Sediment Quality Criteria

William FishCivil Engineering

Environmental Sciences And Resources

Introduction Sediment disposal options often

depend on the chemical and biological quality of the materials

Sediment Quality Criteria hinge on composition and chemistry of sediments

Sediment Characterization

Major Contaminants of Concern in Sediments Persistent toxic organics

(pesticides, PCBs, PAHs) Heavy metals (lead, mercury,

cadmium, etc.)

Reference Approach

Earliest Compare to “natural” or

“background” levels and clean up accordingly

But what to compare with and how?

Sediments Strongly Bind these Pollutants Sediments carry high contaminant

load because pollutants “stick” to them

If pollutants bind extremely well to sediments, they may pose little or no risk

So, how strong is the binding?

Equilibrium Partitoning

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

Must assume chemical equilibrium, not always achieved

Interstitial water is not always the main pathway of biota exposure

Asdorption Modeling:“Organics like 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 Ratio of Water Conc/Sed Conc =

1/(0.02)(200,000) = 0.00025

Virtually ALL PCB stays with sediment, not water

Columbia Slough

Sediment PCB: ~30 ug/kg foc = 0.02 Calculated water PCB: <15 ng/L

(parts per trillion; below detection) But bioconcentration occurs: Carp

tissues contain up to 850 ug/L

Metals and “AVS” AVS = Acid Volatile Sulfides Easy measure of natural reactive

sulfides in sediments AVS combines with toxic metals

and renders them insoluble AVS thus immobilizes metals,

protects biota

AVS and Sediment Quality Criteria If AVS > Total Toxic Metals, then

assume they are all immobilized If Metals > AVS, reduce the metal

mobility in proportion to the AVS Unfortunately AVS may break

down if sediments are disturbed and exposed to air

Interstitial water Approach Directly sample and measure

interstitial water But does not work for sediments

above waterline (tidal) Very hard to do outside a research

settling

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 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