Plots in urban community gardens are commonly divided by either chromated copper arsenate pressure treated lumber or creosote railroad ties, containing polycyclic aromatic hydrocarbons (PAHs). These plot dividers leach contaminants into the surrounding soil. Dr. Wendy Heiger-Bernays of the Research Translation Core has proposed a low-cost intervention after conducting the first study ever to examine the migration of PAHs and arsenic from timbers into garden soil. Importantly, the intervention mitigates the problem without discouraging or interrupting gardening.

A new publication in the International Journal of Soil, Sediment and Water describes the assessments that informed the development of the intervention, as well as the effects of the intervention. In urban community gardens in three Boston neighborhoods, investigators determined how far the contaminants migrate from the timbers and in what concentrations they occur throughout the plots. The investigators also analyzed the levels of contaminants in samples of municipal compost and found that compost that did not include street sweepings contained very low concentrations of PAHs, phthalates, arsenic, lead, and other metals.

The investigators recommend removing the creosote-containing timbers and exchanging them with alternatives such as cinder blocks. The soil 8’’ below and up to 18" immediately adjacent to the dividers should be replaced with new soil and previously tested, non-contaminated municipal compost. The clean compost provides a source of microorganisms that assist in the breakdown of the PAHs and dilutes any remaining PAHs. Concentrations of most PAHs detected were reduced below regulatory levels as a result of the intervention.