The 1,600 acre landfill is in Kettleman City, in the southern San Joaquin Valley.
The King's County Board of Supervisors voted unanimously on Tuesday to allow the expansion. But opponents say the landfill is responsible for a spike in birth defects in the community.
About 400 truckloads of waste -- including lead, asbestos, and arsenic -- are hauled to the dump each day.
"They should have investigated the birth defects before they even considered expanding Chem Waste because they need to know for sure if it's going to cause something or isn't," said Miguel Alatorre, a Kettleman City resident.
The company that owns the site says it will close it if a state investigation links the birth defects to the landfill.
It also still needs state and federal permits to move forward.