First attempt to put up a video from a newly made channel. See next post. {{Edit: Looks like it worked!}}
I would assume gathering moisture??
Water I would hazard. I’ve seen bees lined up along a pool of cows’ poo.
Propolis is made by bees from gathered resin
Water was my first thought, but @Cowgirl thought that there was plenty of water. So option number 2 was propolis - normally from resinous plants, but who knows what goes into compost, unless you make your own. 
Lot of pine trees etc in Pennsylvania.
Bees seem to prefer “dirty” water to clean. Maybe because of minerals in it? Who knows. But the existence of a fresh water source doesn’t mean they won’t look for water in different places.
Option 3 is that they are gathering dust as pollen because there is no pollen available…
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Moisture: Lot’s of pollen available in the Northeast. I’m near you and I have Maples in bloom.
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