A METHODOLOGY FOR ESTABLISHING PHYTOTOXICITY CRITERIA FOR CHROMIUM, COPPER, NICKEL, AND ZINC IN AGRICULTURAL LAND APPLICATION OF MUNICIPAL SEWAGE SLUDGES

This study illustrates a methodology of evaluating phytotoxicity criteria for metals, when sewage sludges are applied on cropland. The analyses are based on data available in technical literature and fundamental principles of soil fertility and plant nutrition. First, data from laboratory experiments, where plants were grown in media treated with only one metal element, were used to develop an empirical cause-and-effect relationship between metal concentration in plant tissue and growth retardation of plants. This relationship is used to select the phytotoxicity threshold (metal concentration of plant tissue corresponding to a given level of plant injury). Then, data from land application of sewage sludge experiments were used and soil metal loading rates were plotted against the corresponding plant tissue metal concentrations. The probability of plants, grown on soils with a given range of metal loadings, exceeding the phytotoxicity threshold may be calculated. Finally, an appropriate loading range is selected based on the risk of exceeding the phytotoxicity threshold that is considered acceptable. In this exercise, the data were extracted without imposing any bias and represent a wide range of conditions where land application of municipal sewage sludge has been practiced nationwide. The reliability of the results, however, remain dependent on the quality of the data. A great deal of information already exists in the published literature. Through this exercise, gaps in the technical data base may be identified.