Propagation characteristics of key sources of impacts

Many of the NSP2 activities with the potential to generate environmental impacts take place in marine waters during the construction phase. In many cases, whether a significant impact will materialise will be influenced by the extent of propagation through the marine environment of the physical changes that arise from such activities. This is particularly relevant to the identification and consideration of transboundary impacts that may be experienced some distance from the location of the source of impact. Therefore an important early task in the Espoo EIA process was the determination of such propagation characteristics as a means to establish the areas of influence and hence the appropriate spatial focus for the baseline studies and subsequent assessments. This was undertaken through a review of the results of targeted modelling and monitoring studies undertaken as part of the national NSP2 EIAs/ES. The main findings that have determined the area of influence are outlined below. Further information is provided in Section 10.1 and Appendix 3, whilst potential impacts are assessed in Chapter 10 – Assessment of environmental impacts.

Physical changes of seabed features and sedimentation on the seabed 8.3.1

Various seabed works, e.g. trenching (pre-lay trenching (dredging), post-lay trenching), rock placement, anchor handling and munitions clearance, will cause physical disturbance of the seabed and may also create new features on the seabed, e.g. spoil heaps (from trenching) and rock piles below and around the pipelines (Chapter 6 – Project description), while settlement of suspended sediment may increase the sediment layer.

The maximum distance on each side of the pipeline within which such direct seabed disturbance may occur will be 100 m for trenching, 100 m for rock placement and 1,000 m for anchor handling. Depending on the size and nature of the munitions being detonated, disturbance of the seabed may extend up to approximately 7-8 m from the detonation location /25/.

Outside the 100 m zone of immediate disturbance (described above), the suspended sediment is predicted to settle in areas close to the pipeline with only very small areas with sediment layers exceeding 1 mm. Further information is provided in Section 10.1 and Appendix 3.

Release of sediments to the water column 8.3.2

Modelling undertaken for the national EIAs/ES indicates that increases in suspended sediment concentration (SSC) during construction of NSP2 will be driven primarily by trenching before pipe-laying (dredging), which takes place in nearshore areas, and after pipe-laying (trenching by ploughing), which would be required in selected sites offshore. Approximately 3.5 km and 50 km

of dredging are anticipated at the Russian and German nearshore areas, respectively. Ploughing is estimated to be required at approximately 7 locations over some 265 km of the route (see Atlas Map PR-02-Espoo to PR-05-Espoo). The release of sediment therefore will be localised to these areas, with its propagation, and subsequent sedimentation, dependent on water depth (which influences, e.g., grain size distribution) and hydrographical conditions.

Dredging activities at the landfalls will give rise to the largest sediment plumes. In the nearshore area along the coastline in Russia, the maximum distance of elevated SSC of 10 mg/l over a period of more than 24 hours is modelled to be 10 km south of and up to 30 km north of the dredging location. Furthermore, increased concentrations close to the dredging location are found up to 5 km from the coastline. Sediment dispersion in Germany varies from 200 m in the Pomeranian Bay to 500 m – 1 km in the Greifswalder Bodden. Further information on the duration and level of increase in SSC for such activities is provided in Section 10.1 and in Appendix 3.

Modelling of a worst-case ploughing scenario predicts that increases in SCC may extend up to 25 km from the ploughing site. However, only very low concentrations of suspended sediments reach that distance.

Rock placement will also release suspended sediment to the water column, but to a much lesser extent than dredging or ploughing activities. Modelling of SSC dispersion for rock placement predicts that while some increase in SSC could occur up to 10 km from the pipeline, the concentration would only be slightly above the average SSC and well within natural variations.

Furthermore, because rock placement activities are restricted to discrete locations, subsequent impacts will similarly be limited to the very immediate vicinity of such activities. Further information is provided in Section 10.1 and Appendix 3.

Anchor handling and the thrusters of DP vessels may also disturb the seabed, resulting in the release of sediment to the water column. However, in the case of DP vessels this impact would be restricted to shallow waters and localised.

Release of sediment-associated contaminants to the water column 8.3.3

Release of sediment-associated contaminants to the marine environment is closely linked to the seabed intervention works undertaken. With regard to SSC, dispersion is dependent on the physical settings. Modelling undertaken in Finland and Russia indicated that munitions clearance in Finland and Russia will result in the greatest area of exceedance of predicted no-effect concentration (PNEC) values for the three modelled contaminants - BaP (PAH), PCDD (dioxins) and Zn. A total area of approximately 163 km², 57.1 km² and 4.82 km² will occur for the three contaminants respectively. The maximum duration of the exceedance will be in the order of 3-19 hours, although this will only apply in an area much smaller than the total and close to the source. In nearshore and shallow waters, dredging will result in the greatest area experiencing an exceedance of PNEC values for the three modelled contaminants. A total area of approximately 172 km², 108 km² and 53 km² respectively will experience an exceedance of PNECBaP, PNECPCDD/F TEQ upper and PNECZn values. The maximum duration of the exceedance will be in the order of 256-374 hours, although this will only apply to an area much smaller than the total and close to the source.

Underwater noise 8.3.4

Underwater noise can potentially arise from a range of NSP2 construction activities notably munitions clearance (by far the loudest activity), followed by rock placement. Beyond the immediate vicinity of the noise-generating activity, the noise level associated with trenching, pipe-laying, anchor handling, construction vessel movements and other construction activities will be generally undistinguishable from the background noise levels in the Baltic Sea, where there is already a large volume of ship traffic.

Noise modelling for munitions clearance, which may take place in Russia and Finland, show that in a worst-case scenario the threshold for impacts on marine mammals may be exceeded up to 23 km and 60 km from the detonation site for permanent and temporary hearing loss, respectively. The distance at which these levels will be experienced, however, depends on numerous parameters, such as water depth and seabed structure. Impacts (injury) on birds in the worst case may be experienced up to approximately 2 km from the munitions detonation site, while those for fish may occur up to 1.5 km from the detonation site.

Underwater noise predictions for rock placement show that thresholds above which receptors can be impacted are exceeded for mammals only in very close vicinity (0-80 m) to the construction activities (with the exception of avoidance reactions). Results from underwater noise modelling of vibro-piling and dredging show that noise propagations are even smaller.

Release of contaminants from anodes 8.3.5

Sacrificial anodes of zinc and aluminium alloy will be attached to the pipeline to prevent corrosion. Beyond the immediate vicinity of the anode (i.e. <5 m), the concentrations of metal ions within the water column as a result of anode degradation during the operation phase will generally be undistinguishable from background concentrations. Within the immediate vicinity of the anode, PNEC values may be exceeded by zinc and aluminium. Monitoring along NSP showed that concentrations of heavy metals were below the detection limit approximately 1-2 m from the pipelines and hence well below the PNEC. The concentrations of cadmium and lead in the water column around both the aluminium and zinc anodes will be so low that they will fall below the ecotoxicological assessment criteria (EAC) and PNEC values. For further information see Appendix 3, Section 2.4.3.