North-east Irish Sea Research Articles

New northerly limit for the sharptail mola in the northeast Atlantic: first sighting of Masturus lanceolatus in the northeast Irish Sea.

The ocean sunfishes are currently represented by five species within three genera: Mola mola (Linnaeus, 1758), Mola alexandrini (Ranzani, 1839), Mola tecta (Nyegaard, Sawai, Gemmell, Gillum, Loneragan, Yamanoue & Stewart, 2017) (Sawai et al., 2018), Masturus lanceolatus (Lienard, 1840), and Ranzania laevis (Pennant, 1776). Growing conservation concerns have led to an ongoing re-examination of their classification status on the International Union for the Conservation of Nature Red Lists, a process requiring the latest data to best support their conservation management on a global scale. Here we report the first sighting of Masturus lanceolatus in the northeast Irish Sea, representing a new northerly range limit for this species in the northeast Atlantic. The species is more commonly reported in tropical seas, and in the Western Atlantic they occur from North Carolina in the United States to southeast Brazil. This sighting increases the most northerly range limit by 2610 km, providing additional insight into the ecology and potentially changing distribution of this generally considered tropical megafaunal species.

Plutonium mobilization from contaminated estuarine sediments, Esk Estuary (UK)

Since 1952, liquid radioactive effluent containing238-242Pu, 241Am, 237Np, 137Cs, and 99Tc has been released with authorization from the Sellafield nuclear complex (UK) into the Irish Sea. This represents the largest source of plutonium (Pu) discharged in all western Europe, with 276 kg having been released. In the Eastern Irish Sea, the majority of the transuranic activity has settled into an area of sediments (Mudpatch) located off the Cumbrian coast. Radionuclides from the Mudpatch have been re-dispersed via particulate transport in fine-grained estuarine and intertidal sediments to the North-East Irish Sea, including the intertidal saltmarsh located at the mouth of the Esk Estuary. Saltmarshes are highly dynamic systems which are vulnerable to external agents (sea level change, erosion, sediment supply, and freshwater inputs), and their stability remains uncertain under current sea level rise projections and possible increases in storm activity. In this work, we examined factors affecting Pu mobility in contaminated sediments collected from the Esk Estuary by conducting leaching experiments under both anoxic and oxic conditions. Leaching experiments were conducted over a 9-month period and were periodically sampled to determine solution phase Pu via multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), and to measure redox indicators (Eh, pH and extractable Fe(II)). Microbial community composition was also characterized in the sediments, and at the beginning and end of the anoxic/oxic experiments. Results show that: 1) Pu leaching is about three times greater in solutions leached under anoxic conditions compared to oxic conditions, 2) the sediment slurry microbial communities shift as conditions change from anoxic to oxic, 3) Pu leaching is enhanced in the shallow sediments (0–10 cm depth), and 4) the magnitude of Pu leached from sediments is not correlated with total Pu, indicating that the biogeochemistry of sediment-associated Pu is spatially heterogeneous. These findings provide constraints on the stability of redox sensitive Pu in biogeochemically dynamic/transient environments on a timescale of months and suggests that anoxic conditions can enhance Pu mobility in estuarine systems.

Capillary Limited Flow Behavior of CO2 in Target Reservoirs in the UK

The flow of supercritical CO2 and brine in the subsurface is predicted to be strongly dependent on both the fluid properties and the heterogeneity of the pore space. However there are few laboratory studies that characterise the interaction between fluid properties and heterogeneity in real reservoir rocks. We explore the sensitivity of CO2 flow paths and relative permeability to pore space capillary heterogeneity in target CO2 storage reservoirs in the UK and Australia. Samples from potential North Sea and East Irish Sea reservoirs and a current CO2 storage site in Australia are compared. The rock samples are all of high permeability (>500mD) and porosity (>12%), and are clean and homogeneous sandstones. Relative permeability is found to be highly sensitive to minor heterogeneities in pore structure at reservoir conditions that give rise to a low CO2 viscosity and in particular when the flow is capillary limited, as will be the case for most of the reservoir. We use a simple capillary number in guiding the measurement of relative permeability and residual trapping under viscous and capillary limited conditions. Observations suggest that to fully characterise the behaviour in a reservoir a range of relative permeability curves must be measured which can be applied as the flow of CO2 slows with distance from the near wellbore and flow behaviour changes from viscous-dominated to capillary-dominated. Experiments are performed at 8-20 MPa, 40-90°C and brine molalities of 0 – 5 mol/kg NaCl. Saturation is measured in situ, using a medical x-ray CT scanner, which allows the fluid arrangement to be observed at a resolution of 0.25x0.25x1 mm.

Temporal Trend in the Transfer of Sellafield-Derived 14C into Different Size Fractions of the Carbonate Component of NE Irish Sea Sediment

From 1994 onwards, radiocarbon discharges from the Sellafield nuclear fuel reprocessing plant have been made largely to the northeast Irish Sea. They represent the largest contributor to UK and European populations of the collective dose commitment derived from the entire nuclear industry discharges. Consequently, it is important to understand the long-term fate of 14C in the marine environment. Research undertaken in 2000 suggested that the carbonate component of northeast Irish Sea sediments would increase in 14C activity as mollusk shells, which have become enriched in Sellafield-derived 14C, are broken down by physical processes including wave action and incorporated into intertidal and subtidal sediments. The current study, undertaken in 2011, tested this hypothesis. The results demonstrate significant increases in 14C enrichments found in whole mussel shells compared to those measured in 2000. Additionally, in 2000, there was an enrichment above ambient background within only the largest size fraction (>500 μm) of the intertidal inorganic sediment at Nethertown and Flimby (north of Sellafield). In comparison, the present study has demonstrated 14C enrichments above ambient background in most size fractions at sites up to 40 km north of Sellafield, confirming the hypothesis set out more than a decade ago.

Estimating estuarine turbidity: An application to estuaries of the Isle of Man and northeast Irish Sea

A simple estimation method for estuarine turbidity that may be helpful for management purposes is used to predict SPM concentrations for the strongly tidal estuaries of the northeast Irish Sea. It demonstrates a systematic tendency for the longer, more slowly flushed estuaries to have greater ‘intrinsic’ SPM concentrations within their high turbidity regions than the shorter estuaries. Predicted maximum SPM concentrations for the mainland estuaries vary from ∼10 mg l−1 (Bladnoch) to ∼10 g l−1 (Solway, Mersey and Dee). The Isle of Man estuaries are predicted to have maximum SPM concentrations that are less than 10 mg l−1. These small predicted concentrations are largely a consequence of their short lengths and short residence times. Geomorphology is the ultimate reason for this difference in Isle of Man and mainland estuarine turbidity. The Isle of Man has steep slopes and small rivers that flow rapidly to sea via short estuaries, whereas the mainland has a number of large, long, coastal plain estuaries that allow strong tidal currents to develop and provide storage for large quantities of fine sediment. Survey data for the Solway Esk and the Forth and Clyde Estuaries are used to illustrate the effects of uncertainty in the definition of an estuary's tidal length, which is a problem both for estuarine science and estuarine management. Despite the dearth of published, good quality field data for the estuaries described herein, there is nevertheless a good qualitative agreement between the predicted values of SPM and the majority of these observations.

Modelling 99Tc concentrations in Fucus vesiculosus from the north-east Irish Sea

In 1994 there were substantial increases in the quantity of 99Tc discharged into the north-east Irish Sea from BNFL Sellafield (UK), concomitant with improvements in waste treatment procedures. As a consequence, the concentration of 99Tc observed in seawater and biota samples, taken from the Irish Sea coastline, increased significantly. Elevated concentrations were also reported in Dutch, Danish, Norwegian, Swedish and Arctic waters in subsequent years. In the present study a simple numerical model was developed and applied to time-series data of 99Tc concentrations in the brown seaweed Fucus vesiculosus, collected from three UK sites in the vicinity of Sellafield (St. Bees, Heysham, Port William). The model considered site-specific scaling effects, lag times, previous discharge history and potential seasonal variation in uptake. In general, there was a good fit between predicted and observed concentrations, but the degree of uncertainty varied inversely with the frequency of sampling. We did not observe a significant seasonal variation. The modelled lag times to the three sites were consistent with transport times based on observations of the water column distribution of 99Tc. The model was applied to a variety of discharge scenarios, reflecting current discussion on the future management of 99Tc releases. Concentrations in Fucus reached asymptotic values in 3–10 years, depending on the scenario and sampling site under consideration.

Long-term irradiation of a marine fish, the plaice Pleuronectes platessa: an assessment of the effects on size and composition of the testes and of possible genotoxic changes in peripheral erythrocytes

Purpose: Previous investigation showed the very significant effects of chronic gamma -radiation on plaice testes at mean absorbed dose rates as low as 1.3mGyh-1 given over a period of 168 days (accumulated dose 4.7Gy). The present paper examines the effects on the testes of exposure to even lower dose rates of gamma -radiation given over periods of 73 days or 197 days. In addition, the use of micronucleus counts and flow-cytometric measurement of nuclear DNA content in samples of peripheral blood for monitoring genotoxic effects has been assessed. Materials and methods: In Experiment 1, adult male plaice were exposed at mean absorbed dose rates of 0.25, 0.5 or 1.2mGyh-1 for 73 days (mean accumulated doses of 0.43, 0.85 and 2.03Gy, respectively) and in Experiment 2 to 0.24, 0.5 or 1.0mGyh-1 for 197 days (mean accumulated doses of 1.07, 2.24 and 4.57Gy, respectively). At termination the testes were removed, weighed and sections were prepared and examined histometrically. In addition, in Experiment 2, blood samples were taken during exposure and at termination. Blood smears were scored for micronuclei and samples processed and examined for nuclear DNA content by flow cytometry. Results: Significant reductions in testis weight were seen in all radiation groups after 197 days of exposure, which were predominantly due to decreased amounts of sperm. In plaice killed after 73 days (at an earlier stage of spermatogenesis), there were no significant differences in weight compared with controls but amounts of spermatogonia were significantly reduced in irradiated fish. Conclusions: Exposure to dose rates as low as 0.24mGyh-1 of gamma -radiation given over a period of 197 days significantly reduced the weights of plaice testes, this being consequent on reductions in the amounts of sperm. Although there was some evidence of radiation affecting the numbers of spermatogonia it was not possible to determine the primary target for radiation damage which eventually caused the sperm reductions. Along with the related work described by Greenwood and Knowles (1996) this is the first investigation of a marine fish and it indicates that plaice testes are probably more radiosensitive than those previously described in tropical fish and of a similar radiosensitivity to mammalian testes. Although significant effects were observed after the lowest dose rate used of 0.24mGyh-1, this is still a factor of about 400 times greater than the estimated absorbed dose rate to plaice testes in the north-east Irish Sea off Sellafield at the present time. Micronucleus counts and flow-cytometric analysis of blood DNA both failed to show any evidence of genotoxic damage.

The influence of mixing timescales and re-dissolution processes on the distribution of radionuclides in Northeast Irish Sea sediments

The sediments of the northeast Irish Sea contain substantial inventories of contaminant radionuclides as a consequence of past discharges of low-level liquid radioactive waste from the Sellafield nuclear fuel reprocessing plant. Previous work has indicated that onshore transport of this contaminated sediment is the dominant mechanism of supply of Sellafield waste radionuclides to surrounding intertidal and floodplain areas, but an inherent, and previously unsubstantiated, assumption in this work was that past discharges from Sellafield are effectively homogenised in the surface offshore sediment, with respect to 238Pu 239,240Pu and 241Am 239,240Pu activity ratios, as the result of mixing on a timescale of a few years. Results are presented here for a study of three cores, of lengths ranging from 10 to 38 cm, collected from the deposit of contaminated offshore sediment in 1992. The activity ratios of the longlived radionuclides 238Pu, 239,240Pu and 241Am in the cores exhibited variations that were small relative to those in the Sellafield discharge, indicative of a high degree of mixing of the sediment. Moreover, the sediment activity ratios were found to be compatible with those of the Sellafield time- integrated discharge and with those of sediment deposited in surrounding coastal areas during the year after sample collection. In view of the heterogeneous nature of the sediments in the northeast Irish Sea, caution must obviously be applied in any generalisation based upon the analysis of only three cores, but for the locations sampled, the results are consistent with the surface sediment acting as a well mixed source of long-lived Sellafield waste radionuclides. 60Co and 134Cs were detected in the sediment to depths ranging from 10 to 18cm, implying transport to these levels by mixing on a timescale commensurate with their half-lives. Activities of the short-half-life nuclides 106Ru and 144Ce decreased with depth in the sediment, suggesting a timescale for mixing of the surface (< 13 cm depth) sediment of the order of 1 year. On the basis of the distribution of these short-half-life radionuclides, biodiffusion coefficients of the order of 10 −7 cm 2 s −1 were derived for the top 10 cm of sediment. The trends in 137Cs 241Am activity ratio with depth were consistent with re-dissolution of radiocaesium that had initially been taken up in the sediment during the period of high discharges from Sellafield. Comparison of the results with 137Cs 241Am activity ratios for Irish Sea sediment in the late 1970s suggests that up to 77% of the 137Cs had been re-dissolved from surface (< 10 cm depth) sediment and some 15% from deeper layers. Finally, it was observed that the 137Cs 241Am activity ratios for the three cores studied implied a K d of the order of 10 4–10 5 for radiocaesium in this system.

Distribution of technetium-99 in UK coastal waters

The concentrations of 99Tc, discharged to the northeast Irish Sea from the nuclear reprocessing plant at Sellafield, have been measured in UK coastal waters. Temporal and spatial distributions of this radionuclide in surface seawater are provided prior to and post the authorized discharge of elevated quantities of 99Tc, arising from the operation of the Enhanced Actinide Removal Plant (EARP). Preceding the commissioning of EARP, concentrations of 99Tc in a large proportion of the Irish Sea were reasonably uniform (1–4 mBq l −1). Increased levels from most recent discharges were observed along the Cumbrian and southern Scottish coastline. However, 99Tc concentrations decreased rapidly away from the coastline and were dispersed in a parallel direction to the shore, consistent with residual surface currents. Following elevated discharge periods, a significant pulse of 99Tc was rapidly transported northwards parallel to the shore. Post-EARP surveys indicate that the most rapid migration of 99Tc from the outfall to the North Channel was less than 3 months and the mean transit time is estimated to be in the order of 6 months. Comparison of UK coastal surveys, prior to and post EARP, suggest that 99Tc had migrated to the northern North Sea within some 9 months following the initial elevated discharge.

Remobilization of Sellafield-derived radionuclides and transport from the north-east Irish Sea

Plutonium-239,240 activities in seawater from the north-east Irish Sea, collected in January 1992, were in the range of 0.17–4.30 mBq l −1 in solution and 7.2–62.3 Bq kg −1 in suspended particulates. The 238Pu: 239,240Pu activity ratios for the soluble fraction were consistent with those of the underlying sediments but inconsistent with contemporary discharges from Sellafield, indicating that most of the plutonium in solution was derived from re-dissolution from the sediment. The activity ratios for the particle associated material were more variable, but again, on average, typical of the sediments. The annual loss of 239,240Pu from the north-east Irish Sea in solution was estimated to be 1.19 TBq and that associated with suspended particulates to be 0.11 TBq. From this, a halving time of 350 years for the loss of 239,240Pu activity from the sediment by dissolution was determined. A similar treatment of 137Cs data yielded an annual loss of 86 TBq in solution and a halving time of 23 years. Calculations using a numerical model of the Irish Sea yielded similar estimates of current loss rates and permitted the estimation of total cumulative losses from the Irish Sea by advection through the North Channel of 7.9 TBq of 238Pu, 48 TBq of 239,240Pu 2.6 × 10 4 TBq of 137Cs.

The Behavior of Sellafield-Derived 14C in the Northeast Irish Sea

Radiocarbon is an important constituent of the low level, liquid, radioactive effluent discharged from the Sellafield nuclear fuel reprocessing plant in northwest England, but despite the fact that it gives the highest collective dose commitment of all the nuclides in the waste, its behavior in the Irish Sea is poorly defined. There is therefore a clear requirement for an improved understanding of 14C behavior in the Irish Sea, to assist with dose evaluation modeling and definition of the mixing and accumulation characteristics of the sediment in this area. In this context, results are presented here for a temporal study of 14C activities in four geochemical fractions of seawater and in a sediment core from the vicinity of the Sellafield effluent outfall. Clear 14C enrichments in the dissolved inorganic carbon (DIC) and particulate organic carbon (POC) components of seawater were observed, with temporal trends in activity that were related to variations in the Sellafield discharge. Smaller, but nevertheless detectable, enrichments were also observed for particulate inorganic carbon (PIC) and dissolved organic carbon (DOC) in the seawater. The distribution of 137Cs and 241Am revealed that the sediment core could be classified into three zones in which the intensity of mixing decreased discontinuously with depth. Bulk carbonate 14C analyses of the core demonstrated the presence of glacial or pre-glacial carbonate in the system, but failed to show any evidence of contaminant 14C input or provide information on sediment accumulation processes. In contrast, analysis of bulk organic matter from the sediment provided clear evidence of the recent perturbation of a well mixed system by input of younger material, consistent with the recent input of contaminant 14C from Sellafield and possibly weapons testing fallout.

The influence of spawning and nursery grounds on the distribution of sole Solea solea (L.) in the Irish Sea, Bristol Channel and adjacent areas

In the coastal waters of west England and Wales, sole, Solea solea (L.), are found in greatest abundance in the north-east Irish Sea and the Bristol Channel. Data from research and charter vessel surveys in these areas showed differences in the spatial distribution of age-groups between the two regions. Juvenile sole (0-group, 1-group and 2-group) were found almost exclusively in the shallow (<20 m deep) parts of the north-east Irish Sea and adults (3-group and older) were also found in the same area and in deeper water. In contrast, while juvenile sole in the Bristol Channel were abundant in and around Carmarthen Bay, Swansea Bay and the Severn estuary, they also occurred frequently in deeper areas of the Bristol Channel down to depths of 40 m. Adult sole in the Bristol Channel were also relatively more abundant at depths >40 m than adults in the Irish Sea. The movements of sole which produce these patterns of distribution in the two areas are also shown by the recaptures of sole tagged in the respective nursery areas. The differences in the distribution of sole in the two areas may be related to the depths of their spawning grounds and the distances that maturing fish must travel to reach them. In the north-east Irish Sea, sole spawning grounds are generally in waters of <40 m depth and within an extensive area of relatively shallow and gently shelving sediments close to the coastal nursery grounds. The bottom topography of the Bristol Channel is more steeply shelving and the spawning grounds of sole are at a depth of 40–75 m off Trevose Head, ≈100 km from the main nursery areas. The hydrographic conditions in the two areas are described, and it is concluded that one of the more important requirements of a successful spawning ground is the presence of suitable hydrographic conditions to transport eggs and larvae to nursery areas.

Natural and Anthropogenic Radioactivity in Coastal Regions of the UK

The radionuclide contents of a range of coastal marine samples (sediment, sea water, seaweed, mussels and winkles) collected from 15 UK sites have been investigated to provide a perspective on the relative contributions to enhanced marine radioactivity in the UK by the nuclear and non-nuclear industries. Sites typifying high levels of anthropogenic radioactivity (Irish Sea area) were influenced by the authorised discharges from the nuclear fuel reprocessing plant at Sellafield into the northeast Irish Sea. Enhanced concentrations of natural radionuclides were due to: (1) discharges from a phosphate ore processing plant (Whitehaven) and (2) dumping of coal spoil directly into the North Sea (Blackhall Colliery). New site-specific concentration factor (CF) values derived here are consistent with existing literature data except for 210Pb and 210Po, for which the results suggest that the upper and lower values of the currently recommended CF ranges should be extended. Radiologically, the highest potential exposure to the public calculated from the results derives from the ingestion of Whitehaven mussels (3.2 mSv.y-1), the greatest single contribution being from technologically enhanced 210Po.

Nearshore shallow gas around the U.K. coast

The occurrence of shallow gas in seafloor sediments within 50 km of the coast of the British Isles is generally attributed to the degradation of organic matter within post glacial deposits. However there is evidence that buried rockhead topography may also be a factor governing the existence of shallow gas in some localities. In such cases gas may be escaping from the rock through fissures. Where channels have been cut into the rockhead and are now filled with unconsolidated sediments, gas often accumulates in the central portion of the channels. This may be due to the greater thickness of sediment in the centre of the channels generating sufficient gas to be detectable in seismic profiling data. The process of compaction causing gas to be squeezed out of the pore spaces may also be a contributory factor. In other localities no such relationship appears to exist and the gas is present at very shallow levels (within 5 m of the seabed) in the superficial sediment column. Detailed analyses of high resolution continuous seismic reflection profiles from Cardigan Bay, the northeast Irish Sea and the Firth of Forth, would appear to permit the categorization of nearshore shallow gas zones around these parts of the U.K. continental shelf and probably elsewhere, in order to assist with the explanation of their occurrence and the likely hazard that the zones present to seabed and sub-seabed structures.

Mercury in flounder ( Platichthys flesus L.) from estuaries and coastal waters of the north-east Irish Sea

This paper reports concentrations of mercury in muscle tissue of flounder ( Platichthys flesus L.) from various estuaries and coastal waters of north Wales and north-west England that encompass the north-east Irish Sea. Mercury concentrations were highest within and offshore the Ribble Estuary (0·476 ± 0·037 rmmg kg−1) 1 but were also high within the Mersey (0·389 ± 0·042 mg kg −1) and Dee Estuaries (0·295 ± 0·065 mg kg −1) and at coastal sites in thevicinity. Values for all three estuaries and their near coastal sites were significantly higher than for sites in the north and central sectors of the study area. Regressions of mercury concentration against fish length showed significant relationships and age accumulation for the majority of sites, particularly within or in close proximity to the Mersey, Dee and Ribble Estuaries.

Mercury in muscle tissue of lesser-spotted dogfish ( Scyliorhinus caniculus L.) from the north-east Irish Sea

This paper reports concentrations of mercury in muscle tissue of lesser-spotted dogfish ( Scyliorhinus caniculus L.) from various locations within Liverpool Bay and the north-east Irish Sea. Concentrations of mercury in fish muscle showed distinct spatial variation, with values from the northern sectors of the Irish Sea significantly lower than those for the southern area, particularly sites in Liverpool Bay and around the mouth of the Mersey Estuary. The data support the hypothesis that there are several major populations of lesser-spotted dogfish within the study area, each of which remains quite distinct during seasonal movements. Consequently, exposure to environmental mercury is reflected in muscle concentrations of mercury. Regressions of mercury concentration against fish length showed significant relationships for the majority of sites, with the slope relating to distance from known sources of mercury contamination. Mean mercury concentrations were higher than in corresponding populations of flatfish from the same areas.

Spatial patterns in accumulation of mercury by fish from the NE Irish Sea

This paper reports concentrations of mercury in muscle tissue of three marine species of fish; plaice ( Pleuronectes platessa L.), dab ( Limanda limanda L.) and lesser-spotted dogfish ( Scyliorhinus caniculus L.), at various locations within Liverpool Bay and the North East Irish Sea. Mean concentrations of mercury in fish muscle within any one sampling area followed the order: lesser spotted dogfish⪢dab>plaice. Concentrations in lesser spotted dogfish were 5–10× higher than in flatfish from the same site. Regressions of mercury concentration against fish length showed significant relationships for the majority of species-site combinations, with the slope of the regression relating to distance from the Mersey Estuary.

Mercury in the stomach contents of dab ( Limanda limanda) from the North East Irish Sea and Mersey Estuary

A study of mercury concentrations in the stomach contents of fish from the north-east Irish Sea and Mersey Estuary has been shown to provide a means for surveillance of geographical and time-based changes in environmental exposure of fish biota to mercury in marine and estuarine ecosystems. This paper describes data for the flatfish dab ( Limanda limanda), caught during the period 1986–1988. The low degree of variability in the data enables confirmation of clear trends in mercury concentration in stomach contents over time. As the inputs of mercury to the sewage sludge dumping ground in Liverpool Bay have decreased, there has been a corresponding decrease in mercury in fish food items. The mean mercury value in stomach contents around the dump site has declined to 100 μg kg −1 (wet weight) which now predominates over the whole of Liverpool Bay. In 1986, mercury concentrations in stomach contents of fish ranged to over 750 μg kg −1 although the majority of values were below 200 μg kg −1. Most of the sites within the Mersey Estuary produced mean concentrations which were similar to those in the open sea, except for Garston which is the site closest to an inland, and principal alternative, source of mercury.

Iron and manganese geochemistry and the distribution of 239,240Pu and 241Am in the sediments of the north east Irish sea

The geochemical cycling of Fe and Mn, within the seabed sediments of the northeast Irish Sea, has been investigated in an attempt to learn more about the factors controlling the distribution and behaviour of plutonium and americium released, under authorisation, from the BNF plc nuclear fuel reprocessing plant at Sellafield, U.K. Enrichment of surface sediment with both Fe and Mn results from active diagenetic redox behaviour which produces characteristic interstitial water concentration profiles at particular sites. The shape of the interstitial water profiles is controlled, in part, by differences in the general bioturbation regime at each site. This factor has been simulated in simple laboratory tank experiments. Evidence from selective chemical leaching suggests that 40% of the Pu and 80% of the Am are associated with the operationally defined Fe/Mn oxide phase. However, in the surface sediment there is no apparent correlation between either Fe or Mn content and either the Pu or Am concentrations determined by strong acid digestion of the samples. Fluxes of Fe [(1.8−29) × 10 −9 μmol cm −2s −1] and Mn [(2.0−2.5) × 10 −8 μmol cm −2 s −1] upward in sediment are similar to those measured at other nearshore sites and indicate a potential for oxide scavenging of trace elements at the sediment/water interface. The behaviour of Pu and Am has probably been dominated by general non-equilibrium conditions resulting from: (i) the recent introduction of the Pu and Am into the environment (Sellafield discharges started in 1952); (ii) the dominating effects of dispersion from the pipeline source, and (iii) the variation with time of the quantities discharged.

The interstitial water chemistry of 239,240Pu and 241Am in the sediments of the north-east Irish Sea

The distribution of Sellafield-derived 239,240 pu and 241 Am in sediments and interstitial water of the north-east Irish Sea has been studied in order to examine the post-depositional-behaviour and to assess the potential for remobilization of the two radionuclides. The solid phase profiles of 239,240 Pu and 241 Am have major subsurface peaks which are not related to the distribution of organic carbon, Fe, or Mn, but may reflect the history of discharge from Sellafield. The interstitial water profiles of reduced Pu and Am also show major subsurface peaks which are not related to the diagenetic processes of organic matter oxidation by oxygen or nitrate, nor to the remobilization of Fe and Mn. The interstitial water radionuclide profiles show a qualitative relationship with the solid phase radionuclide profiles and this suggests an adsorption/desorption process described by apparent K d 's in the range of (1.7–11.8) × 10 5 for reduced Pu and (3.5–8.9) × 10 5 for reduced Am. However, systematic variations of this measured apparent K d with depth for Pu (and possibly Am) suggest that the sediment/interstitial water system is not at equilibrium. Available data do not support the suggestion that dissolved organic carbon is responsible for complexing Pu in the interstitial water. Scavenging of Pu and Am by precipitating oxide phases near the sediment surface is suggested.