Welcome to Chautauqua Park! Home of the WWII toxic waste dump of the Des Moines Ordanance plant (now the Ankeny John Deere Plant). To the slight NorthWest of GZ there is a large mound which is the capped waste landfill. Several soil quality tests put the levels of toxic chemicals such as lead and arsonic at a level right at the safe zone. This area is part of the Prairie Trail developement and is going to be converted to a greenspace park. Here is a link stating that the area is supposedly "safe". http://idph.iowa.gov/Portals/1/Files/EHS/prairie_trail_ankeny.pdf
I don't want to get into the politics of this area but it did delay the expansion of State St. when the put it through years ago and was the reason the road is so curvy. Look up 'Love Canal, NY', there are some similarities.
For the container itself, it should be a really easy find, but the lid is hard to get off and get back on. Please put the lid back on tight when you are through signing the log. There is some swag and a couple items you may want while visiting a "toxic waste dump".