NORTHWOOD, N.H. (AP) – State environmental officials have given a Texas company 90 days to find the source of a leak at a gas station it owns or remove its underground storage tanks.

In a letter to Motiva Enterprises, state Department of Environmental Services officials said testing in May found “extremely elevated concentrations of MTBE” in groundwater near its Route 4 Shell station.

Motiva, which is based in Houston, said in a written statement that it does not believe there is an active leak at the station but is continuing with regular tests and the installation of a permanent treatment system.

MTBE, or methyl tertiary-butyl ether, is a gasoline additive designed to reduce air pollution. Many communities have complained about contamination caused by the additive, which is required by the federal government.

The company first tested water near its storage tanks over a year ago and found MTBE levels exceeded 24,000 parts-per-billion in some spots. State officials consider any concentration over 13 parts-per-billion dangerous.

The company then tested 17 private wells, eventually installing a treatment system at one that showed elevated levels.

Since then, no other private wells have shown concentrations of the additive above what state considers dangerous. The recent elevated levels could be due to older contamination.

The company said the increase in concentration could be due to an increase in groundwater levels in April and May.

Last month, New Hampshire joined a handful of other states in banning MTBE. The law bans the additive as of 2007. The chemical is blamed for polluting 15 percent of the state’s public water supplies and thousands of private wells.

The Environmental Protection Agency has given preliminary acceptance to the state’s plan to opt out of the program.

AP-ES-07-17-04 0851EDT