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Sediment Quality Criteria

Sediment Quality Criteria. William Fish Civil 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.

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Sediment Quality Criteria

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  1. Sediment Quality Criteria William Fish Civil Engineering Environmental Sciences And Resources

  2. 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

  3. Sediment Characterization

  4. Major Contaminants of Concern in Sediments • Persistent toxic organics (pesticides, PCBs, PAHs) • Heavy metals (lead, mercury, cadmium, etc.)

  5. Reference Approach • Earliest • Compare to “natural” or “background” levels and clean up accordingly • But what to compare with and how?

  6. 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?

  7. 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

  8. Asdorption Modeling:“Organics like 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

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