Prediction of phytotoxicity and uptake of metals by models in proposed USEPA 40 CFR part 503 sludge regulations: Comparison with field data for corn and wheat

The United States Environmental Protection Agency (USEPA) has recently (February 6, 1989) proposed the 40 CFR Part 503 regulations which govern sludge use and disposal. The numerical limits on cumulative soil metal loading rates in these regulations were generated through risk assessment analysis based on modeling metal flux through several terrestrial pathways. Field data from a 6300 ha dedicated land application site operated by the Metropolitan Water Reclamation District of Greater Chicago (District) since 1972 indicate that these pathway models do a poor job of predicting bioaccumulation of Cd, Cu, and Zn in corn leaf and grain and wheat grain tissue. The USEPA pathway model does not take soil pH into account and predicts that plant tissue concentration will be linearly related to metal loading rates in soil. District field data indicate that crop tissue metal concentrations are dependent on soil pH and increase at low metal loadings but then become independent of total metal loading rate. No phytotoxicity was observed in corn or wheat despite soil loading rates for Cr, Cu, Ni, and Zn that were 13, 28, 3, and 16 times higher than their respective phytotoxic thresholds predicted by the USEPA pathway model.