Gas Emission and Soil Chemical Properties Associated with Underground Coal Fires, Wuda Coalfield, Inner Mongolia, China

Abstract

The coal fire area in the Wuda coalfield is divided into four parts based on the degree of burning and on surface characteristics: sub-area B is characterized by degraded vegetation, sub-area A has spontaneous combustion but no obvious features, sub-area O has vents and cracks, and sub-area C is characterized by red pyro-metamorphic rocks. Emitted gas contents and temperature at the surface were measured and analyzed. In addition, soil samples were collected for analysis of chemical soil properties. The compositions and concentrations of emitted gases vary from one sub-area to another. The nearly the same correlations among gas contents in sub-area A and in sub-area O indicate that their generation mechanism is the same. SO2, H2S, C2H6, C2H4, and C2H2 can only be detected near the cracks and vents in sub-area O. CO and H2 can still be detected in sub-area C, although the CO2 and CH4 contents in sub-area B are higher. Sub-areas A and B exhibit high organic matter contents due to plant nutrient accumulation and deposition of plant litter, whereas organic matter contents decrease at the vents and cracks in sub-area O and in sub-area C because of pyrolysis, leaching, and erosion. Soil near the vents and cracks in sub-area O show strong acidity, and soil pH is correlated with total salt contents of soil, which are composed mainly of Al-, K-, Mg-, or Fe-bearing sulfates. Sub-area A showed weak soil acidity, which was the result of acidification of pore water in soil due to their long-term exposure to CO2 and SO2. Total salt contents in soil in the coal fire area show no significant difference compared to those in the background area except near the vents and cracks in sub-area O. The findings of this study suggest that gas emission and soil chemical properties are reliably related to the underground coal fires.