Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 3, 1997
Publication Date: N/A
Interpretive Summary: Land application of biosolids, a byproduct of municipal waste water purification, improves soil tilth and provides essential plant nutrients. However, concerns exist over the long-term bioavailability of heavy metals in soils amended with biosolids. This study was conducted to assess the bioavailability of trace metals in agricultural soils with a history of multiple biosolids applications. Soils from former biosolids application sites were extracted using a series of chemical solutions designed to remove increasingly stable chemical forms of heavy metals. Lettuce was grown on the soils and analyzed for heavy metal content after harvesting. Biosolids-applied cadmium was in forms that were easily extracted from soil and readily available for uptake by lettuce more than 15 years after application. The major portions of biosolids-applied chromium, copper, nickel, lead, and zinc were in relatively stable soil chemical forms that were not taken up in large quantities by lettuce. When compared to previous studies of these biosolids-amended soils, the present results show that there is no increase in the bioavailability of biosolids-applied heavy metals more than 15 years after application. These results will be used by regulators, both federal and state, for development of sensible guidelines for agricultural utilization of municipal biosolids.
Technical Abstract: Concerns exist over the long-term availability of trace metals in soils amended with biosolids. The objective of this study was to assess the bioavailability of trace metals in agricultural soils with a history of multiple biosolids applications. Trace metals in surface soils from two biosolids-amended sites were speciated into increasingly stable chemical fractions using a series of sequential extractions that were operationally defined as exchangeable (Exch), specifically adsorbed (SA), Fe-Mn oxide/ acid replaceable (Ox/AR), residual organic (R-Org) and residual inorganic (R-In). Romaine lettuce (Lactuca sativa) was grown on the soils to determine heavy metal bioavailability. In both control and biosolids- amended soils, 60-75 percent of Cd was found in the more easily extracted Exch and SA forms, but the percentage was significantly greater in the biosolids-amended soils. Biosolids applications also increased the percentages of Ni and Zn in the Exch and SA fractions, but generally accounted for less than 25 percent of the total metal. Biosolids applications had little effect on the Exch and SA fractions of Cr and Pb. Greater than 75 percent of Cr, Cu, Ni, Pb, and Zn were found in the relatively stable Ox/AR, R-Org, and R-In fractions of control and biosolids-amended soils, but biosolids applications increased the percentage of Cr, Cu, and Pb in the Ox/AR fraction. Lettuce concentrations of Cd, Ni, and Zn, and to a lesser extent, Cu and Cr were significantly increased by biosolids applications, but Pb uptake was not affected. Cadmium, Cr and Cu concentrations in lettuce were highly correlated (p less than .001) to metal concentrations in one or more of the Exch, SA, and Ox/AR soil chemical fractions.