If they said drilling the floor will solve the problem, it's not a plumbing issue, it would be ground water, probably seeping in through the foundation.

There was a heating oil storage tank which was excavated from the site in 2010, groundwater report was clean, but soil report had findings next to excavated area. 

there can be overfills and spills that migrate into the soil and groundwater beyond the USTgrave.

From environmental standpoint, u may want to collect groundwater samples along boundary of the property with the gas statiob to see if it has impacted groundwater as well as soil gas samples to see if there is potential for vapor intrusion.if groundwater not used, then principal environmental risk would be vapor intrusion which can be effectively managed with radon system and vapor barrier.Note that if the gas station is downgradient from your property (gwvflows towards gas station), vapor intrusion can still be a problem since vapors can migrate thru sewers, utilities and any conduits.

The heat and depth to groundwater should minimize your costs.you should negotiate right to do phase 2 and ur consultant could estimate cleanup costs.condition your obligation to close on getting approved remedial action plan below a certain cost.

Lookup ground water temp in your area during the coldest time of year, for example 50 degrees.

It was ground water, an act of god right?

Further away it shouldn’t have much impact assuming the area is good from an economic standpoint and the environmental impact doesn’t extend off site (like a groundwater plume that extends for miles).

Two of the roommates are moving out. weve got one more roomate to find. the problem:the fourth room area will occasionally flood with ground water when there is a bad enough storm, about once or twice per year. like, last monday when i was showing the room to an optimistic, potential roommate (who then backed out of the deal, after seeing the water starting to puddle.)current roommate living there is a CHAMPION and smilingly puts up with it for the massive discount rent he pays while living this close to downtown dallas (<10m). the landlord:Knows about the flooding, we've told him via email (not certified mail, yet) and he did not do anything to solve the issue. there are also other issues in the house (like i said, old) like the gutters falling apart, balcony wood rotting, foundation issues, etc. he simply will not put in any money unless he has too. 

What ended up being the problem was that ground water was getting vaporized by an electrical charge from the the power supply underground servicing the pool.