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8.6.1.3 Impact 3: Change in water quality due to re-suspension of contaminants within sediment.


86. Disturbance of sea bed sediment has the potential to release sediment-bound contaminants, such as heavy metals and hydrocarbons into the water column. The data in Tables 8.10 and 8.11 show that levels of contaminants observed within the East Anglia THREE site (station 49) are very low, with concentrations of all metals below Cefas Action Level 1 and Canadian Sediment Quality Guideline TEL values. Concentrations of Acenaphthene and Napthalene marginally exceed Cefas Action Level 1 at station 49; however, both results were well below the Canadian Sediment Quality PEL. As stated above (section 8.5.3) contaminant levels were generally low at all stations sampled. Since levels of contaminants within the sediments are relatively low, even though up to 26,000m3 of sediment could be released at each foundation, the magnitude of effect is considered to be low.


87.


In the offshore cable corridor, concentrations of contaminants are generally higher than within the East Anglia THREE site, with a number of sites exceeding the Cefas Action Level 1 and Canadian Sediment Quality Guideline TEL value for a number of metals, as well as several hydrocarbons. Concentrations of arsenic also exceed Cefas Action Level 2 and the Canadian Sediment Quality Guideline PEL value at a number of sites. Increased concentrations of suspended solids resulting from the installation of the offshore export cables are, however, predicted to be significantly less than that for the foundations and short-lived since cable installation in relation to jetting proceeds at a rate of 150 to 450m/hr per hour. As a result, any contamination that is present would be rapidly dispersed and is unlikely to cause an exceedance of water quality standards. For cable installation, the magnitude of effect is also predicted to be low due to the rapid nature of the installation activities.


88.


Overall therefore, the magnitude of effect on levels of contaminants within the water column is predicted to be low. Since the receptor is considered to be low sensitivity, the re-suspension of contaminated sediment from construction activities is expected to have a negligible impact.


8.6.1.4 Impact 4: Change inWater and Sediment Quality due to Accidental Releases or Spills of Construction Materials or Chemicals


89. A wide range of vessels and construction methodologies would be employed during offshore construction ranging fromboats (<24m) to large dredging vessels, tugs and barges, Dynamic Positioning (DP), heavy lift and survey vessels. In addition to the risks regarding the potential for pollution from leaks or spills of fuels carried on- board these vessels, there is also the potential for accidental pollution associated with the use of construction materials in the marine environment.


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Chapter 8 Marine Water and Sediment Quality Page 29


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