Data summarised from the 'National Monitoring Programme Survey of the Quality of UK Coastal Waters' (MPMMG 1998)

The results from this part of the survey have highlighted the major control that sediment characteristics, i.e. grain size and organic carbon, exert on observed contaminant concentrations. They concluded that the reported concentrations appeared to be typical background concentrations and were considerably lower than those reported for contaminated areas, such as some sewage sludge disposal sites.

Concentrations of all metals at offshore sites were relatively low, compared with those in estuaries. However, there were differences between estuaries, with a tendency for higher concentrations to be observed in those estuaries with either current or historical industrial inputs of metals. To some extent, these higher metal concentrations observed in estuaries stem from the presence of fine-grained sediment rich in clay minerals.

For example, relatively high mercury concentrations were observed in the Forth, Tees, Thames and Tamar. The Forth estuary has a history of mercury contamination, and although inputs have reduced dramatically, the turbid nature of the estuary has resulted in considerable retention of mercury in the system. Despite the known historical input of mercury to the Mersey estuary from chloralkali works, and the established contamination of the area, not all sediment samples contained high concentrations of mercury. In some industrialised estuaries, such as the Tees, concentrations of all metals were relatively high. Other estuaries, however, tended to have high concentrations of a smaller number of metals, e.g. Tyne (lead and zinc), Clyde (chromium and nickel) and Tamar (arsenic).

One difficulty in assessing these data is establishing the extent to which concentrations are determined by anthropogenic inputs or local geological sources. For example, the mineralised parts of the Pennines are likely to contribute to the relatively high lead concentrations observed in the Tyne and Tees estuaries. Metal concentrations in the Tamar estuary are also known to be influenced by mineralisation and historical mining activity within the catchment.

The NMP determinands were 11 individual polychlorinated biphenyl (PCB) congeners (PCBs 28, 52, 101, 105, 118, 128, 138, 153, 156, 170 and 180), dieldrin, aldrin and endrin, three DDT group compounds (pp-DDT, pp-TDE and pp-DDE) and hexachlorobenzene (HCB). Organic contaminants are lipophilic and therefore have low water solubilities. They preferentially adsorb onto sediments, particularly where these are fine grained and/or contain a high proportion of organic carbon. Concentrations would, therefore, be expected to be inherently higher in areas with fine-grained organic rich sediments than in areas dominated by coarse sandy sediments.

Data for PCB 153 (a relatively abundant PCB congener) were used by the NMP to give an overall impression of contamination by PCBs.

Although data were presented for a total of 67 sites, very few sites yielded positive results. This was to be expected at some intermediate and most offshore sites, but previously published data suggest that PCB levels in some coastal and riverine sediments can be locally elevated, especially in areas of high industrial activity (CEFAS, 1997). However, two factors seem to have influenced the data set. Firstly the detection limits for many samples were inadequate to detect the very low background concentrations present. Secondly, many of the samples were sandy sediments, i.e. coarse particles with low organic carbon content. These comments are also applicable to many of the other organic contaminants discussed in this part of the report.

The highest reported levels of PCB 153, up to 25 mg kg^-1 median value, were reported at two sites in the Severn estuary; these are in the area of a site of historic manufacture of PCBs and were therefore not unexpected.

The non-systemic insecticide dieldrin was commonly found at concentrations of 0.2-5 mg kg^-1 at estuarine and intermediate sites and these values represent typically background concentrations.

Although use of the insecticide DDT has ceased, its persistence means that it still occurs widely in the environment. In general, environmental levels of the parent compound are lower than its metabolites (ppTDE and pp-DDE). pp-DDT was rarely found, but pp-DDE and pp-TDE were more ubiquitous in their occurrence. Clear gradients of contamination were evident in the fine-grained sediments of the Forth and Clyde estuaries. In contrast, concentrations were very low in the coarse sandy sediments of the Tay estuary. Concentrations of compounds were low and often undetectable at most intermediate and offshore sites.

Apart from the elevated levels from a known point source in the Forth, other positive values represent background concentrations.

The NMP survey only carried out a 'pilot' study on the concentration of PAHs presented in the sediments of estuaries and offshore sites around the UK. Their findings are highlighted below.

Coverage of NMP stations in this pilot study of PAHs in sediments was not exhaustive, but a good spatial coverage was achieved and in some cases replicated over two years. These data suggest that concentrations are low or undetectable at most intermediate and offshore sites, and further work should be concentrated on fine sediments and depositional areas. Significant concentrations of PAH have been found in a number of estuaries. The conclusions of the NMP survey stated that the bioavailability of PAH and their uptake by shellfish (particularly bivalve molluscs) should also be evaluated in the next phase of the NMP programme, so that the significance of these compounds can be fully assessed for both marine animals and human consumers.

Total PAH concentrations ranged from not detected at some offshore sites with a sandy substrate to 35,400 mg kg^-1 dry weight in mud from the River Tyne at Hebburn. The samples from Scotland were at the lower end of this range, 27-488 mg kg^-1 dry weight, with the highest value in the Firth of Clyde. All of the highest concentrations (total PAH >10,000 mg kg^-1 dry weight) were found in the highly industrialised estuaries of north-east England, particularly in muddy sediments from the Rivers Tyne and Wear. Total PAH concentrations between 1,000 and 10,000 mg kg^-1 were found at sites in the Rivers Thames, Tamar and Severn and also at two offshore sites off the Tyne and off the Tees. Additional samples taken at non-NMP sites indicated that fine sediments from the Rivers Blyth and Tees also yielded total PAH concentrations above 10,000 mg kg^-1 dry weight, as did samples from a further site in the River Tyne at Tyne Bridge. One site within Millford Haven sampled in June 1996 yielded the highest recorded concentration of 93,000 mg kg^-1. Millford Haven has been an oil terminal and refinery site since the 1960s and has been subject to discharges and numerous oil spills, the most recent being that from the Sea Empress in February 1996.

Total PAH concentrations between 1,000 and 10,000 mg kg^-1 were found at sites in Poole Harbour, the River Exe, Swansea Bay at Tenby and in the River Mersey. Nine sediments taken on a 3 x 3 grid in the Celtic Deep yielded total PAH concentrations of 366 to 786 mg kg^-1 dry weight, with both total and individual PAH concentrations varying by up to approximately a factor of two.

The highest concentrations of PAH recorded at NMP sites were:

• naphthalene 2,430 mg kg^-1;
• phenanthrene 6,220 mg kg^-1;
• anthracene 1,470 mg kg^-1;
• fluoranthene, 6,610 mg kg^-1;
• pyrene, 5,900 mg kg^-1;
• benz[a]anthracene, 4,130 mg kg^-1;
• chrysene, 3,410 mg kg^-1;
• benzo[e]pyrene 4,940 mg kg^-1;
• benzo[a]pyrene, 3,310mg kg^-1;
• benzo[ghi]perylene, 1,930mg kg^-1 dry weight.

The highest concentration of benzo[e]pyrene was found in the River Wear and of all the other PAH congeners at Hebburn in the River Tyne.

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