Many areas do not allow water wells in floodplains or require certain design features in them, the reason being that they are very susceptible to intake of floodwaters which often cause contamination.

Does anyone have any experience with a contaminated well? 

I also had a loan denied for having contaminated soil underneath the property.

I asked my attorney to request seller to remove oil tank and seller came back that they're prepared to remove oil tank and remediate any contamination but requesting to buyer (myself) to contribute 50% with a cap of 5K.

The only way the sale wouldn't go down would be if there some obvious contamination which there isn't, or a bad cleanup job which could be fixed; the offer's pretty cut and dry in that regard, and I know others who have gone through the process with no issues (none of them sold a residence, they were just holding raw land). 

Dry-cleaners of course can pose a significant contamination issue with a property, but would a regular self-serve laundromat have similar contamination potential?

Paint will not adhere to any surface if it has contaminants like cigarette tar, vaping residue, grease from cooking, etc.

I had to hire a meth remediation company to go in and test for and remove any sign of contamination. 

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The issue if you buy without first having the tank removed is that you may own the environmental liability, including if the oil gets into the groundwater or contaminates the adjacent properties.