The wikepedia article also clearly states:
However, as of late 2013, CCS had never been successfully implemented on a commercial scale due to a number of environmental and health problems caused at test sites. In Australia in 2014 the Government filed charges over alleged serious environmental harm stemming from Linc Energy's pilot Underground Coal Gasification plant near Chinchilla in the Queensland’s foodbowl of the Darling Downs.[28]
Meanwhile, as an article in the Bulletin of Atomic Sciences pointed out in March 2010, UCG could result in massive carbon emissions. “If an additional 4 trillion tonnes [of coal] were extracted without the use of carbon capture or other mitigation technologies atmospheric carbon-dioxide levels could quadruple,” the article says, “resulting in a global mean temperature increase of between 5 and 10 degrees Celsius.”[29][30]
Aquifer contamination is a potential environmental concern.[4][31] Organic and often toxic materials (such as phenol) could remain in the underground chamber after gasification if the chamber is not decommissioned. Site decommissioning and rehabilitation are standard requirements in resources development approvals whether that be UCG, oil and gas, or mining, and decommissioning of UCG chambers is relatively straightforward. Phenol leachate is the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. The US Dept of Energy's Lawrence Livermore Institute conducted an early UCG experiment at very shallow depth and without hydrostatic pressure at Hoe Creek, Wyoming. They did not decommission that site and testing showed contaminants (including the carcinogen benzene) in the chamber. The chamber was later flushed and the site successfully rehabilitated. Some research has shown that the persistence of minor quantities of these contaminants in groundwater is short-lived and that ground water recovers within two years.[26] Even so, proper practice, supported by regulatory requirements, should be to flush and decommission each chamber and to rehabilitate UCG sites.