Biogeochemical characterization of municipal compost to support urban agriculture and limit childhood lead exposure from resuspended urban soils

Chronic low-level lead exposure among low-income minority children is an urgent environmental justice issue. Addressing this ubiquitous urban public health crisis requires a new transdisciplinary paradigm. The primary goals of this work are to inform best practices for urban gardeners working in lead contaminated soils and to reimagine urban organic waste management schemes to produce compost, which when covering or mixed with urban soil, could minimize lead exposure. We investigate bulk and bioaccessible lead from five types of compost used in urban gardens in Boston, MA. We categorized them by feedstock and measured bulk elemental concentrations and physical characteristics. Our results show that different feedstocks exhibit unique geochemical fingerprints. While bulk lead concentrations in compost are a fraction of what is typical for urban soils, the bioaccessible lead fraction in compost is greater than the default parameters for the Integrated Exposure Uptake Biokinetic (IEUBK) model. The lack of geochemical differences across feedstocks for lead sorption to carbon indicates a similar sorption mechanism for all compost. This suggests that municipal compost would be suitable for capping lead contaminated urban soils. Risk assessment models should consider lead bioaccessibility, to prevent the underprediction of exposure risk, and should include compost along with soils as urban matrices. Based on the observed bioaccessibility in our compost samples, 170 mg/kg total lead in compost will yield the same bioaccessible lead as the IEUBK model predicts for the 400 mg/kg EPA soil lead benchmark. Local logistical challenges remain for interdisciplinary teams of city planners, exposure scientists, and urban agricultural communities to design organic waste collection practices to produce compost that will support urban agriculture and primary lead exposure prevention.