Pumpkin Spring is a geological oddity is a limestone formation off the Colorado River. Some water spills over the top, but that which remains has a deadly mixture of arsenic, copper, zinc and lead. It's dangerous, but beautiful. View more pictures at the link.
Link | Photo: Flickr user al_hikesaz
http://www.azdeq.gov/environ/water/assessment/download/gcreport.pdf
While pumpkin itself is absent from the report nearby watersheds are shown, many with arsenic concentrations in the safe zone.
I can't find an immediate source for the arsenic level in Pumpkin Springs, so I've just taken out the entire sentence.