Ronald Hernandez, an environmental investigation specialist with the San Antonio River Authority, said it appears oil dumping on a downtown street is what led to a slick and smelly mess along a section of the San Antonio River Walk known as the Museum Reach.
The spill, which was announced by the agency Tuesday, has turned patches of green grass to brown in an area behind VFW Post 76, near Tenth Street and Avenue B.
"We feel that someone, essentially, just backed up into the street and spilled used motor oil," Hernandez said.
He said patches of oily grass also found along Avenue B indicate that is the site where the spill originated.
The rain that fell during the weekend, Hernandez said, then sent it flowing downhill, through the VFW property and onto the River Walk.
He said it's unclear how much of the oil, if any, made it into the river, itself.
"With the rain event we had this weekend, that kind of helped to dilute things so there wasn't a real hard impact to the river," Hernandez said. "We don't have any impact to the aquatic community. We didn't find any dead fish, any impact to the wildlife. And that's good."
While animals, so far, appear to be unscathed, the situation did not get by some humans.
"I have noticed right through here (that) the grass is kind of dying out. And there's an odor," said Becky Wright, who was jogging along the River Walk Wednesday morning.
Hernandez said the immediate goal is to clean up the mess as quickly as possible.
Workers with the Texas Commission on Environmental Quality, among others, were expected to visit the site Wednesday afternoon to help determine the best way to soak up the spill.