I collect the constant flow of dumped unfired ammo at my range and just toss the stuff into a box. Rimfire, there is no problem with that, I just bend it over and the bullet pops out. The powder is discarded and the brass goes in with the live ammo. Lead in the melt box. That's done with anything that I can tear down.
There are several possible solutions. First, contact your police or county sheriff by email, phone, comment section, however you need to do it, but it would be best if you can do it by email and print out the reply. Ask if they will accept it for disposal and if they will have an officer pick it up at your home or other spot that is not inside the police station. Dropping in on the police unannounced with a box of discard ammo is just a dumb thing to do. Carry it through the metal detector? Oy.
Contact your recycling center, ask about the ammo, contact the hazardous waste facility, contact your trash hauling company, just poke around, asking if anyone is willing to accept whatever bulk of ammunition you have. There are plenty of places, I know that it's done because we have an explosives disposal incinerator here in my town. It is used to burn and detonate even large quantities of old and non functional stuff that goes boom, and that includes a metric fartload of old ammunition that is retired, that can't go into military surplus or be pulled down for salvage.
You may have to wander a few miles to turn the stuff in, but there are places that will accept it and dispose of it. That, simply put, is the only safe and secure way to get rid of a bunch of junk ammo.
I had a bunch of old twenty gauge shotgun shells, old reloads. I just tore the crimps open and dumped the shot into a can, pulled the wads and dumped the powder, stuck the empty destroyed hulls in a bag. So someday I have some number eight shot, or a few pounds of high antimony lead.