Real-time monitoring of carbon dioxide emissions from a shallow carbon dioxide release experiment

This study was conducted to analyze CO₂ migration from a shallow CO₂ release experiment using a continuous soil CO₂ flux measurement system. Approximately 1.8 t CO₂ was injected from 1 to 30 June 2016 through the point sources with perforated release wells laid at 2.5-m soil depth. Using LI-8100A instruments, CO₂ concentration, CO₂ flux, soil temperature, soil moisture, relative humidity, and atmospheric pressure were continuously measured every 30 min at 0, 1.5, 3.0, 4.5, and 6.0 m from the well from 29 May to 4 August 2016. Typically sensors for soil temperature and moisture were installed at 5-cm depth, and CO₂ concentration, relative humidity, and atmospheric pressure were measured at the chambers. The CO₂ flux was not maximum directly above the release well. Carbon dioxide flux at 6.0 m from the well was similar to the background level. The relationship between CO₂ flux and environmental factors, described using a temporal correlation analysis, indicated that CO₂ flux was primarily driven by soil temperature and had the inverse correlation with relative humidity and atmospheric pressure. Heavy rainfall inhibited in-soil CO₂ migration by filling the soil pore with water. The anomalously high CO₂ flux detected at 1.5 m from the well may have been caused by the associated permeability structure, in which a permeability discrepancy leads to the vertical or horizontal flow of in-soil CO₂. These findings from this shallow CO₂ release experiment should be considered as basic information to characterize and model the in-soil CO₂ transport related to CO₂ leakage.