@James Channing if the grading includes moving soil away from the building because it is touching the sill plate (bottom of the exterior wall), then I would do the grading before the fumigation. 

@Rob D.I keep recommending my clients have an- Attorney approval- Contractor review- Radon test- Lead based paint test- Asbestos test- Septic inspection- Finance contingency for FHA- $2500 repair expectation for any repairs FHA demands- $1 EMD- Engineers inspection- Furnace inspector- Partner approval- Environmental impact study at sellers expense- Neighbor interview- Plumber inspection- Assignment of contract- Well test- HOA rules approval- Certificate of Compliance acceptance- Marketability of title- Town code review- Survey- Soil testFor some reason they keep complaining they are not getting a response, beat out or end up spending thousands in tests and fees only to lose to some cash buyer when they request a repair.

Say, there's an oil tank in ground that spilled and contaminated the soil. 

What roles do easements, drain lines, utilities and soil samples play in determining the value?

If the soil near the foundation is not getting enough water and dries out, it may cause issues with the foundation settling. 

My background is in ecological design, which you can find out more about in my profile, but in short it means I design and help create abundant edible landscapes that not only grow healthy food, but also enhance the ecology by purifying water, providing habitat for wildlife, and actually sequestering Carbon dioxide from the atmosphere in soil and plants.I have much gratitude to the creators of this space for creating something that is truly inspiring and brings people together.  

If it's a slab, it depends on the cause of the problems and the underlying soil. 

I will go to the building department. regarding the property I am looking at what would my bank do prior to closing a loan secured by the property to make sure there was no contamination in the soil?

I've disturbed the soil frequently.

The house was built in 1980 and the soil in the area is bentonite.