The group Greenbelt Alliance filed papers opposing the 4,500 home development six feet below sea level in Oakley.
The alliance claims 1,600 acres of farm land would be lost and Dr. Inking water could be exposed to pollutants if the houses are built.
They also claim residents would be in danger if the levees failed.
"By putting homes there if any sort of disaster would happen it could flush pollutants into the delta and really damage the eco-system there, as well as potentially impact water quality," said Carey Knecht from Greenbelt Alliance.
/*Oakley*/ officials say environmental concerns raised by Greenbelt, dating back to 2006, have all been addressed, and they believe the development should go forward.
They also claim greenbelts concerns about the levees are unfounded, because city law requires new levees be built and old ones re-fortified.