Susan,
 
Since the cistern is deeper than four feet, you will need to work with  
California OSHA for safety planning. They may require wearing a safety harness  and 
having some sort of crane on hand to pluck you out if the sides cave-in.  
They may also require Hazardous Material training, breathing apparatus, steel  
toe shoes, and possibly shoring. These things can get complicated, but better  
that than a fatal accident.
 
The U.S. Navy required me to retain a civil engineer to design a safe  
excavation that went below four feet. We had shoring, sump pumps, and wore hard  
hats, but CAL OSHA never inspected that site because it was on federal  property.
 
Ron May
Legacy 106, Inc.