Decision making criteria
The main considerations for the selection of response methods the physical behaviour, it’s the persistency, and the hazards associated with the substances released.
The response options, in the form of decision trees, are therefore divided into groups applicable to groups of chemicals with the same behaviour and hazard characteristics.
Substances released in the aquatic environment may pass into the air as gases or vapours while those which float on the water surface or sink to the seabed, may subsequently dissolve or disperse as droplets into the water column. Again, some substances and mixtures of substances may transiently exhibit combinations on the water bottom (sinkers), or could be a combination of these processes. Each compartment (atmosphere, water surface, water column, seabed) has its own relevant hazard considerations. For example, toxicity and explosion hazards must be considered for those substances, which pass into the air after release.
To select and decide on the most appropriate response action, chemicals can be divided into nine groups with similar response approaches based on the behaviour and the relevant hazard.
The hazards are:
- Aquatic toxicity
- Bioaccumulation
- Persistence
- Toxicity by inhalation
- Explosiveness
- Flammability
- Radioactivity
- Corrosiveness
- Carcinogenicity
The aim of release response action is to reduce or eliminate hazards not expected to self-correct by natural dilution and oxidative degradation in convenient timescales. Nine decision trees have been developed for this purpose, choice depending on:
- The behaviour group of the substance spilled
- The hazard profile of the substance.
The choice of the most appropriate response action depends on the behaviour and hazard of the substance released.
It is clear that the choice of decision tree depends also on the compartment(s) into which a substance will pass on release. Substances passing into more than one compartment require more than one decision tree to be consulted for selection of the applicable response.
An other approach often used is to distinguish three response options e.g.:
- No intervention possible
- Non intervention
- Intervention
General approach
Check whether any remedial measures can eliminate or minimize the release. If not, then:
1.Determine into which compartment(s) and to what extents the released substance will pass by reference to the behaviour class expressed as SEBC (%).
2.Determine the relevant hazard aspects. (See section 5.4 of this manual) All scores > 0 for a hazard aspect are relevant. The higher the relative hazard values the higher response the priority should be.
3.Determine with the help of Table 1 and the result of steps 1 and 2, which decision tree(s) are applicable and subsequently which response method(s) may be applicable.
The decision trees are found on pages 98 to 106. Explanation of the response options is given in paragraphs 6.1.1 and 6.1.3. To explain the use of this table, which in fact is the working method to support decision-making; some examples are given. It should be noted that more than one response method is often applicable. For instance, "remove ignition sources" and "consider partial evacuation".
Example: a spill of toluene.
Information from Chemsheet: see Figure 72 and Figure 73
1. Behaviour (SEBC):
E = 50 % (in the air )
F = 50 % (on the water surface)
2. Relevant hazard aspects:
Persistence (PE) = 2,2
Toxicity by inhalation (AT) = 0,9
Explosiveness (EX) = 1,6
Flammability (FI) = 1.4
3. Use of Table 1 to select the response methods applicable to a toluene spill by reference to the following prioritisation of decision trees:
Tree 2 - (E = 50 and EX = 1,6)
Tree 3 - (F = 50 and FI = 1,4)
Tree 4 - (F = 50 and PE = 2,2)
Tree 1 - (E = 50 and AT = 0,9)
General remedial measures
There are some general measures to minimize the effects of a spill regardless of the substance and its hazards.
Figure 74 shows some general options for counter pollution.
Decision tree 1 – Evaporators or gases toxic or carcinogenic by inhalation
- A sub lethal effect: for example, inhibition of growth
- A lethal effect: concentration > 3 times MAC or TLV.
The area affected depends on many factors, such as organism type, condition of organism, concentration, exposure time etc).
Gas and vapour plumes are difficult to impossible to combat at sea or anywhere else. Response to such plumes when necessary is mainly to reduce exposure by issue of warnings and ultimately by evacuation of the area likely to be affected, where the gas cloud is expected to be or to pass.
Appropriate computer models to predict the size, concentration and trajectory of such plumes can be devised though measurement of actual concentrations is more reliable.
Decision tree 2 - Explosive gases / vapours
As a rule of thumb, when the concentration of a gas in the air is over 10% of the LEL (lower explosion limit) care must be taken in order to avoid an explosion.
Gas and vapour plumes are difficult to impossible are to combat directly at sea or anywhere else. Response to such plumes is mainly to remove ignition sources and to evacuate the area likely to be affected until the danger has passed.
Appropriate computer models to predict the size and trajectory of such a gas cloud can be devised, though direct measurement of concentration is more reliable.
Decision tree 3 – Flammable (floating) substances
This group of chemicals can be combated with the so-called first line oil recovery vessels. As this group of chemicals presents fire and possibly explosion hazard, the LEL needs to be continuously measured during recovery operations.
In most cases, such spills will disappear from the water surface by evaporation. Computer models may be devised to predict the degree of hazard, the size, potential trajectory, and the lifetime of the slick in the absence of remedial measures, though all of these are readily predictable from the distillation profile, the viscosity-controlled spreading rate and extent, the known movement of such slicks on wind and tide vectors, and the viscosity-controlled half-life of the slick.
Decision tree 4 - Floaters, which are persistent or carcinogenic
As long as the slick is detectable its position and its layer thickness can be monitored by direct observation sampling and analysis, though models can be devised to e estimate the size of the slick and of the threatened area if such is believed more desirable.
Floating substances can cause harm to individual birds and to coastlines prior to their natural degradation.
As many floaters are invisible to the naked eye, remote sensing techniques independent of the visible spectrum need to be used.
Again, computer models could be devised to predict the seriousness, size and trajectory of the slick should this be believed necessary.
Decision tree 5 - Dissolvers toxic in water, carcinogenic or bio accumulative
A concentration of a toxic substance can have one of three effects on organisms:
- No effect: concentration normally taken as 1% of the LC50(96);
- A sub lethal effect: for example, inhibition of growth;
- A lethal effect.
The effects depend on many factors: organism type, exposure time, condition of the organism, etc.
This group of chemicals, once released into the water and dissolved, cannot be recovered at sea. Response to this group of chemicals is thus focussed more on reduction of the toxic exposure by restriction of user-access to the area affected until dilution, migration or natural degradation removes the offending substance.
Appropriate computer models could be devised are to predict the size and trajectory of such a dissolved substance in the water column, the time over which it might be expected to affect benthic habitats, though in situ measurements of concentration are likely to be more reliable.
Using mussels as a bio monitor of dissolved substances in the water column has produced very promising results.
Chapter 6 provides more information on measurement techniques..
Decision tree 6 - Corrosive substances
Corrosive substances are solids or liquids capable of direct erosion of living tissue more or less severely, though their corrosive activity is reduced by their corrosive action.
This group of chemicals, once released into the water and dissolved, cannot be recovered at sea. Response to this group of chemicals is focussed more on reducing exposure to the substance by restriction of user-access to the affected area until corrosivity is reduced by natural dilution or by natural neutralisation.
Appropriate computer models could be devised are to predict the size and trajectory of such substances in the water column, the time over which it might affect benthic habitats, though direct measurement would be more reliable. Again, we should recall that the inorganic constituents of seawater act as a buffer in neutralising both acids and bases..
Decision tree 7 - Persistent or carcinogenic sinkers
As long as the pool blanketing the seabed is detectable, its position should be monitored, while the area covered and the area affected by solution and dispersion should be assessed by sampling and analysis. As to its possible effect on the environment.
Substance in this group will stay on the sea floor giving some time to react and to determine the best combating option. This group of chemicals can theoretically be combated with existing dredging equipment. Best solution is usually determined on an ad hoc basis. Depth, substance type, accessibility and many other factors determine the most suitable dredging equipment and techniques.
Detection itself is a problem; they’re being of sunken no techniques capable of detecting sinkers at unknown locations. Even the operations of ROV, and/or divers need a starting position, and while computer models could be devised predict the movement of a sunken substance, it too would need to know where to start. Again it would appear that even direct sampling would need a preliminary focus.
A good example of the difficulty is that the chlorine cylinders lost by the Sindbad were located only when accidentally retrieved from the seabed by fishermen at great danger to themselves months after being lost.
Decision tree 8 - Explosive substances (IMDG class 1)
Apart from substances that emit explosive gases, explosive substances themselves must be considered. Again, with explosives being transported in packaged form, the type of packaging itself contributes to the explosive damage.
The danger with packaged explosives is, of course, that they might accidentally detonate on release to the environment.
Most explosive packaged goods lost in the marine environment, remain a danger until they are recovered.
Decision tree 9 - Radioactive substances
Radioactive substances emit radiation. Contamination of a human or marine organism with even a small amount of magnifies the effect by close proximity, though the specific effect depends on the intensity of the source and on whether the radiation is in the form of alpha particles, beta particles or gamma rays, each having different energy levels.
When dealing with such substances, experts in the field of radioactivity should be consulted.
