Dispersion of geogenic CO2 in the lower atmosphere: Statistical analysis and application to the Syabru-Bensi Hydrothermal System in the Nepal Himalaya

Highly concentrated geogenic CO₂ emissions have been reported worldwide. Although atmospheric CO₂ dispersion is the most common occurrence, specific topographic and meteorological conditions can lead to surface accumulation in the form of “CO₂ rivers”. Although catastrophic events such as the deadly limnic eruption of Lake Nyos in 1986 are well documented, the behavior of these CO₂ rivers is not well understood. This lack of understanding poses challenges for hazard assessment and mitigation. While computational models such as computational fluid dynamics (CFD) and integral models provide analytical insights, their practical application in risk management is limited by computational cost and accuracy constraints. To address these limitations, we simulate the behavior of CO₂ rivers using TWODEE, a depth-averaged numerical model that is a computationally efficient alternative for simulating dense flows. We test the model at the Syabru-Bensi Hydrothermal System (SBHS) in central Nepal, where high seismic activity and significant CO₂ degassing have been observed. In the field, we measure the airborne CO₂ concentration, wind velocity and direction using autonomous sensors at 0, 50, 150, and 300 cm above the ground at each measurement point, as well as surface CO₂ flux using the accumulation chamber method. Our results demonstrate the robustness of the statistical approach by providing well-constrained maps of CO₂ concentration in the lowest atmospheric layers over large distances from the emission source. This method can be applied to other non-volcanic and volcanic sites. Additionally, we assess the impact of the 2015 M_w 7.9 Gorkha earthquake in Nepal, which triggered additional CO₂ degassing vents and changes in surface CO₂ flux across the SBHS. Our work aims to improve our understanding of how dense gases disperse in the lower atmospheric layers. We are developing an operational hazard assessment tool with potential applications in real-time risk management. This tool will quantify the CO₂ budget of CO₂ rivers in various geodynamic contexts and estimate health hazards in volcanic and non-volcanic environments.