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Assessing and Cleaning Up Contaminated Sediments. ESR 410/510 Environmental Cleanup and Restoration. Introduction. Sediment contamination problems have been recognized for many years Widespread concern did not appears until 1970s Sediments of virtually all active harbors are contaminated

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Assessing and cleaning up contaminated sediments

Assessing and Cleaning Up Contaminated Sediments

ESR 410/510

Environmental Cleanup and Restoration


  • Sediment contamination problems have been recognized for many years

  • Widespread concern did not appears until 1970s

  • Sediments of virtually all active harbors are contaminated

  • Also, many other major waterways such as the Great Lakes, Puget Sound, the Hudson River

Sediment characterization
Sediment Characterization

  • Grab sampling at low tide

  • Sediment surface showing biological activity

Sediment dwelling benthic organisms
Sediment Dwelling (Benthic) Organisms

  • Eel Grass

    Zostera marina

    Requires abundant light and clear water. Excess algae kills it off.

Sediment dwelling benthic organisms1
Sediment Dwelling (Benthic) Organisms

  • Polychaete worms

    Burrow to 20-30 cm into sediments, mixing shallow and deep sediment material and associated contaminants. (Bioturbation)

Sediment dwelling benthic organisms3
Sediment Dwelling (Benthic) Organisms

  • Burrowing shrimp, Callianassa californiensis

  • Southern rock lobster, Jasus novahollandiae

  • Mole crab, Emerita analoga

Sediment dwelling benthic organisms4
Sediment Dwelling (Benthic) Organisms

  • Mole crab (Emerita)

  • Horseshoe crabs (Limulus polyphemus)

The benthos we eat
The Benthos We Eat

  • Rock sole

  • Softshell clams

  • Blue crab

  • Mussel

Major contaminants of concern in sediments
Major Contaminants of Concern in Sediments

  • Persistent toxic organics (pesticides, PCBs, PAHs)

  • Heavy metals (lead, mercury, cadmium, etc.)

Sediment sampling mackereth pneumatic corer
Sediment Sampling:Mackereth Pneumatic Corer

What constitutes contaminated sediments
What Constitutes “Contaminated Sediments”

  • ALL sediments contain at least traces of metals, including some very toxic metals

  • Many persistent organic compounds like DDT and PCBs are globally distributed at low levels

  • So the mere presence of “contaminants” does not mean a sediment is contaminated by local sources or to a worrisome level.

Defining problem contamination the reference approach
Defining problem Contamination: The Reference Approach

  • Earliest scientific approach

  • Compare levels in site samples to “natural” or “background” levels

  • Background levels drawn from what are believed to be relatively “pristine” locations

  • If samples > background, clean up accordingly

  • But what to compare with and how?

  • Also, the method does not account for mixtures of chemicals nor the “bioavailability” of the chemical


  • Just because a chemical is found in the environment does not mean it is available for uptake by organisms (bioavailable)

  • E.g., toxic metal ions trapped inside rock particles (like ore) are not bioavailable unless the ore dissolves

  • Many other natural processes limit the bioavailability of certain forms, particularly with metals and with hydrophobic organic compounds

Sediments strongly bind many pollutants
Sediments Strongly Bind Many Pollutants

  • Sediments carry high contaminant load because pollutants “stick” to them

  • If a contaminant has little tendency to “stick”, then of course, it probably will not end up or persist in the sediment

  • But, if pollutants bind extremely well to sediments, they may pose little or no risk

  • So, how strong is the binding?

Partitioning of contaminants between sediments and water
“Partitioning” of Contaminants Between Sediments and Water

Dissolved chemical (mobile, available)

Sediment particles

Interstitial or Pore Water

Adsorbed chemical (immobile, less available)

Equilibrium partitoning
Equilibrium Partitoning Water

  • What’s in the interstitial water is in proportion to what’s on the solids

  • E.g., suppose100 molecules on the solid and 10 in the water. Double the contamination and its 200 on the solids and 20 in the water

Asdorption modeling organics like other organics
Asdorption Modeling: Water“Organics like other Organics”

  • Sediments typically contain some natural organic matter (“humus”-like material)

  • Organic contaminants bind strongestto natural organic matter in the sediment

  • Binding strength depends on compound AND on organic content of the sediment

Predicting binding of organic contaminants
Predicting Binding of Organic Contaminants Water

  • “Kp” = overall partition coefficient

  • Sed. Conc. = Kp x Water Conc.

  • Koc = “generic” coefficient for compound (tabulated)

  • Kp =focx Koc

  • focis the fraction of organic carbon in sediment

Example: Water

  • PCB: Typical Koc = 200,000 L/kg

  • Sediment with 2% organic content (foc = 0.02)

  • Sed Conc = 0.02(200,000) x Water Conc

  • Ratio of Water Conc/Sed Conc = 1/(0.02)(200,000) = 0.00025

  • Virtually ALL the PCB stays with the sediment, not in the water

Pcb in the columbia slough
PCB in the Columbia Slough Water

  • Sediment PCB: ~30 ug/kg

  • foc = 0.02

  • Calculated water PCB: <15 ng/L (parts per trillion; below detection)

  • But bioconcentration occurs later: Carp tissues contain up to 850 ug/L (magnification of 57,000X !!)

Interstitial water approach
Interstitial water Approach Water

  • Directly sample and measure interstitial water

  • But does not work for sediments above waterline (tidal zones, floodplains)

  • Very hard to do outside a research settling

Sediment bioassays
Sediment Bioassays Water

  • Various means of testing in lab the organisms with sediment samples

  • Costly, but useful

  • Hard to say connection to actual system in some cases

Pathways for contaminant transport
Pathways for Contaminant Transport Water

  • Desorption during dredging: sediment --> water

  • Mobilization of non-settling particles during dredging

  • Desorption or mobilization during handling/dewatering

  • Desorption or mobilization during final confinement

Sediment cleanup options
Sediment Cleanup Options Water

  • No action: Let nature cover over or dilute contaminated sediments with fresh sedimentation

    • Works only if all sources of contaminant are shut off, so some “action” often required

    • Good only if natural processes fast enough to mitigate danger

    • Not good if severe imminent harm

Dredging as a remediation method
Dredging as a Remediation Method Water

  • Appropriate where environmental impacts are severe

  • Where physical disruptions like strong currents, flooding, navigational dredging are likely to occur

  • Biggest problems are

    • Resuspension of dirty sediments

    • Severe disruption of the benthos

    • Where to put the spoils?

Quantities of dredged sediments in the northwest alone
Quantities of Dredged Sediments in the Northwest Alone Water

  • 250 million cubic yards from 25,000 miles of navigation channels

  • 75 million cubic yards from permits

  • 325 million cubic yards each year

How much dredged material
How Much Dredged Material? Water

> 5½ feet deep over

Washington, D.C.

> 1½ feet deep

over Chicago

Authorization Water

  • Rivers and Harbors Act of 1899,

    • Section 10

  • Clean Water Act

    • Section 404

  • Marine Protection, Research, and Sanctuaries Act

    • Section 103

Regulatory authorities
Regulatory Authorities Water

  • U.S. National Marine Fisheries Service

  • U.S. Fish and Wildlife Service

  • U.S. Environmental Protection Agency

  • State Fish and Game Agencies

  • State Water Quality Certifying Agencies

  • State Coastal Zone Management Agencies

  • Other Federal and State Agencies

TIER I Water

  • Existing

  • Data




  • Physical/Chem. data

  • Screening Tests

  • Predictive models


  • Toxicity Tests

  • Bioaccumulation Tests


  • Chronic Sublethal Tests

  • Steady-State Bioaccumulation Tests

  • Risk Assessment

Basic dredge types
Basic Dredge Types Water

  • Hydraulic

    • Pipeline

    • Hopper

  • Mechanical

    • Clamshell

  • Other / Combinations

Factors in selection of dredging equipment
Factors in Selection of WaterDredging Equipment

  • Physical characteristics of sediments

  • Quantities to be dredged

  • Dredging depth

  • Distance to disposal area

  • Physical environment of and between areas

  • Contamination level of sediments

  • Method of disposal

  • Production required

  • Types of dredges available (US or foreign)

Disposal of spoils
Disposal of Spoils Water

  • Simple in-water disposal usually not an option for contaminated sediments

  • Upland disposal gets it safely out of the water but presents many problems

Upland disposal
Upland Disposal Water

  • If not “hazardous” under RCRA, may be able to just pile it up on the shore

  • Near-shore upland sites are cheaper and can use a slurry pipeline, but may not be available (NIMBY or other limitations on land use)

  • If not near the shore, transportation adds big costs (truck or rail; pipeline not usually feasible)

Barview Water

  • Slurry inlet pipe

Barview Water

  • Variable-depth outlet weir

Hazardous waste disposal
Hazardous Waste Disposal Water

  • If sediment fails RCRA test, then must either:

    • Treat it on-site to make it pass the haz waste test, then ship to disposal

    • Take it off-site for treatment and disposal

    • Ship directly it to a Subtitle C (Haz Waste) landfill

  • For large loads the first is the cheapest, for smaller loads, the last 2 may be best

Safe in water disposal confined disposal facilities
Safe In-Water Disposal: WaterConfined Disposal Facilities

  • Like an “underwater landfill”

  • Sediments are disposed in the water but are confined inside a secure facility

  • Can be right next to shoreline

  • Might be out in deeper water

  • If you don’t move the sediments, but just confine them in-situ, it’s called “capping”


  • Site Assessment tells us if we have a problem that must be dealt with

  • We may conclude that the no-action approach may be the best

  • If action is required we must decide whether dredging is required or if in-situ capping will work

Conclusions cont d
Conclusions, cont’d Water

  • Dredging has a whole lot of safety issues, especially controlling resuspended sediments

  • Disposal methods must be assessed with respect to safety, reliability, costs, and public acceptance

Conclusions cont d1
Conclusions, cont’d Water

  • In-situ capping can be a good alternative to dredging

  • But there are many situations where it is not feasible

    • Very shallow waters

    • Fast or unpredictable currents

    • Powerful tides

    • Navigational dredging

    • Anchor weighing by large ships