Distribution of Methane and Carbon Dioxide in the Soil Gas of an Undermined Area

Abstract

The aim of the study was to determine the content of methane and carbon dioxide in soil gas in undermined areas of the Upper Silesian Basin. Measurements and sampling were carried out directly in the field. The Geotech BIOGAS 5000 analyzer was used to determine methane, carbon dioxide, and oxygen. The 16 measurements were carried out at the Ema heap, and 60 measurements were carried out at the construction site of the "New Moravian-Silesian Scientific Library" in Ostrava. Measurements were carried out at both sites during two periods. The results showed a consistently low concentration of methane in the soil gas, not exceeding 0.3%, which corresponds to 300 ppm. The concentration of CO₂ in the soil gas was highly variable, reaching values up to 12.5% in places. A strong negative correlation between carbon dioxide and oxygen was found, indicating active biochemical processes and gas exchange. Vertical measurements showed that with increasing depth, the CO₂ concentration increases, while the oxygen content decreases, indicating that the diffusion of gases is limited and their sources are in deeper layers. Seasonal changes affect the behavior of gases in warmer periods. More CO₂ is produced due to increased biochemical activity and the decomposition of methane in the presence of oxygen, which releases CO₂ into the atmosphere. According to the study, areas with underground storage function mainly as oxidizing environments, where CO2 buildup poses greater risks to the environment and public safety than methane. In post-mining areas, these findings are crucial for risk assessment, land-use planning, and environmental monitoring.