The Sawyer Street de-sanding facility is the first of two contaminated sediment processing areas. Dredging sediment is pumped to a building at the end of Sawyer Street to filter out the larger more coarse material from the dredged sediment such as sand, shells and small rocks.
The coarser sand is filtered out of the finer grain, silty sediment which the majority of the PCBs are attached to. The remaining slurry of silt, sea water, and PCB contamination is then pumped again to a de-watering facility further down the harbor, where the water is pressed out, treated to stringent standards, and the resulting de-watered sediment (which is similar to hard packed soil in consistency) is properly disposed of at licensed off site landfills.
Science Fun Fact (or at least interesting to some):
Why do PCBs like silt better than sand?
If you were to compare a cupful of sand versus a cupful of silt, there would be more particle surface area in the cupful of silt. Since PCBs 'stick' to the surface area of something, there would be more PCBs in the cupful of silt.
More particles = more surface area = more PCBs.
Deeper into chemistry:
Silt comes from the breakdown of organic materials (plant and organisms). Chemical compounds that are made up of hydrocarbons (PCBs - polychlorinated biphenyls) are attracted to these organic surfaces because of the carbon content. Even more reason for the PCBs to attach themselves moreso to silt than to sand.
May 22, 2014
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