As far as I know any arsenic in soil will not usually have any effect on plants. It does break down and will not affect the quality of any plants

My suggestion is to keep the pH neutral, or slightly above 7, in the part of your garden where the shed was located. Alkaline soils tend to sequester, or trap, metal contaminants. If there is any arsenic in the soil, it should remain in the soil & not affect the plant, or the fruit. Since blueberries do best with soil pH between 4 and 5, they will have to be planted somewhere else.

Arsenic is a heavy metal and will come from soil to plants to berries to you. You can replace soil or (I have read on Internet or some magazine) you can plant special fern that will remove arsenic from soil and then you need to dig and dispose these ferns. Just to be on safe side, you always can test level of arsenic in soil in labs.
Yelena

Another plant you can try for removing the arsenic is datura. It's used often to remove metals from the soil, and it's easy to grow, too.

Hi guys,

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.

Daturas can be annuals or perennials, depending on the climate. Zone 6 they might regrow from roots. They reseed prolifically, and they are considered a weed, but they have beautiful flowers that release a sweet scent at night. I have grown them just because I like them. They're easy to grow. Datura stramonium (Jimsonweed) is the easiest, but lots of the studies of metal uptake were with D. innoxia, which the Army has even been used to remove TNT from the soil.

From my experience with a lumber mill & yard and a shallow bored well...arsenic was undetectable despite yearly samples from a certified laboratory. Heavy metals do not migrate easily. I'd analyze the soil before going to any great effort for treatment. You might find that arsenic is in greater quantity (naturally) in the subsurface anyhow.

The pressure treating was probably CCA, copper-chromium-arsenic.

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.