Tommy E. Myers

Centrifuge simulation of the consolidation characteristics of capped marine sediment beds

Abstract Marine sediment capping is a technique where clean sand or sediment is placed over contaminated sediment to reduce the migration of contaminants to the environment. Environmental regulations have limited the use of in situ sediment capping due to concerns about the contaminant migration through the cap. A series of centrifuge tests were conducted to simulate the effects of consolidation settlement of capped marine sediment. This study describes the testing and monitoring of the centrifuge tests. The results from the centrifuge tests are interpreted and compared to predictions made by the PSDDF computer program, which can qualitatively estimate the consolidation settlement of capped marine sediment. Centrifuge tests were utilized to predict the consolidation of marine sediment caused by the placement of a capping layer. The centrifuge tests used the modeling of models technique to verify that correct modeling procedures were utilized. In this study, the maximum deviation between the centrifuge test results and PSDDF prediction was 20%. Thus, designers should utilize PSDDF consolidation settlement results with caution. Dye tracer studies showed the importance of consolidation-induced advective transport of contaminants. Thus, the capping layer must be appropriately designed to reduce the effects of consolidation-induced advective transport. This may be accomplished by adding a reactive barrier or geosynthetic barrier layer to the cap design.

Determination of the environmental impact of consolidation induced convective transport through capped sediment

The presence of contaminated sediment poses a barrier to essential waterway maintenance and construction in many ports and harbors, which support 95% of US foreign trade. Cost effective solutions to remediate contaminated sediments in waterways need to be applied. Capping is the least expensive remediation alternative available for marine sediments that is unsuitable for open water disposal. Dredged material capping and in situ capping alternatives, however, are not widely used because regulatory agencies are concerned about the potential for contaminant migration through the caps. Numerous studies have been conducted on the effects of diffusion through caps, however, there is a lack of experimental data documenting the effects of consolidation induced transport of contaminants through caps. This study examines consolidation induced convective contaminant transport in capped sediment utilizing a research centrifuge. In this study, consolidation induced convective transport was modeled for 7h at 100 x g, which modeled a contaminant migration time of 8 years for a prototype that was 100 times larger than the centrifuge model. In this study, hydrodynamic dispersion was a function of the seepage velocity. And, advection and dispersion dominated the migration of contaminants. Centrifuge model results were compared to an analytical solution for advection and dispersion. The advection-dispersion equation demonstrated that the centrifuge test is a conservative estimate for predicting contaminant transport. In conducting sensitivity analysis of the advection-dispersion equation to the centrifuge modeling, as hydrodynamic dispersion decreased, the time for contaminant breakthrough increased. Moreover, as the sediment to water distribution coefficient increased, the contaminant concentration into the overlying water decreased.

The migration of contaminants through geosynthetic fabric containers utilized in dredging operations

Abstract Recent changes in environmental regulations to protect the water column have prohibited the open water disposal of dredged sediment from the New York Harbor. These restrictions have decreased the average amount of dredging by the New York Port Authority by 70% for 1996. As a result, the New York Harbor will lose about a foot of depth each year if the contaminated sediments are not dredged. Decreases in the depth will have a severe economic impact, as larger cargo ships will dock in deeper ports. Because of the restrictions and perceived political problems with dredging and open water disposal of the contaminated materials, the New York Port Authority and the Corps of Engineers are investigating the use of geosynthetic fabric containers (GFCs) to reduce the movement of contaminated sediments outside the boundary of the disposal site and to decrease the impact of the sediment on the water column. During the dredging operation, the barge will be lined with the appropriate GFC to filter the dredge sediments. This laboratory study investigates the migration of fines and contaminants through GFCs. Contaminated dredges sediment was characterized for the physical and chemical properties. Bench scale filtration and barge simulation tests were conducted on the contaminated sediment and GFC configurations to determine the amount of total suspended solids that would be released to the water column.

Modeling Contaminant Transport Through Capped Dredged Sediment Using a Centrifuge

The presence of contaminated sediment poses a barrier to essential waterway maintenance and construction in many ports and harbors, which support 95% of U.S. foreign trade. Cost effective solutions to remediate contaminated sediments in waterways need to be applied. Capping is the least expensive remediation alternative available for marine sediments that is unsuitable for open water disposal. Dredged material capping and in situ capping alternatives, however, are not widely used because regulatory agencies are concerned about the potential for contaminant migration through the caps.Numerous studies have been conducted on the effects of diffusion through caps, however, there is a lack of experimental data documenting the effects of consolidation induced transport of contaminants through caps. This study examines consolidation induced advective contaminant transport in capped sediment utilizing a research centrifuge. Centrifuge modeling simulates the increase in the gravitational acceleration (g) of a prototype, which is N times larger than the model, where N is gravitational acceleration factor. For contaminant migration, the time of transport in the model is inversely proportional to the square of the acceleration factor in the prototype. In this study, consolidation induced advective transport was modeled for 22.5 hours at 100-g, which modeled a contaminant migration time of 25 years for a prototype that was 100 times larger than the centrifuge model. Thus, advection and dispersion dominated the migration of contaminants. The centrifuge modeling results were compared to an analytical solution for advection and dispersion.

Recent Developments in Formulating Model Descriptors for Subsurface Transformation and Sorption of Trinitrotoluenea

Recent published data show that transformation and sorption are key processes involved in the subsurface transport of TNT. The state-of-the-art understanding of TNT soil transformation and sorption phenomena is summarized below: There is unequivocal evidence of reductive transformation of TNT in soils. However, soil properties affecting TNT transformation are only partially understood. Edaphic factors such as redox are probably important since highest reductive transformation rates occur under anaerobic conditions. TNT transformation products include 2A-DNT, 4A-DNT, 2,4-DANT, and 2,6-DANT. Azoxytoluenes have also been reported. Triaminotoluene may be an important TNT transformation product, but this product has not been routinely measured. TNT soil sorption is rapid and can be modeled as equilibrium controlled. Poor mass balances are difficult to interpret owing to lack of information on transformation products. Various irreversible disappearance mechanisms other than transformation to elutable transformation products are possible.