Deposition of artificial radionuclides in sediments of Loch Etive, Scotland.

The nuclear fuel reprocessing plants on the Sellafield site (UK) have released low-level effluents into the Irish Sea under authorisation since 1952. This has led to the labelling of nearby offshore sediments with a range of artificial radionuclides. In turn, these sediments act as a long-term secondary source of both soluble and particle-associated radionuclides to coastal areas. These radionuclides are of interest both in assessing possible environmental impacts and as tracers for marine processes. Here we present results from a study of the geochemistry of natural (234, 238U) and artificial (137Cs, 241Am, 238Pu, 239+240Pu, and 236U) radionuclides and their accumulation in sediments from Loch Etive, Scotland. The data are interpreted in the context of the historical radioactive discharges to the Irish Sea and biogeochemical processes in marine sediments. Loch Etive is divided into two basins; a lower, seaward basin where the sedimentation rate (∼0.6 cm/yr) is about twice that of the more isolated upper basin (∼0.3 cm/yr). These accumulation rates are consistent with the broad distribution of 137Cs in the sediment profiles which can be related to the maximum Sellafield discharges of 137Cs in the mid-1970s and suggest that 137Cs was mainly transported in solution to Loch Etive during that period. Enrichments of Mn, Fe, and Mo in sediment and porewater from both Loch Etive basins result from contemporary biogeochemical redox processes. Enrichments of 238U and 234U in the lower basin may be a result of the cycling of natural U. By contrast, the Sellafield-derived artificial isotope 236U does not seem to be affected by the redox-driven reactions in the lower basin. The 238Pu/239,240Pu ratios suggest contributions from both historical Sellafield discharges and global fallout Pu. The uniform sediment distributions of Pu and Am, which do not reflect Sellafield historical discharges, suggest the existence of a homogenous secondary source. This could be the offshore 'mud patch' in the vicinity of Sellafield from which the supply of radionuclides reflects time-integrated Sellafield discharges. This source could also account for the continuing supply of Cs to Loch Etive, even after substantial reductions in discharge from the Sellafield site.

Discriminatory classification of natural and anthropogenic waters in two U.K. estuaries.

The ability to distinguish water inputs from both natural and anthropogenic sources was investigated in the complex environment of an urban estuary (Tyne) and a relatively pristine estuary (Tweed). We used a data set from a total of 11 estuarine transects, comprising measurements of bulk dissolved organic matter (dissolved organic carbon and nitrogen), dissolved nitrogen (total dissolved nitrogen, ammonium, nitrate+nitrite and dissolved organic nitrogen), optical absorbance measurements (a350, S290-350) and fluorescence excitation emission matrix measurements (fluorophores A, H, B and T intensity and A and H emission wavelength maxima). In order to investigate trends within the numerous parameters measured, multivariate statistics were employed. Principal components analyses showed 63.4% of the variability in the total data set can be explained by two sets of components and 74.9% of the variability by the spectrophotometric measurements alone. In both analyses the first component correlated to the mixing of terrestrial and marine waters and the second component was correlated to sources of pollution such as domestic sewage. Within the data set, river flow and terrestrially derived DOM were significantly correlated, and situations with high river input showed an increase in terrestrial signature in the estuary. Discriminant analyses were also carried out and indicated that 59.8% (total data set) and 53.3% (solely spectrophotometric data) of the samples can be correctly classified into their respective groups (water categories) assigned on the basis of salinity and sampling location. Overall the results clearly show the potential of spectrophotometric techniques to discriminate distinct water categories with different DOM characteristics. In particular, measurement of the fluorophore H emission maxima, the spectral slope parameter, S290-350, and fluorophores T and B intensity enabled discrimination of DOM from riverine, estuarine, marine, and sewage affected water categories. The results presented here indicate the ability of spectrophotometric data alone to distinguish between marine, anthropogenic and terrestrial DOM and distinguish terrestrial DOM from different catchments (Tyne vs. Tweed). With current advances in the in-situ deployment of absorbance and fluorescence spectroscopy it is anticipated that multivariate statistics will gain importance as a cost effective, powerful and diagnostic approach to assessing the distributions of water types and their associated DOM characteristics and fluxes at the land-ocean interface.