In late 2009, federal inspectors collected samples of coal tar sludge from in and around two decaying storage tanks at Tonawanda Coke.

An expert testified Thursday that most of the samples – 20 of 26 – contained hazardous levels of benzene, a chemical known to cause cancer in humans.

Philip Flax, an official at the U.S. Environmental Protection Agency, also told a federal court jury that Tonawanda Coke’s practice of mixing its sludge with outdoor piles of coal violates a federal law governing the handling of hazardous waste.

“Facilities are required to minimize releases into the environment," he told the jury.

Flax, who both sides acknowledged is an expert in his field, was the latest witness to testify about Tonawanda Coke’s hazardous waste management, one aspect of the government’s criminal prosecution of the Town of Tonawanda manufacturer.

The company is facing 19 separate charges of polluting the air and ground in and around its River Road plant, and then trying to hide it from federal officials.

For the first time in the two-week-old trial, the chemical benzene became a focal point of the testimony being heard by the jury and Chief U.S, District Judge William M. Skretny.

Benzene, which has been linked to Tonawanda Coke’s operations in the past, is commonly used by chemical, oil and gas industries and is known to cause cancers, especially leukemia, in people exposed to the substance.

The benzene discovered in the coal tar at Tonawanda Coke was found, in some cases, at levels far above the regulatory standard for what is considered non-hazardous levels.

Flax, under questioning by prosecutor Rocky Piaggione, acknowledged the higher-than-acceptable levels in his testimony and was critical of the company’s handling of sludge containing the dangerous ingredient.

He was particularly critical of Tonawanda Coke’s practice of mixing sludge into coal piles at the plant, in part because there was no “impermeable surface” to prevent contaminants from making their way into the ground.

“It’s that critical time period when the problem occurred," he said of the company’s practice of mixing sludge with coal.

The company, in contrast, has argued that the sludge was handled responsibly and points to the coal and “40 feet of impermeable clay” that lie beneath the coal piles.

“At no time does the sludge touch the ground,” the company said in a 2009 letter to the EPA.

As they have throughout the trial, defense lawyers argued that Tonawanda Coke’s practice of mixing sludge and coal dates back years and, until recently, has been allowed by state inspectors.

“Did you know the facility had been mixing coal tar into the coal fields for decades?" Gregory F. Linsin, a lawyer for the company, asked Flax at one point Thursday.

Linsin also suggested that Tonawanda Coke’s strategy of recycling its sludge should be considered preferable to sending it off site for treatment and disposal.

“Isn’t it fair and reasonable ... to look at the intent of the people conducting the recycling?" he asked Flax. “Don’t you have to look at the big picture?”

Flax said the EPA encourages recycling but only when it’s done properly and within the guidelines established by the agency.