An old, underground drainage structure caused erosion at the foot of the lagoon's wall that led to the breach, according to reports by the U.S. Department of Agriculture and the state Department of Ecology, officials said.
The Natural Resources Conservation Service, a subsidiary of the USDA, supervised construction of the lagoon in the 1990s and did not locate or remove the drain, according to the agency's own report.
The spill at the Bartelheimer Brothers dairy farm southeast of Snohomish occurred late April 11 or early April 12, unleashing 27 million gallons of liquid manure onto adjacent farmland. Some of it spilled directly into French Slough, an arm of the river roughly 100 yards from the lagoon, and some got in through drainage pipes, officials said.
The spill caused fish to have to surface to survive, and water samples showed high levels of fecal coliform bacteria in the slough, according to the Natural Resources Conservation Service. The public was advised to stay out the slough and the river downstream.
It took 17 days after the spill for the slough to meet state water quality standards and four days for the river, according to conservation service.
"We were fortunate the incident had no long-term impact on the environment," said Larry Johnson, a conservation engineer with the conservation service in Spokane.
"None of us wants to see this happen again," Johnson said in a written statement. "And we're going to do everything we can to ensure that it won't."
The Natural Resources Conservation Service supervised the design of the manure lagoon in 1995 and its construction in 1997. When it was built, workers dug down 5 feet below the surface and built up the walls from there.
Two feet farther below the surface, a wooden "box" drain had been built in the 1940s, a structure designed to funnel water away to help the land dry out, said Larry Altose, a spokesman for the state Department of Ecology.
"They basically draw water into them and as they draw water, they can also pull soil into them," Johnson said. The breach in the lagoon wall probably happened over a long period of time, he said.
According to the agency's investigation, the workers knew there was an old box drain in the vicinity but did not know its precise location. Usually the drains are installed only a few feet deep, so when workers dug the hole for the lagoon and didn't find one, they assumed it wasn't there, Johnson said.
"It happened to be 7 feet deep," Johnson said.
The report concludes that "the wooden box drain tiles' satisfactory removal and relocation should have been confirmed prior to giving final approval to the construction," and that the responsibility for the failure of the waste structure is "assigned to planning and design."
The Natural Resources Conservation Service is updating its design and planning methods for these structures, according to Johnson.
It has yet to be decided whether the lagoon will be rebuilt, Johnson said. The farm has other ways to store manure, farm owner Jason Bartelheimer said in April.
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