Understanding phytotoxicity thresholds for trace elements in land-applied sewage sludge

Sewage sludge contains trace elements. With repeated sludge applications as crop fertilizer, these trace elements may accumulate in the soil to phytotoxic concentrations. My objective was to assess the scientific basis for the current cumulative loading limits for Cu, Ni, and Zn in land-applied sewage sludge. An overview of the USEPA's phytotoxicity risk assessment, the basis for these loading limits, was presented, discussed, and compared in the context of the underlying principles of phytotoxicity thresholds. I concluded that EPA phytotoxicity thresholds in immature plants were arbitrarily selected, and extrapolations to field applications of sewage sludge were not substantiated. Variability in plant accumulation rates and soil sorption capacities of trace elements were not adequately considered in the EPA analysis. Because of the assumptions, simplifications, and extrapolations introduced into the phytotoxicity analysis, the final results, that is, EPA's cumulative loading limits for Cu, Ni, and Zn, are not substantiated by the analysis. The phytotoxicity analysis does not represent an appropriate application of underlying scientific principles. A more appropriate basis for regulating trace element applications in sewage sludge would reflect an understanding of site-specific characteristics, possibly including concentration limits for trace elements in plant tissue, soil, and/or groundwater. Additional research would be required to substantiate EPA's conclusions about the cumulative pollutant loading limits, as derived from EPA's phytotoxicity risk assessment.