BfR-Wissenschaft 
23 
5 Oil and dispersants - marine sensitivity aspects 
K.H. Carlo van Bernem 
Helmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research,Geesthacht, Germany 
Whenever a decision-maker has to decide which kind of action could be applied to respond 
to an oil spill, a risk evaluation is necessary. Belluck et al. (1993) defined three classes for 
ecological risk assessment (scientific, regulatory and planning) that lie along a continuum 
from most to least quantitative. Because costs (and usually time) increase with the level of 
scientific detail that can be obtained, the desire to improve the analysis must always be 
weighted against the effort of the additional information. 
The behaviour of oil in water should be assessed before the questions “Will dispersants work 
effectively with a particular oil in the environment of interest?“ and “What are the ecological 
consequences of dispersant use?“ can be answered. 
The main processes involved when oil (especially crude oil) is spilled on sea water, summa 
rized as “weathering“ (Daling et al., 1990) are spreading, evaporation, dissolution, formation 
of emulsions, dispersion in the water column, sedimentation and, biodegradation. 
Direction and speed of a driven oil slick depend on current conditions and on wind velocity. 
Consequently a drift model for coastal waters is a good tool for use as part of a conceptual 
model to predict the areas at risk. A spreading slick itself forms a large region of „sheen“ 
about 1 pm thick containing less than 5 % of the total oil volume. The majority of oil is bound 
to a much smaller area with a thickness of several millimetres in case of a stable emulsion. 
Within the first few hours or days most crude oils will loose up to 40 % of their volume by 
evaporation. This process, driven by temperature and wind speed, reduces the portion of 
lighter components of the oil leaving a smaller pollution volume with a higher viscosity and 
minor toxicity. This loss of oil components to the atmosphere is supplemented by a much 
smaller rate of dissolution. The amount of water soluble hydrocarbons around an oil slick is 
generally in the ppb range but remains toxic and bioavailable for marine organisms. On the 
other hand the incorporation of water into the oil residue left by evaporation and dissolution 
leads to a large increase of pollutant volume raising the viscosity once again. Very stable 
emulsions formed by some oils are resistant to chemical treatments or heating. Under rough 
sea conditions low viscous oils disperse naturally into the water column to a large extent, 
forming droplets of a wide range of sizes. While larger oil droplets resurface, only the smaller 
(< 70 pm) are found in permanent dispersion. Clay and particles of similar size (1-100 pm 
diameter), and microscopic organisms, interact with dispersed oil droplets by adsorption and 
ingestion. In waters of high turbidity, as for example in estuaries, the resulting flocculation 
(oil-mineral-microbial complexes) can reach high levels and obey the characteristic environ 
mental processes of sedimentation and accumulation in areas of low hydraulic energy like 
nearshore tidal flats and watersheds. 
The final weathering-process of spilled oil is biodegradation. All but the most refractory com 
ponents of a crude oil can be degraded by biological actions in the water column as well as 
in sediments. The rates depend on temperature and the availability of oxygen. They range 
from 1-50 mg/m 3 /day to years in very cold or anaerobic environments. 
The advantages of using chemical dispersants are twofold: in the first place they reduce the 
pollutant volume on the water surface. Secondly they increase the rate of biodegradation 
processes by increasing the reactive surface of the oil. Their effectiveness depends mainly 
on the kind of the oil, its state of weathering (viscosity and degree of water-in-oil emulsions) 
and on the hydraulic energy in the area of concern. Other factors of gradual influence are: 
salinity, turbidity and temperature.