State and local environmental officials say they've traced the cause of a Saturday morning gasoline spill into Malletts Creek to two underground storage tanks at the Ann Arbor Shell gas station at 3240 Washtenaw Ave.

Matthew Naud, environmental coordinator for the city of Ann Arbor, said there may have been a history of leaks or spills - possibly during fuel fill-ups - that built up a quantity of gasoline around the tanks, which are surrounded by gravel and clay. Because the tanks are at a low spot on the property, Naud said Saturday's rains could have brought the accumulated gasoline to the surface.

"My guess is it's in the gallons and probably not more than tens of gallons, but it was a pretty heavy odor,” Naud said of the leak, adding that it was hard to estimate the quantity of gasoline that reached the creek because even a small amount can create a sheen when mixed with water.

The incident at the service station at Huron Parkway and Washtenaw Avenue brought out the Ann Arbor Fire Department’s hazardous materials unit to contain the gasoline. The department’s haz-mat unit placed booms in the creek to keep the fuel from flowing downstream to the Huron River.

Personnel from the city’s Emergency Operations Center assisted. The city used a large vacuum machine to draw the fuel from the creek.

"My understanding was the DEQ was pretty pleased with what we were able to maintain, but it's the Shell station's problem. They're responsible for cleaning it up,” Naud said. “It turns out they're pretty old tanks on the site.”

Station owner Dianne Gonzalez, who rents the property from Safeway Oil, said the incident has been “blown out of proportion.” She said state Department of Environmental Quality inspectors came out to her property on Monday and told her there were no problems with the site.

“I honestly think somebody overfilled their gas tank,” she said. “It was really no big deal. I was there at the station when it happened.”

Kevin Lund, senior geologist with the DEQ's District Office in Jackson, responded to the scene Saturday. He said the agency is still evaluating enforcement options, but the gas station mostly likely won't be penalized.

“Ideally you upgrade the tanks, but the owners are within the law operating those tanks,” Lund said.

"It appeared to me when I got there that the result of the spill was likely due to the geology of the area, the setting of the tanks and the age of the tanks," Lund added. “It's a low spot on the west side of the property and the property adjacent is about five or six feet higher. What we can do is try to understand the problem.”

Gonzalez said the gas station is scheduled for complete demolition in the near future, though she couldn't say exactly when. She said the plans call for building a new convenience store with a national food tenant. At that time, the gas tanks will be removed and relocated to another part of the property, she said.

Ryan Stanton covers government for AnnArbor.com. He can be reached at (734) 623-2529 and ryanstanton@annarbor.com.