Water leaches from the soil into the main sewer lines (street lines).

When it breaks replace it.4) Every single kitchen and bathroom sink has mold underneath it.

But now, as reservoir levels hit new lows and mountain snowpack from California’s Sierra Nevada Mountains—which normally releases water through spring and summer—already nearing depletion, the exceptionally parched soil is also putting the region at increased risk for fires.

Somewhere without earthquakes,  tornadoes, hurricanes, tons of rain or smothering humidity, floods, ice/snow, high property taxes, any state income tax or loose soil slab foundations but reasonable landlord/tenant laws.    

I'm about to go under contract to purchase a SFH with an extra lot that happens to have an old oil/gas tank underground.I've done some preliminary searching and all I could find in terms of cost to remedy is anywhere from $2,500 for a basic removal, all the way up to $100k for a major contamination problem (replacing all the contaminated soil).The sellers are selling the property as-is, but this is an item that has a massive range of risk so I'm wondering how to better mitigate and reduce that risk.Thank you!

If you are in Texas/Clay soil, every foundation pipe sinks/settles.

She would have the peace of mind to know you aren't going to sell form underneath her and you would have the knowledge that you "could" have a buyer by a certain date. 

What I would be willing to spend money on is marketable research that doesn't expire...soils report, biological resource report, traffic analysis (just the existing ADT etc and even this could end up out of date), allowable land uses per the zone etc.

My landlord had a agreement that after 2 years of renting we could do rent to own, but 4 yrs later he sold the house from underneath us...

Soil Suitability.