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.

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