Risk Assessment of Contaminated Sediments in Anoxic Fjord Basins

摘要

Background/Objectives. Flekkefjord, a small coastal town situated in southern Norway, has traditionally been a center of industrial activity. It is located along an 18 km-long fjord that consists of four interconnected basins, extending from the inner-city area towards the North Sea. Like other fjord systems in Norway, the threshold fjord basins of Flekkefjord are a result of earlier glacial activity. Typically these basins are 30–50 m deep and separated by shallow thresholds 3–10 m deep. Due to the thresholds, there is poor water exchange in the deeper parts of the fjord basins. Over time, organic matter accumulates in these basins, consuming all oxygen in the deeper water. This results in anoxic conditions in the deeper water levels and sediments. In Flekkefjord this process has been accelerated by an extra supply of organic matter to the fjord from municipal sewage outlets, tannery waste products, and leaching of fertilizer from agricultural land. In addition the fjord basins are contaminated by emissions from various industrial activities, such as ship yards and tannery factories. Approach/Activities. In 2011, a sediment survey was conducted in the four fjord basins around Flekkefjord. The purpose of the study was to identify the areas with the highest level of contamination so that they can be prioritized for remediation. Sediment was sampled in 37 stations, of which 6 were core samples. The samples were analyzed for pollutants, and toxicity tests were conducted on a representative sample from each basin. Porewater measurements were taken for a more site-specific assessment of the partition coefficient, Kd. Based on these results a risk assessment was carried out according to the analysis spreadsheet developed by Klif (the Norwegian Climate and Pollution Agency). Results/Lessons Learned. The 2011 survey shows that the Grisefjorden and Tj?rsv?gbukta fjord basins are especially contaminated with respect to chromium, lead, copper, mercury, PAHs, PCBs and TBT. This is also reflected in the toxicity tests, amongst others for dioxin-like PCBs. It has been challenging to conduct a risk assessment for these two basins as they are anoxic in their deeper parts resulting in little life on the sea floor. This requires a re-evaluation of the standard input parameters in the risk calculations. Lack of benthic organisms reduces the possibility for bio-diffusion, which in turn results in little to no leaching of pollutants from sediments to the surrounding environment. Analysis of the pore water samples showed, however, that they were contaminated. The high level of pollutants in the sediments suggests a need for remediation so that dietary restrictions on sea food can be removed in the future. However, the anoxic conditions in the fjord basins indicate that there currently is little risk of leaching of contamination from the sediments. Anoxic basins isolate contaminated sediment, while increased oxygen levels in these basins would probably lead to increased