MALTBY – Two planned chemical storage buildings at the Brightwater Sewage Treatment Plant are not threatened by active earthquake faults.
That’s the conclusion of a King County engineering consultant’s study released Thursday.
Soil samples from seismic trenches dug near the building sites found dense soil deposits that were compacted by the weight of glacial ice, the study by Atlanta-based Mactec says.
“Basically, what we found is the glaciers were really busy on our site 16,000 years ago,” said Christie True, Brightwater project director.
The international building code defines an active earthquake fault as a place that has evidence of movement within the past 11,000 years, True wrote Snohomish County officials this week.
In King County’s eyes, the study helps make the case the project is on safe ground.
The study, by a team of engineering geologists, was conducted as part of a $70 million development agreement with Snohomish County. King County is building the $1.62 billion regional plant on 114 acres on Highway 9 in Maltby in Snohomish County.
The report doesn’t necessarily conflict with the U.S. Geological Survey’s finding this summer that the Brightwater site is above an active earthquake fault.
Geologists found evidence of a fault on the property, but located away from the chemical buildings.
“We know that there’s an active fault strand running across the northern part of the property,” said Brian Sherrod, USGS research geologist.
Sherrod said an initial survey of the area showed evidence of an earthquake on the property in the past 2,700 years.
He said based on the length of the fault, it could produce an earthquake up to 7.5 on the Richter scale.
“Obviously if an earthquake ruptures the ground under that facility that would be a pretty dire situation,” he said.
USGS geologists were not invited to participate in the latest study, he said.
King County knows the property contains a fault and is building the plant to seismic standards that exceed current standards for sewer treatment plants in the Pacific Northwest. The plant is designed to withstand a 7.3 earthquake, True said.
It will include re-enforced holding tanks, and holding ponds down slope from the treatment plant to capture material if a major earthquake caused a spill.
Opponents contend that it is dangerous to build a wastewater treatment plant along a fault.
King County broke ground on the project in April and plans to begin operations in 2010.
The plant will process 36 million gallons of sewage a day from Snohomish and King counties, and eventually up to 54 million gallons a day.
Project opponent Emma Dixon, a Woodinville resident with Sno-King Environmental Alliance, said she didn’t have a copy of the report on Thursday. She said she is disappointed the USGS wasn’t included in the study.
Her group and the city of Woodinville have tried in court to block the construction of the plant. The group will still explore its options after reviewing the report.
“It’s very troubling that they wouldn’t include the experts in this field,” she said. “It’s completely within their ability to resolve this 100 percent.”
Reporter Lukas Velush: 425-339-3449 or lvelush@heraldnet.com.
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