Ex Situ Treatments Following Dredging and/or Excavation Method Comments Dewatering Extraction of water from removed sediment Particle separation Selective removal of sediments (e.g., fine particles) that contain relatively high concentrations of trace metals Soil washing Extraction of metals from the sediments using a water-based solvent which may or may not be combined with other reagents Vitrification Heating of contaminated materials to high temperatures to produce a glass-like non-leachable material with low-permeability
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Ex Situ TreatmentsFollowing Dredging and/or Excavation
Method Comments
Dewatering Extraction of water from removed sediment
Particle separation Selective removal of sediments (e.g., fine particles) that contain relatively high concentrations of trace metals
Soil washing Extraction of metals from the sediments using a water-based solvent which may or may not be combined with other reagents
Vitrification Heating of contaminated materials to high temperatures to produce a glass-like non-leachable material with low-permeability
Solidification-stabilization Addition of binding agents to produce a hardened material of low-permeability
In Situ Treatments
Phytoremediation Use of plants to extract trace metals from the soils and sediments
In situ vitrification Same as above, but heat source is typical produced by an electrical current delivered through electrodes
In situ soil washing Same as above, with the exception that solutions are applied and extracted to in situ materials
Encapsulation Encasing of contaminated material with a low-permeability substance
Electrokinetics Use of an electric current to concentrate and remove ions
In situ (subaqueous) capping The 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 capping The 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 yds3 of material (Romgnoli et al., 2002)
• The volume of material removed during these operations can vary dramatically, ranging from 101 to 106 m3.
• Following the Aznalcóllar tailings dam failure in Spain, for example, more than 4.7 x 106 m3 of tailings and contaminated sediment was removed from along the Rios Agrio and Guadiamar in 1998-1999 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-05-012, OSWER 9355.0-85. No permission required.
DebrisRemoval
SedimentRemoval
Transport Staging Pretreatment Treatment
Air or Gas ResidueTreatment
Solids
Contaminated Solids
Contaminated
Solids
Water EffluentTreatment and/or
Disposal
Disposal and/or Reuse
Disposal and/or Reuse
Flow Diagram for Dredging and Excavation
Figure 3 and 4, page 186, from Romagnoli, R., Doody, J.P., VanDewalker, H.M., and Hill, S.A., 2002. Environmental dredging effectiveness: Lessons Learned. In: A. Porta, R.E. Hinchee, and M. Pellei (eds.), Management of Contaminated Sediments, Battelle Press, Columbus, Ohio.
Maximum = 33 on 1/20/1999
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10/2
2/98
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6/98
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1/6/
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8/24
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(A)
(B)
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 00-4245.
Figure 1, page 183, from Romagnoli, R., Doody, J.P., VanDewalker, H.M., and Hill, S.A., 2002. Environmental dredging effectiveness: Lessons Learned. In: A. Porta, R.E. Hinchee, and M. Pellei (eds.), Management of Contaminated Sediments, Battelle Press, Columbus, Ohio.
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Ave
rag
e C
on
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(mg
/kg
)
Pre-dredging During dredging Post-dredging
DownstreamNear Short
DownstreamNear Short
DownstreamFar Shore
DownstreamFar Shore
UpstreamNear Shore
UpstreamNear Shore
UpstreamFar Shore
UpstreamFar Shore
3-Week Caged Fish 6-Week Caged Fish
41.3
6.5
3.1
54
35
11 1111
0.62.8
0.22.0
1.1 0.131.1
0.39
5.02.7 2.20.5 0.38 0.64
0.230.22
Change in Concentration (μg/g)
Site ContaminantPre-
remediationPost-
RemediationPercent Change
Grasse River, NY PCBs--Average--Maximum
5181,780
75260
8685
Sheboygan River, WI PCBs--Average--Maximum
6404,500
39295
9493
River Raisin, MI PCBs--Maximum 1,400 136 90
St. Lawrence River, NY PCBs--Average--Maximum
2008,800
9.2<100
9599
Lower Fox River, WI(Deposit N)
PCBs--Average--Maximum
16-13061-186
14130
12-890-30
Lower Fox River, WISMU 56/57
PCBs---Maximum 710 17 98
Manistique River, MI PCBs---Maximum 4,200 1300 69
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-05-012.
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., 1996. Electrode assisted soil washing. Journal of Hazardous Materials 48:111-119.
Requires that the soils are permeable, thus favors coarser-grained sediments.
