Data summarised from the 'National
Monitoring Programme Survey of the Quality of UK
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
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
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
DDT group compounds
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
Apart from the elevated levels from a known point
source in the Forth, other positive values represent
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
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
- naphthalene 2,430 mg
- phenanthrene 6,220 mg
- anthracene 1,470 mg
- fluoranthene, 6,610 mg
- pyrene, 5,900 mg
- benz[a]anthracene, 4,130 mg
- chrysene, 3,410 mg
- benzo[e]pyrene 4,940 mg kg-1;
- benzo[a]pyrene, 3,310mg
- 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.