Investigators believe they are honing in on a potential cause of an oil sheen on the Cady Marsh Ditch.
"Our concern is it is a Clean Water Act violation," said Jacob Hassan, on-scene coordinator for the U.S. Environmental Protection Agency. "Any sheen on a natural waterway is a concern for us."
Federal, state and local officials were called to the 7-mile drainage ditch, which traverses Calumet Township, Griffith, Highland and Munster, Thursday afternoon after it was reported to the U.S. EPA Thursday morning.
Hassan said the booms placed in the waterway Thursday to contain the oil appeared to be working early Friday afternoon.
"It looks like it has dramatically dropped off," Hassan said.
Lake County Surveyor George Van Til said he and his staff surveyed the water on Friday and determined "whatever was going into the ditch has now stopped."
"We're not talking about a huge amount of petroleum product here," Van Til said. "...This is a modest incident in terms of scope."
Van Til said his staff will be working to take samples daily over the weekend farther downstream to ensure it is dissipating.
Cady Marsh is a county-regulated stormwater drain that runs from Grant Street in Gary to Hart Ditch in Munster. The water ultimately flows into the Little Calumet River at Griffith but was stopped before doing so.
Any trace amount that may have reached the river, Van Til said, will be negligible with dilution and dissipation from the added rainwater on Thursday and Friday.
Van Til said in spite of the small scope of the spill, "everyone reacted the same way they would if it were a large incident."
Officials on Thursday said the source of the oil was not believed to be one of the many petroleum pipelines running through the region.
Hassan would not elaborate on the potential source, saying investigators planned to work throughout the weekend to keep the oil contained and identify its source.
Van Til said he is confident the source has been found.
"The people that are potentially involved are as concerned about it and eager to deal with it as the regulators are," Van Til said, adding that the potentially responsible party is "not a major player around here."
"We all agree this wasn't anything intentional," Van Til said.