As plastics sit there being compressed amongst layers of other junk, rainwater flows through the waste and absorbs the water-soluble  compounds it contains, and some of those are highly toxic.
Plastmasai guļot iespiestai starp citu mēslu slāņiem, lietus ūdens plūst cauri atkritumiem un absorbē tajos esošos ūdenī šķīstošos savienojumus, no kuriem daļa ir ļoti indīgi.