Diffuse gases in soil of Araró-Simirao geothermal system, Michoacán, Mexico

Abstract

The Araró-Simirao geothermal system is located in the southeastern part of the Cuitzeo Lake depression, in the central part of the TransMexican Volcanic Belt (TMVB) province. It is a convective hydrothermal system dominated by sodium chloride water and high boron content. The thermal springs mainly release CO2 and lower concentrations of H2S, H2, and noble gases (He, Ne, and Ar). The aim of this study was to delimit the upflow areas in this geothermal system by determining the relationships between diffuse gas emission concentrations in soils and the zones of the greatest plausible permeability. Three sampling campaigns were carried out in 2018 (August and November) and 2019 (May). In these campaigns, diffuse gas emission in soils (CO2, Rn, and Hg vapors) and soil temperature were measured. The diffuse gas emission measurements ranged from 0.8 to 1,421 g m-2d-1 for CO2 flux, from 0.49 to 2,914 ng/m3 for gaseous elemental mercury (GEM), from 1,060 to 124,100 Bq/m3 for 222Rn, and from 0 to 7,511 Bq/m3 for 220Rn. The highest values were obtained in the dry season (May). Several anomalous zones of CO2 fluxes, GEM, and 222Rn were found to match faults and lineaments, interpreted as the greatest permeability zones. The zone with the highest values of these three parameters was located in the so-called mud pool at the crossing of the Araró-Simirao fault with a NW-SE (south zone) lineament, which is associated with the upflow zone of the system. Other anomalous zones were located in fracture and/or lineament zones in the central and northern parts of the system, which are associated with the outflow zone. According to the data obtained herein, CO2 could work as a carrier of Hg and 222Rn in the upflow zone; and this gas may be dissolved in the groundwater in the outflow zone.