Questions about pollution have bubbled up in the Vermillion River in Hastings.

White foam has twice been spotted floating downstream from the spot where a storm sewer pipe empties into the river near 18th Street. But the source of the foam is a mystery.

Officials with the Vermillion River Watershed Joint Powers Organization have been unable to get a sample of the water because it flows from the storm sewer only during heavy rains or rapid snow melt. And without a sample to be tested, watershed manager Travis Thiel said, it's hard to know what is causing it.

"As far as the foam goes, I can't speculate," he said, noting that potential reasons could vary widely.

The nearly 50-foot drop from the storm sewer to the water could cause the bubbles and foam to form, or it could be caused by some sort of pollutant, such as soap that gets washed into the storm sewer.

Thiel surveyed the watershed, which includes a mostly residential area and a milk plant, after resident Ken Larson first reported seeing the foam in fall 2008. Thiel didn't find any obvious contaminant at that time or during an inspection of the area in late March after Larson reported the foam again.

Larson, who walks across the river bridge every day to get from the Veterans Home to the library where he volunteers, said he is going to keep an eye on the drainage pipe and call the minute he sees the foam again.

He said the foam spread out like a carpet and lingered far downstream from the bridge, and he said it would be a shame if it damages the river, known for its small-mouth bass fishing.

"It spread like a foam carpet," Larson said. "I can't prove it's going to kill fish, but it doesn't look good."