166
meaning that the treated oil slick will still have substantial size and lifetime. The
response decision should include whether the result is worth the effort and costs for
the specific situation.
As expected, a certain reduction in interfacial tension is most effective on lowviscosity oil, as the relative change in droplet size is higher (Sect. 10.3.5). Together
with the observation that high viscosity hinders dispersant incorporation into the oil
slick (Canevari 1984), this means that chemical dispersion is relatively and absolutely less effective on higher-viscosity than on low-viscosity oils.
Sub-optimal natural dispersion causes the oil slick to elongate substantially in
the wind direction, where the thick slick portion is followed by a long tail in the
upwind direction. Dispersants can be beneficial in these situations, accelerating
the spreading of the thick slick part and the subsequent transition into the phase
where size is slowly reduced as more mass is moved to the water column. The
chemical as well as operational effectiveness of dispersants does decrease with
increasing viscosity.
The added benefit of dispersants is limited in conditions that are optimal for
natural dispersion, resulting in only slight slick elongation and with a slick thickness that is symmetrical in the wind direction.
10.4.2 Effects
As explained, the environmental benefit of dispersant application lies mainly in
removing oil from the water surface: Depending on the effectiveness, dispersants
can have a positive influence on mitigation of surface effects. However, the enhanced
volumes of suspended/dissolved oil can enhance adverse effects in other compartments of the environment (O’Sullivan and Jacques 2001).
10.4.2.1 Water Column
Although the dispersants themselves are designed to be low in toxicity, the resulting
enhanced exposure to hydrocarbons can potentially cause toxic effects to water column biota. Where physical surface oil impacts are mass independent for thicknesses
above the effects threshold (Jongbloed et al. 2002), potential water column effects
are more transient and are determined by exposure time and maximum concentration (Lee et al. 2015).
The increased exposure to hydrocarbons by enhanced dispersion is only brief,
as concentrations are expected to diminish quickly due to dilution (Lee et al. 2015).
This dilution is less evident in case of continuous dispersion of large or prolonged
spills such as the Deepwater Horizon incident or in shallow or enclosed waters
(BfR Wissenschaft 2016). Assessment of potential toxic effects encompasses
uncertainties such as species-dependent sensitivity. Moreover, the precise exposure
of the organisms over time depends on the specific spill conditions, oil qualities, as
M. Zeinstra-Helfrich and A. J. Murk
meaning that the treated oil slick will still have substantial size and lifetime. The
response decision should include whether the result is worth the effort and costs for
the specific situation.
As expected, a certain reduction in interfacial tension is most effective on lowviscosity oil, as the relative change in droplet size is higher (Sect. 10.3.5). Together
with the observation that high viscosity hinders dispersant incorporation into the oil
slick (Canevari 1984), this means that chemical dispersion is relatively and absolutely less effective on higher-viscosity than on low-viscosity oils.
Sub-optimal natural dispersion causes the oil slick to elongate substantially in
the wind direction, where the thick slick portion is followed by a long tail in the
upwind direction. Dispersants can be beneficial in these situations, accelerating
the spreading of the thick slick part and the subsequent transition into the phase
where size is slowly reduced as more mass is moved to the water column. The
chemical as well as operational effectiveness of dispersants does decrease with
increasing viscosity.
The added benefit of dispersants is limited in conditions that are optimal for
natural dispersion, resulting in only slight slick elongation and with a slick thickness that is symmetrical in the wind direction.
10.4.2 Effects
As explained, the environmental benefit of dispersant application lies mainly in
removing oil from the water surface: Depending on the effectiveness, dispersants
can have a positive influence on mitigation of surface effects. However, the enhanced
volumes of suspended/dissolved oil can enhance adverse effects in other compartments of the environment (O’Sullivan and Jacques 2001).
10.4.2.1 Water Column
Although the dispersants themselves are designed to be low in toxicity, the resulting
enhanced exposure to hydrocarbons can potentially cause toxic effects to water column biota. Where physical surface oil impacts are mass independent for thicknesses
above the effects threshold (Jongbloed et al. 2002), potential water column effects
are more transient and are determined by exposure time and maximum concentration (Lee et al. 2015).
The increased exposure to hydrocarbons by enhanced dispersion is only brief,
as concentrations are expected to diminish quickly due to dilution (Lee et al. 2015).
This dilution is less evident in case of continuous dispersion of large or prolonged
spills such as the Deepwater Horizon incident or in shallow or enclosed waters
(BfR Wissenschaft 2016). Assessment of potential toxic effects encompasses
uncertainties such as species-dependent sensitivity. Moreover, the precise exposure
of the organisms over time depends on the specific spill conditions, oil qualities, as
M. Zeinstra-Helfrich and A. J. Murk