I live in AZ, too, and I agree with Rimdig Chuck about the pulltabs. Someone finally gave me an explanation of why there are so many in the parks here, though I'm not sure of it's veracity. It seems we have a very large population of people of the Hispanic persuasion, of which most of the males have mustaches which tend to get caught in the tab of soda or beer cans. It's just habit then to pull the tab off before drinking, saving a little hair-pulling pain. This was told to me by a mustachioed gentleman, so I guess it's true in his case at least.
I'm really happy that there is no machine as yet that can distinguish gold from tabs. We have thousands of Snowbirds (winter visitors) here in the winter months, many of which have metal detectors and are scouring the parks, and most everywhere else. They tend to run their discrimination really high just to avoid the tabs and aluminum pieces and only find coins. This leaves nearly all the nickels and gold that has ever been lost still in the ground after they pass over it. Our parks are littered with tabs and junk, but also with nickels and gold for anyone willing to put forth the effort to dig every low signal. Not many will do this....including myself most of the time.
Marc