DOVER, N.H. (AP) – High levels of MTBE contamination have been found in monitoring wells near wells that provide half the drinking water for the city, officials said.

The gasoline additive was found in several monitoring wells that offer early warnings of contaminants before they reach the drinking water.

City officials said there is no public heath concern yet, and the city has hired an engineering firm to see what can be done to prevent the contamination from spreading to the drinking water.

MTBE has been added to gasoline to help prevent air pollution, but has been found to contaminate water .

In the fall, one well showed MTBE contamination of 400 parts per billion, several times the allowable limit of 13 parts per billion for drinking water.

City Manager Mike Joyal said the city is working with the State Department of Environmental Services on the issue.