Estimating annual CO₂ emissions and building a leakage scenario in a tectonically quiescent area

Soil CO₂ flux (FCO₂) was intensively measured in a natural CO₂ emission site located in a tectonically quiescent area, with topographic and geophysical investigation, to estimate annual emissions and causes of focused leaks. FCO₂ varied between 6.5 and 2155.4 g/m²/d (median 76.1 g/m²/d; n = 67) within a 612 m² area situated close to a CO₂-rich groundwater well. The threshold value for geogenic FCO₂ was determined to be 23 g/m²/d using a Q-Q plot. A FCO₂ hotspot was delineated where FCO₂ exceeded the threshold. Based on linear regression using air pressure and temperature, the annual CO₂ emissions from the hotspot in 2019 were approximately 19 tons/y. According to the 2D electrical resistivity distribution, the subsurface around the well was highly weathered, showing low electrical resistivity (<2000 Ω-m) down to a depth of 50 m, whereas the top layer around the hotspot showed a relatively high electrical resistivity (> 500 Ω-m), implying a resistant unsaturated zone. High signal attenuation and discontinuous and dipping reflectors of ground-penetrating radar signals suggested gaseous CO₂ transport through preferential pathways in the resistant near-surface. Based on these and previous hydrochemical results, the extremely high FCO₂ in the narrow extent can be explained by a leakage scenario in which the frequent usage of CO₂-rich water through the well enhances CO₂ degassing, while defects in the well casing accelerate the upward movement of CO₂ gas, which flows out through a preferential pathway in the resistant unsaturated zone. This study provides insights into focused leakages through faulty wells and near-surface preferential pathways. In addition, the quantification of annual CO₂ emissions through point sources is applicable for assessing natural gas emissions produced by hotspots. The non-negligible emission rate in the hotspot (about 92 g/m²/d) emphasizes the need to quantify natural CO₂ emissions in tectonically quiescent areas for risk assessments and carbon budgets.