A truck driver was killed Monday in a single-vehicle rollover that his stepson watched from a vehicle that was trailing the truck, the Washington State Patrol said.

Troopers said the truck, a septic vacuum truck with a trailer, was headed westbound on State Route 702 approaching State Route 507 when the wreck happened. One of the tanks opened up during the wreck and spilled its entire load -- about 3,000 gallons of landfill wastewater.

Troopers said there was heavy fog in the area at the time of the rollover, but it wasn’t immediately clear if that was what led to the crash.

The truck was the only vehicle involved.

The driver was pronounced dead at the scene. No one else was hurt.

Cleanups near the scene were expected to take several hours. The State Patrol said traffic was getting around the crash scene "just fine" with "very small if any backups."

Updates on the crash may be found on Trooper Guy Gill's Twitter page: twitter.com/wspd1pio