CONCORD, N.H. — A jury found Exxon Mobil liable Tuesday in a long-running lawsuit over groundwater contamination by the gasoline additive MTBE, and ordered the oil giant to pay $236 million to New Hampshire to clean it up.
The jurors reached their verdicts in less than 90 minutes, after sitting through nearly three months of testimony in the longest state trial in New Hampshire history.
The state seeks $236 million to monitor and remediate groundwater contaminated by MTBE — which travels farther and faster in groundwater than gasoline without the additive.
‘‘We appreciate the jurors’ service during this long trial, but erroneous rulings prevented them from hearing all the evidence and deprived us of a fair trial,’’ said Exxon Mobil lawyer David Lender.
Jurors found that Exxon Mobil was negligent in adding MTBE to its gasoline and that it was a defective product. They also found Exxon Mobil liable for failing to warn distributors and consumers of the product about its contaminating characteristics.
The jury determined that the hazards of using MTBE gasoline was not obvious to state officials, who opted into the reformulated gasoline (RFG) program in 1991 to help reduce smog in the state’s four southernmost counties.
Lawyers for Exxon Mobil argued the company used MTBE to meet federal Clean Air Act mandates to reduce air pollution and should not be held liable for sites contaminated by unnamed third parties, such as junk yard owners and independent gas station owners.
The state says more than 600 wells in New Hampshire are known to be contaminated with MTBE and an expert witness estimated the number could exceed 5,000.
Jurors had more than 400 exhibits to sift through, including memos and reports dating back decades. Those memos included some dating back to 1984 in which Exxon Mobil researchers warned against using MTBE gasoline.
Jessica Grant, representing the state, said they were pleased the jury held Exxon Mobil accountable for widespread ground water contamination.
‘‘The finding of Exxon’s negligence is particularly important because it shows the jury understood that this problem could have been avoided,’’ she said.