Environmental Statement - Maersk Oil

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Balloch Field Development Environmental Statement
Section 3 Baseline Environment
as odd carbon n‐alkanes from terrestrial plants are removed in this region. Pristine sediments
exhibiting a predominance of odd number biogenic n‐alkanes might be expected to have a CPI value
greater than 2.0, while crude oil or refined products show no preference for odd or even n‐alkanes
and achieve a CPI close to unity (1.0) (McDougall, 2000).
The CPI ratio indicated that the Balloch survey area showed a dominance of odd‐numbered alkanes,
with values relatively constant ranging between 2.05 and 2.39 throughout the survey area (stations 4
and 2 respectively) (Fugro, 2005). High CPI ratios (>2) of longer chained (nC12‐36) alkanes are usually
taken to indicate input of cuticular waxes from higher terrestrial plants. The CPI levels were higher
than the UKOOA (2001) mean background levels for this region of the North Sea, indicating that the
sediment is typical of the CNS as it does not show any evidence of point source contamination.
Polycyclic Aromatic Hydrocarbons (PAHs)
Polycyclic Aromatic Hydrocarbons (PAHs) are evident throughout the marine environment (Laflamme
and Hites, 1978), with natural sources including plant synthesis and natural petroleum seepage.
However, these natural inputs are dwarfed in comparison to the volume of PAHs arising from the
combustion of organic material such as forest fires and the burning of fossil fuels (Youngblood and
Blumer, 1975). These pyrolytic sources tend to result in the production of heavier weight 4‐6 ring
aromatics (but not their alkyl derivatives) (Nelson‐Smith, 1972).
Another PAH source is petroleum hydrocarbons, often associated with localised drilling activities.
These are rich in the lighter, more volatile 2‐3 ring aromatics (NPD; naphthalene (128), phenanthrene,
anthracene (178) and dibenzothiophene (DBT) with their alkyl derivatives). As the lightest and most
volatile fraction, NPD is the dominant PAH in petrogenic hydrocarbons but is also the quickest to
degrade and weather over time.
The 2‐6 ring PAH concentrations ranged from 76 ng/g (station 4) to 289 ng/g (station 6). No spatial
pattern of distribution was seen across the site. The mean total PAH concentration at all stations was
higher than that found during the previous Fugro (2005) survey (33 ng/g). The majority of stations
were marginally lower than the mean background levels for the CNS (233 ng/g, UKOOA, 2001), with
the exception of station 6 (289 ng/g). Significant positive autocorrelations (p