Environmental Statement - Maersk Oil

3 ‐ 6
Balloch Field Development Environmental Statement
Table 3‐4 Average water temperature for the Balloch development (BODC, 1998).
Section 3 Baseline Environment
Location Mean temperature in winter ( o C) Mean temperature in summer ( o C)
Sea surface 2.5 19.9
Sea bottom 4.5 11
During late spring, the water column begins to stratify due to increased solar radiation and calmer
conditions. This results in the formation of a thermocline in the water column, separating a warm less
dense surface layer from the rest of the water column, where winter temperatures remain. The
thermocline increases in depth between May and September and is typically between 20 m to 50 m
during summer (OSPAR Commission, 2000). In late August/early September, stratification begins to
break down due to decreased solar heating and increased wind and wave action. Water temperature
remains relatively uniform through the water column during the winter months (Doody et al., 1993).
3.3.6. SALINITY
Like temperature, salinity affects the properties of seawater and the marine organisms inhabiting it.
Fluctuations in salinity are largely caused by the addition or removal of freshwater from seawater
through natural processes such as rainfall and evaporation. The salinity of seawater around an
installation has a direct influence on the initial dilution of aqueous effluents, such that the solubility of
effluents increases as salinity decreases. Salinity in the development area shows little seasonal
variation, with water salinities of approximately 35 throughout the year (BODC, 1998).
3.3.7. AMBIENT AIR QUALITY
Air quality measurements are not measured on offshore sites; however, regular monitoring of
onshore sites is carried out by local authorities in many rural areas. Air quality from rural locations
can be used as an approximation of the air quality that is likely to apply in a nearby offshore location.
As such, information for a rural Scotland location (Strath Vaich) has been used as a proxy for the
Balloch location and is presented in Table 3‐5.
Table 3‐5 Ambient concentration of NO 2 for Strath Vaich (AEA Energy and Environment, 2008).
3.3.8. WATER QUALITY
Averaging time µg/m 3
Average 1.0 1
98 th Percentile 7.5
99.9 th Percentile 16.0
100 th Percentile 16.8
1 Converted assuming an ambient air temperature of 10 o C
Regional inputs from coastal discharges and localised inputs from existing oil and gas developments
may affect water quality in different areas of the North Sea. Water samples with the highest levels of
chemical contamination within the North Sea are generally found at inshore estuary and coastal sites
that are subject to high industrial usage. Where concentrations of total hydrocarbons are found to be
high offshore, this is normally in the immediate vicinity of installations. Concentrations generally fall
to background levels within a very short distance of the point of discharge (CEFAS, 2001).
The North Sea Quality Status Reports (North Sea Task Force, 1993) state that, although the waters of
the NNS as a whole do not contain contamination above normal background levels, slightly higher
levels of some contaminants (e.g. copper, iron and vanadium) are typically found in the shallower
SNS. Lead is an exception as dissolved lead is quickly removed onto the surfaces of suspended
particulate matter (SPM) which is relatively high in the coastal SNS area (apart from in the Dogger
Bank region). It therefore does not get transported in the dissolved phase to the SNS by coastal