Residents in the nearby Sheffield Manor neighborhood found the sludge, as well as a green slime, in an unnamed creek near a water treatment plant off Falls of Neuse Road.
The green slime, state tests show, is also algae -- but it is alive.
"That is what really is the mystery," said Mark Vander Borgh, an environmental biologist with the state water quality laboratory. "That's what's the intriguing part for me -- how we can have two types of algae, both green algae, and one of them has died off and the other one is completely healthy."
Vander Borgh says the two types of algae are fairly common in cool water. By themselves, they are harmless, but he has never seen it die off like the white sludge in the creek has.
Raleigh's Water Treatment Plant discharges into the effected creek. A tributary that joins it shows no sign of death. So, what is killing one kind of algae and not the other?
"This isn't natural," said Ron Gregory, of the Sheffield Manor Homeowner's Association. "It has never occurred before."
Sheffield Manor residents have focused on the colorful creek, partly because it flows into Falls Lake, Raleigh's main drinking water source. Their relief is measured.
"There's always a relief to know that something is not, at least on the surface, toxic," Gregory said. "What we still don't know is what caused it."
Chemists are still conducting tests to determine if chemicals from the water plant contributed to the limited algae kill. Vander Borgh admits that could be a possibility.
"I would say, yes. But why wouldn't it have effected both (types of algae)?" Vander Borgh asked.
City tests on samples collected Monday show calcium, which is used to remove chlorine from the water that treatment plants put into the creek, but it is still unclear whether calcium, or something else, led to the unusual environmental changes.
Test results are expected next week.
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