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Title: Water quality impacts of aquatic dredged material disposal (laboratory investigations)
Authors: Burks, Sterling A.
Engler, Robert M.
Keywords: Aquatic ecology
Dredging spoil
Dredged material
Publisher: U. S. Army Engineer Waterways Experiment Station
Series/Report no.: Technical Report (Dredged Material Research Program (U.S.)) ; no. DS-78-4
Abstract: Natural processes in aquatic ecosystems tend to concentrate heavy metals, chlorinated hydrocarbons, pesticides, nutrients, and oil and grease compounds in bottom sediments. These contaminants are not very soluble in water under the conditions that normally occur in oxygenated uncontaminated surface waters. Therefore, introducing high concentrations of these contaminants into aquatic ecosystems will generally result in an equilibrium condition where most of the contaminant will be sorbed (adsorbed and absorbed) by suspended particulate material and then deposited on the bottom when the suspended material settles. The time necessary to achieve the equilibrium condition depends upon the physicochemical conditions in the aquatic system and the quantity and duration of the contaminant introduction. There has been concern that dredging and open-water disposal operations may release these trapped contaminants to the overlying water column again and thus have the potential to damage aquatic environments. Dredged Material Research Program (DMRP) reports and other literature indicate that dredging operations have the potential to temporarily mobilize or release some contaminants from the sediments. During disposal operations, the anaerobic sediments are mixed with aerated surface water, and a complex chemical interaction occurs. Heavy metals, such as cadmium, copper, chromium, lead, and zinc, are stabilized in the oxygen-free sediments as insoluble sulfides. When the sediments are oxygenated, these metals form oxides that are slightly more soluble than the metal sulfides. The reduced forms of iron and manganese in oxygen-free sediments are, however, more soluble than the oxides formed in the overlying water. As a result, immediately after disposal of sediments in oxygenated surface waters, iron and manganese hydrous oxides begin to coagulate and precipitate. The surface of the iron hydrous oxide coagulates has a strong affinity for other heavy metals and effectively sorbs or removes the slightly soluble metal oxides of cadmium, copper, chromium, lead, and zinc from the water column. The cycle of phosphorus between the oxygenated surface waters and the reduced sediment closely parallels that of iron. Therefore, during disposal of dredged material in open waters, soluble forms of phosphorus may be temporarily released until they are scavenged from the water column by ferric hydrous oxide coagulates. The other major nutrient, nitrogen, is accumulated in sediments in the reduced form ammonia, which is highly water soluble and thus can be readily released to the water column during disposal operations in open water. Chlorinated hydrocarbon pesticides and oil and grease compounds in sediments are not released in significant amounts during disposal in open waters. These compounds are generally not very water soluble and thus are rapidly sorbed by suspended solids. Disposal of dredged material containing high concentrations of chlorinated hydrocarbon pesticides will probably not cause any significant effects on water column organisms. However, the pesticide and oil and grease compounds could have an adverse effect on benthic organisms attempting to recolonize the new sediments. These potential impacts were investigated in the DMRP and the results are synthesized in another DMRP report. The effects of dredged material disposal in open water are temporary and primarily restricted to the immediate vicinity of the disposal area. Toxic levels of ammonia, and possibly some of the heavy metals, may be released immediately after disposal. However, within a few hours, the levels of metals should be decreased below toxic concentrations by dilution and sorption onto iron hydrous oxide coagulates. Ammonia is usually rapidly reduced below toxic concentrations by dilution; biooxidation of ammonia may also occur but is a slower process, usually requiring several days. Therefore, careful monitoring of ammonia concentrations at dredged material disposal sites should be performed to prevent buildup of toxic concentrations of this compound.
Description: Technical Report
Gov't Doc #: Technical Report DS-78-4
Rights: Approved for Public Release; Distribution is Unlimited
Appears in Collections:Technical Report

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