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.

While the size of individual particles is a single value, a particle size distribution can be displayed in many different ways. If you are interested in mass of material then it is appropriate to display the fraction of the weight that is within a certain particle size range. However, as contaminants are adsorbed to the surface of particles, the weight particle size distribution is not appropriate to understand how much surface area is available to adsorb contaminants. Instead the particle size should be displayed as the fraction of surface area which is within a certain particle size range.

The weight particle size distribution shows that how the different sediments are composed of a mixture of largely sand and silt. It also shows that some sediments are mainly slit, while some are mainly sand.

Rather than displaying a particle size distribution in terms of percent of total area, by displaying it in units of area, it becomes how different the surface areas of the different sediments are.

The split of the sediments into types of particles by weight fraction shows the differing composition of the different sediments.

Displaying the split of the surface area by particle types, shows how the surface area is dominated by the silt fraction

Plotting cumulative particle size distributions makes it clear how much the surface area is dominated by the smallest particles, the silt.

Particles below 1um in size account for less than 10% of the weight of the sediment in all but one sample.

Particles less than 1um in size account for between 75 and 95% of the surface area of the sediment.