On the transport of CO2 and Radon in the geothermal area of Acoculco, Puebla, Mexico

Abstract

We present a conceptual and numerical description of CO${}_2$ and Radon (²²²Rn) transport in the geothermal system of Acoculco, Mexico. We build upon extensive CO${}_2$ flux measurements that have been conducted in the past and new CO${}_2$ flux and Rn concentration data collected during this investigation. The accumulation chamber method was employed for CO${}_2$ flux measurements, whereas Rn concentration derives from analysis of gas samples collected in active scintillation cells. Additionally, the structural setting in the study area is reinterpreted based on a geomorphological assessment. As a result, a 3D conceptual model of the CO${}_2$ and Rn is proposed and numerically modeled with the aid of TOUGHREACT V4. Our collected data confirm the existence of advective transport of CO${}_2$ and Rn within two sites known as Azufres and Alcaparrosa, with soil CO${}_2$ fluxes that can reach 30,000 g m⁻²day⁻¹, and Rn concentrations as high as $\sim$12,000 Bq/m${}^3$. In view of the relatively short half-life of ²²²Rn ($\sim$3.8 days), the measured concentrations are indicative of localized permeable paths that connect with deep structures. Our numerical model represents adequately the non-isothermal diffusive transport of CO${}_2$, while advective fluxes are not attained under simple darcian flow. Likewise, under a hypothesis of a deep Rn source ($\sim$1200 m), with an initial molar ratio Rn/CO${}_2$ of 1$\times 10^{-4}$, Rn concentrations drop rapidly (within $\sim$100 m from the source). Non-porous flow (channeling) and distributed sources of Rn arise as likely hypotheses to explain cold degassing in Acoculco.