Collection, pre-treatment and analyses of Cs-137 and Tc-99 in marine samples at the Institute of Marine Research (IMR), Norway

Full text: The Institute of Marine Research (IMR) is an important contributor to the Norwegian marine monitoring programme RAME (Radioactivity in the Marine Environment). RAME is funded by the Ministry of the Environment and coordinated by the Norwegian Radiation Protection Authority (NRPA). Sample collection is performed from IMRs research vessels in the open sea areas of the North-, Norwegian- and Barents Seas and in Norwegian fjords. The samples consist of biota (fish and other marine organisms), sediments and seawater. Biota samples are frozen onboard the ship and transported to IMR where the samples are subsequently ground up, freeze dried, homogenized and aliquoted into polyethylene counting boxes of appropriate size prior to analysis. Attempts are made to collect filets from 25 fish for each sample of large fish such as cod, haddock, saithe, red-fish and Greenland halibut. For smaller fish (e.g. blue whiting, polar cod, capelin and Atlantic herring) and other organisms such as amphipods, krill, and deep-sea shrimps, a sample of 2-3 kg of each species is taken. These samples are ground up whole. Sediment samples are collected using a Smoegen boxcorer, from where both surface samples and cores are taken. The samples are frozen onboard the ship. While half-frozen, the cores are cut into slices of 1 or 2 cm thickness on board the ship, then frozen again and transported to IMR where they are treated as described above for the biota samples. Large volumes (typically 50-200 L) of seawater are needed in order to get enough material for analysis. Pre-treatment of the samples in the field is therefore an advantage. Surface samples (5 m) of seawater are collected from a shipboard pump, while a CTD-rosette multi-bottle sampler with 12 10 L samplers is used to collect seawater from depths below 5 meters. For the analysis of Cs-137, Cu2[Fe(CN)6]-impregnated cotton filters are used for the pre-concentration. One pre-filter without impregnation, two Cu2[Fe(CN)6]-impregnated filters and a flow-meter are mounted in a series directly on the ships water intake. About 200 L seawater are passed through the filters at a flow rate of 5 L/min. The filters are then dried at 60 deg. C for about 24 h and brought back to IMR where they are ashed at 450 deg. C and filled into 60 ml polyethylene counting boxes prior to analysis. Samples from the CTD-rosette multi-bottle sampler are shipped back to the laboratory and analysed using the ammonium molybdophosphate (AMP)-method. Biota, seawater and sediment samples are gamma-counted on Ortec HPGe-detectors with efficiencies ranging from 30 to 60 %, electric cryostat cooling systems, and 10 cm lead shieldings. For the analysis of Tc-99, 50-100 L of seawater is collected from the shipboard pump (surface samples) or CTD-rosette multi-bottle sampler (samples from the water column) and filled into 25 L polyethylene carboys. A known amount of rhenium (Re) is added to each sample as a carrier and yield monitor on board the ship. Further, the samples are passed through anion exchange resins made of BioRad AGx8 (100-200 mesh) packed into 60 ml disposable polystyrene syringes. The syringes containing the anion exchange resin are brought back to IMR for further radiochemical treatment. A RISOe beta-counter is used for beta-counting for Tc-99. Information about salinity and temperature of the seawater is normally available from the stations we collect samples. This information is important while interpreting the results. Samples of biota and sediments are collected for analysis of different hydrocarbons and persistent organic pollutants (POPs) from the same stations as the radionuclide samples. Combining these results gives us a more overall picture of the pollution load. (author)