It happened between Monday night and Tuesday morning when the failure of sewage plant pumps caused an overflow.
Plant operators learned of the spill at 7 a.m. Tuesday, and reported the incident at 8:50. The Tiburon sanitary district manager says the spilled sewage contained no solids and, they believe, poses a low environmental risk because it had already been chlorinated and dechlorinated.
This is the latest in a string of Marin sewage spills this year. The largest occurred Jan. 31, when 2.7 million gallons of partially treated sewage was discharged from the sewerage agency of southern Marin's Mill Valley plant.