VOLCANIC SOIL GAS DIFFUSING AT ZUNIL-CERRO QUEMADO-SANTA MARIA VOLCANIC COMPLEX, GUATEMALA

The Zunil hydrothermal system is located in the Samala River valley in Guatemala, Central America. This hydrothermal system is exploited to generate electrical energy with 24 MW of installed power. Zunil volcano located to the SE of the geothermal field has been proposed as the heat source for this field, even when two more active volcanoes (Santa Maria to SW and Cerro Quemado to the NW) are also close to the system. In order to understand how gases are discharging in the soils of this system as they relate to the movement of fluids, soil gas samples were collected and CO2 efflux was measured along a transect from Zunil to Cerro Quemado to Santa Maria volcanoes, crossing the Samala River Valley. Several faults are located along the river valley striking NW and WNW. Several caldera structures are also present in the area. Our results show that soil temperatures (at 40 cm depth) were higher at the Samala River valley and at the summit and SE slope of Cerro Quemado. Radon and thoron were higher at a zone located at the NW slope of Zunil volcano suggesting the presence of a fault. However, the highest thoron/radon ratios occur close to the interception of NW and WNW faults located in the river valley. He, Ar, He/Ar, and CO2 concentrations were higher at the valley floor than at the volcanic edifices, especially at the points close to the fault. However, high CO2 concentration and slightly higher CO2 efflux values were observed along the SE slope of Cerro Quemado, in a similar trend as the soil temperature. Mean CO2 efflux was 21 g.m-2.day-1 with a maximum of 125 g.m-2.day-1. These values are within the range of other hydrothermal areas in Central America. Relatively higher concentrations of sulfur dioxide, probably derived from hydrogen sulfide oxidation, were found at the NW slope of Santa Maria Volcano, at an older caldera border. These results suggest that soil gas concentrations along the NW slope of Zunil volcano are generally low, except for radon. For the studied transect, the volcanic edifice that has the higher concentrations of gases is Cerro Quemado, except for sulfur dioxide. Faults located at the Samala River valley and related to the regional tectonics and caldera structures seem to channelize the gases in this system.