Modeling the fate of 226 Ra present in produced water discharged to the North Sea (Norwegian sector) Henrik Rye 1 , D.Ø. Eriksen 2 , R. Sidhu 2 , E. Strålberg 2 , K.I. Iden 2 , K. Hylland 3 , A. Ruus 3 and M.H.G. Berntsen 4 1 The Foundation for Scientific and Industrial Research (SINTEF), NO-7465 Trondheim, Norway 2 Institute for Energy Technology (IFE), Instituttveien 18, 2007 Kjeller, Norway 3 Norwegian Institute for Water Research (NIVA), Gaustadaleen 21, NO-0349 Oslo, Norway 4 National Institute for Nutrition and Seafood Research (NIFES), P.O. Box 2029 Nordnes, Bergen, Norway INTRODUCTION As a part of the NFR (The Research Council of Norway) project “Radioactivity in produced water from Norwegian oil and gas installations – concentrations, bioavailability, and doses to marine biota”, added radiation levels in the sea have been calculated with a numerical model. The purpose of the simulations was to reveal the size of the added radiation levels in the ambient sea, compared to the natural background radiation levels. Produced water discharges contain radionuclides of various types. 226 Ra, 228 Ra, 210 Pb and 210 Po are some of the most important ones with respect to potential environmental impact and doses to man. Compared to natural background in sea water (around 1 - 2 mBq/L for all of these, NRPA 2004), the 226 Ra is considered to be the most important one. The reason for this is that it appears to be more abundant than the others (order 1 – 12 Bq/L in produced water, NRPA 2004) and also because it is an α emitter. MATERIALS AND METHODS The 226 Ra nuclides are generally present in the produced water stream in dissolved state (ionic form). Radium is chemically similar to Barium (Ba), which is also present (as dissolved) in the produced water stream in relatively large amounts. However, when discharged into the sea, the produced water will mix with sea water (which is rich in SO 4 2- - ions) to form BaSO 4 (barium sulphate or barite) and RaSO 4 . These reactions will transform Ba and Ra into solid state. However, dilution of the produced water will tend to slow down or reverse the particle formation process. Because the sea water (except for very deep waters) is generally unsaturated with barite, BaSO 4 will tend to dissolve again for sufficiently large dilutions (Monnin et. al, 1999). Radium is shown to behave similarly (Adloff and Guillaumont, 1993), the discharge of 226 Ra into the sea is therefore assumed to be present as partly particle and partly dissolved state. Another process that can impact on the fate of 226 Ra in the recipient is the adsorbtion to organic (particle) matter naturally present in the recipient. Thus, three different paths can be envisaged for the fate of 226 Ra in the recipient after the radium has left the discharge pipe: 1): 226 Ra remains in dissolved state in the recipient and dilutes there along with other water soluble constituents in the discharges. 2): 226 Ra remains in dissolved state and adsorbs to natural organic particle matter present in the recipient. The natural particles descend through the water column and down on the sea floor. 3): 226 Ra forms particle matter (together with Ba) consisting of Ba(Ra)SO 4 (radium sulphate and barite) and then descends down on the sea floor as particles (sinking rates to be dependent on the particle size distribution).