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Pollution Characteristics of Toxic and Harmful Elements and Its Environmental Impact in Water Co-Produced from Coalbed Methane Wells in the CBM Development Block in Western Guizhou

Li Qingguang;Wu Pan;Gu Shangyi;Liu Pei;Hu Haiyang;Gao Wei;Gong Chaobing;Chang Xixi;Key Lab of Karst Environment and Geohazard of Ministry of Land and Resources,College of Resource and Environmental Engineering,Guizhou University;Guizhou Research Center of Shale Gas and CBM Engineering Technology;Guizhou Bureau of Coal Geology;  
Coal seams are often enriched in various toxic and harmful elements such as F, As, Cd, Cr, Ni, Pb, Hg and Sb, so the ecosystem might be faced with serious environmental threats when the co-produced water is brought to the surface in the production of coalbed methane(CBM). In this paper, we present a systematic water chemistry study of co-produced water from Songhe coalbed methane development block in western Guizhou Province. The results show that, compared to Class Ⅲ of Groundwater Quality Standard(GB/T 14848-2017), Ba and Hg in co-produced water in the study area exceeded the standard seriously, and the average concentrations were 10 621.6 μg/L and 16.5 μg/L, respectively. The Ba anomaly in co-produced water was attributed to the dissolution of Ba-bearing carbonates and the depletion of sulfate induced by sulfate reduction, and Hg was from sulfide minerals. The sources for Cr, Mo, Cd and Sb were sulfides and carbonates in coal seams. The high content of Mo is associated with ion exchange reactions under high salinity, and molybdate and metallic sulfides are the main forms for Mo. Under weak alkaline conditions, the hydroxide adsorption state of Al is the main occurrence form of Cr, Mo, Mn, Cd, Sb, Cd, Cu, Pb and Ba in the co-produced water. The sources of Ni were obviously different from those of other elements, and it most likely came from the mineralization of coal organic matters. The results of water quality evaluation showed that the pollution of the water coproduced from coalbed methane wells in the study area is very serious, which is mainly due to the serious overstandard of Hg and Ba. In SH9, mercury is 27 times the legal limits and that is 23 times for barium in SH8.
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