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

Welcome back, everyone, and in case you just tuned in, we’re talking with Jen about her vegetable garden and the unpleasant possibility that she may be growing toxic vegetables and feeding them to her family.

Unfortunately, the best place for the garden turned out to be right by an old telephone pole, which was probably treated with the wood-preservative copper chromate arsenate (CCA), and we all know what that means: it means that arsenic (along with smaller quantities of copper and chrome) could be leaching from the pole into the soil, where it would get taken up by vegetables. Which are eaten by her children. That’s pretty scary.

Before starting “real” research in this area, I’d read a number of short articles that seemed to pooh-pooh the whole CCA thing, which I was inclined to take quite seriously. I’m a pessimist by nature, and tend to assume that They are out to get Us; give me another ten years, and I’ll probably be a bona-fide conspiracy theorist.

Yet after a fair amount of research, I spent my first post on the topic explaining why I thought Jen probably didn’t have a serious arsenic problem in her vegetable garden. I’m going to expand on that today, then I’ll spend another post or two (this thing keeps changing shape—i.e., getting longer—as I work on it) outlining a couple of very different paths she could follow to deal with it.

Soil Test and Arsenic Availability: As Daphne (of Daphne’s Dandelions) mentioned in comments on yesterday's post, getting a soil test is always a good start. Make sure it’s one that includes arsenic levels, and insist on getting an explanation of the results. Your local extension office (there’s one in every county, I believe) should be able to recommend a lab, or you can search them out on line. Fifty bucks or less usually covers the cost.

When you look at the report, remember that the total amount of arsenic (or any other mineral) in the soil isn’t the same thing as the amount that’s actually available for uptake by plants.

This point, along with many others, is covered by Ruth Lively in her Fine Gardening article, “Does Pressure-Treated Wood Belong in Your Garden?” Lively’s article is the most thorough treatment of the subject I’ve found that’s pitched to gardeners, and it’s readable to boot. (I cannot for the life of me find dates on Fine Gardening articles; if anyone knows where they're hidden, could you please enlighten me? Thanks.)

On arsenic availability, Lively cites a 1977 study by two British scientists, C. Grant and A.J. Dobbs, whom she says found that “soil with 24 ppm [parts per million] total arsenic had 7 ppm available arsenic. At 14 ppm total arsenic, available arsenic was undetectable.” A much more recent Chinese study I looked at said that available arsenic amounted to only 0.7–38.2% of total arsenic in the soil.*

Lively conducted her own small-scale but helpful test: she took samples from the soil at three different distances from the CCA-treated wood of a raised garden bed and sent them off to a lab along with a control sample. She reports that “Only the soil very close to the treated wood shows a higher-than-background level of arsenic.”

This is encouraging, especially when you consider that the soil she tested was surrounded by wood that was only three years old, whereas your telephone pole, Jen, is considerably older than that.

The Bad News: In this whole scenario, then, only two things indicate that you might have a serious contamination problem: first, the fact that the soil has been tilled, which spreads the contaminated soil throughout the garden; and second, the fact that this is a telephone pole, not just a post. Telephone poles are big.

In a 2004 downloadable article,** Paul Cooper (a researcher mentioned by Lively who’s now a forestry professor at the University of Toronto) and two colleagues examine the various factors that influence and limit arsenic leaching from CCA-treated wood. Surface area is one such factor. And telephone poles have plenty of that.

It wouldn’t be so bad if one only had to take into account the buried part of the pole. But as Cooper puts it, “utility poles… have a large aboveground surface that drains into a small volume of soil at the base of the pole, and it is not surprising that relatively high levels of preservative components have been detected in soil adjacent to poles” (p.7).

The Good News: On just about every other criteria Cooper mentions, your situation comes out looking good. So let’s go down the list.

Age: As mentioned in the first post, old wood leaches less. This is an old pole. Check off number one.

Exposure: Submerged wood leaches far more than wood that’s only exposed to precipitation. This wood isn’t submerged. Check.

Debris: Construction debris such as shavings and sawdust contribute inordinately to contamination because they have such a large surface area. Telephone poles don’t generally get shaved, drilled, and shaped; they just get stuck in the ground. Check.

Maintenance: “Aggressive” cleaning that involves scrubbing, power sanding, or the use of harsh chemicals, can accelerate leaching or loosen wood chips, which then contaminate the soil below. I’ve never seen anyone cleaning a telephone pole, have you? Check.

Finishes:
Finishes such as paint slow chemical leaching from CCA-treated wood, which is an advantage when one is dealing with newly treated lumber. However, in this case, where the pole is so old, it’s hardly a factor. So while we can’t check this off, we can cross it off.

Two other points not mentioned by Cooper et al:

Age again: The pole’s age means not only that the current rate of leaching is low; it also means that the chemicals that have leached from it have been exposed to various neutralizing factors for decades. The organic matter already in the soil before Jen ever started her garden has been working to convert the arsenic to less toxic forms for thirty to sixty years. Check.

Acids: Acids such as those produced during composting increase leaching rates. But no composting has gone on right next to that pole for at least two years. Finished compost is not significantly acidic, so applying mature compost doesn’t cause problems. Check.

So I repeat: I very much doubt that there are toxic levels of arsenic in your garden, Jen. However, that's an opinion, and the opinion of a non-scientist and non-expert. Which is why the soil test might still be a good idea.

More tomorrow, folks.

_________________________________________

“Soil arsenic availability and the transfer of soil arsenic to crops in suburban areas in Fujian Province, southeast China”
Rui-Qing Huanga, et al. Science of The Total Environment. Volume 368, Issues 2-3, 15 September 2006.

**  “Variability in Evaluating Environmental Impacts of Treated Wood” (USDA PDF version)
Stan T. Lebow, Research Forest Products Technologist, Forest Products Laboratory, Madison, Wisconsin
Paul Cooper, Professor, Faculty of Forestry, University of Toronto, Toronto, Canada
Patricia K. Lebow, Mathematical Statistician Forest Products Laboratory, Madison, Wisconsin
Paper presented at the Environmental Impacts of Preservative-Treated Wood Conference, Orlando, Florida, 2004.

13 Responses to Readers’ Questions: Arsenic from pressure-treated wood, Part II

  1. ON the whole arsenic thing. When I was a kid I lived in a small farming town. Everyone had a well. As some point (I think because of IPP buying all the water rights) a lot of people applied for loans to build new houses or upgrade. That resulted in water tests, and everyone failed because of arsenic levels. The Feds came and tested us all (felt like a lab rat) because we had such high arsenic concentrations in the water, and people were dying. Of course they were all well into their 90s and dying of things completely unrelated to arsenic poisoning. Turns out the “safe” levels had been extrapolated from rodent tests and then reduced considerably to be “safe” for humans. No science (other than what they were doing on us). My advice is to be cautious but not overly so. (I don’t have two heads or anything…anymore, so you should be fine.)

  2. First: on being a conspiracy theorist, all I have to say is: Just because you are a Conspiracy Theorist (CT), doesn’t mean that Monsanto is not out to get patents on EVERY vegetable out there.(that being the most recent conspiracy I follow). Also, I am a CT from way back and I found a woman foolish enough to marry me and have children with me, proving that even CTs can lead moderately normal lives. :-)
    Second: When I bought my house, I found evidence in the yard that the original owner (an avid gardener) had garden timbers lining up all his beds and those timbers had now become buried. I know these were old timbers of treated wood so probably had the arsenic stuff and had been leeching it out for years. I dug all of them up. Unfortunately they were all over the place. My point is, you may have arsenic in your yard and not even know it for the treated wood is not always as obvious as a telephone pole. I am glad for your post since it gives me SOME peace of mind (or as much peace of mind as an old CT can have).
    :-) Happy First Day of Spring!

  3. Hi Kate, Have we met?! I’ve seen your blog a bunch-sorry I haven’t left a comment before;-) I asked my husband about all of our wood, thankfully he doesn’t think we have any of the ‘older’ treated wood–although it’s ‘ALL’ treated. We have a fence but we put that in ourselves about 10 yrs ago; the deck is new; and we do have buried landscape timbers (buried over time by adding layer upon layer of mulch)…but he bought those in the last 10 yrs. too. It is a scary thing, though. I’m glad you wrote about this subject…it really is a relevent issue for many people–and many of them have no idea there’s any possibility of danger.

  4. Alan–“My advice is to be cautious but not overly so.” Agreed. The other thing I’ve learned about arsenic is that it comes in dozens of different forms and compounds, and toxicity varies hugely. The stuff in fish and shrimp, for instance, is essentially benign.
    Good job getting rid of the extra head. I find one more than enough to deal with, myself. If I had two, they’d be arguing over how to run my digestive system.
    David Tee hee about the CT stuff. As for the timbers, you’re right–a telephone pole is hard to miss, but an old fence or old timbers could have been leaching for years and then been removed and how would you know? I am incredibly pleased, myself, to find that it probably isn’t an issue. Glad I could give someone a bit of peace of mind. (Not something I’m known for…)
    Hi,Jan, and welcome; I suspect we’ve crossed paths at Blotanical, but we can now consider ourselves formally introduced. How-do-you-do.
    I’d be most “concerned” about the deck, except that you say it’s new. If it was built since 2003, then it shouldn’t be treated with CCA. The newer treatments, which skip the arsenic, can leach small amounts of chromium and copper, but not at toxic levels.
    A deck built of CCA-treated wood is of concern because even touching the wood is not a good idea; gloves are advised. The fence is a gardening issue particularly if it was built in place, as sawdust and shavings have enormous surface area, so they leach heavily in the first few years.
    However, at ten years old, the fence is already beyond its heaviest leaching period. If plants are growing next to its supports, then you don’t have a serious toxicity problem, though I think I’d avoid planting vegetables within a couple of feet of them.
    Kate

  5. Thanks for doing the in-depth thought on this crucial issue! We all probably have some contaminant fears – in the city, it’s maybe more likely to be lead or other heavy metals. I have meant to get my soil analyzed for donkeys years. Maybe this is the time.

  6. I recently got a load of compost from my local landfill (free of charge). I was concerned about arsenic from pesticides and treated wood. I sent a sample to a lab. It only cost $15 to analyze the sample. The results were less than the detection limits.

  7. Karen, as long as you promise not to tell anyone, I’ll admit that I, too, have not yet gotten my soil analyzed. It was only this winter that I started researching all this stuff. Like you, I think it may be time. Here’s hoping the news is good for both of us.
    Philip–Well done! What lab did you use? Anyway, I’m glad the results were favorable.
    –Kate

  8. I used a local lab (Arkansas Analyitical). I am an environmental consultant so I routinely use this lab for work. Most large cities probably have an environmental lab.

  9. Thanks, Philip. I asked because that charge—$15—was much lower than the $35 or so I’ve heard quoted as normal. Anyway, thanks for getting back to me on that.
    –Kate

  10. I’m writing for my best friend Robin who is at work today. Can any of you brilliant gardeners (said with respect, not sarcasm) tell her whether the cedar she spent $400 on has to be painted before she builds her raised beds? She will use a food safe p-aint, if necessary. She just wanted to help the wood survive for years. What do you think?

  11. Suzanne, you can assure your friend Robin that no painting or treatment of any kind is necessary. That’s what’s so great about cedar and redwood–they carry their own protection in the wood itself. The paint would chip off long before the wood will degrade.
    Cut it and use it.
    One caution: if she’s indeed cutting it up at home, keep the sawdust off the lawn. (Lay down a tarp or cut elsewhere.) A bit is no problem, but lots of sawdust will deprive soil of nitrogen, starving plants. (There’s a post on this from last June; if you type “sawdust” in the search box in the right sidebar, it comes right up.)
    Thanks for bringing your question to the Manic!
    –Kate

  12. Good reasoning – also I’m not a scientist – was just contemplating a small vegetable garden where a 30 year old pressure treated deck had been removed. It had also been stained. I’m thinking I’m reasonably safe in going ahead. As this is a slightly shaded area – I will likely just be planting herbs, spinach and some lettuce – no root crops.

  13. Sounds good, Janet. Thirty years old? Stained? Feel free. One study cited in the Fine Gardening articls pointed out that even with root crops, arsenic doesn’t tend to accumulate in the edible parts. Very good of it, I’d say.
    –Kate

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