@Jennifer Lauren - if you are responsible for the oil and keeping the unit heated then I would recommend continuing to fill the tank as you may risk being in the wrong or her stating she isn't paying because you are not holding up your end of the lease.

I have a rental property in Emmaus, PA (near Allentown), and I paid a plumber to remove the tank as part of converting the system to natural gas.

Usually, only heating oil USTs associated with commercial properties are regulated. thus, most home heating oil tanks may and often are taken out of service without any oversight from state regulators.Second- the rules that are in effect for heating oil tanks are usually less stringent than for diesel or gasoline tanks since heating oil tends to thicker and may not migrate as far. as a result, many states just require tanks to be cleaned out. they may also require the tanks to be filled with sand or concrete to prevent collapse. but sampling may not be required.Third- the key to determining if a tank has leaked is to collect soil samples from around and beneath the tank (groundwater also if groundwater is shallow). another quick way to see if a tank is leaking is to "dip" it with a stick that has a paste that turns color in the presence of water. generally, if water has gotten into a tank, it can mean oil has leaked out of the tank (although a certain amount of condensation may be present even where there is no leak so this is not a infallible test).Fourth- if there is currently a tank in the basement, ask the owner if there used to be a buried tank (a/k/a underground storage tank).Fifth, If the tank has impacted the soil, the cleanup generally varies from $25K to $50K though the costs will depend ont he depth of the contamination. i once had a home with a heated pool that had pressurized piping and used diesel. the contamination went down 40 feet and the consultant went crazy excavating the soil to the tune of $400K!!!

No appriaser is going to tank a sale by $2k. 

It has city water and sewer, not septic tank as many areas around this neighborhood have.

A common septic tank?  A

I would bet he is on a septic tank as well.  

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Did you ask to yank the tank as part of the test?

If it has a bad odor, it is probably contaminated.Joe makes a good point on the expansion tank as well.