Negotiations between Enbridge Inc. and the U.S. Environmental Protection Agency have been extended again over a fine that could exceed $100 million for violations under the Clean Water Act in the pipeline operator’s 2010 Kalamazoo River disaster.

The spill of highly toxic tar sands oil fouled a 40-mile stretch of the river in Michigan. It was the biggest inland oil spill in U.S. history and resulted in a massive cleanup that kept the river closed for nearly two years. The cleanup has cost the company more than $1.2 billion. In addition, Enbridge has already been assessed almost $83 million in penalties by other state and federal authorities.

Enbridge, an energy company based in Calgary, disclosed the extension of talks with the EPA in its annual report filed with the U.S. Securities and Exchange Commission. The deadline for reaching a settlement had been extended once before. Management cited the intricate nature of the talks.

“Given the complexity of settlement negotiations, which we expect will continue, and the limited information available to assess the matter, we are unable to reasonably estimate the final penalty which might be incurred or to reasonably estimate a range of outcomes at this time,” the company said in the filing.

Since the spill, Enbridge’s estimates of the penalty it faces from the EPA have mounted. In 2013, the company said it expected a $22 million fine. Last year it nearly doubled the number, and this year didn’t predict the amount.

The EPA said it has agreed to extend the deadline though the middle of June. Last year the EPA agreed to extend the five-year statute of limitations for imposition of fines by six months until January.

Representatives of the EPA and Enbridge declined to discuss the ongoing negotiations.

More than 1 million gallons of diluted bitumen, tar sands oil from Canada, spilled into the Kalamazoo River near Marshall, Mich., after Enbridge pipeline 6B ruptured on July 25, 2010. Because of the nature of the oil, it quickly sank to the river bottom and created a cleanup nightmare.  

Enbridge faces the highest penalty imposed for a Clean Water Act violation involving oil pipeline spills in the last 15 years, according to an InsideClimate News review of EPA data.

The Clean Water Act, the principal federal law governing water pollution, allows for a fine of as much as $4,300 for each barrel of oil spilled. At one point the EPA estimated the spill at 27,339 barrels, which would put the maximum fine at more than $117 million.

Andy Levine, a former EPA lawyer now in private practice in Philadelphia, said the new delay signals the multifaceted nature of the Enbridge case. 

“When you have a spill of this magnitude, there are numerous aspects that have to be considered,” he said. “The government will want to make sure it has done its due diligence before reaching a settlement. And that takes time.”

Factors that must be considered include immediate harm, including how the oily contamination affected wildlife, the threat to human health and the degradation to the marine environment, Levine said.

“What the government must also factor in is what is down the line: What long-term residual impacts may be because of the contamination,” Levine said. “What the government doesn’t want to do is resolve the matter too quickly and not account for future impacts.”

Carl Weimer, executive director of the Pipeline Safety Trust, a nonprofit watchdog organization based in Bellingham, Wash., said there could be two dramatically different interpretations of the delay.

One is that the government is meticulously piecing together the facts so that Enbridge will feel compelled to settle without being dragged into court. The other might be that negotiations are failing and the matter is heading to court.

“I’m hoping that it’s getting down to the brass tacks of identifying how many violations there are and how high the fine is,” Weimer said.

In addition to the clean water penalty, Enbridge disclosed in its filing that it will most likely face additional costs by being required to implement new spill prevention, leak detection and emergency response measures.