The Syabru‐Bensi hydrothermal system in central Nepal: 1. Characterization of carbon dioxide and radon fluxes

Abstract

The Syabru-Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30-62°C) water springs and cold (<35°C) carbon dioxide (CO 2 ) degassing areas. From 2007 to 2011, five gas zones (GZ1-GZ5) were studied, with more than 1600 CO 2 and 850 radon flux measurements, with complementary self-potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO 2 and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m À2 d À1 and 38.5 ± 8.0 Bq m À2 s À1 , with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t d À1 and 140 ± 30 MBq d À1 , respectively. Soil-gas radon concentration is 40 × 10 3 Bq m À3 in GZ1-GZ2 and 70 × 10 3 Bq m À3 in GZ3-GZ4. Strong relationships between CO 2 and radon fluxes in all gas zones (correlation coefficient R = 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO 2 . CO 2 carbon isotopic ratios (δ 13 C from À1.7 ± 0.1 to À0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/R A < 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO 2 discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface.

Large-scale organization of carbon dioxide discharge in the Nepal Himalayas

“…Significant radon flux was observed in association with the CO 2 at all sites (Figure and Table ). Radon fluxes >1 Bq m −2 s −1 , similar to fluxes in the SBHS [ Girault et al ., ], were observed at Timure and Chilime, yielding radon discharges of 840 ± 170 and 290 ± 70 Bq s −1 , respectively, significant compared with the discharge in the SBHS (1600 ± 300 Bq s −1 ). In Dolpo, radon discharges were also significant (180 ± 40 and 70 ± 15 Bq s −1 ).…”

“…In Figure , we compare these two parameters for our sites with more familiar instances of mofette sites and a selection of volcanic and hydrothermal sites (reviewed in Girault et al . []). The points of group I (SBHS, Timure, and Chilime) with significant discharge over a comparatively small surface area fall at the lower edge of the trend of volcanic and hydrothermal sites.…”

“…Surface area versus estimated total CO 2 degassing from the ground for our sites together with other selected sites (see review in Girault et al . []). Solid lines are power law best fits: black line for volcanic sites and gray line for fault‐related sites.…”

“…Furthermore, the high carbon isotopic ratios (δ 13 C) of the hot springs, with values reaching +13‰, suggested a metamorphic decarbonation source at depth and a massive CO 2 degassing near the water table, larger than 75% in the Trisuli and Marsyandi Valleys [ Becker et al ., ; Evans et al ., ]. Second, explicit CO 2 emission from the ground was discovered near hot springs of the Trisuli Valley, in the Syabru‐Bensi hydrothermal system (SBHS) [ Perrier et al ., ], where it was found to be associated with a radon‐222 signature and was subsequently mapped in detail [ Girault et al ., ], providing a valuable baseline for long‐term monitoring [ Girault et al ., ]. In this paper, we present the results of systematic search and measurement of gaseous CO 2 release in the vicinity of other significant hot springs from western to eastern Nepal.…”

Long-term temporal variability of the radon-222 exhalation flux from a landform covered by low uranium grade waste rock

“…Soil moisture, porosity, temperature and atmospheric pressure affect transport within the soil pore space (Schery et al, 1984;Griffiths et al 2010;Sakoda et al, 2011;Barbosa et al, 2015) and Tanner (1964) showed that in a porous medium any combination of diffusion and convection will satisfy a simple diffusion model. Continuous, time resolved measurement of 222 Rn flux density and 222 Rn activity concentrations at several depths using active devices, coupled with simultaneous monitoring of other (meteorological) parameters, allow the influence of these parameters or the relative contributions of advection and diffusion to be evaluated (Ferry et al, 2001;Barbosa et al, 2015;Girault et al, 2014). In contrast passive, integrating devices are typically used in uranium exploration (Corner et al, 2009), for population dose assessment studies (Ramachandan and Sathish, 2011) or when longer term averages over large areas of heterogenous substrates are required the decay of 222 Rn in the open atmosphere, the progeny radionuclides may attach to aerosol particles or remain as unattached atoms.…”