Presentation on theme: "Ex Situ Treatments Following Dredging and/or Excavation MethodComments DewateringExtraction of water from removed sediment Particle separationSelective."— Presentation transcript:
Ex Situ Treatments Following Dredging and/or Excavation MethodComments DewateringExtraction of water from removed sediment Particle separationSelective removal of sediments (e.g., fine particles) that contain relatively high concentrations of trace metals Soil washingExtraction of metals from the sediments using a water-based solvent which may or may not be combined with other reagents VitrificationHeating of contaminated materials to high temperatures to produce a glass-like non- leachable material with low-permeability Solidification-stabilizationAddition of binding agents to produce a hardened material of low-permeability
In Situ Treatments PhytoremediationUse of plants to extract trace metals from the soils and sediments In situ vitrificationSame as above, but heat source is typical produced by an electrical current delivered through electrodes In situ soil washingSame as above, with the exception that solutions are applied and extracted to in situ materials EncapsulationEncasing of contaminated material with a low- permeability substance ElectrokineticsUse of an electric current to concentrate and remove ions In situ (subaqueous) cappingThe placement of a clean, isolating material over contaminated sediment in a subaqueous environment without relocating or causing a major disruption to the original channel bed material Soil and sediment cappingThe placement of clean material over contaminated sediment out in a subareal environment
Site Decontamination through Excavation/Dredging The most commonly used remediation strategy Favored because: –(1) it has been shown to be an effective remedial technology in a wide range of riverine and marine environments. –(2) it is consistent with recent legislation which favors remedial technologies that permanently reduce the volume, toxicity or mobility of the contaminant of concern.
Definitions Excavation is defined as the subareal extraction of sediment using earthmoving equipment (e.g., as backhoes and front-end loaders) Dredging refers to the extraction of sediment from an underwater environment (NRC 1997). –Environmental Dredging: conducted to remove contaminated sediment –Navigational Dredging: conducted to maintain navigable channels and other facilities
Use Has been used at about 100 Superfund Sites across the country; most have targeted sites with less than 50,000 yds 3 of material (Romgnoli et al., 2002) The volume of material removed during these operations can vary dramatically, ranging from 10 1 to 10 6 m 3. Following the Aznalcóllar tailings dam failure in Spain, for example, more than 4.7 x 10 6 m 3 of tailings and contaminated sediment was removed from along the Rios Agrio and Guadiamar in as part of an emergency cleanup operation (Hudson-Edwards et al. 2003). In the U.S., Cleland (2000) found in a study of 89 completed, ongoing, or planned sediment cleanup projects in the U.S., that approximately 1.4 million cubic yards of material had been removed by dry excavation methods.
Advantages of Dredging/Excavation Decontamination of the site generally; Allows for greater flexibility in land-use. Perceived as having a lower risk of failure than other methods; Can be conducted in a predictable time frame; Costs can be predicted reasonably well.
Other Considerations of Dredging/Excavation Quantity of material to be excavated – it can be very expensive; Where will the material go (CDF, CAD, existing landfill)? Permission and license requirements Resuspension and environmental degradation problems Questions regarding effectiveness of decontamination process
Figure 6-1, page 6.1, in USEPA, 2005, Contaminated sediment remediation guidance for Hazardous Waste Sites. EPA-540-R , OSWER No permission required.
Figure 3 and 4, page 186, from Romagnoli, R., Doody, J.P., VanDewalker, H.M., and Hill, S.A., Environmental dredging effectiveness: Lessons Learned. In: A. Porta, R.E. Hinchee, and M. Pellei (eds.), Management of Contaminated Sediments, Battelle Press, Columbus, Ohio.
Figure 1, pg. 1 in J.J. Steuer (2000). A mass-balance approach for assessing PCB movement during remediation of a PCB-contaminated deposit on the Fox River, Wisconsin. USGS Water-Resources Investigations Report
Figure 1, page 183, from Romagnoli, R., Doody, J.P., VanDewalker, H.M., and Hill, S.A., Environmental dredging effectiveness: Lessons Learned. In: A. Porta, R.E. Hinchee, and M. Pellei (eds.), Management of Contaminated Sediments, Battelle Press, Columbus, Ohio.
Change in Concentration (μg/g) SiteContaminant Pre- remediation Post- Remediation Percent Change Grasse River, NYPCBs --Average --Maximum 518 1, Sheboygan River, WIPCBs --Average --Maximum 640 4, River Raisin, MIPCBs --Maximum1, St. Lawrence River, NYPCBs --Average --Maximum 200 8, < Lower Fox River, WI (Deposit N) PCBs --Average --Maximum Lower Fox River, WI SMU 56/57 PCBs ---Maximum Manistique River, MIPCBs ---Maximum4, Degree of Residual Contamination From Miller JR and Orbock Miller, SM: 2007, Contaminated Rivers, Springer
Figure 6-8, p. 6-28, USEPA (2005) Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. EPA-540-R
Soil Washing Soil washing is a general term used for the extraction of a wide range of organic and inorganic contaminants from soils and sediment using a water- based fluid as a solvent Two Basic Components –Particle separation of excavated materials; –Leaching of contaminants from sediments/soils in situ or that have been excavated.
Particle Separation of Excavated Materials Assumes that contaminants are segregated with one size fraction of the alluvium which can be separated and disposed of separately; General idea is to remove the contaminated fraction – which in most cases is the fine-grained sediment – and return the coarser, uncontaminated sediment to the site. This reduces the overall amount of material that must be disposed of in a landfill or other type of disposal facility; These methods can be relatively expensive - $1.5 M per hectare.
Particle Separation of Excavated Materials Variety of engineering methods exist to remove the fine fraction; these include sieving, flotation techniques, hydrocyclones, fluidized-bed separation, or spiral classifiers. Rulkens et al., 2003
Leaching Methods of Soil Washing Method is not all that commonly used in U.S., but is used in EU. Likely to be used in U.S. more in future. Can be expensive. Chemicals that are used include inorganic acids, organic acids, and complexing agents such as EDTA, or a combination of these chemicals.
Figure 1, page 112, in Krishnan, R. Parker, H.W., and Tock, R.W., Electrode assisted soil washing. Journal of Hazardous Materials 48: Requires that the soils are permeable, thus favors coarser-grained sediments.