Environmental monitoring during survey by submarine U-864 outside Fedje in 2014

Sammendrag

NIVA has done environmental monitoring during a survey by the wreck of submarine U-864 outside Fedje. The survey was done in January 2014 on the vessel Geosund from DOF Subsea Norway. The main task for NIVA was to monitor if the work by the submarine caused any spreading of contaminated sediment. This was done by monitoring of turbidity around the wreck and analysis of mercury in water samples. A water current profiler was deployed in the mid area between the two main wreck sections. NIVA also did measurements for mercury in air on the vessel. The digging and dredging in the contaminated area caused some spreading of particles contaminated with mercury, but the concentrations in the water samples were low (class II (good) or less). One sediment trap (10 m above the seabed) recovered from a position ca 200m northwest of the operation center contained 4.2 mg Hg/kg dry weight. This is classified as class V (very bad). The high concentration of mercury in the sediment trap shows that the digging and dredging has caused spreading of contamination. Model calculations based on the measurements available from the survey, showed that particle spreading will occur mainly towards northwest. Sediment fractions with size of more than 100 μm will settle within the 50 m model grid surrounding the operation. Particles with size 63 μm will not have travelled far beyond 200 m northwest of the operation center, but fine clay fractions (20 μm) may be transported up to 1 km away. It can be assumed that smaller sized particles will not constitute any significant fraction of the bulk sediment.