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.

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