Probabilistic hazard analysis of the gas emission of Mefite d'Ansanto, southern Italy

The emission of gas species dangerous to human health and life is a widespread source of hazard in various natural contexts. These mainly include volcanic areas but also non-volcanic geological contexts. A notable example of the latter occurrence is the Mefite d'Ansanto area in the southern Apennines in Italy. Here, large emissions of carbon dioxide (CO2) occur at rates that make this the largest non-volcanic CO2 gas emissions area in Italy and probably on Earth. Given the topography of the area, in certain meteorological conditions a cold-gas stream forms in the valleys surrounding the emission zone, which has proved to be potentially lethal to humans and animals in the past. In this study, we present a gas hazard modelling study that considers the main species, CO2, and the potential effect of another notable species, hydrogen sulfide (H2S). For these purposes, we used VolcanIc Gas dIspersion modeLling v1.3.7 (VIGIL), a tool that manages the workflow of gas dispersion simulations in both the dense- and dilute-gas regimes and is specifically optimised for probabilistic hazard applications. In its latest version, VIGIL can automatically detect the most appropriate regime to simulate based on the gas emission properties and meteorological conditions at the source. Results are discussed and presented in the form of maps of CO2 and H2S concentration and persistence at various exceedance probabilities, which consider the gas emission rates and their possible ranges of variation defined in previous studies. The effect of seasonal variations is also presented and discussed.