The current problem really wasn’t a fault of his, just a consequence of the soil.  

Both of the neighboring houses have these as well so perhaps it's the soil?

This ”multiplying the acreage in each soil group and farm woodland by the applicable agricultural assessment value.

I'd look for deals in the suburbs South of there and be aware of clay soil and foundation issues.

-Costs of street development and utility extensions (public water and septic)-Approval process for subdividing, what should I make contract contingent on to protect myself (approvals, soil tests, stormwater etc)-Is there a form of financing people generally use?

Sandy Bicharay, excellent agent.

He recently started his small business for deliveries (i.e. mulch, soil,  sand, gravel, caliche, etc.) and lawn service. 

On the flip side, I could tear down the screen porch, which was a poorly-done addition anyway and have the tank removed.The cost of the abandonment is $2500 + soil testing which may be up to $4,000 out of pocket costs for a state-mandated site test.

The FHA says that examples of such problems include but are not limited to the following: Missing handrailsCracked or damaged exit doors that are otherwise operableCracked window glassDefective paint surfaces in homes constructed post-1978 (because of lead paint hazards)Minor plumbing leaks (such as dripping faucets)Defective floor finishes or coverings (worn through the finish, badly soiled carpeting)Evidence of previous (non-active) wood-destroying insect/organism damage where there is no evidence of unrepaired structural damageRotten or worn-out counter topsDamaged plaster, sheetrock or other wall and ceiling materials in homes constructed post-1978Poor workmanshipTrip hazards (cracked or partially heaving sidewalks, poorly installed carpeting)Crawl spaces with debris and trashLack of an all-weather driveway surface=================There are many areas, however, where the FHA does require problems to be remedied in order for the sale to close.Electrical and Heating The electrical box should not have any frayed or exposed wires.All habitable rooms must have a functioning heat source (except in a few select cities with mild winters).Roofs and Attics The roofing must keep moisture out.The roofing must be expected to last for at least two more years.The appraiser must inspect the attic for evidence of possible roof problems.The roof cannot have more than three layers of roofing.If the inspection reveals the need for roof repairs and the roof already has three or more layers of roofing, the FHA requires a new roof.Water HeatersThe water heater must meet local building codes and must convey with the property.Hazards and NuisancesA number of conditions fall under this category.

There is a lot to learn, you have to avoid prairie soil, you'll want to stay away from the Coliseum  Plume  (an area that experienced soil contamination) as well as sections of town that have a lot of violence but look like cheap deals (so many out of state investors get really burned).