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LITTLETON, N.H. - A New Hampshire jury on Tuesday found Exxon Mobil liable for $236.4 million in a civil lawsuit that charged the oil company had polluted groundwater in the state with a gasoline additive used to reduce smog in the 1970s and 1980s.

Following a three-month trial, jurors deliberated less than two hours before finding that the world's largest publicly traded oil company acted negligently in contaminating the groundwater with the additive MTBE, said Jessica Grant, a lawyer who represented the state.

"We're very pleased that the jury held Exxon accountable for the harm its defective product caused to the state's groundwater resources and that they also held Exxon responsible for its negligence," she said.

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Originally filed in New Hampshire court in 2003, the state charged that Exxon and other major oil companies knew that the additive was likely to contaminate groundwater and was more difficult to clean up than other pollutants. Some damages from the suit will help pay for the costs of testing and cleaning affected water supplies.

Exxon vowed to appeal.

"MTBE worked as intended to improve our air quality and the benefits of its use substantially outweighed the known risks," said spokeswoman Rachael Moore. "MTBE contamination in New Hampshire is rapidly decreasing and the state's current system for cleaning up gasoline spills ensures safe drinking water."

The U.S. Environmental Protection Agency today considers the additive a potential human carcinogen, though much of the research on the chemical has focused on the health effects of inhaling it rather than drinking it. New Hampshire banned the substance in 2007.

Exxon was the only one of the 22 original defendants in the original suit to go to trial. Other defendants either had the suits against them dismissed or agreed to settlements.

Those included Canada-based Irving Oil, which agreed to pay $57 million last year, and Venezuela's state-owned Citgo Petroleum, which struck a $16 million agreement as the trial began.

The three-month trial on the suit, filed in state court, was moved to the state's federal courthouse in Concord to accommodate the large number of witnesses, lawyers and exhibits. The jury found that contamination had caused $816 million in damages in the state. Exxon's market share of 29 percent was used to compute damages, Grant said.