Felix,
On one hand, I envy your situation. You can go to public parks, and dig Civil War relics. There is no chance of that, here in Oklahoma, and I envy that at any time, while digging in your city park, you have at least the CHANCE at a really special find!
ON THE OTHER HAND, you have what I would consider to be a nightmare scenario, there. In my experience, hunting Civil War-era relics is often on farmland, and so there is not usually an abnormally large amount of "modern" trash to deal with. As such, ANY mid tone in that type of site runs a good chance, probabilistically, of being a worthwhile target. At a park, those same targets are most often going to be a square or rectangular tab, etc.
The last time I was on a Civil War relic hunt, the targets I was focused on (bullets and buttons) were all in the 14 to 18 range on the Equinox. That is the EXACT SAME RANGE that 99% of my square and rectangular tabs fall into -- i.e. they are targets that, unless I am gold ring hunting -- I avoid LIKE THE PLAGUE in an average public park. I can't imagine a scenario where I am trying to dig three ringers or eagle buttons, in a loaded-with-modern-trash public park! I don't think I'd have the patience for that!
No WONDER the trash frustrates you!
Steve