Readers’ Questions: Arsenic from pressure-treated wood, Part I

(My most recent absence can be attributed at least in part to the fact that I, along with most of Bozeman's Symphonic Choir, got up at an appallingly early hour on Saturday morning to catch the bus to Billings for a joint concert. We acquitted ourselves honorably (three standing ovations, though one being for Bach's 4th Brandenburg Concerto which features not a note for singers, it's hard to take credit for all three. I'll give it a try, though: we were eloquent even in silence? No? Anyway, after the reception some of us sang all the way home, arriving at those homes about nineteen hours after we'd left them. Such excursions require a day or two to catch one's breath.)

Last week Jen in Maryland left the following comment on my January post “Compost vs. Arsenic: And the winner is–compost!” about dealing with arsenic in garden soil:

As a gardening novice, I placed a small veggie garden next to an old telephone pole in my backyard. It then dawned on me that maybe the soil could be contaminated with whatever they treated the pole with. The house is 60 years old, so I don’t know, but now I’m tempted to give up, for fear of contaminating my family. Advice?

The stuff we are talking about here is the copper chromate arsenate (CCA) that was used as a wood preservative from the 1930s on. By the 70s it had became the outdoor wood-treatment of choice across the United States until concerns that matched Jen's led to its being banned by the EPA, save for a few rare exceptions, in 2003.

While CCA is certainly toxic, a single sixty-year-old telephone pole is not going to be releasing much of it. The greatest surge in leaching from CCA-treated wood occurs with the first heavy rains after it is installed; after that, the release rate is very low. If the pole is as old as the house, then it's lost most of its umph. Also, since arsenic is fairly immobile, any arsenic that’s leached into the soil will remain near the pole unless the earth has been mixed and disturbed, meaning that the problem, which is not very severe, is probably not very widespread either.

Of course, as you’ve had a garden there, a certain amount of mixing has occurred. However, you've been adding topsoil and mulch, both high in organic matter. Compost would have been even better, as it contains a higher proportion of humus and beneficial microbes, but any of these things helps to defang the toxic monster in two ways. First, compost provides chemical attachment points to which arsenic compounds can bind. Once bound, they don’t tend to enter plant roots: they stay in the soil.

Secondly, though the arsenic in CCA is the more toxic inorganic arsenite form, compost provides the chemical and microbial environment that converts it to the less lethal organic arsenate forms, so if you’ve been adding organic material to the plot, then much of the arsenic in it has been converted to the organic form. The scientific studies I've seen focus on compost, but a rich soil and good mulch would probably have similar, if less potent, impact.

Finally, unless you have a very strange garden, your pH is well above 4. Very low pH encourages arsenic uptake by plants, but at normal garden pHs, uptake is slowed significantly, which means, again, that  most arsenic stays in the soil rather than migrating into your vegetables.

Taking all of this into account, I doubt you’re facing a particularly serious situation. It does help to remember that arsenic is everywhere already. It's a natural part of our soil and water, and at low concentrations, it's a non-issue. However, in the interests of thoroughness, let’s look at all the available options—short of moving to the North Pole, where growing vegetables will be the least of your problems.

There are four things you can do. The two passive approaches are either to abandon the site or to grow stuff there that you don’t plan to eat.

The two active ones are to immobilize the arsenic in the soil or to remove it using phytoremediation. These are the two I’ll discuss–tomorrow. At the moment a small spot of continued recovery is in order.

12 Responses to Readers’ Questions: Arsenic from pressure-treated wood, Part I

  1. Very good information Kate, I think a lot of people wonder about this stuff. Well written with accurate and insightful info. Lots of us gardeners aren’t new but would still consider ourselves beginners so this is great!
    Thank you for sharing!

  2. You can also get a soil test to see if you should be worried at all. I’m fairly certain my extension service offers a test that gives you the concentrations of all heavy metals. You could take a couple of samples and see how far away you need to plant to be safe.

  3. Seems like I remember learning in a past hort class that apple seeds contain small amounts of naturally-occurring arsenic. So it is all around . . . VW

  4. Kate,
    Well thought out article and good reading for everyone that gardens. We just built a huge addition and the good spruce sawdust got added to the garden the bad treated wood sawdust went into the soil in a few places. Under the deck and at the low spot in the back yard. I’m guessing nearly all of us have treated wood sawdust in their yard somewhere.

  5. Heather–Glad you liked it!
    Daphne–Exactly. That’s part of today’s post.
    VW–Well, I’m glad I checked that, because I was about to say, Yes, they do, and actually they don’t. They do contain cyanide, as do pear and elderberry seeds, but in such low concentrations that apparently you’d have to eat all the seeds from a bushel of apples at one time to do yourself any harm. Oh, and you’d have to chew them thoroughly, too.
    Randy–Thanks. Good luck with the sawdusted yard.

  6. I run into questions like this a fair bit. It’s pretty interesting, good to know as much as possible, tough to resolve. I didn’t know that compost would bind with it. Carpenters make a point not to touch treated wood without gloves. That’s the main thing I try to remember.

  7. Ryan–Gloves sound like a a good idea. The binding thing is what I love….

  8. Okay…I’m so confused. We buildt a shop using old telephone poles which were sawed at a mill into 6 x 6 posts. I had several of those left which I used as retaining walls., etc. We buildt a small garden 10 x 20. On the right side and back side of the garden we used two of these cut poles to make a border and attached a small lattice fence with gate. I have planed cucmbers, etc in this small area. Do I need to worry about any chemicals coming into the soil and being taken in by the plant? If so, can I wrap the post (which a lying on the ground) in plactic and replant. We had heavy rain recently and I hate to throw out what I have alreafy planted. I have no idea how old these ples are. They look like cedar after they were cut. HELP PLEASE!!

  9. Hang in there, Suzie and keep breathing.
    First of all, the fact that these posts were sawed at the mill, not at home, is good news; the sawdust is a much bigger problem than the posts.
    Now for the questions: You say the posts are old, but you don’t know how old. Did they appear weathered? Because if so, my bet is they’re over ten years old, and they did most of their serious leaching long ago.
    Remember, CCA-treated wood leaches most in its first year, and less and less after that. Articles I looked at didn’t even bother recording what they lost after the first three years.
    So–I don’t think you have a problem. To be absolutely sure, you can get a soil test, because then you’d really be able to sleep at night. In your case, I’d take the soil from right by the buried poles. If that comes back clean, you know the whole plot is okay.
    Good luck–I hope this was helpful.

  10. Great post! I never even stopped to think about things like that having chemicals which would affect gardens. I wonder if those pretreated landscape posts have chemicals in them too? Probably so.

  11. Greetings, J Jacobsen, and welcome to the Manic. Having taken a look at your article about soil pH (follow the name link, folks) I’m surprised to find that this is new to you. But there’s always something out there that we didn’t think of!
    Yes, any treated wood has “chemicals,” but since the US banned CCA in 2003, a landscape post these days is a much safer bet than it was ten years ago or more.

  12. So, thank you for your humor and encouragement not to panic. I only recently learned what is done to pressure treated wood to make it ‘pressure treated’ and was subsequently appalled. Appalled because I recycled some from a dismantled deck to create…you guessed it…a compost bin.
    This article (which I am confident you have come across before)
    had me in a tizzy. I am still quite concerned about it’s suggestion that the composting PROCESS leeches the arsenic so much more efficiently than the wood simply sitting in the soil.
    Have you come across any research specifically on arsenic levels in compost which was created in direct contact with the pressure treated wood?
    My current plan is to shovel out all the compost and line the bin with untreated wood….but the compost has been in there for about two weeks…I’m wondering if I should just shovel it into the trash.
    Again, thank you for your, um, grounded approach to things!

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