Sediment Quality Criteria William Fish Civil Engineering Environmental Sciences And Resources

Introduction n Sediment disposal options often depend on the chemical and biological quality of the materials n Sediment Quality Criteria hinge on composition and chemistry of sediments

Sediment Characterization

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

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

Sediments Strongly Bind these Pollutants n Sediments carry high contaminant load because pollutants “stick” to them n If pollutants bind extremely well to sediments, they may pose little or no risk n So, how strong is the binding?

Equilibrium Partitoning n What’s actually in the interstitial water is in proportion to what’s on the solids n Must assume chemical equilibrium, not always achieved n Interstitial water is not always the main pathway of biota exposure

Asdorption Modeling: “Organics like Organics” n Sediments typically contain some natural organic matter (“humus”- like material) n Organic contaminants bind strongest to natural organic matter in the sediment n Binding strength depends on compound AND on organic content of the sediment

Predicting Binding of Organic Contaminants n “K p ” = overall partition coefficient n Sed. Conc. = K p x Water Conc. n K oc = “generic” coefficient for compound (tabulated) n K p =f oc x K oc n f oc is the fraction of organic carbon in sediment

Example: n PCB: Typical K oc = 200,000 L/kg n Sediment with 2% organic content n Ratio of Water Conc/Sed Conc = 1/(0.02)(200,000) = n Virtually ALL PCB stays with sediment, not water

Columbia Slough n Sediment PCB: ~30 ug/kg n f oc = 0.02 n Calculated water PCB: <15 ng/L (parts per trillion; below detection) n But bioconcentration occurs: Carp tissues contain up to 850 ug/L

Metals and “AVS” n AVS = Acid Volatile Sulfides n Easy measure of natural reactive sulfides in sediments n AVS combines with toxic metals and renders them insoluble n AVS thus immobilizes metals, protects biota

AVS and Sediment Quality Criteria n If AVS > Total Toxic Metals, then assume they are all immobilized n If Metals > AVS, reduce the metal mobility in proportion to the AVS n Unfortunately AVS may break down if sediments are disturbed and exposed to air

Interstitial water Approach n Directly sample and measure interstitial water n But does not work for sediments above waterline (tidal) n Very hard to do outside a research settling

Pathways for Contaminant Transport n Desorption during dredging: sediment --> water n Mobilization of non-settling particles during dredging n Desorption or mobilization during handling/dewatering n Desorption or mobilization during final confinement

Sediment Bioassays n Various means of testing in lab the organisms with sediment samples n Costly, but useful n Hard to say connection to actual system in some cases