Geochemistry and budgetary considerations of 14C in the Irish Sea

Sellafield nuclear fuel reprocessing plant has been discharging 14C as a component of the low level liquid radioactive waste into the Irish Sea since 1952. This 14C is of radiological importance due to its long half-life, mobility in the environment and propensity for entering the food chain. Unlike the overall tend towards lower discharge activities since the peak in discharge activity during the 1970s (for example 137Cs and 241Am), the discharge activities for 14C have increased since 1994/95 due to a change in discharge policy from mainly atmospheric discharges to mainly marine discharges. In contrast with other Sellafield derived radionuclides (for example 137Cs and 241Am), the behaviour of 14C in the Irish Sea is, however, poorly understood despite an increase in research effort in recent years. This study was therefore undertaken to improve our understanding of temporal variations in specific activities in the biogeochemical fractions of the water column and biota in the Irish Sea and North Channel in relation to the Sellafield discharges. The project also determined a budget of Sellafield derived 14C in the sediments of the NE Irish Sea and measured the flux of this 14C from the Irish Sea. Before the contribution of Sellafield derived 14C to any carbon fraction can be made the activity of the marine background must first be estimated. The dissolved inorganic carbon (DIC) fraction of the water column, mussel, seaweed, crab, flatfish and roundfish samples were collected annually from the west coast of Co. Donegal, Ireland, and from these, the average current marine background activity was estimated to be approximately 253 Bq kg-1 C. There were clear signs of incorporation of Sellafield derived 14C to the biogeochemical fractions (dissolved inorganic carbon or DIC, dissolved organic carbon or DOC, particulate inorganic carbon or PIC and particulate organic carbon or POC) of the water column in seawater from the NE Irish Sea (Nethertown and St. Bees Head site) and North Channel (Portpatrick), although 14C specific activities of the biogeochemical fractions of the water column in the North Channel were lower than those found close to the point of discharge. Temporal trends in DIG specific activities at Nethertown bore no relationship to temporal trends in the discharge activities. In contrast with the Nethertown site temporal trends in the biogeochemical fractions (DIG, PIG and POG) at the St. Bees Head site rapidly responded to temporal trends in the discharge activity, with the most rapid changes in specific activity being exhibited by the DIG, followed by the POG. In the North Ghannel trends in the DIG specific activities were similar to those for the discharge activities, although a 1-2 month lag time was apparent before peaks in the discharge activity were seen in the DIG specific activities at the North Ghannel. Temporal trends of 14C specific activities for the other biogeochemical fractions (DOG, POG and PIG) at Portpatrick did not bear any relationship to the temporal trends in the discharge. Using the monthly discharge data and monthly specific activities in the DIG at Portpatrick the data can be modelled to give a flow rate of 4.6 km3 d-1 through the North Channel (David Prandle Pers. Comm.). This is in good agreement with the flow rate of 5 km3 d-1 used in previous studies to estimate the removal of Sellafield derived radionuclides from the Irish Sea through the North Channel. Using the flow rate of 5 km3 d-1 an estimated removal from the Irish Sea during the 2 year study period of 11.4 +/- 0.7 TBq in the DIG fraction, 0.9 TBq in the DOG fraction, <0.01 TBq in the PIG fraction and 0.07 TBq in the POG fraction was calculated. This gave a total of 12.4 TBq of 14C removed from the Irish Sea through the North Channel, approximately 100% of the total Sellafield discharges during the study period.