The state Health Department is investigating a spill in a drainage canal near Aina Haina that turned the water purple.
State Health Department investigators on Wednesday took samples from the ditch next to Kawaikui Beach Park.
Liz Galvez, of the state Health Department, said the sample would be analyzed to "help determine how we would do the clean up."
She added, "There will be further investigation on this. We have our Clean Water branch because this is a pathway to the ocean."
The ditch empties into Maunalua Bay, which is part of the Humpback Whale Sanctuary.
It's unclear whether the unknown substance poses a threat to wildlife. In the drainage ditch, Hawaii News Now spotted two dead ducklings
"It's reckless and first of all inconsiderate of the environment. Whoever dumped this, spilled this or whatever, should have alerted the government," said environmental activist Carroll Cox.
Cox first reported the spill to the state on Wednesday. A source told him that the water had been purple at least since Monday.
The state is cleaning up the canal with a pumper truck. The lab tests to find out what's in the water could be completed in a week.
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King tides are expected to begin Thursday.