Ask-a-Dave's-Gardener: Dirt smelled of two cycle oil when dug and now problems!, 1 by NEtransplant

NEtransplant wrote:
I just moved to a new apt and have a little backyard so I quickly turned it into a garden! Upon digging into the ground (underneath some planters that are on legs so this is full shade) I got a strong wiff of two cycle oil, the likes of which you use for a chain saw. Later I dug up a little fuel primer button so I'm assuming once upon a time a gas tank tipped over. Now I poured in about 5 inches of nice new soil into the ground after digging a 5"x5"x3' trough and planted some Corsican mint which did fine for 2 or 3 weeks and now is all slowly dying from the center/sides turning brown then black and progressing across the entire plant! Does anyone have any ideas as to what this might be? Is it a case of contamination or could it be the damp soil it sits in? I thought that mint had no problem with damp soil (no standing water just moist to the touch) but perhaps its a fungal problem and if so should I try to spray for it or just replace all the soil?
PS: there are a few vinca plants that are planted in between the mint and seem to be growing slowly but other than that show no problems.