The tanker overturned around 10 p.m. Monday about three miles above Angelus Oaks.
Highway 38 remains closed to all traffic from Lake Williams Road to Angelus Oaks.
It's unknown when the highway will reopen, but authorities said it could be shut down for two days while crews clean up the mess.
The California Highway Patrol recommends using Highway 18 or Highway 330 instead.
The fuel spill was contained and did not go into any streams or lakes, officials said. The big concern was that Forsee Creek, a key source of drinking water for Highland and Redlands, was less than 100 feet away.
Even though about 3,500 gallons had been siphoned out, officials said about 5,000 gallons of fuel seeped into the ground.
Because of this, the East Valley Water District decided to temporarily cut off the source of water.
"The water we are delivering to our customers is completely safe and free of any contaminants," the district said in a statement. "We are holding water in reservoirs and having it tested by a certified lab now. We believe it's fine but are proceeding with an overabundance of caution to make sure it's free from any of the contaminants."
Officials with the city of Redlands were at the scene of the spill Tuesday morning to see for themselves.
"The water is safe to drink, we're testing it and we'll continue to test it until all the work is finished here," said Chris Diggs, Redland's assistant utilities director.