Contractors finished Monday cleaning up some 20 gallons of suspected motor oil that wound up in Bayou Lafourche sometime Sunday, a spill that’s being blamed on a northern Assumption Parish car wash, parish officials said.

Never posing a serious threat to drinking water systems a few miles downstream, the oil was removed Sunday from the water’s surface while waterside vegetation with oil sheen on it was removed and bagged by midafternoon Monday, officials said.

John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said the oil was discovered about noon Sunday, and crews dredging the bayou downstream laid out containment booms in the Bertrandville area north of Napoleonville to prevent the oil from spreading.

Boudreaux said the oil is believed to have come from the Bayou Lafourche Carwash at 5734 La. 1, Napoleonville, which is in the unincorporated community of Plattenville.

“Looks like it was dumped down a car wash drain,” Boudreaux said.

La. 1 runs along the west bank of Bayou Lafourche, and, Boudreaux said, the car wash drain empties into the bayou without treatment for the wastewater. By some estimates, the bayou is the drinking water source for more than 300,000 people in four parishes.

Roman Rivere, who manages the car wash for his elderly mother, said Monday the wash has existed since 1975. He admitted the car wash does not treat its discharge, but he said the soapy wastewater dissolves somewhat before it reaches the bayou.

Rivere said he also has been informed by authorities that someone must have dumped the oil down the drain at his car wash, though he doesn’t know who.

“It’s never happened before in the 30-something years” his family has operated the car wash, he added.

Rivere said the state Department of Environmental Quality has permitted the car wash’s normal discharges into the bayou.

Online DEQ records show inspectors found the car wash did not have a wastewater permit in 2007, but the wash received a back-dated permit in December 2007 with discharge limits.

DEQ renewed the five-year permit in 2009 and again in 2014. DEQ also began requiring the car wash to use biodegradable detergents in 2009.

The latest report available from online DEQ records regarding the car wash’s required wastewater discharge monitoring, however, dates from the first quarter of 2011.

Boudreaux said the oil spill happened about four to five miles upstream of the nearest drinking water intake.

He said he believed Assumption Parish sheriff’s deputies were investigating who dumped the oil, but a spokesman for the sheriff said he had not been made aware of such a probe.

DEQ officials were not able to comment immediately about the spill but suggested it would be violation of agency rules.