Water sampling shows the massive Ohio River algal bloom is dissipating, which could mean the Ironman swim scheduled for Sunday will go ahead as planned.

About 636 miles of the Ohio River — from Wheeling, W.Va., to Hancock County, Ky. — has been under a recreation advisory for the past month. The harmful algal bloom (HAB) is made up of blue-green algae, a toxic cyanobacteria. Regulators say coming into contact with the water could make people sick.

DEP spokesman Lanny Brannock said last week’s testing shows lower levels of microcystin toxins, which are produced by the bloom.

“The results show us that we have HAB microcystin toxic levels that are four parts per billion or below throughout the racecourse,” he said. “Our advisory threshold is 20 [parts per billion].”

It will take two consecutive tests that are below the advisory threshold for the DEP to lift the warning.

The Ironman triathlon is scheduled for Sunday. The three-part race includes 112 miles of biking and a 26.2 mile marathon. But it’s the 2.4 mile swim that’s worrying race organizers.

In a DEP blog post, Ironman Louisville race director Eric Atnip said the swimming segment of the race would only happen if the water is safe.

“We want the swim portion of the race to take place, but only if it’s deemed safe for the athletes as determined through testing and visual confirmation that HABs are at safe levels in the river,” he said.

State officials will take more water samples tomorrow and expect results Thursday.