State water regulators have ordered Shell Oil to clean up and monitor pollution in soil under the Carousel neighborhood of Carson.
Testing found benzene and hydrocarbons - chemicals known to cause cancer - four years ago.
The neighborhood sits where Shell maintained crude oil in reservoirs for decades.
The Los Angeles Regional Water Quality Control Board says Shell must make a plan to clean soil under some of the nearly 300 homes to a depth of 10 feet.
Shell also has to investigate how far the pollution has spread, and monitor pollution levels in soil and air. Water regulators have emphasized that they're moving quickly; they say they're seeking public input every step of the way.
Remediation efforts could force temporary relocation of people in the Carousel neighborhood as early as this summer.
But some who have signed up to sue Shell over the pollution have raised questions about the extent of the cleanup - and about how far the pollution has already spread.
Now the company must turn in a cleanup plan within two months. Water board representatives say no current plans prevent deeper cleanups if they're needed in the future.