Incompletely treated sewage is being discharged into Auke Bay because leaking oil has turned active treatment plant bacteria into inactive scum.
As a result, the city's Auke Bay Wastewater Treatment Plant has been completely shut down since Monday.
``This is a pretty big issue,'' said Brad Hahn of the state's Department of Environmental Conservation in Anchorage. ``But there's a very strong mixing zone in Auke Bay, and that helps.''
When operators inside the Auke Bay plant first sniffed a slight diesel odor on Saturday, they dubbed it the ``mystery spill'' because they could not locate a source or even a sheen.
Sunday, the diesel odor was stronger. Monday, operators found a significant amount of heating oil in the plant's clarifier, and began collecting it.
Inspection of sewer lines leading into the plant pinpointed an underground fuel tank serving the Auke Bay Post Office and an apartment complex on Glacier Highway.
At least 25 gallons of heating oil contaminated the plant. Seeping continued Wednesday, said city Public Works Director Ernie Mueller.
``The oil is very poisonous to the (beneficial) microbes used in the treatment process. It killed off our activated sludge'' and created an emulsified scum, Mueller said today.
``With a biological treatment plant, when you make a change like this, it takes three weeks to fully recover,'' Mueller said.
As soon as chemical tests show the level of poisonous oil has dropped sufficiently, fresh microbes will be brought from the Juneau-Douglas treatment plant at the Thane Road rock dump to reseed the Auke Bay sludge.
The Auke Bay plant handles sewage from the National Marine Fisheries Lab and the University of Alaska Southeast as well as residences on Back Loop Road. It normally processes 70,000 to 100,000 gallons a day.
Until the plant is fully functional, the effluent will be disinfected - ``a process that occurs no matter what,'' Mueller said. And some solids will be trapped and held. But what is discharged into the ocean will not be completely treated.
``We have warned the Environmental Protection Agency that, because of this (microbe) die-off, this will be happening,'' Mueller said.
Located slightly uphill from the treatment plant, the underground fuel tank has been removed, together with saturated soil. Both the U.S. Coast Guard and DEC are supervising, Mueller said.
The fuel tank itself has proved to be a red herring, said Scot Tiernan, an environmental specialist with DEC.
``We have a contractor out there right now drilling to find the actual source,'' Tiernan said. ``A fuel tank was removed but turned out not to be responsible for this particular leak.'' The true source might be an old tank nobody knew about, or an ongoing leak just surfacing from a broken fuel line.
``We just have to keep probing,'' Tiernan said. In the meantime, sumps have been dug to collect the oil and keep it away from the treatment plant. As of about 11 this morning, some 80 gallons had been recovered outside the plant.