A DECADAL STUDY OF MUNICIPAL WASTE MANAGEMENT COMPOST PROGRAM: GEOMOBILE LEAD AND THE CONSEQUENCES FOR URBAN AGRICULTURE

Compostable materials make up almost half of global waste generated, yet, composting accounts for 5% of the waste treatment according to the World Bank. Compostable waste feedstocks, if properly collected and processed, have many value added end use options including landscaping, remediation, and agriculture. Since 2006, our lab has partnered with The Food Project (TFP), an urban agriculture NGO focused on youth leadership through urban farming. We analyzed a suite of biogeochemical properties of TFP compost including lead (Pb), organic carbon, pH, and grain size. Our initial goal was to aid TFP with lead screening of the compost materials received on site. However, through participatory research our study evolved to better understand the fate and transport processes of fugitive lead in the context of urban farming and to develop best practices to reduce risk.

Municipal compost Pb concentrations consistently surpassed the German compost benchmark of 150 ug/g by an average of 1.6x in the first six years of data collection. These results led to the halting of the municipal compost program. Since then, TFP has sourced compost from suburban suppliers at greater cost. Several suppliers have been used, with Pb concentrations ranging from 150 to 55 ug/g Pb, and with the newest supplier having concentrations 4.5x lower than that of City of Boston compost pre-2012. Notably, the newest iteration of the City of Boston composting program delivered to TFP’s urban farm contained 185 ug/g Pb. This suggests that over a 15 year period and through several vendor contracts, municipal compost Pb concentration has yet to meet international benchmarks.

For both municipal compost and compost from TFP, fine grain size fractions (<37 microns) have 2.25x the Pb concentration of the bulk compost. This finding suggests that Pb in this system is geomobile and must be managed to minimize resuspension and transport. Managing collection schemes and limiting soil resuspension on site (e.g. covering the compost and adding raw leaf material from the farm) are easy to implement measures that reduce the geomobility of the fugitive Pb. The long residence time of Pb, coupled with our findings, mean that unmindful residential development may set legacy Pb onto human exposure pathways.