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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 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
Sediment Characterization • Grab sampling at low tide • Sediment surface showing biological activity
Sediment Dwelling (Benthic) Organisms • Eel Grass Zostera 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
Sediment Dwelling (Benthic) Organisms • Mole crab (Emerita) • Horseshoe crabs (Limulus polyphemus)
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.)
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
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 Dissolved chemical (mobile, available) Sediment particles Interstitial or Pore Water 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 strongestto 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 =focx Koc • focis 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 • 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? > 5½ feet deep over Washington, D.C. > 1½ feet deep over Chicago
Authorization • 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 INCREASING COMPLEXITY/ COST ENHANCED RESOLUTION • Physical/Chem. data • Screening Tests • Predictive models TIER III • Toxicity Tests • Bioaccumulation Tests TIER IV • Chronic Sublethal Tests • Steady-State Bioaccumulation Tests • Risk Assessment
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)
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