CONCORD, N.H. (AP) — Nearly a decade after it was first brought, a lawsuit accusing two oil giants of widespread groundwater contamination in New Hampshire is expected to present jurors with the most complex and time-consuming trial in the state’s history.

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.