Readers Q&A

USING PRESSURE TREATED WOOD IN COMPOSTING BIN CONSTRUCTION

Q:

If a composter wants to build a compost bin for yard trimmings and/or manure and/or yard trimmings, and eventually wants to put that compost on a vegetable garden, should they avoid using pressure treated lumber?

A:

Prudence suggests that you should not use pressure treated lumber for constructing a composting bin. It is now well demonstrated that chemical components of the pesticide do leach from treated lumber. The compost is likely to retain a good share of those chemicals (although some would also be carried with water into the soil or drains below). Potentially, this could affect the compost's quality, if not its safety and performance.

The predominant type of pressure treatment chemical currently used is chromated copper arsenate or CCA. Copper (Cu), chromium (Cr) and Arsenic (As) are the chemical elements that can leach from the wood. General ly, the greatest concern is the leaching of As, which is a carcinogen and an acute poison in certain forms and in sufficiently high amounts.

Recent research has demonstrated that CCA-treated lumber does leach Cu, Cr and As over time. Its been found that soil beneath CCA-treated wood decks has higher than background levels of these chemicals. The research demonstrates that the chemi. cals move with rain from the wood into the soil. The chemical concentrations are highest near the drip line and decrease with soil depth. Similarly, soil adjacent to raised garden beds made from CCA-treated wood contains higher than normal levels of As, and those levels decrease with distance from the wood (to background levels at roughly six to 15 inches away). The situation is possibly worse for CCA-treated wood used for a composting bin. At least one study found greater Cu losses from CCA-treated blocks of wood (jack pine exposed to composting vegetable matter, compared to the Cu losses from similar blocks that were soaked in wa. ter or buried in soil (Cooper and Ung, 1992; Forest Products Journal, 57(42v.9)57-59). The authors suggested that the organic acids that developed during composting enhance leaching of Cu. However, it also was reported that the losses were not large enough to affect the "efficacy" of the compost. If you already have a composting bin or storage unit that has CCA-treated lumber, and you are concerned about it, you can paint the wood or apply a sealant or line it with plastic to minimize the leaching of the chemicals in the wood.

Cu, Cr, and As are all regulated elements within the U.S. EPA Part 503 regulations on biosolids, which are often used as a standard for other organic residues. Therefore, adding more Cu, Cr, or As via leaching from bin lumber could be a problem if the compost is on the borderline of meeting the regulatory limits. However, yard trimmings and manure composts are rarely in this situation (although livestock manure can have high copper levels).

Unlike the soil next to a deck, the elements leached out of wood will not accumulate for a long period in a given batch of compost. The different batches of compost that move through the bin will disperse the elements into the environment. However, if the compost is consistently used on the same ground, like a vegetable garden, the leached chemicals will accumulate in the soil.

Research conducted in Florida indicates that compost can influence arsenic leaching positively or negatively, depending on the characteristics of the compost. Aziz Shiralipour, the principal investigator, reports that "the municipal solid waste compost (MSW) reduced the leachability of As in soil, especially in the native soil, whereas the biosolids compost greatly increased the leachability of native As" (as opposed to the chemical As added for the experiment). The difference appears to be due to the electrical conductivity (EC) and dissolved organic carbon (DOC) of the composts. More leaching occurring with the biosolids compost that had higher EC and DOC. According to Shiralipour, "these findings indicate that composts with high EC and/or high DOC are not suitable for reducing arsenic leachability. On the other hand, composts with low EC and/or low DOC are effective in reducing arsenic leachability in contaminated soils, therefore reducing the chance of contaminating the groundwater." Although much depends on the specifics, in general, yard trimmings compost would tend to act more like the MSW and manure compost more like the biosolids product. The Florida research is expected to be reported in Compost Science and Utilization this year.

An answer to the question of using pressure treated lumber for composting bins would not be complete or balanced without some mention of risk. Pressure treated wood proponents and some scientists argue that As is naturally ubiquitous in the environment and that the amount lost from the wood does not increase the health risks. In fact, the most critical pathway of As exposure from CCAtreated wood appears to be hand to mouth contact for children playing on CCA-treated wood playgrounds (neglecting worker exposure at construction sites). Many proponents would advise you to use the pressure treated wood for the bins because the risks are small.

In any case, the debate may soon be irrelevant. The U.S. EPA and the wood preservative industry recently agreed to phase out the use of arsenic-based wood preservatives in two years. The announcement is careful to state that the agreement does not imply that CCA-treated lumber poses unreasonable health risks. In fact, EPA will continue with the risk assessment for CCA-treated lumber that is currently in progress.