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|Development of a simplified column test for evaluation of thickness of capping material required to isolate contaminated dredged material
|Long-Term Effects of Dredging Operations Program (U.S.)
Brannon, James M.
Sturgis, Thomas C.
|Environmental Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Abstract: A procedure using a small-scale (22.6 l) leaching column was developed for predicting the cap thickness required to chemically seal contaminated dredged sediment from the overlying water column. Several parameters, including dissolved oxygen depletion rates and release rates of ammonium-nitrogen, orthophosphate-phosphorus, and manganese, were evaluated as tracers for use in the predictive test. In addition, several tests were run using potassium chloride amendments to the contaminated sediment prior to capping to determine if this salt would be a suitable tracer. Dissolved oxygen depletion and ammonium-nitrogen release were generally the best tracers to use for predictive purposes. On occasion, orthophosphate-phosphorus release was also an effective tracer. Use of manganese or potassium chloride as tracers proved ineffective. When large-scale (250 l) verification tests were conducted using various capping materials and cap thicknesses that were less than the thickness predicted by the small-scale tests, it was found that these thicknesses were generally unsatisfactory as chemical and biological seals for the contaminated sediments. Tests conducted with cap thicknesses greater than the predicted thicknesses demonstrated that these thicknesses were effective in preventing transfer of contaminants into the overlying water column and the biota in the water column. Sandworms (Nereis virens) were able to burrow through the cap material and into the underlying contaminated sediment. As a result, these organisms accumulated contaminants from the underlying sediment. Overall, the verification tests indicated that the small-scale predictive tests provided the information required to determine the cap thickness necessary to obtain a satisfactory chemical seal; however, it was apparent that additional cap material was necessary to obtain the most effective seal against biological activity. It is important that the cap thickness selected with the small-scale tests be shown to seal a minimum of two of the three parameters used to measure cap effectiveness. To prevent exposure of burrowing benthic organisms to contaminated sediment, it is recommended that a safety margin be added to the thickness required to achieve a chemical seal. This safety margin is determined by the depth to which the deepest burrowing benthic organism within the region can reach. The sum of the thickness required to achieve the safety margin and the thickness required for the chemical seal is the thickness necessary to isolate the contaminated sediment chemically and biologically from the overlying water column and the aquatic biota. This work does not include hydrodynamic effects that may result in resuspension and/or scouring of cap material. Procedures to counter the effects of hydrodynamic processes require engineering considerations that will be addressed in later guidance.
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