Anthropogenic 14C in the Natural (Aquatic) Environment

Begg, Fiona H (1992) Anthropogenic 14C in the Natural (Aquatic) Environment. PhD thesis, University of Glasgow.

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Abstract

Increasing global awareness of the radiological significance of 14C releases from the nuclear and radiochemical industries has resulted in a number of studies within the last decade investigating the atmospheric releases and their effect on the terrestrial biosphere. However, liquid discharges also occur, with preliminary studies indicating enriched 14C specific activities present in fish and shellfish harvested in the vicinity of discharge locations. The basis of this study was to determine the behaviour and environmental distribution of anthropogenically produced 14C released to the aquatic environment from the nuclear fuel reprocessing plant at Sellafield, owned by British Nuclear Fuels and the radiochemical plants in Cardiff and Buckinghamshire owned by Amersham International plc. Most sampling was undertaken in the Irish Sea with smaller scale studies being carried out in the Bristol Channel and the Grand Union Canal. Within the study area, from Earnse Point 40 km south of Sellafield, northwards to the Clyde Sea area, preliminary studies on intertidal biota samples ie. mussels, winkles and seaweed indicated enriched 14C specific activities in all the samples relative to the current ambient level of 115.4 pM. The highest activities were observed in the immediate vicinity of the discharge location; mussels with a measured activity of 787 pM, winkles of 613 pM and seaweed of 415 pM. The 14C specific activity observed at most sites appeared to be organism dependent with mussels>winkles>seaweed. This is the result of differences in the uptake mechanisms of the organisms and indicates that the dissolved inorganic carbon and the particulate material within the water column are enriched in 14C. However, on analysis of the biogeochemical fractions of the water column, enriched 14C activities were observed only in the DIC fraction which could explain those activities found in the seaweed but not those in the mussels and winkles. Enriched 14C activities were found in the phytoplankton, indicating that there is a source of enriched organic particulate material within the water column as a result of photosynthetic uptake of enriched DIC, however this will be a seasonal effect. Nevertheless, this enrichment is still not high enough to support the activities observed in the mussels and winkles, although, this was only a single sample and may not be a true reflection of the activities present. A similar anomaly is present in the activities found in bottom-dwelling fish and the sediments on which they feed; the fish are more highly enriched than the organic fraction of the sediments. These discrepancies may point to higher discharges having occurred in the past and/or to areas within the Irish Sea which are of a more enriched nature and were not investigated during this study. Prior to 1985, the discharges of 14C from Sellafield were not monitored and hence are merely best estimates. A temporal study of Nori (Porphyra umbilicalis) samples collected within the period 1967 - 1988 were analysed in an attempt to confirm these estimates. To ensure that Nori did reflect past discharges, 137Cs and 241Am were also analysed. The agreement observed between the 137Cs and 241Am discharges and measured activities was excellent, indicating that Nori did reflect Sellafield discharges. However, a similar agreement was not obtained for 14C, suggesting that either the estimated discharges are incorrect or that due to biological activity, Nori does not reflect 14C discharges in the same way that it does for 137Cs and 241Am. The geographical distribution of Sellafield-derived C in the DIC was determined by extensive sampling within the Irish Sea and Scottish coastal water areas. 137Cs, a known conservative radioactive tracer of water movement, was also analysed at the sites to allow comparison with the chemical behaviour of 14C. The results indicate that the behaviour of 14C in seawater, like that of 137Cs, is largely conservative. There was, however, a slight increase in the 14C/137Cs ratio with increasing distance from Sellafield. This may be a reflection of biological uptake of carbon or the desorption of 137Cs from the sediments. A more complex treatment of the data was carried out using a compartmental model, based on the hydrography of the study area, which was initially developed from 137Cs data. The agreement between the predicted and observed values indicates that the 14C distribution (as DIC) is being determined by water movement and the local current system ie. 14C in the DIC is behaving in a relatively conservative manner in the water column.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Marian Scott
Keywords: Biogeochemistry, Nuclear chemistry, Environmental science
Date of Award: 1992
Depositing User: Enlighten Team
Unique ID: glathesis:1992-76369
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 15:20
Last Modified: 19 Nov 2019 15:20
URI: http://theses.gla.ac.uk/id/eprint/76369

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