The spill happened last Friday and Saturday and a smaller one also occurred on Wednesday. According to the The Ross Valley Sanitary District, 840,000 gallons of sewage spilled into the creek and the district director says someone plugged up the sewer pipe.
The sewage spilled out of a manhole next to Kent Middle School. The Ross Valley Sanitary District had just completed a construction project replacing sewer pipe and after the rain stopped and they were able to get down into the 30-inch sewer pipe, the district's general manager says it was clogged with construction debris.
"We found large pieces of asphalt in the sewer system. We found helmets, large plastic helmets," Brett Richards from the Ross Valley Sanitary District said.
There was rubber from the running track and bundles of wire and concrete.
"There is just no reason for that to be there. It's either severe negligence, which is possible, or it's intentional and malicious and at that point it becomes probably a case of environmental terrorism," Richards said.
JMB Construction in South San Francisco is the company that did the construction project this summer and they did not return ABC7's calls. The district sent them a letter saying they should contact their insurance company.
But a worker for the district, who declined to go on camera for fear of being fired, said the reason for the spill had nothing to do with debris in the pipe. On Friday night, the districts' pump station was turned off without the pump station pipes couldn't handle the rain.
"I'm absolutely sure it wasn't because the pump station was down. I'm absolutely sure that my engineers told me that system could handle all of the rain fall that we had if there was not a blockage," Richards said.
The reason that matters is of course liability the sanitary district has called the FBI to investigate, but residents seem skeptical, saying the sanitary district has a reputation.
"They've been reluctant and resistant to disclose problems and to make necessary corrections," Mill Valley resident Larry Alboher said.
The head of the Marin County's Environmental Health Department says the district explanation is unusual
"I have never heard of construction debris blocking a sewer main before," Rebecca Ng said.
On the other hand, pump stations failing on being down is a rather common cause.
"That is very possible. Yes, yes," Ng said.
ABC7 spoke with the were talking with the health department to see if there is a health risk and they are warning residents not to go into the water of Corte Madera Creek, don't let your dog go in it and boaters should avoid the creek and area around the Larkspur ferry where the creek empties into the bay.
Testing just begun and the results are expected after Christmas.