Tires are relatively inert, they don’t leach toxic chemicals, (at least not quickly). All those millions of tires that wear out every year, all that rubber that disappears from your wheels has to go somewhere. It’s deposited on the pavement and the rain and snow washes it into creeks and rivers, and into the ground water. After a hundred years of driving no one yet has raised an alarm about rubber in the water.
The big sin of scrap tires is that they hold water. They’re black and absorb heat from the sun and make an ideal mosquito hatchery. Also, in a pile they are a fire hazard, they’re unsightly and if buried gradually work their way to the surface with every freeze/thaw cycle.
A volunteer effort by the RCCRC collected 16 tires in just a two mile stretch of Campbell road and brought them to the pick-up site. One can only wonder about the enlightened citizens who take it upon themselves to deposit these in the deepest ditches and most overgrown areas on backcountry roads.