularly
observed in areas of active faults and thrusts of the Himalayan region. Rock
types and their structural setting and degree of weathering immensely influence
the stability of the hill slopes in all the four major litho tectonic belts of
the
The studies in the Himalayan region have demonstrated that the tectonic
processes, types of rocks and geohydrological
characteristics of rock mass control the concentration of radon in soil and
water (Choubey et.al. 1998,
1999,
2001).
Landslides, which result due to geodynamic processes, have been considered as a
source of radon (Purtschller et. al. 1994,
1995).
The indoor measurement in Koefels landslide (
Geology of the area
The study area lies in the north central part of the Kumaun Lesser Himalaya. The geology of this area has been studied in detail by various workers (Gansser 1964; Saklani 1972; Agarwal and Kumar 1973; Valdiya 1978, 1980). Geologically, the area belongs to the Almora Group (also known as Almora Crystalline) of Lesser Himalaya (Valdiya 1980). The rock consists of over-thrusted succession of a variety of schists, micaceous quartzite and gneisses belonging to the lower amphibolite facies of regional metamorphism and tectonically emplaced bodies of granite and granodiorites. The Almora Group is further divided into three units, namely Suryu formation (with Champawat granodiorite in the upper part), the Gumalikhet formation and the Munsari formation. The lithology of the landslide zone that was studied belongs to the Saryu formation (Fig. 1). The rocks of the slide zone consist of alternate bands of garnetiferous muscovite schists and gneisses. At places, mylonitized quartz porphyry and weathered micaceous quartzite are also present. Bands of augon gneisses are noticed further east near the Gwaldam area. The rocks of the Almora group are separated from underlying sedimentaries of the Berinag formation of the Jaunsar group by the Munsiari thrust. The dip of the bedrocks of the slide area varies from 20° to 30° in NE and NNE direction. The rocks are characterized by multiple deformations resulting in superimposed folding and repeated faulting and thrusting. The rocks are cut by numerous NW to SE- and NE to SW-trending faults and fractures.
Fig. 1 Geological map of the study area showing the location of the
slide
The bedrocks of the slide area have evidently been affected by severe tectonic movements, which have occurred along the Almora thrust. A number of shear planes are developed parallel, oblique and perpendicular to the bedding. The more competent granite gneisses are highly fractured, jointed and sheared, whereas, the less competent schists are extensively sheared and pulverized. The joints and fractures are generally open near the surface and their openness can be partially attributed to the de-compression, which accompanied the valley formation. Prominent sets of joints and fractures are noticed and are dipping in 60–65° towards the NE, 35° towards NW and 75° due ESE direction.
Description of the
slide zone
Figure 2
shows the geomorphologic features of the slide zone. The slide is present on
the northeast-facing slope of the
Fig. 2 Map of the landslide zone showing
geomorphic features of the landslide zone and location of the samples for radon
measurement
The slide measures about 150 m in length and 300 m in width. Several scars of slump and debris fall and slide are present in the crown portion and main body of the slide. The exposed slide scars dip at 60–65° towards the NE direction. There are at least three sets of colluvial benches present in old landslide deposits. Two 1st order streams (the eastern one is lineament controlled) cutting the slide on either side are present in the slide zone. The fall of debris scars the middle portion of the slide near the vertical dipping at 65°NE and about 10 m in length and 8 m in width. Scars caused by the fall of debris in the toe portion of the slide are present along the road and shown as road-cut scar (Fig. 3). Because the whole rock mass laden with debris from the head to toe region has slided down towards the Pindar river, lateral scars of 3 m in height, sloping 60° towards N, 30°E and 65°NW are present on either side of the slided mass.
Fig. 3 Photograph of the landslide showing deris
slide scar along the Ist order stream on the western
flank and along the road in the toe portion of the slide
Due to the slide, several 2–3-m long and 0.5-m-deep E–W and NW to SE-oriented transverse cracks and 0.3–0.5-m wide and about 1-m-deep N to S-oriented longitudinal cracks have developed in the upper and middle portion of the slide mass during the distress movements. Besides, several semi-circular radial cracks are also present in the crown portion of the slide. These cracks were open near the surface. At places, particularly towards the toe portion, secondary scars of 0.5–1 m height developed subsequently, indicating continued slow downward movement (creeping) of the sliding mass.
Causes and mechanism
of the slide
It is a compound slide i.e. a slump in the crown portion and the fall of
debris, and slides in the middle and lower portion. The rocks consist of highly
fractured and jointed schists and gneisses and old
landslide debris. The intense fracturing and joints converted the rocks into
loose blocks. Water has played a major role in triggering the movement. Exposed
backward-rotated scars and radial cracks in the crown portion in gneisses and schists suggest slumping. The disposition of fractures in
joint planes (trending NE-SW, NW-SE, ESE-WNW) and schistosity
(30° toward NE) create conditions for sliding, apart from the presence of old
landslide debris and slumped mass. The presence of highly sheared, fractured
and jointed rocks due to the close proximity of major tectonic plains (north
and south Almora thrust) have also contributed to the
slope failure. The presence of drainage on either side of the slide and spring
and seepages in the main body of the slide indicates continuous percolation of
rainwater. The schist rocks acted like a viscous body after absorbing water and
facilitating mass movement by lubricating the slip surface. The
Fig. 4 Cross section of the landslide showing the mechanism of
the slide
Materials and methods
The radon measurements were made in water (spring and seepage) and soil gas of the debris in the landslide zone. About 750 ml of the water sample was taken in a radon-tight reagent bottle of 1-litre capacity connected in a close circuit with a ZnS-coated detection chamber through a hand-operated rubber pump and a glass bulb containing CaCl2 to absorb the moisture. Air was then circulated in close circuit for a period of ten min till the radon formed a uniform mixture with the air and the resulting alpha activity was recorded.
In soil-radon emanometry, auger holes, each 50 cm in depth and 6 cm in diameter, were left covered for 24 h so that the amount of radon and thoron became stable. The soil-gas probe was fixed in the auger hole and formed an airtight compartment (Ghosh and Bhalla 1981). The rubber pump, soil-gas probe and alpha detector were connected in a close circuit. Air was circulated through a ZnS-coated chamber for a period of 15 min till the radon formed a uniform mixture with the air.
The alpha particles produce scintillations on the sample chamber, which are sensed by the photomultiplier tube and converted to electronic pulses counted on a digital display. Clamping both the ends then isolated the detector and observations were taken after 4 h. A time gap of 4 h is necessary to allow alpha-emitting daughters of radon to come into equilibrium with their own daughters. The resulting number of the alpha counts is converted into Bq/m3 by using the calibration factor 1 count/min=0.11 KBq/m3, determined by Singh et al. (1986) under similar conditions.
Results and
discussions
The results of radon measurements in spring water and soil are given in Table 1 and their location in Fig. 2. The radon concentration measured varies from 3.1 Bq/l to 18.4 Bq/l in spring water, whereas in soil-gas samples, it varies from 2.3 kBq/m3 to 12.2 kBq/m3. The average (background) value in soil and water in the surrounding area from similar lithology varies from 0.2 kBq/m3 to 0.9 kBq/m3 and 0.7 Bq/l to 1.2 Bq/l, respectively (Choubey et al. 2004) in preparation). The radon concentration across a cross section of the landslide (Figs. 2, 4) shows that the radon values in soil are slightly higher (4.7–12.2 kBq/m3) in the upper slopes i.e. towards the crown portion of the landslide as compared to the toe portion (2.3 kBq/m3 to 6.3 kBq/m3). The water samples also show a similar trend (15.3 Bq/l to18.3 Bq/l in crown portion and 3.1 Bq/l to 4.2 Bq/l in toe portion, except sample# SW 1). The relatively high concentration of radon (18.4 Bq/l) in sample# SW1 in the toe portion of the slide is possibly because the spring is present close to a lineament-controlled stream on the side of the slide mass. Several fractures and joints are present in the freshly exposed rocks along the stream and may also contribute to the enhanced radon in this sample.
Table 1 Radon concentration in soil gas and water
in the land slide area
|
Sample no. |
Location of sample in slide area |
Radon concentration |
|
In soil |
||
|
SL1 |
Crown area |
10.3+0.83 kBq/m3 |
|
SL2 |
Crown area |
9.1+0.78 kBq/m3 |
|
SL3 |
Crown area |
12.2±0.90 kBq/m3 |
|
SL4 |
Crown area |
11.7±0.88 kBq/m3 |
|
SL4a |
Crown area |
4.7±0.56 kBq/m3 |
|
SL5 |
Crown area |
12.1±0.89 kBq/m3 |
|
SL6 |
Toe area |
2.8±0.43 kBq/m3 |
|
SL7 |
Toe area |
2.3±0.39 kBq/m3 |
|
SL8 |
Toe area |
6.3±0.65 kBq/m3 |
|
SL9 |
Toe area |
5.7±0.61 kBq/m3 |
|
SL11 |
Away from the slide |
4.1±0.52 kBq/m3 |
|
SL12 |
Away from the slide |
2.3±0.39 kBq/m3 |
|
In spring water |
||
|
Sw1 |
Toe area |
18.4±1.10 Bq/l |
|
Sw2 |
Toe area |
3.1±0.45 Bq/l |
|
Sw3 |
Toe area |
4.2±0.53 Bq/l |
|
Sw4 |
Main body of slide |
8.1±0.73 Bq/l |
|
Sw5 |
Main body of slide |
9.8±0.81 Bq/l |
|
Sw6 |
Main body of slide |
8.4±0.75 Bq/l |
|
Sw7 |
Crown area |
15.3±1.01 Bq/l |
|
Sw8 |
Crown area |
18.3±1.10 Bq/l |
The radon concentrations (8.1–9.8 Bq/l) in spring water of the middle portion of the slide are in between the radon values of the crown and toe portion of the slide. The radon in soil/debris of the middle portion of the slide could not be measured due to inaccessibility to the site. Towards the crown portion, the increased intensity of fractures and near absence of debris enhanced the radon emanation in soil. The low concentration of radon both in soil and water in the toe portion of the landslide may be due to the high porosity and permeability of the debris, which does not allow radon to accumulate in the soil and water. Schery et al. 1982 observed that the permeability/porosity of the sub-surface material plays an important role in the escape of radon to the atmosphere.
The radon concentration measured in the present landslide is relatively low
when compared to other parts of the world (Kemsi et
al. 2001),
and also, there are not many references related to landslides as possible
sources of radon; a comparison with the reported studies of Ennemoser
et.al. (1994)
and Partscheller et al. (1995) of the Koefels (
Measurements of radon in the landslide studied have shown that higher radon levels are in accordance with the frequency of fractures in augen gneisses and schists exposed in the landslide zone. In the basal part of the sliding plane, the highest degree of fracturing occurs. As a result, the radon concentration in soil gas and water show higher values near the sliding plane. The grain-size reduction due to high shear stress near the slide scar and further weathering of the slided mass expose the uranium-rich mineral (apatite, zircon), thus giving rise to high values of radon concentration in this portion of the slide.
However, Surbeck (1992) considered rockslides as an important factor controlling radon distribution but mainly related to permeabilities and not as a possible source. However, as pointed out by Ennemoser et al. (1993), the emanation rate of radon from soil depends not only on the concentration of uranium and radium content but to some extent on emanating power and the diffusion coefficient for radon in soil. The freshly fallen landslide debris, compared to solid rocks, has an increased emanating power and diffusion coefficient due to intense fracturing, and crushing down to the grain and sub-grain size, resulting in an increased surface area and porosity (Purtschller et al. 1995). Semkov (1990), Sutherland (1994) and Pirchl et al. (1994) have also suggested correlation between specific surface area and emanating powers of radon. Thus, particle-size fractions due to fracturing and crushing close to the landslide scar in the crown area in Garhwal Himalaya have possibly played a major role in increased radon emanation. Between the slide scar and toe of the slide, the distribution of radon takes place through a circulation pathway that exists between slided masses. But the concentration decreases towards the toe portion as radon possibly escapes into the air through highly porous, permeable and pulverized loose colluvial debris
Inference
In our study, radon concentrations are not high when compared to Koefels (
Acknowledgements We are thankful to the
Director, Wadia Institute of
Radon and 
held at