Regulators are still trying to figure out what caused a dairy-farm lagoon to fail earlier this week, spilling millions of gallons of manure that wound up in the Snohomish River.

Regulators are still trying to figure out what caused a dairy-farm lagoon to fail earlier this week, spilling millions of gallons of manure that wound up in the Snohomish River.

A breach sometime Sunday or early Monday at Bartelheimer Brothers’ waste lagoon near Snohomish is the worst such incident in regulators’ memories. As much as 15 million gallons of manure, water and other organic matter may have flowed into a slough that drains to the river.

But so far consequences might not be as bad as first feared — at least for the river.

Early test results found no evidence of the biggest potential threat to the river’s health — an explosion of bacteria that increases dissolved oxygen, which can suffocate fish. There are no results yet from tests of the river’s pH and fecal coliform levels, but environmental health officials don’t expect long-term damage.

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“This isn’t good,” said Ecology Department spokesman Larry Altose. “But it’s an isolated case. The ecological systems can and will process the nutrients that came into the river.”

But Altose said spilling that much waste at once, rather than a steady leaching over days or months, allows rough water to flush the manure quickly downstream as it’s being diluted. Unlike chemical pollutants, like oil, the danger with organic matter is the possibility of long-term changes in water chemistry.

“Dealing with organic matter like this is different than dealing with a toxic spill like chlorine, which is a poison and will kill stuff right away,” said Bob Everitt, with the Washington Department of Fish and Wildlife.

Still, it’s too early to know what impact the spill will have on the estuary at the river’s mouth where water slows and waste may have a chance to settle. Regulators are still focused on cleaning up the farm.

The Washington Department of Agriculture learned of the spill Monday morning, sometime after a berm that holds back 21 million gallons of cow waste had blown out. The lagoon is 20 feet deep — five feet of it below grade — and was full. Roughly three-quarters of its capacity is believed to have spilled.

Bartelheimer typically uses the waste to fertilize corn and hay fields. On Wednesday crews were pumping manure from damp fields and spreading it elsewhere around the farm to keep more from getting into the river.

State agriculture officials inspect each of the state’s 500 dairies to make sure they’re safely managing their animal waste. This dairy’s last major inspection, in April 2009, found “minor issues,” but nothing warranting state action, said agency spokesman Jason Kelly.

“This was an example of an operation that had not had any significant manure-management problems in the last five years,” he said.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com