Bednarik said the substance appeared to be mixing with the water, so it was hard to know just how much was there.
Crews deployed booms into the river to try to contain and absorb the substance, which had reportedly encompassed areas from about 2800 South to at least 1500 South.
Bednarik said some booms were put in place to divert and collect more concentrated samples of the substance so the Salt Lake Valley Health Department could conduct more testing Monday to try to determine what it is. He said the Environmental Protection Agency had been notified.
Firefighters didn't yet know what caused the spill or where it was leaking from, other than that it was coming from south of Salt Lake City, Bednarik said. He said officials have shut down gates that allow a surplus canal to feed the river.
He said officials on Sunday will be working to determine the substance's source.