KENTFIELD (BCN) — A 369-page draft report released by the Ross Valley Sanitary District Thursday alleges that “severe contractor negligence” was to blame for two December 2010 sewer spills that belched 2.6 million gallons of rainwater containing sewages into Corte Madera Creek and San Francisco Bay.
The sanitary district presented its report to the Regional Water Quality Control Board and concluded that the spills were due to negligence by JMB Construction, of South San Francisco, and possibly vandalism.
On Dec. 17, about 842,000 gallons of rainwater and sewage spilled out of manhole covers at College Avenue and Stadium Way and other locations in Kentfield.
A second spill on Dec. 22 that leaked 1.8 millions of gallons of sewage was originally blamed on the failure of a 30-year-old pipe, but the district report alleges that JMB is responsible for both spills.
The district said that the system had performed “flawlessly” during an Oct. rainstorm that dumped more than five inches of rain in Ross Valley, which was twice as much precipitation as during the Dec. 17 spill. No rain fell during the Dec. 22 spill, according to the report.
The report, the district said, provides engineering documentation that proves the affected part of the system was capable of handling at least 12.5 million gallons per day, which they said is much more than what was needed to accommodate rainwater and sewage flow during the December spills.
According to the report, “excessive and unusual” amounts of construction debris in the sewer system blocked a siphon. The district alleges that the contractor was “at least partially responsible” for the blocked siphon and a crushed 27-inch sewer main whose capacity was critically restricted.
“Either or both of these factors together were ultimately responsible” for the spills, according to a statement released by the sanitary district Thursday.
Brett Richards, the Ross Valley Sanitary District general manager, said, “The cause of the spills became clear once we discovered the construction debris, crushed pipe, and reviewed the chronology of all events leading up to the spills.”
A preliminary investigation of the troubled system revealed large chunks of asphalt, pieces of wood, and large construction nail-gun cartridges, among other debris, that Richards said prevented the siphon from properly functioning.
A closer inspection of the sewer mains after the Dec. 22 spill revealed damage to the 27-inch pipe, which the district said was severely crushed in two places “next to where JMB had dug two large pits as part of its work on the district’s Woodland-College-Goodhill Capital Improvement Project.”
Calls to JMB Construction for comment were not returned Sunday.
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