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|Title:||Effectiveness of capping in isolating contaminated dredged material from biota and the overlying water|
|Authors:||Long-Term Effects of Dredging Operations Program (U.S.)|
Brannon, James M.
Hoeppel, Ronald E.
Sturgis, Thomas C.
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: The effectiveness of capping in chemically and biologically isolating contaminated dredged material was investigated using large- (250 𝓁) and small- (22.6 𝓁) scale laboratory reactor units. The ability of various cap materials to isolate contaminated dredged mat erial was assessed in the large reactor units by following the movement of chemical contaminants and microbial spores contained in the capped dredged material into the overlying water column and by monitoring the biological uptake of chemical contaminants by clams and polychaetes. The depth of cap material needed to chemically isolate contaminated dredged material was assessed in the small-scale reactor units. Changes in overlying water concentrations of dissolved oxygen (DO), ammonium nitrogen, manganese, and orthophosphate were monitored following isolation of the water column from air by placing a 4-cm layer of mineral oil on the surface. The constituents analyzed were selected due to their mobility under anaerobic conditions, ease of measurement, and generally high concentrations in contaminated dredged material compared to clean cap materials. Three capping materials, sand, clay (New Haven Harbor sediment), and silt (Vicksburg silt), were evaluated for their efficiency in preventing transfer of contaminants from a contaminated sediment into the overlying water column and biota. In the presence of bioturbating polychaetes (Nereis virens) at densities of 100 large animals per square metre, a 50-cm cap of any of the three materials tested in the large chamber experiments was effective in preventing the transfer of chemical constituents and microbial spores to the overlying water and nonburrowing biota. Chemical analysis of polychaete tissue and visual observation showed that the polychaetes penetrated both the 5-cm and 50-cm caps of all materials tested. A 5-cm cap in the presence of polychaetes was not completely effective in preventing the transfer of contaminants and microbial spores in the dredged material into the overlying water and biota. However, a 5-cm New Haven Harbor sediment or Vicksburg silt cap was relatively more effective than a 5-cm sand cap in preventing the movement of polychlorinated biphenyl or polyaromatic hydrocarbon compounds through the cap and into biota (clams). These bioaccumulation results were in relatively close agreement with results obtained in small reactor units for DO depletion and ammonium nitrogen release. The efficiency of cap materials in preventing DO depletion and ammonium nitrogen releases attributable to the capped dredged material was in the order: New Haven sediment > Vicksburg silt > sand. Cap materials with higher proportions of clay and silt should, therefore, be relatively more effective than cap materials consisting predominately of sand in preventing contaminant movement into the overlying water and biota. However, a thick cap (50 em or more) of any of the materials tested effectively isolated the overlying water and nonburrowing biota from the contaminated sediment.
|Appears in Collections:||Technical Report|