North East File Collection - NEFC

Much of the contamination within sediment is due to the adsorption of waterborne contamination onto the surface of the particles within the sediment.

Smaller particles have a higher area per unit weight of material:

• 1 cm diameter spheres: Total surface area ≈ 0.6m²
• 1 mm diameter spheres: Total surface area ≈ 6m²
• 1 micrometer diameter spheres: Total surface area ≈ 6,0000m²

For sediments the particle size ranges from gravels of many millimetres to silts of only a few microns in size, meaning that the surface of a predominantly gravel sediment will be orders of magnitude smaller than a predominantly silt sediment.

Contaminants which are adsorbed require surface to be adsorbed to, so the higher the surface area, the more contaminant will be contained in the sediment.

The level of contamination is controlled by many factors, local contamination release, flow of the river etc.. So the role of surface area can be difficult to determine in the chemical analyses of different sediments.

The overspill from dredging barges and the resuspension which happens during dredging is mainly composed of the finest particles within the sediment. The finest particles are likely to contain the highest levels of contaminants.

The set of samples taken as part of the mid-licence sampling of the River Tees dredge disposal licence show a strong correlation between relative total surface area and the level of polycyclic aromatic hydrocarbon contamination. MMO_Results_ - MAR02085 UPDATED.xlsm - MLA/2015/00088 19th October 2023.

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