CalTrans is reporting a 90-minute delay on southbound Highway 99 near Buhach Road in Atwater.
The California Highway Patrol says the driver of a big rig fell asleep at the wheel and slammed into a partition, spilling diesel fuel onto the highway.
One lane is open on the southbound side of Highway 99.
The accident was reported shortly after midnight on Friday. Approximately 80 gallons of fuel spilled, according to the CHP.