The oil, which is actually a waste product of the hydraulic plant, was being stored by Caterpillar in two large pits. When a pump failed, the oil overflowed into a large parking lot and a lot of it made it into the river.
Sixty-five-thousand gallons of waste oil spilled from the Rockdale Caterpillar plant over the weekend. Of that, approximately 10 percent of it poured into the Des Plaines River.
While private contractors, the EPA and the Coast Guard have been working to contain the spill, the oil has been spotted much farther down stream.
"We're looking at about a nine-mile area now we've seen affected. Not seeing any real heavy pools of oil, more a light sheening," said Paul Mehler, U.S. Coast Guard.
"This oil, while less harmful because it is so light and thin, is also virtually impossible to take out of the river," said Lindsey Flanagan, U.S. Coast Guard.
Caterpillar is claiming the cause of the spill is mechanical, not human error.
"Right now everything is being investigated to make sure it doesn't happen again. It was a mechanical pump failure," said Kate Kenny, Caterpillar spokesperson.
Orange booms have been placed in the river to trap the spill. The oil is then removed by special absorbent towels.
While traffic on the river should reopen shortly, it may take weeks to know if there is any significant harm to wildlife.
So far it appears the oil has not made a major environmental impact.
"There's no indications right now that any fish or wildlife is being affected," said Jim Mitchell, U.S. EPA.
The Department of Natural Resources and the Illinois EPA is going to be out in Rockdale for much of this week and possibly into next week to see if there are any fish kills or any other signs of environmental damage.