If it is underground you must be extremely careful (especially with a 100 year old tank) that it did not fail (leak) and cause contamination.

The concern is generally that the property may have been used in the past and that the property may be contaminated due to a manufacturing process or storage of materials used.

I was thinking about asking for credit to the bank, but I don't the costs involved on removing the tank and cleaning any potential contamination. 

I've used the fogging system for mold and other contaminants.

A corrective action plan was put in place during 2013, and February of 2017 the soil contamination was said to be below the "site specific cleanup levels".

What is the likely hood of it being contaminated if it has already been decommissioned?

If at any point the property owner pays off the overdue property taxes before the lien expires (period may vary by state), the county is the party that earns the interest.Properties that are owner-occupied (as indicated by a homestead exemption showing up on the annual tax bills) or have a mortgage are most likely to be bid on because they have a higher likelihood of being paid off either by the homeowner or the mortgagee (the lender); the homeowner probably doesn't want to lose his/her place of residence and the lender probably doesn't want to lose its investment.If no one has bid on a lien, there are likely issue(s) with the property that, in the eyes of bidders with investor mindsets, render the property valueless, for example:the property has no direct access (landlocked or waterlocked),the property is too small to be built on as-of-right per the municipality's zoning codes,the property is contaminated (a Phase I environmental report would scour records on the property to see if contamination is likely, and if so, a Phase II environmental report would be done and soil, etc. samples would be taken to confirm the contamination),the market fundamentals indicate little probability for profit given the level of risk, such as high vacancy rates, low rents, or slow sale/rental velocity,the property is in a "poor" location due to many of the types of things that turn off people looking for a home in which to live, such as high crime; poor or nonexistent infrastructure, such a streets, water & sewer, etc.; too rural/too urban; neighboring uses detrimental to the property's value, e.g. railroad, warehousing/industrial district, jail/prison, cemetery, etc.I hope that helps!

He was nice enoughto forward me the results from the reports.The phase 2 involved soil samplesfrom various areas in a radius from the contaminated property.

There are many postage stamp-size lots, irregular, unbuildable, brownfield (contaminated) property, etc. 

What is the process if removed a underground oil tank and the soil is contaminated?