I never recommend this but, I would either take some soil in to be tested and/or dig out the soil and replace it, or, grow in raised beds with a barrier between the bed and the soil.

15 years is a long time to barely have grass growing back do you have any idea what was dumped there?
Turpintine, lead based paint, solvents?

I think turpentine, water used to wash off brushes. No lead based paints were used. I'll take a picture when the sun's out and upload.

Where could I submit the soil samples to get tested?

Turpentine is basicly pine sap distilled. not a bad thing. remember the best solvent we know is water. MEK a scarry solvent is Hydrogen peroxide with an extra oxygen molecule.

Almost all oil based paint requires oil based solvent to clean brushes and tools. Turpentine will work but is much more expensive and not likely to be used. Oil based solvent in the soil is best treated by turning it up and allowing it to vent into the air. When an old service station site is tested and found to have fuel from leaking tanks in the soil, the soil is usually hauled away to make the site immediately usable. This contaminated soil is allowed to vent away contaminates in a remote area(or sometimes on the original site)until it passes the test. Al

Al - not completely true on the uses for contaminated soil, petroleum contaminated soils are usually thermally treated. Even when turning it up and allowing it to vent, semi-volatile organic compounds (PAHs) will remain. I would be careful about testing as then you would have knowledge of an issue you may need to report when selling. I don't know about VOC uptake by fruit trees. If the grass has a hard time growing, i would think there may be significant contamination remaining.

Why take a chance, i would plant the trees elsewhere.

But old gas also contained a very dangerous and persistent chemical whose name escapes me and is a serious contaminant of ground water. Is that stuff also a volatile?

lead?

MTBE

there are many VOC components to gasoline, some break down quickly in sols and others can remain for decades, depending on the soil conditions.

Itilton got it right- that is the contaminant in gas that's gotten the most press around here.

However, paint thinner is not gasoline and I don't think it represents a health threat to people through fruit. I suspect the soil would have a pretty strong petroleum smell if that is the cause. I've encountered contaminated soil in sites I've planted before- had that smell. I moved the offensive soil to an unused part of the property and replaced it with soil from there. You may find that the contamination isn't very deep.

My goal is to grow fruits- nectarines, persimmon, grapefruit. If the soil turns out to be not usable, then could i grow the fruit trees in a raised garden bed as suggested? Maybe I'm misunderstanding the concept of raised garden beds, but the retail raised garden beds sold at places like Lowe's don't seem to have floors. Or do they? Seems like roots would simply reach into the contaminated soil.

I can probably find out some laboratories for you if you actually want to run tests (if you let me know where you are located). My email is in my page. Also, i would suggest you do the test at 3 depths to pin down where the contamination may reside, maybe 1 ft down, 2.5 ft down, and 5 ft down. VOC samples can cost around $80 bucks a piece. There may be some more inexpensive test kits that can provide an indication of contamination without specific results of the individual compounds. You can call pine environmental, who is an equipment supplier to the environmental remediation industry and pick their brains. They also have field screening equipment available for rent (and they ship).

Possible options:

Put plants in large pots on top of that section.
Test soil in that section by planting tomato plants from seed and if they grow well with no plant disfiguring soil is probably ok. (Also dig down 3 feet in spots to get soil samples to put in a pots to test by growing tomato plants).
Test the store purchased foods you currently eat to see if they already contain "man made" chemicals and run the same tests on the fruits grown from trees in that soil and then decide!
If the tree rats eat the fruits grown on that soil then it is probably safe for human consumption!

Almost certainly the soil can be remediated. You can google "bioremediation by fungi". Virtually anything organic lasts less or of order a few months in the presence of fungi, organic matter and water. A reasonably cheap plan is to order some mushroom spawn, get some fresh chips, till it all in, water and cover with cardboard. If you use fungi.com they could even advise you on which spawn to use.

You could even plant right away, if you fill the hole with soil from a different spot in the yard. but a 15X15 area will not support three trees, unless their canopy can expand several feet in each direction.