Ex Situ Treatments Following Dredging and/or Excavation

1
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
2
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
3
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.

4
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

5
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.

6
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.

7
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

8
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.
9
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.
10
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.
11
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.
12
Degree of Residual Contamination
Change in Concentration (µg/g) Change in Concentration (µg/g)
Site Contaminant Pre-remediation Post-Remediation Percent 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 9.2lt100 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
From Miller JR and Orbock Miller, SM 2007,
Contaminated Rivers, Springer
13
Figure 6-8, p. 6-28, USEPA (2005) Contaminated
Sediment Remediation Guidance for Hazardous Waste
Sites. EPA-540-R-05-012.
14
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.

15
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.

16
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
17
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.

18
Requires that the soils are permeable, thus
favors coarser-grained sediments.
Figure 1, page 112, in Krishnan, R. Parker, H.W.,
and Tock, R.W., 1996. Electrode assisted soil
washing. Journal of Hazardous Materials
48111-119.