Hydrogeological characterization and extended seasonal hydrogeochemical monitoring of a CO₂ controlled-release experimental site: the case of TECNOPUC-Viamão, Rio Grande do Sul state, Southern Brazil

Abstract

Abstract Reduced-scale CO2 release experiments in shallow aquifers serve as crucial monitoring strategies for detecting unintended CO2 leakage into potable aquifers within Carbon Capture and Storage (CCS) projects. Understanding site-specific geological, hydrogeological, and climatic features is essential. However, accurately tracing changes in groundwater quality due to this process and using hydrochemical parameters for CO2 leakage diagnosis require establishing a solid, seasonally relevant baseline to avoid misinterpretation. This study focuses on detailing the geological, hydrogeological, and geophysical characteristics of the TECNOPUC-Viamão CO2 controlled-release experimental site in Rio Grande do Sul, Brazil. It presents a comprehensive three-year investigation into the seasonal natural background hydrochemistry. Field characterization involved recognition boreholes, in-situ infiltration and Slug Tests, topographic surveys, DC resistivity measurements, and groundwater sampling campaigns for physicochemical, major, minor, trace elements, and δ¹³C-DIC evaluation. Results indicate the area comprises a granite-derived multilayer phreatic aquifer with two distinct hydrostratigraphic units (St and Aa). These units differ in lithological composition, hydraulic conductivities (St: 10− 4 m/s, Aa: 10− 8 m/s), apparent resistivities, and physicochemical and hydrochemical compositions. The St unit shows slightly neutral pH, higher temperature, EC, ORP, DO, Ca, Mg, K, Fe, Mn, Sr, B, HCO3−, and DIC concentrations, with δ¹³C-DIC between − 3 to -8‰. Conversely, the Aa unit displays slightly acidic pH, lower temperature, EC, ORP, higher DO fluctuation, Na, SO42−, Cl−, NO3−, Zn, Al, Ni concentrations, lower HCO3−, DIC levels, with δ¹³C-DIC ranging between − 6 to -11‰. Additionally, seasonal monitoring campaign revealed that there is a clear temperature-related influence on Ca, Mg, K, Na, NO₃⁻, and trace elements (Fe, Ba, Sr, Mn, Al, B, Ni, and Zn) behavior, which could impact further interpretation of the results during the upcoming CO2 injection phase, and that the Control Charts can confidently serve as a valuable tool in understanding the inherent natural hydrochemical trends.