“OLD” FUGITIVE LEAD IN NEW URBAN COMPOST: A DECADAL STUDY OF GEOMOBILE PB IN CITY OF BOSTON COMPOST

Locally sourced compost is the life blood of urban agriculture. It's used to fill raised beds, added as a soil amendment to increase carbon contents, and helps manage soil moisture. In the greater Boston area, consistent access to compost low in fugitive Pb and rich in nutrients has been limited. In Roxbury, the average yard/garden soil Pb is 900 µg/g, with maximum levels reaching 3700 µg/g Pb. Ratios of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb implicate gasoline and paint as the dominant sources of urban soil Pb in Boston, demonstrating that old, fugitive Pb is contaminating new compost. The long residence time of Pb in urban soils and compost make this issue a persistent problem even though lead gasoline and paint sources have not been used for decades.

In response to these findings, The Food Project has constructed over 1,500 raised beds in Roxbury, Dorchester, and Mattapan to promote safer cultivation methods in the context of such elevated lead levels, which is vital in an area where food insecurity is 20%. These raised beds and the larger urban farms are in constant need of locally sourced compost to sustain their activities. Fugitive lead concentrations in municipal compost from the City of Boston ranged from ~150-420 ug/g over the past decade. Compost tends to have lower total Pb than soil but greater Pb bioaccessibility, where 170 µg/g total Pb in compost is equivalent in bioaccessibility to the 400 µg/g EPA soil lead benchmark, as based on the Integrated Exposure Uptake Biokinetic (IEUBK) model. However, the new CDC BLRV of 3.5 µg/dL would require total soil Pb to be closer to 50 µg/g, and 30 µg/g in compost. While low concentrations are difficult to obtain for compost produced in urban systems, new commercial management practices have been able to produce compost with 40-50 ug/g total Pb in suburban settings, but these suppliers are expensive and often inaccessible to urban farmers.

For all tested sources of compost, fine grain size fractions (<37µm) have 2.25x the Pb concentration of the bulk compost. This suggests that particles with the highest Pb concentrations are also the most geomobile and must be managed to minimize resuspension. By evaluating lead fate and transport pathways and collaborating with urban farmers, it is possible to develop best-practices to foster safe urban agriculture.