sediment quality criteria william fish civil engineering environmental sciences and resources
TRANSCRIPT
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