Geostatistical analyses of environmental contaminants in the dredging area Borg 1

Summary

In connection with the planned dredging of the area Borg 1 (the approach to Borg Harbour, city of Fredrikstad, Norway) Rambøll AS has performed chemical analyses of 227 sediment samples from 96 surfaces samples and sediment cores in the dredging area. Based on this, a 3D map of the sediment has been made, showing the sediments that can be deposited in a deep sea area (assumed to be in pollution class 1-3) and the sediments that should be deposited in a landfill (assumed to be in pollution class 4-5). Such a map assumes that the pollution is correlated both horizontally and vertically, i.e., that samples taken close to each other are chemically more similar than samples separated by a larger distance. We found that this assumption holds in principle, but the spatial scale of variation is small compared to the typical distance between samples, leading to large uncertainty. That is especially true for sediments at 1 2 m depth below the existing sea bottom. Of 9 cores that has been analysed for all compounds at all or almost all depths, 4 showed strong contamination at depth (below 1 m). The 10-30 cm layer appears to be particularly contaminated, with 67% of the samples showing high (class 4-5) contamination (the number is very uncertain due to low sample size). Among the other depth layers, there was relatively little variation in the fraction of sediment samples that were substantially contaminated (from 28% at 0.5-1 and 1-2 m depth to 37% at 30-50 cm depth). Calculations of volume indicate that the degree of contamination is very uncertain for a part of the sediment that is planned to be deposited in the sea (around 33% of the total volume that will be dredged). If these sediments have the same fraction of class 4-5 contamination as the samples, a realistic worst-case scenario is that around 10% of the total dredging volume is "misclassified" for deposition in sea. It is recommended that more chemical analyses are performed from the existing sediment cores, and new cores should be collected for selected areas. The contamination consists of both metals, PAHs, PCB and TBT. Contamination by heavy metals is not a good indicator of contamination in general: if heavy metals is used as a general indicator of contamination, this method will capture only around half of the sediment with class 4-5 contamination.