The Queensland government says Linc “wilfully and unlawfully” caused serious environmental harm during trials of its UGC operations.

The latest charge is the fifth for Linc, with four other similar charges filed in April last year.

What has been described as the largest and most protracted investigation in the history of the Queensland Environment Department concluded that Linc’s UCG operations at Hopeland caused irreversible damage, potentially to the atmosphere, vegetation, water and soil.

Queensland Environmental Regulator director-general Jon Black said that after the detailed investigation the Department of Environment and Heritage Protection alleges Linc operated its trial UCG plant outside its environmental authority, causing gas to escape off the site.

Work by the department concluded that the Chinchilla plant operated above hydrostatic pressure, fracturing the landform, and allowing contamination to escape.

“These fugitive gases polluted a widespread area by following underground pathways, between two and six metres underground,” Black said.

“The polluting gases included carbon monoxide, hydrogen and hydrogen sulphide.”

“There is no immediate harm caused to agriculture, including grazing or cropping,” Black said, as extensive testing and monitoring has confirmed that the regional air quality and the drinking, stock and underground water supplies remain safe.

Linc’s UCG process involved igniting coal seams under the ground and extracting the gas, a process used for decades internationally.

Linc was attempting to refine the technology and convert the syngas into liquids.

Other UCG hopefuls caught up in a 2010 contamination scare use different methods, such as oxygen injection that does not burn the coal seams.

Black said that during the course of the investigation, the 100-strong investigation taskforce also conducted farm environmental assessments on a number of properties in the area that were considered to have potentially been impacted by the gas contaminants.

“The assessments were designed to test properties for potential risks to residents and impacts to crops or grazing. Government officers tested bores, surface soils, tank water and dam water at each property.

“At the conclusion of the assessments, each landholder was provided with a comprehensive scientific report specific to their property.”

Initial advice was that as the gases were found below the root zone in the soil, immediate impacts to grazing animals and stock are also considered very unlikely, however landholders have been told not to excavate below two metres without having their soil assessed.

Black said EHP remained committed to continuing scientific investigations to determine an appropriate environmental management strategy for the gas contamination and associated issues to ensure necessary rehabilitation and removal of the excavation exclusion zone.

“EHP understands that this has not been an easy period for the Hopeland community and I express EHP’s gratitude for the community’s cooperation, patience and understanding as our investigations proceeded.”

In March, the ABC revealed that the department's investigation backed claims by former workers at Linc's UCG plant that they have suffered ill health as a result of a series of "uncontrolled releases" of gas at the site between 2007 and 2013.

The symptoms were described as consistent with exposure to the known chemical constituents of syngas.

Linc says it is confident that good science and appropriate legal process will result in its vindication and validation of the underground coal technology.

“In particular, it is Linc Energy’s contention that the department has not properly considered alternative processes (including radiolysis of water and natural gas, biological sources and the CSG industry),’’ the company said in a statement.

Cleaning up the Chinchilla site would present enormous challenges, with no known precedents for remediation of gas contamination from UCG operations.