High carbon dioxide flux associated with radon-222 gas exhalation and dipolar self-potential anomaly at the Syabru-Bensi hot springs in central Nepal

Abstract

Gas discharges have been identified at the Syabru-Bensi hot springs, located at the Main Central Thrust zone in Central Nepal and characterized by a water temperature reaching 61°C, high salinity and high alkalinity. The gas is mainly dry carbon dioxide, marked by a ?¹³C isotopic anomaly of -0.8‰. The diffuse carbon dioxide exhalation flux, mapped by the accumulation chamber method, reaches 19 000 g×m^-2×day^-1, comparable with values measured on active volcanoes. Radon exhalation flux at the soil surface has been measured at more than sixty points in the vicinity of the main gas discharge. Extreme values, larger than 2 Bq×m^-2×s^-1, similar to peak values measured in volcanic areas or above uranium waste piles, are observed in association with the larger values of the carbon dioxide exhalation flux. This high radon exhalation thus results from emanation at depth, producing a radon concentration in the pore space varying from 25 000 to more than 50 000 Bq×m^-3, transported to the surface by the flow of carbon dioxide. The high radon-222 content of the carbon dioxide offers an interesting tracing method and an additional practical tool for long term monitoring, for example to study transient changes preceding large earthquakes. An extended dipolar self-potential anomaly has also been found, with a negative pole reaching -180 mV at the main gas discharge, and a wide positive lobe on the terrace above. This dipolar anomaly, the largest reported so far, is interpreted in a hydroelectrical numerical model assuming a primary upward fluid flow associated with the gas, coupled with a secondary flow towards the springs, taking into account the resistivity structure obtained from profiles of electrical resistivity tomography. Thus, the Syabru-Bensi hot springs provide a unique opportunity to study the generation of electrical currents associated with biphasic fluid flow in a geodynamically active area. A pilot multidisciplinary team has now undertaken a multidisciplinary study of the geological, geophysical and geochemical properties of the Syabru-Bensi geothermal system. Studying the spatial and temporal variations of the gas discharges and the associated properties of the hot springs may lead to important clues on the presence and displacements of crustal fluids in relation with the nucleation of large earthquakes in the Nepal Himalayas. Journal of Nepal Geological Society, 2007, Vol. 36 (Sp. Issue) p.15