Oil and Gas
Development on the Sakhalin Island Shelf: An Assessment of Changes in
the Okhotsk Sea Ecosystem
Alexander Leonov
Oil
Transformation in a Marine Environment
Knowledge about the
processes of oil decomposition in a marine environment serves as a
scientific base on which we can construct a strategy for dealing with
oil pollution of the seas and oceans. This knowledge determines the
efficiency of chemical and microbiological measures to counteract oil
spills. Oil from different origins differs in structure, and these
differences increase when oil contacts water and air: the oil's
structure begins to vary owing to the loss of part of its hydrocarbons
which have minimum molecular mass, density and viscosity, as well as
maximum volatility and solubility in the water. At the same time, the
properties of oil remaining in a water environment vary in the opposite
direction [53].
The complex transformations of oil and its
products begin immediately after contact with a marine environment. The
course, duration, and the results of the oil's transformation depend on
the properties and structure of the oil and on the particular situation
and parameters of the environment. The main features of the oil's
transformation are: the dynamic (especially in the initial stages) and
close interactions of physical, chemical and biological processes in
the dispersion of all the oil's components down to their complete
disappearance in the initial substrate.
Thus, it is
possible to group the following major processes in the transformation
of oil when it enters a marine environment [1, 40, 42, 52-66]:
Oil spilled on the
sea surface is initially influenced by the action of gravitational
forces and then is controlled by its viscosity and forces of surface
tension. A one ton oil spill distributed in a 50 m radius has a
thickness of up to 10 mm; the formation of a thinner film (less than 1
mm) covers an area up to 12 square kilometers [64]. As the crude oil
spreads, it quickly loses its volatile and water-dissolved components,
and the remaining viscous fractions retard the spilling process. The
oil film drifts predominantly in the wind's direction with a speed
equal to 3 to 4% of the wind's speed, frequently exceeding the rate of
water motion [62]. In the course of time, the oil film on the surface
becomes thinner and as it approaches critical thickness, (about 0.1
mm), it begins to breakdown into fragments, which are transported over
extensive areas. In strong winds, the residues of the oil film are
quickly dispersed in a layer of active mixing. Thus, the essential
components of the oil turn into emulsified forms and are transported
significant distances by currents.
This process is
especially important for less dissolved saturated hydrocarbons. During
contact with the air, the volatilization of hydrocarbons occurs from
the water surface into the atmosphere [53]. During the first several
days after an oil spill, a significant amount transforms into a gaseous
phase. This amount can make up to 75, 40 and 5 % respectively for easy,
mean and heavy oils [66]. The evaporation of low molecular alkanes,
cycloalkanes and benzene are the quickest (from minutes up to hours).
Polycyclic aromatic hydrocarbons (PAH) (anthracene and pyran types) do
not transform into a gaseous phase; they remain in a water environment
and are exposed to complex transformations as a result of oxidation,
biodegradation and photochemical processes which usually result in the
formation of more polar and dissolved compounds. A combination of
meteorological and hydrological effects (the power and direction of the
wind, waves and currents) determines the specific characteristics of
the distribution and the subsequent state of oil in a marine
environment.
The solubility of
oil hydrocarbons depends on their molecular structure and mass.
Aromatic hydrocarbons are mostly dissolved, actively passing into a
water environment and behaving like truly dissolved substances [53].
Naphthene hydrocarbons seldom dissolve in water. As a rule, when a
hydrocarbon's mass increases, its solubility in water is reduced. After
oil enters a water environment, the relative enrichment of the
dissolved fraction by the most dissolved low molecular aromatic and
aliphatic hydrocarbons with their subsequent and rather fast
volatilization and increasing of the contribution of less volatile
(less dissolved) fractions of aromatic hydrocarbons take place. About 1
to 3% (sometimes up to 15%) of crude oil can pass into a dissolved
state. First of all, it concerns the low molecular hydrocarbons of
aliphatic order and aromatic structure, as well as polar compounds
appearing as a result of oxidizing transformations of some initial
petroleum fractions in the marine environment. The transition process
into a dissolved state is spread over time and depends on the
hydrodynamic and physiochemical conditions of the surface waters. The
concentration of dissolved fractions under the oil film in the sea is
made up of 0.1 up to 0.3 to 0.4 mg/l [61]. An excess of these
concentrations is usually accompanied by the formation of decomposable
oil-water emulsions.
Many parameters affect the formation of
water-dissolved oil product fractions. The most important among them
are: the oil type; the degree and duration of oil mixing with water;
the ratio of mixed volumes of oil and water; and the sedimentation time
required for the achievement of stable hydrocarbon distributions
between water and petroleum phases.
Emulsified oil is often the dominant form of
chronic oil pollution. This fact is stipulated by the extended action
of hydrodynamic factors (wind and others), by receipt of the oil into a
marine environment in the form of emulsions, and by the presence of
high molecular compounds in the oil pollution's structure (promoting
self-emulsification). The formation of oil emulsions in a marine
environment depends on the oil's structure and the water's turbulence.
The most stable emulsions ("water oil" types) contain between 30 to 80%
water. They are usually formed after strong storms in zones of heavy
oil spills with an increase in nonvolatile fractions (for example,
naphthenes) and can exist in a marine environment for more than 100
days as a peculiar "emulsion" of brown and other tones. The stability
of emulsions increases with temperature decreases. "Oil in water" type
emulsions are unstable because of the action of inter-surface tension
forces which quickly reduce the oil's dispersion. This process can be
slowed down with emulsifiers - surfactant substances with strong
water-receptive properties. These substances are used for eliminating
the consequences of petroleum pollution. Thus, stabilization of the
petroleum emulsion, its dispersion in the formation of microscopic
drops and the acceleration of oil decomposition in the water column
takes place.
Hydrometeorological
conditions are a determining influence on the fate
of different oil products at all stages in their distribution in a
marine environment. The role of hydrological and meteorological
conditions is especially important in the first hours after oil enters
a marine environment, when the oil still has low viscous volatility and
dissolved fractions. Only in this period is the effective dispersion of
oil products possible; small dispersed fractions will not be formed
later [53].
Oil aggregates may
be frequently found in a marine environment in the form of resinous and
mazut lumps and balls (petroleum lumps, tar balls, pelagic tar). They
are formed by about 5 to 10% of spilled crude oil and up to 20 to 50%
of settled oil and oil products in the ballast and flush waters of
tanker holds. The chemical structure of aggregates is rather changeable
but its basis is usually made of asphaltic (up to 50%) and high
molecular compounds of heavy oil fractions.
The chemical
oxidation of oil in a water environment begins only a day after its
entering into the sea. The chemical oxidation of oil is often
accompanied by its photochemical decomposition under the impact of an
ultraviolet part of solar spectrum. This process is catalyzed by
vanadium and is inhibited by sulfur. The final products of oil
oxidation (hydroperoxides, phenols, carboxyl acids, ketones, aldehydes
and others) usually possess increased solubility in water and increased
toxicity.
Microbiological
decay defines the final fate of oil products in a marine environment.
There are about 100 species of bacteria and fungus capable of using oil
products for their growth. Their number does not exceed 0.1 to 1% of
the number of heterotrophic bacterial communities in clean water areas
and this figure increases up to 1 to 10% in polluted water [54]. The
mechanisms of oil hydrocarbon uptake by microorganisms are the subjects
of special laboratory studies [40, 42, 63].
The ability of hydrocarbons to biodegrade
depends on the structure of their molecules. Compounds of the paraffin
order (alkanes) have this ability to a greater degree in comparison to
aromatic and naphthene substances. The rate of microbiological
destruction of hydrocarbons usually decreases as the complexity of
their molecular structure increases. For example, the biodegradation
rate is tens or hundreds of times lower for anthracene and
benzo(a)pyrene than for benzene [59, 65]. The biodegradation rate of
oil depends on the degree of oil dispersion, on the water's
temperature, on the content of biogenic substances and oxygen, as well
as on the species' structure and the number of the oil-oxidizing
microflora [55, 60].
Oil-based drill
solutions impregnated by drill slimes are rather stable in a marine
environment. Experiments simulating natural conditions have shown that
the biodegradation of oil-based drill waste after 180 days did not
exceed 5%, whereas other drill solutions (prepared on the basis of
fatty acid esters) were nearly completely degraded (99%) due to
microbiological processes and physical-chemical decomposition [57].
Part of the oil
(up to 10 to 30%) is sorbed in suspended matter and settles on the
bottom of the seabed. Sedimentation occurs more in narrow coastal zones
and in shallow water where the amount of suspended matter is
significant and water mixing occurs more frequently. At a greater
distance from the coast, sedimentation occurs extremely slowly, except
for heavy oils. Suspended oil and its components are subjected to
intensive chemical and biological decomposition. On the sea-bottom, the
degree of oil decay is sharply reduced since the oxidizing processes
slow down due to anaerobic conditions. The fractions of heavy oil
accumulated in sediments may be stored there for months and years.
The ratio of dissolved and suspended forms of
oil and its components in a marine environment varies in an extremely
wide range depending on the particular combination of environmental
factors, the structure, properties and the oil's origins. For example,
in the Baltic Sea, this ratio varied in the range of 0.2 to 2.1 [52]. A
study of oil sedimentation in the Caspian Sea showed that a significant
amount of marine salt (possibly in the form of concentrated brine) was
found adhering to suspended oil particles. In two samples, 0.3 and 0.1
mg of salt were found respectively in 4.4 and 2.1 mg of oil
hydrocarbons [53].
The general
conclusion from all the studied processes is that oil quickly loses its
initial properties. It is divided into groups of hydrocarbons and
fractions of different forms whose composition and chemical structure
are considerably transformed. The content decreases owing to the
dispersion and decay of the initial and intermediate compounds and the
formation of carbon dioxide gas and water. The purification of a water
environment polluted by hydrocarbons takes place once the indicated
processes are completed.
The oil
transformation process in ice-covered water areas is slower due to the
following factors: an increase of the viscosity of crude oil at low
temperatures; restrictions in the oil's distribution owing its
adsorption on the ice surface; accumulation in the porous stratum and
interstices of the ice cover; and a slow down of the oil's decay by
bacterial and photochemical processes in conditions of lower
temperatures and restricted oxygen input [1].
During the spring-summer period, the migration
of oil into ice capillaries varies from 1 to 49 sm/day. The mean rate
of vertical movement of oil into ice is equal to 8 sm/day. Strong winds
and currents break up the ice cover and allow the ice to drift. In the
Bering Sea, for example, the typical rate of ice drift is 7.4 km/day,
and it may increase up to 33-44 km/day during storms [67].
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