Radon emanation of heterogeneous basin deposits in Kathmandu Valley, Nepal
Résumé
Effective radium-226 concentration ( EC Ra) has been measured in soil samples from seven horizontal and vertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong to the 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. EC Ra 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 concentration of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of 3.8 days, is measured using scintillation flasks. The EC Ra values from the seven different profiles of the terrace 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 demonstrating that these values can be used for long distance correlations of the sediment horizons. Widely separated sediment profiles, representing similar stratigraphic positions, exhibit consistent EC Ra values in corresponding stratigraphic sediment layers. EC Ra measurements therefore appear particularly useful for lithologic and stratigraphic discriminations. For comparison, EC Ra values of soils from different localities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg -1 in soils of Syabru-Bensi (Central 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 EC Ra values for these various soils is important for modelling radon exhalation at the ground surface, in particular in the vicinity of 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; a fact that should be seriously taken into account by the governmental and non-governmental agencies as well as building authorities.