Jennie -
Are you familiar with the effects of calcium and magnesium on the soil? Calcium will loosen or "open up" the soil whereas magnesium will make it much tighter. I would suggest getting a soil test to determine the "base saturation percentage" of your soil. This will tell you precisely how much calcium and magnesium you have (in addition to other macro and micro nutrients). Not all labs to a base saturation test, so make sure you find one that does - I use Logan Labs in Ohio.
The reason I make this recommendation is that, in your situation, it's critically important to have enough calcium so that the soil can let go of contaminants. If you have too much magnesium, it will hold tight and you'll have a very difficult time ridding yourself of them. Typically you need around 65% calcium. You might even want to go to 70 or 75% in the short term, until you're sure the soil is clear.
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