The Syabru-Bensi hydrothermal system in central Nepal: 2. Modeling and significance of the radon signature

The Syabru-Bensi hydrothermal system (SBHS), located in the Nepal Himalayas, is characterized by numerous hot (>30 degrees C) springs and the release of dry, cold (<35 degrees C) CO2 associated with radon-222, detailed in the companion paper. In the SBHS, CO2 and radon fluxes on the ground vary over 5-6 orders of magnitude, reaching exceptional mean values of 100 kg m(-2) d(-1) and 12 Bq m(-2) s(-1), respectively. This paper extends the companion paper by developing three quantitative models for the radon signature of CO2 based on measurements of radon and radium concentrations in the spring waters and effective radium concentration of rocks and soils. The first model considers near-surface radon and CO2 degassing from water, considered unlikely unless there exist currently unidentified large discharges of hydrothermal water. The second model considers CO2, arising from deeper hydrothermal sources, incorporating radon from shallow radium sources as it percolates upward toward the surface, considered more likely as a percolation depth of 100 m is sufficient to account for the observed radon discharge. The third model considers the observed peak radon concentrations in the gas zones and assumes that gaseous CO2 can be transported from kilometer-scale depths through a fault network connected to the zones. This latter model affords the possibility that variations of physical parameters at depths associated with earthquake nucleation might be detectable at the surface. Gas-dominated transport might operate in other locations in Himalayas and elsewhere and may be an important aspect of the coupled mechanisms associated with seismically active orogens.