For instance imagine if the site has some environmental contaminants below grade.

Possible Title V and ground water contamination issues, may need a new foundation.

Every Google answer I found was it shouldn’t be done because it can contaminate the soils and groundwater.

If the dwelling has been damaged and needs repairs that most of the buildings seem like they need.. sheetrock, flooring,insulation, wiring,  due from water damage,, which was contaminated water.. they need to also comply and give tenants notice to vacate or be fined if they don't comply.. 

Even worse, what if your well becomes contaminated?

You have to bring suit to find out.For those that recall the environmental contamination days banks or lenders faced, they didn't want to touch the collateral until after the laws changed relieving them of clean-up obligations.

The sale price may not account for things like hidden title defects and/or liens, chemical contamination in the soils, foundations needing multiple piers sunk into bed rock, hidden damage in the walls from termites, one street over from the "War zone" side of town, etc. 

Do you bring masks to avoid mold spore contamination?

ServPro quoted $3000 to have his items moved into a pod while work is done and to have his items moved back, but he refuses to pay a penny for anything (including rent for a year and a half I might add).To make matters worse, he threatened to unplug all water mitigation equipment left running in the condo used to remove moisture and possible contaminants from the air.

And depending on the amount of contaminated water flowing into the concrete, and the composition of the concrete itself--fillered or not fillered--mold could grow in the concrete.