Diffuse gases in soil of Araro-Simirao geothermal system, Michoacan, Mexico

The Araró-Simirao geothermal system is in the southeast part of the Cuitzeo Lake depression. Physiographically, it is located in the eastern portion of the central part of the TransMexican Volcanic Belt province (TMVB). It is a convective hydrothermal system of dominant liquid, with chlorinated sodium waters and high boron content. The principal gas emitted by the thermal springs is CO2 and, in lower concentrations, H2S, H2 and noble gases (He, Ne, Ar). The aim of this study was to delimit fluid ascent areas in the geothermal system by determining the relationships between diffuse gas emission concentrations in soils and the greatest permeability plausible zones (principal faults). Three sampling campaigns were carried out during 2018 (August and November) and 2019 (May), in which diffuse gas emission measurements (CO2, Rn and mercury vapors) across the soils and soil temperature measurements were carried out. For a sampling campaign, the CO2 flux ranged between 4.38 and 94.61 g m-2 d-1 and the gaseous elemental mercury (GEM) concentration ranged between 0.5 and 365 ng/m3. For the other sampling campaign, the CO2 flux ranged between 0.8 and 1,421 g m-2 d-1 and the GEM values ranged between 0.49 and 2,914ng/m3. In the last sampling campaign the222Rn and 220Rn were also measured, and these values oscillated between 1,060 and 124,100 Bq/m3 and from 0 to 7,511 Bq7m3, respectively. Several anomalous zones of CO2 fluxes, GEM and 222Rn concentrations were found, which match the greatest permeability zones (faults and lineaments). The zone with the highest values in these three parameters is located in the so-called «mud pool», at the crossing of the Araró-Simirao fault with a lineament NW-SE (south zone), which is associated with the upflow zone of the system. Other anomalous zones were also located in the fracture and/or lineaments zones in the central and north parts of the system, which are associated with the outflow zone. According to the data obtained, CO2 could be functioning as a carrier of Hg and 222Rn in the area of vertical rise of gases and, in the outflow zone this gas may be found dissolved in the groundwater.