If you have raised garden beds, a deck or a wooden play structure in your back yard that's more than a few years old, you may be exposing your family to toxins such as arsenic and hexavalent chromium through the soil around it.
If you're growing a garden in that soil, it can be especially worrisome. Why garden organically and then grow your veggies in polluted soil?
What are the concerns in CCA-treated wood?
Pressure treated wood is wood that has been treated to protect it from being destroyed by insects, fungus or moisture. Prior to 2004, most pressure treated wood was treated with chromated copper arsenate (CCA), a combination of hazardous chemicals such as arsenic and hexavalent chromium that was used on almost all the pressure-treated lumber in the United States.
At the end of 2003, the EPA banned the sale of lumber treated with CCA for residential use. However, stores were allowed to sell existing stocks, and many residential back yards still contain play structures, garden beds, decks, garden borders, outdoor furniture and other materials made of CCA-treated lumber.
Back then, CCA-treated wood was easy to identify because of its characteristic light green tint. However, that tint fades with time. If you have older wooden structures in your landscape that is not a long-lasting wood such as redwood, there's a high chance that it's CCA-treated lumber.
Studies have shown that toxins do leach from the lumber over time into surrounding soil. One University of Minnesota study found that plants accumulated higher levels of arsenic the closer they were to the lumber. While the plants still contained levels that were considered safe for consumption, there are easy ways to lower your family's exposure.
Here are some easy ways to detox your soil and minimize arsenic exposure from CCA-treated wood.
Plant some ferns
Scientists have found that ferns are excellent at removing arsenic from polluted soil. One study published in the February 2001 issue of the journal Nature found that the fern Pteris vittata, also known as the Chinese brake fern, extracted significant amounts of arsenic from the soil. The fern extracted 200 times more arsenic than any other plant did, mostly into its fronds. The same team later noted other arsenic-loving species in the same genus.
A Virginia-based company known as Edenspace now owns the patent for these ferns, marketing them under the moniker "Edenfern." They note that Edenferns can significantly reduce the arsenic levels in polluted soil in as little as four months. Other ferns are likely to have a similar effect. Another study found that Pteris cretica cv Mayii (Moonlight fern) was just as good as Chinese brake fern for extracting arsenic and LariAnn Garner of Dave's Garden reports, "Edenspace calls their product "Edenfern", but their 'Victory' version is the same Pteris that grows wild in my yard!".
Because most of the arsenic concentrates in the fronds of the plant, harvested fronds should be stored in a plastic bag or other watertight container and then disposed with regular trash or as hazardous waste. Do not compost trimmed or pulled ferns from contaminated areas, as doing so would just add arsenic back into your garden.
Here's lots of information on how ferns remove arsenic for more in-depth exploration.
Plant non-edibles in a buffer zone
The University of Minnesota researchers found that soil by CCA-treated lumber had significantly higher arsenic levels a few inches from the lumber. Consider planting beneficial flowers next to the lumber and planting your edibles farther away (the U of M researchers recommended 15 inches to be safest).
Flowering plants such as dwarf marigolds will greatly help your garden veggies by attracting pollinators to increase your yields and beneficial insects that will feed on the harmful insects threatening those veggies.
Don't mix soil near the treated lumber
Because toxins are highest in the few inches directly next to the treated wood, it's best to leave that soil and not mix it in with the soil farther in the beds.
Many garden experts say this is a good time to increase your compost practices. Fine Gardening reports on the subject, "Brown says that if you already have the older, arsenic-treated wood in your garden, don’t panic. Plants will not take up arsenic unless the soils are deficient in phosphorus. That is not a problem for gardeners who use compost generously."
Think about what you plant
Researchers have found that some plants accumulate arsenic much more than others.
Plants that accumulate more arsenic include radishes and spinach, among others. Plants also accumulate arsenic in different parts. Beets accumulate a fair amount of arsenic but almost all of it is concentrated in the thin, fibrous taproot at the end of the root and not in the part that is eaten.
Some crops have made the news recently because of how well they absorb arsenic from the soil. Rice absorbs heavy amounts of arsenic, which is why there are concerns about arsenic levels in rice these days (here is information about how to minimize exposure from rice). Apple and grape juice have been found to have alarming arsenic levels in some contaminated areas (Consumer Reports found that 10% of these juices contained arsenic levels higher than legal limits in drinking water).
Leafy greens can contain more arsenic, as well. WebMD reports, "“All plants pick up arsenic... Concentrations in leaves of plants are much higher than in grains of plants. Thus, leafy vegetables can contain higher levels of arsenic than rice, especially when they are grown on arsenic-contaminated soils."
Peel root crops like carrots
One easy way to reduce arsenic exposure is to peel root vegetables such as carrots, beets and potatoes. Thoroughly scrubbing these crops also reduces exposure.
Wash hands after contact with CCA-treated lumber
Minimize exposure to the toxins in treated lumber by washing hands after gardening around raised beds and reminding kids to do the same after playing on treated play equipment or decks.
Also keep perspective. The levels that are leached into the soil are already small, and these strategies are quite protective. The benefits of eating fresh, organically grown fruits and vegetables are immense. By implementing a combination of these easy precautions, we can effectively minimize the levels of any harmful chemicals in our foods and maximize the health of the produce we grow in our yards and gardens.
This article is part of a series on naturally removing toxins from soil. Stay tuned in this column for more articles highlighting plants and garden methods that will remove other toxins, such as pesticides, uranium and mercury.