When I was growing up "the dump" was nothing more than an area that was out of town a mile and EVERYTHING went in there. In fact in a few years it was leveled and subdivisions were built over top of it.
Now landfills are carefully managed and have layers of covering put over the soil, then the waste, depending on the type is put in there. Hazard type materials are place in separate places and managed as well. In fact I believe that some are used to generate heat to run generators.