However, in the late 60's there was a MAJOR train derailment there and it took 52 years for the final cleanup of the benzene in the groundwater.       

Many areas are updating their regulations because septic systems are at best rudimentary treatment systems and in areas of high concentration (>1 system per 5 acres) and high ground water, they are not adequate.

I have done countless environmental surveys at countless gas stations in my five years in this profession, and I would never ever purchase a station due to the emense cost/ liability (when contaminants end up in someone elses ground water supply) of remediation.

You can also call Southwest Groundwater Surveys (800-394-6207) and schedule a sonar test to get a more accurate quote.

"The Torrance refinery has allowed contaminated groundwater beneath surrounding residential properties to “store toxic chemicals” emanating from the plant for years, a hydrologist and Cal State Fullerton professor testified in a Los Angeles Superior Court lawsuit." 

Hopefully it didn't get in the groundwater and it's only in the soil.

The deep hole is to expose mineral deposits that indicate the seasonal high ground water level.

Worst of all, it was ground water--coming up from underneath the foundation of the house, and there was no way to stop the flooding...I eventually discovered that the entire house was built on top of a subterranean river. 

Ground Water contamination4. 

Nothing to cancel a deal over but you could say something like “this efflorescence is a sign the exterior waterproofing is insufficient for the hydrostatic pressure of ground water in the area - we are going to need a sellers concession equal to the value of the middle of three independent quotes for remedy” - so just use it to get cheaper price.