233
A . NELSON-SMITH
be catalysed by sunlight or by trace metals, such as vanadium, which
are present in the oil. The products may be water-soluble or surface
active and may thus reduce the bulk of the slick or contribute to its
emulsification. Compounds originally present in crude oil may also
contribute to the formation or stability of emulsions, for example
resinous “ asphaltic ” particles and sulphonic acids (Pilpel, 1954, 1968).
B. Behaviour of spilt oil on sea and shore
When a thin oil is spilt on a clean water surface it will rapidly
spread until, at least under theoretically perfect conditions, it has
become a monomolecular layer. Crude oils on natural waters probably
never achieve this, but the typical iridescence of a small slick indicates
that its thickness is 1-5-10 x
mm, which represents about 1501 000 litres/kmz. Quantities less than this produce a silvery sheen,
colours begin to appear at the lower limit and become dull a t the upper
limit. A thicker slick is dark, without interference colours (Stroop,
1930; American Petroleum Institute, 1963). As the oil spreads, its
more volatile constituents evaporate and the water-soluble materials
are leached out. The remaining residue will have an increased viscosity
and pour-point, thus a lessening tendency to spread further, so that
spreading is a self-retarding phenomenon. Blokker (1964, 1966) has
given equations for the spread of an oil-slick which are elaborated €or
various crude oils by Berridge et al. (1968a) in an account of their field
experiments.
Evaporation plays a considerable part in reducing the bulk of a
crude-oil slick; it is estimated that about one-third of the “ Torrey
Canyon’s ” cargo of Kuwait crude was lost by evaporation following
the spill (Brunnock et al., 1968), equivalent in effect to the removal
of all fractions boiling below about 300°C (see Table I, p. 235).
McKay (in discussion following Brunnock’s paper) pointed out that
this quantity of hydrocarbons equals the average total amounts of
sulphur dioxide and smoke in the atmosphere over Great Britain, so
that the “ Torrey Canyon ” stranding contributed substantially to air
as well as water pollution. Although oil traditionally calms rough
waters, appreciable amounts are still carried away by wind from the
tops of breakers at sea or from waves as they strike the shore (ZoBell,
1964). Considerable damage was caused to lichens and flowering plants
on Cornish cliff tops by wind-blown oil-spray from below (Ranwell,
1968a, b).
Very large amounts may also disappear by the sinking of heavy
residues, often aided by the incorporation of water during the formation
A . NELSON-SMITH
be catalysed by sunlight or by trace metals, such as vanadium, which
are present in the oil. The products may be water-soluble or surface
active and may thus reduce the bulk of the slick or contribute to its
emulsification. Compounds originally present in crude oil may also
contribute to the formation or stability of emulsions, for example
resinous “ asphaltic ” particles and sulphonic acids (Pilpel, 1954, 1968).
B. Behaviour of spilt oil on sea and shore
When a thin oil is spilt on a clean water surface it will rapidly
spread until, at least under theoretically perfect conditions, it has
become a monomolecular layer. Crude oils on natural waters probably
never achieve this, but the typical iridescence of a small slick indicates
that its thickness is 1-5-10 x
mm, which represents about 1501 000 litres/kmz. Quantities less than this produce a silvery sheen,
colours begin to appear at the lower limit and become dull a t the upper
limit. A thicker slick is dark, without interference colours (Stroop,
1930; American Petroleum Institute, 1963). As the oil spreads, its
more volatile constituents evaporate and the water-soluble materials
are leached out. The remaining residue will have an increased viscosity
and pour-point, thus a lessening tendency to spread further, so that
spreading is a self-retarding phenomenon. Blokker (1964, 1966) has
given equations for the spread of an oil-slick which are elaborated €or
various crude oils by Berridge et al. (1968a) in an account of their field
experiments.
Evaporation plays a considerable part in reducing the bulk of a
crude-oil slick; it is estimated that about one-third of the “ Torrey
Canyon’s ” cargo of Kuwait crude was lost by evaporation following
the spill (Brunnock et al., 1968), equivalent in effect to the removal
of all fractions boiling below about 300°C (see Table I, p. 235).
McKay (in discussion following Brunnock’s paper) pointed out that
this quantity of hydrocarbons equals the average total amounts of
sulphur dioxide and smoke in the atmosphere over Great Britain, so
that the “ Torrey Canyon ” stranding contributed substantially to air
as well as water pollution. Although oil traditionally calms rough
waters, appreciable amounts are still carried away by wind from the
tops of breakers at sea or from waves as they strike the shore (ZoBell,
1964). Considerable damage was caused to lichens and flowering plants
on Cornish cliff tops by wind-blown oil-spray from below (Ranwell,
1968a, b).
Very large amounts may also disappear by the sinking of heavy
residues, often aided by the incorporation of water during the formation