I am planting blueberries, raspberries, strawberries, grapes and watermelons.

I heard that the arsenic levels don't rise over the years, but because it has been 20 years, has the arsenic been diluted, washed away or gone deep into the ground?

I contacted a chemist in my state's agricultural experiment station who had done arsenic tests. I wanted to post his answer in case anyone else was interested in this subject. He said digging deeper into the soil than what I planned to, going out more than one foot, combined with the fact that the feeder roots of the items I am planting will be in the new soil, was "sound". He said he would be more concerned if I was planting, say lettuce, where the leaf itself is the edible. He also gave me some examples of studies using different vegetables growing in arsenic soil and what their arsenic levels in the edible parts would be.

Now if I could just get the snow to melt so I can start digging out all this icky dirt!

By the way, where can I get "datura"? Is it a perennial in our zone?

Thanks guys for taking the time to answer me.

Unless the shed was extraordinarily leaky it is likely the only place that any leaching of arsenic from wood took place was at the edges where the footing got rained on, or especially at any footing or posts on or into the ground, but only where the ground could get wet. Thus the interior area may be fine. It might be worth your while just to dig up where soil was in contact with, say, a post, or just beneath the exposed (rained on) outer edge of the flooring/footing.

However, arsenic is a lot worse than we often credit to be (arsenic in food and drinking water has triggered the research). The foods are mostly animal though, I believe, or marine plants.

There is a lot of interest in this now: I'll bet a fair amout of information is posted on the Net.

In city gardening where lead is a big problem it is said that an superabundance of organic matter helps inhibit uptake, but I haven't seen a scientific study.