do soil testing

I had the same situation some years ago.  I hired a soil testing firm to do testing around the tank for a few hundred bucks.  I made an agreement with the seller: if there is no contamination, I will pay for the soil testing.  If contamination is found, she will pay for the soil testing, and also the clean up.  It turned out, there was contamination (tank leakage).  So she paid for clean up (removal of dirty soil and repalcing with clean soil) as well as the soil testing.