Mining company Linc Energy is alleged to have exposed workers to dangerous gases at an experimental plant in Queensland.

According to an internal government briefing document seen by the ABC, an investigation has backed claims by workers that they have suffered ill health as a result of a series of "uncontrolled releases" of gas at the plant between 2007 and 2013.

A medical expert commissioned by the department reviewed workers' statements and medical records and found that "the symptoms presented are consistent with exposure to the known chemical constituents of syngas".

Syngas, or synthesis gas, was produced at the plant near Chinchilla west of Brisbane, by burning subterranean coal seams in a controversial process called Underground Coal Gasification (UCG).

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The gas contains carbon monoxide, benzene, toluene and other substances that are potentially toxic or carcinogenic.

Sorry, this video has expired Linc Energy workers were allegedly exposed to dangerous gases

The plant was a demonstration facility that operated from 2007 and Linc began decommissioning in 2013.

The ABC can reveal that the environment department's probe found "irreversible damage" to the environment at the site.

Linc Energy is already facing criminal charges over allegations of serious environmental harm at Chinchilla.

The environment department filed four such charges against the company in April 2014. The company described the charges at the time as "misguided".

The briefing document reveals that departmental investigators raided Linc's head office in Brisbane and its Chinchilla premises under court-issued search warrants in October 2013, seizing documents and electronic files.

More than 90 former Linc employees and consultants employed by Linc were subsequently interviewed, according to the document.

The internal document states that the raids were triggered after the environment department received "information from a number of confidential informants" about incidents at the Chinchilla site dating back to 2007.

The subsequent investigation, the "largest and most protracted" in the department's history, has substantiated the informants' claims, it says.

"All of the evidence obtained through warrants and witness statements has confirmed the information from the original informants," it states.

Department has not ruled out underground fire in coal seam

The briefing document describes continuing intensive monitoring of the site by the environment department, with officials in the last few weeks finding unexpectedly high levels of toxic gas and hydrocarbon in the soil on adjacent properties.

Last month reports of such findings caused speculation about an underground fire in the coal seam, something the environment department has not ruled out.

According to the briefing document, informants alleged that rock fractures had allowed the release of the gas into the surrounding air and also caused "the contamination of groundwater by syngas and syngas by-products such as benzene [which is a known human carcinogen]".

What is UCG? Underground coal gasification is a so-called "unconventional" means of extracting gas from coal seams that are too deep to mine. Coal is burned in situ underground and the gas produced is siphoned off through wells.

Underground coal gasification is a so-called "unconventional" means of extracting gas from coal seams that are too deep to mine. Coal is burned in situ underground and the gas produced is siphoned off through wells. It differs significantly from CSG production, which involves drawing enough water to reduce the pressure underground to release the naturally occurring methane in the coal seams.

It differs significantly from CSG production, which involves drawing enough water to reduce the pressure underground to release the naturally occurring methane in the coal seams. UCG technology was first proposed in the 19th century but was only adopted seriously after World War II in Russia and former Soviet client states.

UCG technology was first proposed in the 19th century but was only adopted seriously after World War II in Russia and former Soviet client states. Benefits of UCG include the fact that landscapes remain relatively unscathed compared with conventional mining, emissions are low and the gas produced can be used for power generation or condensed to make liquid fuels such as diesel.

Benefits of UCG include the fact that landscapes remain relatively unscathed compared with conventional mining, emissions are low and the gas produced can be used for power generation or condensed to make liquid fuels such as diesel. Disadvantages of UCG are that it is sensitive to local geological conditions, expensive to set up, and the technology is highly specialised and poorly understood by regulators.

It says an external firm commissioned by the department found that "serious environmental harm is apparent, with irreversible damage to more than one environmental receptor [which includes the atmosphere, vegetation, water and soil] in receiving environment evident".

The document describes the department finding "scientific evidence of operation above hydrostatic pressure, fracturing of the landform, excursion of contaminants from gasifier cavities and contaminants at locations throughout the site which exceed licensed trigger values".

Test drilling on the Linc site found "extremely high levels of hydrocarbon in the soil ... and gas associated with the UCG process at shallow depths".

In addition, it says the discovery of carbon monoxide at "unexpected" locations on an adjacent property in the last few weeks suggests that "gas contaminants may be or have been migrating from the Linc site to adjacent landholders".

Local property owners have told the ABC they have received little information from the Government or the company.

Notices have been issued warning landholders not to excavate deeper than two metres because of gas emissions.

"We'd like to know what's going on, because we want to find out where it's coming from and if they can fix it," farmer George Bender told the ABC.

"A fortnight ago, representatives of the department turned up with a letter, gave it to us and said 'we're investigating'.

"They'd drilled holes on a neighbouring farm and found carbon monoxide, hydrogen sulphide and hydrogen.

"It said you can only drill down, dig a hole, two metres. Any more than that you've got to notify them and they'll come down and assess to see if there's any danger.

George Bender says farmers would like to know what is going on. ( ABC )

"None of this should've been allowed in the first place."

Another landholder, Shay Dougall, said locals were upset and questioning their futures in the district.

"People are very worried about all of that — their crops, their animals, their drinking water, their air, their own health, the health of their children, the future of their properties, the security of their properties as to what does this mean," she said.

Linc Energy said it was unaware of the medical report commissioned by the environment department.

In a statement, the company said there had been complaints by staff about ill health as a result of exposure to contaminants, without providing details.

However, it added: "Since 2008, when Linc Energy installed its current workplace, health and safety monitoring program, Linc Energy has not received any complaints from workers about ill health related to exposure to contaminates."

It said there was "no apparent need" to communicate with former staff because it was unaware of the content of the Government's report.

The company said it took its responsibilities seriously and would contact former staff if necessary.

Asked about the carbon monoxide findings on adjacent properties, the company said:

"As the department has not communicated the specific results with us, we have been unable to ascertain what gases were detected where and in what concentrations. Therefore, we cannot make any conclusions about the results."

Linc said its own surveys of air quality since 2009 "confirm compliance with workplace air standards".

Safe operation requires gases produced are contained underground

In the UCG process, a deep coal seam is ignited and burned with the assistance of injected oxygen.

By adding steam, a gas is produced that can be used in power generation or condensed to produce liquid fuels.

Variants of the technology have been in use in former Soviet states and South Africa for several decades.

The safe operation of the technology requires that the gases produced are contained underground.

In 2013, an independent scientific panel on UCG for the State Government questioned Linc Energy's selection of the Chinchilla site because of the unsuitability of the local geology.

It recommended no further UCG activity in the state until it could be shown that such plants could be safely decommissioned.

A UCG plant at Kingaroy, north-west of Brisbane, operated by Cougar Energy, was shut down in 2010 after contaminants were found in groundwater.

Linc Energy ceased operations at the plant in 2013 after spending $200 million developing the project, accusing the former LNP Queensland government of imposing a regulatory regime that unfairly favoured coal seam gas extraction over UCG.

The company has been one of the largest financial donors to the LNP, giving $100,000 to the party in the 2012-13 financial year.

Linc founder Peter Bond is one of Australia's richest men with a fortune estimated at $280 million. ( AAP: Andrew Taylor, file photo )

It gave the same amount to the Australian Liberal Party the following year.

It has also given smaller amounts to the Queensland and federal branches of the Australian Labor Party.

Linc Energy was listed on the Australian Stock Exchange until November 2013, when it shifted its listing to Singapore.

The $700 million company has oil, gas and coal gasification interests in the US and Poland, as well as conventional coal mines in Queensland.

Linc Energy holds exploration licences over 24 million acres in South Australia, where it is searching for shale oil.

The company also owns what it says is the world's only commercial UCG plant, the Yerostigaz facility in Uzbekistan, which has operated since 1961.

Its founder and largest shareholder, Peter Bond, is one of Australia's richest men, with a fortune estimated at $280m by Business Review Weekly in 2014, down from $450m the previous year.

The ABC has sought comment from the environment department.

Below is a letter delivered to landholders in the affected area.

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