GEOCHEMICALLY FINGERPRINTING SOURCES IN THE URBAN COMPOST STREAM: TOWARDS DESIGNING BEST PRACTICES FOR MANAGING FACILITIES AND APPLICATION

Sustaining and supporting the growing interest in urban agriculture requires gardeners, non-profits, and scientists to meet the challenge of providing the benefits of this enterprise, including access to quality produce, increased food security, and community building, without increasing the risk of contaminant mobilization associated with changing land use. Compost is a readily accessible soil amendment that can dilute and possibly immobilize soil contaminants while enhancing soil quality. Lead, for instance, averages 950 µg/g in the Dudley Square neighborhood in Boston, but can be brought below the EPA benchmark of 400 µg/g by the addition of compost to raised beds. We evaluate the physicochemical properties of a range of commonly applied composts to determine if differing sources and management strategies produce different geochemical fingerprints. Our goal is to recommend source management procedures that will increase the utility of the compost by maximizing nutrients and metal-complexing ions while minimizing inputs of metals. The seven compost feedstocks we have analyzed range from municipal yard waste, to residential and commercial food sources, to biosolids. Biosolids contain 4 times more Zn and 14 times more Cu than other compost waste streams while having similar lead concentration. Phosphorous and S are 4 to 5 times higher in biosolids and the pH is one log unit lower. As binding site availability in compost will relate to P, Fe, and C abundance and speciation, assessing how compost matrix varies across sources is essential for evaluating mobility potential for metals.

Using the SBET protocol to evaluate bioaccessibility, we have observed that non-geological elements such as Zn and Pb are more bioaccessible than Ca, K, and Fe and display a wider range of values. Because bioaccessibility does not always scale linearly with concentration, we are examining the relationship between compost source and bioaccessibility. For example, while Zn in biosolids is on average 4x higher than other sources, it is 50% less bioaccessible. Thus, understanding the complex relationships between bulk chemistry, matrix constituents, and bioaccessibility is necessary to design recommendations that safely capitalize on this readily available soil amendment.