> Is your math right? 90 million tons of water/10 m gets me 1.7 km2, so a 1 sq mi landfill would last around ... 1 year, not 287.

You're right, looks like I forgot to convert liters to tons.

>On the other hand, I'm not sure you can just assume that waste has the same density as water; from a cursory glance it seems to about 1/3.

is this before or after compaction? Intuitively speaking, most things I throw out would sink in water. A garbage bin/trash bag might float because it has a bunch of air, but presumably that will all get pushed out when buried 10 meters deep.