Boise Fire officials say a valve in the system suffered a mechanical failure, causing the leak.
The leak was contained within a pit around the farm, which was installed for that purpose. The 50,000 gallons equates to about 1,000 barrels, Sinclair officials tell KBOI News.
Firefighters say the cold weather helped to contain fuel vapors during the spill, making the response easier to deal with.
"Boise Products Terminal regrets and takes full responsibility for the incident and has initiated immediate clean up efforts," Sinclair, which owns the facility, said in a news release.
No injuries were reported.