Arsenic does not dissipate and is a natural part of most soils. The only way to tell if you have a problem is to have that soil tested.
Plants are known to uptake some arsenic as, apparently, part of the plants defense against insect pests, but that is not usually enough to cause humans harm. The major problem with arsenic, or any other heavy metal, in soil is that it might accumulate on the plant, or your hands, and if those are not properly washed before use you might ingest some.

Probably a bigger concern is how much of the smoke you and others inhaled.

I would excavate carefully and remove as much ash as possible; that should be quite possible since it was in a pit and you didn't spread it around?

not supposed to burn treated timber, very dangerous, many here gotten very sick and some deaths, it is more toxic, believe the arsenic becomes like cyanide, dig deep hole and bury it.

len

Here is a link that might be useful: lens garden page

If you chew on the wood, lots of wood over a long period of time, yes.
If you eat the ashes, maybe.
If you inhale the smoke from the burning treated wood, maybe.

If you grow trees 5 years later and eat the fruit ... I doubt it.

Have a soil test and see what the arsenic content is after the area has been tilled.

Len, I can find nothing that indicates that Arsenic will change into Cyanide, is there something, some research, you can post that tells us that?

I think he is suggesting that breathing in arsenic laden smoke has similar effects to inhaling cyanide gas.

I would agree with lazygardens, generally it's a small incremental increase in health risk, not a screaming emergency. Breathing the smoke would be a lot worse.

How much ash we talking about, spread over how much area? 1 wheelbarrow of ash in a 20x40 garden, or what?

It's actually not that hard to calculate (or at least estimate) the increase in As concentration in the soil, if we have something to start with.

"Old" pressure treated wood was CCA - chromated copper arsenate, and two of those elements you definitely don't want in you. From a chemical standpoint, the smoke might be the most dangerous aspect, unless you are eating the stuff.... Because of the dangers associated with these chemicals, "new" pressure treated wood is ACQ - alkaline copper quaternary.

I appraised a former lumber treatment plant site. It was a "superfund" property.

The CCA really does last, I often rebuild steps where the lowest step has been buried in the dirt for many years and still the stuff is not even soft.

It's all in the dose, folks. The Greeks figured that out a long time ago. :-] All of those metals have background levels in soil, so the question is, how much did they increase as a result of the ash, and what do those levels mean with respect to living and gardening there.

Thank you for your replies everyone.

We buried the ash and embers after the bonfire cooled down back in 11/2008. Probably not deep enough. I'd say the burning area was maybe a 5'x5' area and the fire burned for at least an hour. With that said, we did not breathe in any of the smoke as I wore a dust mask (not sure if that helped but I'm still healthy and have my hair). Will I ever do that again? No. I thought burning and burying it would be okay as I didn't have any plans for utilizing that space of my yard.

I'll get a soil test kit and take some measurements after I till.

Since we're on this subject, I do burn organic yard scraps, old untreated wood, dried branches and office copy paper. Was told and also read that ask is great for soil as it can add much needed nitrogen. I spread the ash over the entire garden before I till. I'll water the soil for a week each day then till again. Is that okay to do or should I just stop burning?

Thanks.

Was told and also read that ask is great for soil as it can add much needed nitrogen.

Uh ... no. Any nitrogen was in the proteins. What's left is phosphate and other minerals.

It's not "good for soil" unless you have acidic soil that is low in whatever ashes are high in. Here in AZ, and much of the arid west, it just makes alkaline soils worse.

Ok, here's a crude calculation just to give you an order of magnitude. Let's suppose you had 10 lb of ash, and it was dug into a 5x5 ft area to a depth of 1 ft.

While searching for the actual formula of CCA, I ran across a paper on the metals content of CCA treated wood ash, so I could skip some steps. CCA was made in two grades, 0.25 lb CCA/cu ft of wood and a higher grade 0.4 lb/cu ft. Their sample was 4 kg/m3 which works out to about the 0.4 type. The ash was about 20,000 mg/kg (ppm) Cr, 10,000 ppm Cu and 11,000 As.

After a bunch of conversion, that comes out to 36 ppm of additional arsenic added to the 5x5 area if distributed a foot deep (using 1.5 tons/cu yd of soil). I picked As because it's the most toxic of the three metals.

Now this is above health-based guidelines for residential soil, which are in the range of 5-15 ppm. Note, those numbers are based on your entire yard being at that level, and living on it for a very long time, with a resulting risk of death from cancer or non-cancer health effects reduced to essentially nothing amongst all the noise of other risks in life (i.e. a rate of 1 in 100,000 or 1 in 1,000,000 excess deaths).

You will not experience arsenic poisoning from this soil in the way that you're thinking. It's more like a small incremental increase in health risk, much like smoking a cigarrette (or a whole pack ) won't kill you, but it's not particularly good for you.

Here is a link that might be useful: CCA Ash Analysis Paper

This post was edited by toxcrusadr on Wed, Nov 20, 13 at 12:00

In agreement with above , The other point to be considered is the age of the treated wood. OP mentioned the deck was torn down 5 years ago. Probably the deck was built 25 years ago. During the 20 years, most or some portion of the chemicals had already been leached or broken down. So what you burned 5 years ago did not have anything poison near the freshly treated lumber. AND NOW, after 5 years, that ash being exposed to rain, how much poison do you think is still remaining in it ?

Dust masks are meant to protect ones lungs from particulate matter, not vapors. Dust masks are not protective of things such as smoke.

Certainly true for the cheapie paper masks you can buy at the hardware store. A rubber mask with replaceable cartridges, equipped with HEPA filters, will get out even smoke-sized particles. Vapors are a whole different thing though. Mine has carbon cartridges for that - love to use it when painting. The dog can pert-near pass out and I'm just hummin along.