A sewage spill estimated at half a million gallons prompted closure of commercial shellfish harvesting in Coos Bay and South Slough this week.

But recreational clamming continued, and clam diggers apparently were not warned about the sewage.

The spill was caused by a malfunction in two pumps at the Coos Bay sewage treatment plant in Empire. It dumped an estimated 500,000 gallons of partially treated wastewater into the bay.

The city notified state authorities who promptly closed oyster harvesting. The state worries about commercial oysters because some people eat them raw, said Dawn Smith, food program manager with the Food Safety Division at the Oregon Department of Agriculture.

“There are places up and down the state that have very, very good clamming areas right under the outfalls,” Smith said. “When there are spills, they are supposed to be posting signs.”

When it comes to clams gathered recreationally, the state assumes people cook them and cooking kills fecal bacteria. State officials leave posting notices to wastewater operators since they don’t have the manpower to post signs around the bay and then return to remove them when a closure is lifted, she said. 

An operator discovered wastewater in the plant’s parking lot when he arrived for work at about 7 a.m. Tuesday. The pumps in the intermediate lift station weren’t working, and pumping did not resume until 10:45 a.m.

Several trucks with vacuum hoses were able to recapture about 130,000 gallons of liquid, said Steve Simpson, project manager of CH2M HILL OMI, the city’s treatment plant operator.

The spilled wastewater went from the parking lot to an adjacent pond, and from there into a stream. It entered the bay about 3.8 miles upstream of the Coos Bay bar.

Simpson said the wastewater had been partially treated, though some solids remained in the water when it escaped the plant.

There hasn’t been a spill at the plant for many years, according to Smith and Andy Ullrich, a compliance engineer with the Oregon Department of Environmental Quality.

The lift station that failed normally has three pumps. One had been removed in December for repairs. Simpson said one of the other pumps had been making a suspicious noise, though it was working properly when employees left the plant at 4 p.m. Monday.

Of the two that failed, one of the pumps appeared to have a damaged heater block. The other pump’s motor is no longer operable and has been shipped to Roseburg to be examined. Simpson said he isn’t sure what caused the pump failure, though an inconsistent electrical supply could be to blame.

As of Friday afternoon, the lift station was operating on two pumps and a portable backup.

Clausen Oysters had to return some oysters it had harvested after learning of the closure and expects to lose at least one wholesale order because of it, according to an employee.

Smith said the state will sample water around the bay Monday before lifting the closure. Tides have flushed the bay repeatedly with saltwater since the spill, but officials set a minimum five-day closure to give oysters time to clean out.

The state does enforce shellfish harvest bans at the beaches due to paralytic shellfish poisoning by algae. That toxin is not eliminated by cooking.