The products liability case against the oil companies, Exxon Mobil and Citgo, is to go to trial in mid-January. In 2003, New Hampshire sued 26 oil companies and subsidiaries, claiming the gasoline additive M.T.B.E., or methyl tertiary butyl ether, caused groundwater contamination in a state where 60 percent of the population relies on private wells for drinking water.

New Hampshire is seeking more than $700 million in damages to test and monitor every private well and public drinking water system in the state and to cover cleanup costs where needed, according to court documents.

It is the only state to have reached the trial stage in a suit over M.T.B.E.

Other lawsuits have been brought by municipalities, water districts or individual well owners, and most filed in the past decade have ended in settlements. New York City won a $105 million federal jury verdict against Exxon Mobil in 2009 for M.T.B.E. contamination of city wells; that verdict has been appealed.

M.T.B.E. had been used in gasoline since the 1970s to increase octane and to reduce smog-causing emissions. While it was credited with cutting air pollution, it was found in the late 1990s to contaminate drinking water when gasoline is spilled or leaks into surface water or groundwater.

The federal Environmental Protection Agency had classified it as a “possible human carcinogen.” New Hampshire banned its use in 2007.

All the sued oil companies except Exxon Mobil, based in Irving, Tex., and Citgo, based in Houston and owned by Venezuela, have reached settlement agreements with the state. Just last month, Shell Oil and Sunoco agreed to pay the state a total of $35 million.

When the suit was filed, the state attorney general at the time, Peter W. Heed, said M.T.B.E. contamination had caused an “unprecedented environmental problem.” He said M.T.B.E. “has been associated with adverse health consequences and can render water unpalatable.”

The case was tied up for years on jurisdiction issues in federal courts before being sent back to state court.

Lawyers for the defendants say that they were not liable and that M.T.B.E. functioned as it should. They also stress that the companies have cleaned up their own sites and that contamination elsewhere was caused by third parties that have not been sued.

“They haven’t suffered the injury they claim they did,” James Quinn, a lawyer for ExxonMobil, said during a pretrial hearing in November. He said pre-existing contaminants — including iron, radon and E. coli — could unfairly drive up damages.

Jessica Grant, a lawyer representing the state at the same pretrial hearing, said the case is about whether the oil companies designed a defective product, failed to warn consumers of the dangers of M.T.B.E. “and ignored their own experts who said don’t use M.T.B.E.”

Court officials in October sent a 22-page questionnaire to 500 potential jurors. It asked them whether they believed that oil companies value profits over safety and whether the companies do not fully disclose the dangers associated with their products.

After eliminating those who got their drinking water from a well and those with hardships or deep biases, lawyers this month chose 12 jurors and four alternates who were told to report to Federal District Court here in Concord on Jan. 14. They have been told to expect a four-month trial.

“Everybody who sits on this case is going to be inconvenienced,” Judge Peter Fauver told prospective jurors during jury selection. “We will do everything we can to minimize the impact.”

More than 50,000 exhibits have been marked for identification, and there are upward of 100 lawyers on record in the case. The witness list numbers 230.

Court officials had to improvise a special docketing system because of the number of participants and documents involved. It is one of only a handful of state court cases that have gone fully electronic, with all motions and orders being e-mailed.

Bill McGraw, the chief clerk, noted that the only other case that comes close to it in complexity is a school financing challenge of the 1990s, “and that pales in comparison to this.”

“It’s been a unique experience,” Mr. McGraw said.