Because of groundwater ingression via the backyard (another issue altogether), tenants have had to pull up the laminate flooring, exposing the original vinyl.

Are you going to have soil,ground water, etc around the tank tested to make sure there wasn't any leaks or spills?

has their been any activity in the area that would affect ground water? 

I am hoping on our next deal they will come down on interest a bit.Ok, so here's what the house looked like before we bought it:So far, we have painted the exterior, all new vinyl windows, new exterior doors, new garage door and opener, installing French drains and a sump pump due to there being a bad ground water issue in the crawl space last spring, new roof, opened up the kitchen, put in a french door, replaced all the wood flooring, tiled in bathrooms, framed in laundry and mechanical, shelves in the pantry, all new interior paint, heated flooring in master bath, can lighting.

Perhaps is sits on an area that was an old local drainage and the ground is slightly more pervious allowing the local ground water to collect and move down gradiant until it hits this house's basement.One thing that puzzles me is the why the existing sump pump is not dealing with the issue.

I spoke with a friend who is an environmental engineer who said that it could be the ground water (rare but possible).

I've been told by tank removal companies that if there is a high water table or a lot of ground water in the area and the tank was not properly decommissioned your chance of having a leak could be high.  

Our biggest issue right now is severe drought and the BOR unilaterally deciding not to release water to our irrigation district this year so groundwater rechard did not happen and the water table has lowered dramatically.

Hi Eron,Well you could test it out by laying a big plastic sheet over the area and seeing if it works.But if the water is coming from a general raising of ground water that won't stop it.Good Luck!

Also ask about ground water mitigation.