The specific exclusions listed on this binder state the policy doesn't cover for: Earth Movement or Earthquake, Lead Contamination, Injury resulting from Trampoline, Mold & Fungus, Domestic or Sexual Violence, War & Terrorism, Injury resulting from Pool, limited injury from Dog & Wild Animal.

When steam boilers are replaced, contaminants very often are washed down into the new boiler after it begins to operate.

Lots of industries need to do observations for maintenance, contaminated areas, tower inspections, oil & gas, marine uses, security areas, the list is endless, large companies will have their own but small ones or contractors that have limited uses may rent or hire an operator.

http://www.doh.wa.gov/CommunityandEnvironment/Contaminants/DrugLabs

The site is probably seriously contaminated. 

There is no documentation or list of addresses with meth contamination that it appears on, as it was removed after the remediation, which is supposed to give the house a fresh start, at a cost I believe of $18K.

Usually the State has clear guidelines on this and you don't want to be held responsible in the event of soil contamination or environmental hazards to the property or the property owner could come back afterwards on this issue.

Is it enough if the seller has a policy that covers contamination?

The issue is to confirm if the soil is contaminated and the extent of the soil contamination.

Ultimately if the tank is no longer in service, it should be removed to mitigate the risk of future problems (contamination or simple an inability to sell the property).