Radon and carbon dioxide around remote Himalayan thermal springs

Abstract

Abstract Radon-222 and carbon dioxide (CO 2 ) 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 CO 2 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 CO 2 fluxes (18–32 g m −2 day −1 ), and integrated radon and CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 charged in radon along a fault network. Radon can thus give precious information on the gas transport properties of the shallow continental crust.