Radon and carbon dioxide around remote Himalayan thermal springs

Radon-222 and carbon dioxide (CO₂) emissions were studied around four remote Nepalese thermal springs near the Main Central Thrust: Timure and Chilime in the upper Trisuli Valley, central Nepal; and Sulighad and Tarakot in Lower Dolpo, western Nepal. A total of 279 radon fluxes and 670 CO₂ fluxes were measured on the ground, complemented by radon concentration measurements in soil and water, and assisted by thermal infrared imaging. In Lower Dolpo, mean radon fluxes ranging from 270×10^-3 to 450×10^-3 Bq m^-2 s^-1, radon concentration in water greater than 100 Bq l^-1, low mean CO₂ fluxes (18–32 g m^-2 day^-1), and integrated radon and CO₂ discharges of 70–180 Bq s^-1 and (2.3–3.8)×10^-3 mol s^-1, respectively, suggest shallow-water-dominated transport with simultaneous radon and CO₂ degassing from the hydrothermal water. In the upper Trisuli Valley, mean radon fluxes ranging from 140×10^-3 to 570×10^-3 Bq m^-2 s^-1, larger mean CO₂ fluxes that range from 430 to 2930 g m^-2 day^-1, radon concentration in water of less than 6 Bq l^-1, and integrated radon and CO₂ discharges of 290–840 Bq s^-1 and (390–830)×10^-3 mol s^-1, respectively, indicate fast gas-dominated transport of deep metamorphic-origin CO₂ charged in radon along a fault network. Radon can thus give precious information on the gas transport properties of the shallow continental crust.