Several major oil companies have agreed to pay more than $423 million in cleanup costs to settle groundwater contamination litigation involving the gasoline additive MTBE.

The molecular structure of MTBEThe New York Times reports that the agreement has the potential to be the largest settlement of its kind to date and requires the oil companies fund cleanup efforts over the next 30 years.

MTBE, a potential human carcinogen, was first added to gasoline in 1979 to boost octane level and cut air pollution, but has since been found invading drinking water supplies in at least 17 states. The agreement is not yet final and is still pending review by the federal court in the Southern District of New York this September.

The plaintiffs include 153 public water systems in New York state.