The oil is pollution and seeps into the soil, possibly the groundwater. 

Areas with more senior water rights will probably fare better and areas that rely on groundwater (Pinal County and many of the newer developments that don't get allocations from CAP) may fare worse, assuming current water rights remain in place and play a role.

Plus about $100K clean-up cost (luckily for them their insurance paid because it had satisfied the insurance condition of hitting the water line or ground water)

Do not buy the property until that tank has been inspected, decommissioned and removed (and any soil or groundwater contamination cleaned up / remediated).TRUISM: If there's an underground oil tank, and it's steel, it leaked.