Assessing and Cleaning Up Contaminated Sediments ESR 410/510 Environmental Cleanup and Restoration

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

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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