CHAPTER 8 • Quality Status, Appropriate Monitoring and Legisiation of the North Sea
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drilling activities used water-based drilling muds, but oil-based muds proved more suitable. However, due to evidence of the toxicity of the latter, the oil content of cuttings discharged by platforms in the Norwegian sector was reduced to less than 6% in 1987. In 1993
their disposal was prohibited (Olsgard and Gray 1995). As a consequence there has been
a move back to water-based muds. These in turn require larger amounts of weighting
agents (up to 10 times more) than oil-based muds, which usually take the form of a barium
mineral (barite) which typically contains impurities including a range of other heavy
metals (GESAMP 1993). Oil-based drilling muds continue to be used in the UK sector.
Initial evidence from chemical monitoring programmes indicated raised levels of
total oil hydrocarbons in sediments sampled in the vicinity of drilling platforms. These
can be as much as 10 000 times background levels close to the platforms, but typically
decline rapidly at distances greater than 500-1000 m (Kingston 1992). Oil exploration
and ex;ploitation companies are required to conduct both chemical and biological environmental monitoring of their activities. While raised concentrations of contaminants
(oil hydrocarbons) from drilling muds can be detected at distances up to 10 km from
platforms (Davies and Kingston 1992; GESAMP 1993), evidence for major impacts on
benthic animals (reductions in species diversity) were generally limited to 750 to 1500 m
from the platforms. The Norwegian government has a policy of open access to its quality-controlled environmental monitoring data. This has allowed the application of sophisticated multivariate statistical analyses of both the chemical and biological data
(Gray et al. 1990; Olsgard and Gray 1995). These analyses have shown that the effects
of contamination by drill cuttings of the sea-bed containing a mixture of oil hydrocarbons, barium and other heavy metals, bentonitic clays, together with other bioactive
materials, can have a significant impact on the structure of benthic animal communities for areas between 10 and 100 km 2 around individual platforms. Indeed, these effects can extend beyond the original sampling area, including designated reference sites
(Olsgard and Gray 1995). Drilling sites may show significant differences in benthic
macrofauna 3 years after drilling activity ended, particularly in the deep burrowing
and long-lived species. Where such sediments had been covered by several centimetres
of clean and well-mixed sediments on top of the contaminated sediments, the meiofauna
communities were remarkably uniform (Heip 1992). In acute toxicity studies barite has
not been found to be particularly toxic to marine organisms (see Starczak et al.1992), and
Olsgard and Gray (1995) concluded that the major adverse effects on the biota were
probably associated with the hydrocarbons contained in discharged drilling muds, although other components probably make a significant contribution. Given the number and spatial distribution of oil and gas platforms throughout the North Sea, it is
likely that benthic communities over a substantial area of the seabed have been affected.
8.6.2
Estuarine Inputs
It is well established that the major route for inputs of contaminants to the North Sea
is byway of the river estuaries. Perhaps the most significant pollution gradient in coastal
waters is that due to the plumes of the rivers Elbe and Weser which flow into the German Bight. This gradient was intensively studied during an ICES/IOC Bremerhaven
Workshop in March 1990 along a transect of stations extending 200 km northwest out
to the eastern side of the Dogger Bank (Stebbing et al. 1992). During the workshop,
171
drilling activities used water-based drilling muds, but oil-based muds proved more suitable. However, due to evidence of the toxicity of the latter, the oil content of cuttings discharged by platforms in the Norwegian sector was reduced to less than 6% in 1987. In 1993
their disposal was prohibited (Olsgard and Gray 1995). As a consequence there has been
a move back to water-based muds. These in turn require larger amounts of weighting
agents (up to 10 times more) than oil-based muds, which usually take the form of a barium
mineral (barite) which typically contains impurities including a range of other heavy
metals (GESAMP 1993). Oil-based drilling muds continue to be used in the UK sector.
Initial evidence from chemical monitoring programmes indicated raised levels of
total oil hydrocarbons in sediments sampled in the vicinity of drilling platforms. These
can be as much as 10 000 times background levels close to the platforms, but typically
decline rapidly at distances greater than 500-1000 m (Kingston 1992). Oil exploration
and ex;ploitation companies are required to conduct both chemical and biological environmental monitoring of their activities. While raised concentrations of contaminants
(oil hydrocarbons) from drilling muds can be detected at distances up to 10 km from
platforms (Davies and Kingston 1992; GESAMP 1993), evidence for major impacts on
benthic animals (reductions in species diversity) were generally limited to 750 to 1500 m
from the platforms. The Norwegian government has a policy of open access to its quality-controlled environmental monitoring data. This has allowed the application of sophisticated multivariate statistical analyses of both the chemical and biological data
(Gray et al. 1990; Olsgard and Gray 1995). These analyses have shown that the effects
of contamination by drill cuttings of the sea-bed containing a mixture of oil hydrocarbons, barium and other heavy metals, bentonitic clays, together with other bioactive
materials, can have a significant impact on the structure of benthic animal communities for areas between 10 and 100 km 2 around individual platforms. Indeed, these effects can extend beyond the original sampling area, including designated reference sites
(Olsgard and Gray 1995). Drilling sites may show significant differences in benthic
macrofauna 3 years after drilling activity ended, particularly in the deep burrowing
and long-lived species. Where such sediments had been covered by several centimetres
of clean and well-mixed sediments on top of the contaminated sediments, the meiofauna
communities were remarkably uniform (Heip 1992). In acute toxicity studies barite has
not been found to be particularly toxic to marine organisms (see Starczak et al.1992), and
Olsgard and Gray (1995) concluded that the major adverse effects on the biota were
probably associated with the hydrocarbons contained in discharged drilling muds, although other components probably make a significant contribution. Given the number and spatial distribution of oil and gas platforms throughout the North Sea, it is
likely that benthic communities over a substantial area of the seabed have been affected.
8.6.2
Estuarine Inputs
It is well established that the major route for inputs of contaminants to the North Sea
is byway of the river estuaries. Perhaps the most significant pollution gradient in coastal
waters is that due to the plumes of the rivers Elbe and Weser which flow into the German Bight. This gradient was intensively studied during an ICES/IOC Bremerhaven
Workshop in March 1990 along a transect of stations extending 200 km northwest out
to the eastern side of the Dogger Bank (Stebbing et al. 1992). During the workshop,