Read my Previous Post on the Kingsbury, Arizona Sites first.
Phoenix Memorial Veteran's Cemetery. The supervisor came out to talk to the people filming and told them , the plastic vaults aka "FEMA Coffins" were used when the conditions were too muddy for the cement vaults.
Phoenix gets 7 inches of rainfall per year.
26/5/09 -Update on The Phoenix Memorial Veterans Cememtary.
Mass Graves PDF
Screenshot of one of the pages from the above PDF