Transportation fuels provider Tesoro Corp. has agreed to pay $4 million to settle allegations of air pollution violations at its Golden Eagle Refinery in Martinez, California.

The Bay Area Air Quality Management District said that one of its officials discovered during a routine inspection that Tesoro was periodically draining process waste fluid to its on-site sewer and water treatment system. This was done without first removing compounds that could evaporate and pollute the atmosphere.

As a consequence, volatile air contaminants such as butane and propane would evaporate off of the liquid and pollute the air, potentially contributing to the formation of ozone and smog.

A full investigation by the Air District revealed draining violations from 2007 through 2014.

According to the agency, this draining caused a significant amount of air pollution and was in violation of Tesoro’s permit and local regulations which require refineries to treat process waste so that volatile compounds are removed prior to draining fluids to the on-site sewer.

Sewer drains must also be controlled to prevent the release of air contaminants.

“We require Bay Area refineries to control their emissions at every step of their process,” commented Jack Broadbent, executive officer of the Air District. “Through the Air District’s aggressive enforcement program, these violations were discovered and this uncontrolled release of air pollution stopped.”

Tesoro no longer drains process fluids into its on-site sewer system.