"We can't fingerprint where a nitrate is coming from," said Audrey Eldridge, a state Department of Environmental Quality hydrogeologist who led a local 17-member committee made up of Lane, Linn and Benton county commissioners, business and agricultural interests, and others representing public water supply agencies, homebuilders, natural resource concerns and others.

Many farmers already have made changes that reduce nitrate discharge, Eldridge said, switching from irrigation and fertilizer- intensive crops - such as mint - to crops such as grass seed.

That includes helping Coburg - the last area city still reliant on septic tanks - to move to a centralized wastewater system. Coburg Mayor Judy Volta is a committee member, Eldridge noted.