We had a business like that nearby -- a tire recycler that went up a few years ago. They had half a million tires -- it burned for days. Nearby residents have been running periodic checks on their groundwater since. It's in a semi-rural area, and lots of people rely on wells. Here's a link: Watertown Tire Fire

A big pile of sawdust could spontaneously combust, I suppose. $5 mil a year suggests that pallets aren't worth much, considering the scale of the operation. I know the pallet factories in my area seem to gravitate to run-down industrial properties.