Responding to oil spills, like that in New Zealand’s Bay of Plenty, is a very complex, high pressure situation. Decisions must be made based on whatever data are available at the time.
One of the difficulties in assessing the environmental impacts of oil spills is the lack of environmental baselines against which to measure the changes. There is a lack of scientific data on the most appropriate response options in different situations.
This is a situation that needs to change if we are to improve our capacity to reduce and measure the environmental impacts of future spills.
What do oil spills do to the environment?
Hydrocarbons affect the environment and plants and animals through several different pathways. They are affected physically through smothering or the external oiling of birds and marine mammals. Chemically, toxic compounds enter the animal itself, causing different levels of short and long-term poisoning.
The most visible impact of oil spill is through smothering. This is often the one that gets most attention. The chemical impacts are harder to quantify, being less visible. But tainting (or the perception of tainting) in commercial species such as shellfish can have immediate commercial impacts.
Oil spills are visually alarming and have high local impact. But far more oil enters the global oceans through other mechanisms. Thus if oil spills can be broken up or dispersed over a broader area, their impacts will be reduced.
Once oil enters the local environment, impacts will continue after the obvious tarry oil has been removed or dispersed. This is at least in part because some oil seems to usually remain hidden at depth in the sediments and can’t be physically removed. The time this oil remains depends on the environment, with cold, low energy environments being the slowest to recover.
How do dispersants work?
When dispersants are applied on reasonably fresh oil they can disperse the oil though the water column. Dispersants do not change the amount of oil but they redistribute it. They can be used to alter the parts of the environment that are affected, moving the oil from surface creatures and sensitive shorelines to the water column and bottom creatures.
Where they are used in deep water and high energy environments they also serve to spread the oil over a wider area (or volume), diluting it and reducing its immediate impact.
Dispersants used to be quite toxic but now are considered to be less toxic than the oil itself, although there is still some debate on this issue. The main environmental decision regarding their use is determining where the oil will have least harm: should it be concentrated at the surface and on sensitive shorelines, or dispersed through the water column?
Dispersant use in open waters has the benefit of diluting the oil, thus reducing its local impacts. But dispersant use in shallow water, enclosed areas and shorelines can increase local environmental impacts and is unlikely to be considered these days.
Once oil has reached the shoreline, physical removal and letting nature take its course are some of the few options available.
There does not seem to be much evidence to indicate that dispersing oil leads to greater uptake by organisms, although this would be very hard to measure. The key result is that the oil is diluted so may have less immediate toxicity.
Letting nature take its course
It seems that oil will eventually be broken down by natural processes including microbial activity.
Microbial activity may be especially important after oil has entered habitats such as sub-surface sediments where physical weathering can no longer occur. It seems to be a long-term process as oil has been detected in sediments a decade after oil spills have occurred.
The more volatile components of the oil are typically considered to be the most toxic, but they are also the components that will boil off or evaporate most rapidly. Typically, heavier crudes hang around longer, are harder to disperse and have a greater visual and aesthetic impact.
Oil evaporation will be increased in warmer temperatures and impacts reduced. Dispersion in the water column will be increased in high energy environments (such as high wave action) which will dilute the oil and its local impact. High tidal energy will also spread and dilute the oil, but may make it harder to prevent the oil reaching sensitive areas.
In low energy environments, there is little opportunity for physical processes to operate. Spilled oil that remains at the surface will gradually be dispersed by natural physical processes especially in high energy environments. But oil that reaches low energy environments or gets buried in sediments may persist for a decade or more.
This is why it is so important to try and prevent the oil reaching these sheltered and vulnerable environments.