Methods for waste load allocation of municipal sewage sludge at the 106-mile ocean disposal site

Abstract

Methods for modeling short-term (time-variable) and long-term (time-averaged) exposures in the upper mixed layer that may result from the proposed disposal of sewage sludge at the 106-Mile Ocean Disposal Site are presented. The models have not been field validated at the 106-Mile Site, but they do provide decision makers with “upper-bound” estimates of risk to the upper water column from municipal sludge loadings currently projected for disposal at the site. The methods are believed to be useful in managing disposal site operations and in determining if future loadings to the site would be permissible. The models are two-dimensional in the horizontal plane and assume complete mixing over a prescribed mixed-layer depth. Upper-bound estimates of sludge contaminant concentrations in the upper mixed layer were based on summer hydrographic conditions and an annual disposal rate of 7.2 million metric wet tons of sludge. Preliminary assessments of the potential effects of the sludge disposal were made using U.S. Environmental Protection Agency marine water quality criteria, the Food and Drug Administration tolerance level for polychlorinated biphenyl residues in edible portions of fish tissue and whole-sludge toxicity test results. Within the framework of the assumptions and limitations of this analysis, and based on the projected loading rate, the overall results indicate that there would be no violations of these criteria. The margins of safety relative to the specified criteria were estimated.