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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
Also, many other major waterways such as the Great Lakes, Puget Sound, the Hudson River
Early Evidence of Sediment
Contamination
Replicate studies reveal widespread problems
Sediment Characterization
Grab sampling at low tide
Sediment surface showing biological activity
Sediment Dwelling (Benthic) Organisms
Eel GrassZostera marina
Requires abundant light and clear water. Excess algae kills it off.
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) Organisms
Nereids (Polychaeta)
Sediment Dwelling (Benthic) Organisms
Burrowing shrimp, Callianassa californiensis
Southern rock lobster, Jasus novahollandiae
Mole crab, Emerita analoga
Mole crab (Emerita)
Horseshoe crabs (Limulus polyphemus)
Sediment Dwelling (Benthic) Organisms
The Benthos We Eat
Rock sole
Softshell clams
Blue crab
Mussel
Major Contaminants of Concern in Sediments
Persistent toxic organics (pesticides, PCBs, PAHs)
Heavy metals (lead, mercury, cadmium, etc.)
Van Veen Hand Dredge
Ponar Box Corer
Ekman Box Corer
Large, Ekman-type Corer
Sediment Sampling:Mackereth Pneumatic Corer
Multi-Corer Array
Benthic Biota Samplers (sleds)
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
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
Bioavailability 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
Bioavailability of Pb (thanks Mark)
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
Sediment particles
Interstitial or Pore Water
Dissolved chemical (mobile, available)
Adsorbed chemical (immobile, less available)
Equilibrium Partitoning
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”
Sediments typically contain some natural organic matter (“humus”-like material)
Organic contaminants bind strongest to natural organic matter in the sediment
Binding strength depends on compound AND on organic content of the sediment
Predicting Binding of Organic Contaminants
“Kp” = overall partition coefficient Sed. Conc. = Kp x Water Conc. Koc = “generic” coefficient for
compound (tabulated) Kp =foc x Koc
foc is the fraction of organic carbon in sediment
Example:
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 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 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
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 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
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 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
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?
> 1½ feet deepover Chicago
> 5½ feet deep overWashington, D.C.
AuthorizationAuthorization
Rivers and Harbors Act of 1899, Section 10
Clean Water Act Section 404
Marine Protection, Research, and Sanctuaries Act Section 103
Regulatory Authorities
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• Existing Data
TIER II• Physical/Chem. data• Screening Tests•Predictive models
TIER III• Toxicity Tests• Bioaccumulation Tests
TIER IV• Chronic Sublethal Tests• Steady-State Bioaccumulation Tests• Risk Assessment
EN
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https://www.nwp.usace.army.mil/ec/h/hr/
Basic Dredge Types Hydraulic
Pipeline Hopper
Mechanical Clamshell
Other / Combinations
Factors in Selection ofDredging 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
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
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)
Upland Sediment Disposal: Barview, OR
Barview, OR
Barview Slurry inlet pipe
Barview Variable-depth outlet weir
Hazardous Waste Disposal 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
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”
Confined Disposal Alternatives
CONCLUSIONS
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
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’d
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