The Significance of Dredging and Dredged Material Disposal as a Source of Nitrogen and Phosphorus for Estuarine Waters

Abstract

Several hundred million cubic meters of waterway sediment are dredged each year in the United States to maintain adequate navigation depth. A significant part of this dredging and dredged sediment disposal takes place in freshwater tidal, estuarine, and marine waters. U.S. waterway sediments contain sufficient quantities of nitrogen and phosphorus compounds which, if released in available forms during dredged material disposal, could stimulate the growth of excessive amounts of algae and other aquatic plants in estuaries. This paper discusses the results of that portion of a comprehensive study conducted by the authors which assessed the significance of dredged sediments as a source of nutrients for U.S. estuarine waters.

Laboratory studies were conducted on waterway sediments from over 20 marine, estuarine, and freshwater tidal areas to evaluate the suitability of the elutriate test for estimating contaminant release from dredged sediments during open water disposal. Disposal operations involving sediments from nine of these areas were monitored for nutrient release to the water column.

Sediment kjeldahl nitrogen concentration generally ranged from ∿ 200 to 4,000 mgN/kg with a mean concentration of ∿ 1,550 mgN/kg. Elutriate tests generally showed a release of soluble nitrogen (ammonia plus nitrate) to 0.5 to 35 mgN/1. Total phosphorus concentrations of the sediments studied averaged ∿ 950 mgP/kg and ranged from ∿ 100 to 3,750 mgP/kg. Response of sediment-associated and soluble phosphorus to elutriation ranged from a decrease in soluble ortho P concentration by 0.6 mgP/1 to an increase by 1.6 mgP/1. Generally, relatively small amounts of nitrogen and phosphorus are released during elutriation and would be expected to be released during open water disposal of hydraulically dredged sediments; this was substantiated by intensive field monitoring of disposal operations. While there is no relationship between the bulk sediment content of N or P and the amounts of available forms released in elutriate tests, the maximum sediment nitrogen released in available forms was about 1 percent; maximum soluble orthophosphate released was less than 0.1 percent of the total P content of the sediment. Even less release would be expected during barge-scow disposal of mechanically dredged sediment.

This study demonstrated that modified elutriate tests provide an indication of sediment-associated contaminant behavior during open water disposal of hydraulically dredged sediment. The key to the proper use of these procedures is having a knowledge of the hydrodynamic characteristics of the disposal area, the limiting nutrient in the area of concern, and, for phosphorus, a knowledge of the iron system in the sediments being dumped.