Ö Springer 
Geo-Mar Lett (2017) 37:163-170 
DOI 10.1007/s00367-016-0454-6 
CrossMark 
ORIGINAL 
Effects of chemical dispersants on oil spill drift paths 
in the German Bight—probabilistic assessment based 
on numerical ensemble simulations 
Fabian Schwichtenberg * 1 • Ulrich Callies 1 • Nikolaus Groll 1 • Silvia Maßmann 2 
Received: 15 February 2016/Accepted: 7 June 2016/Published online: 21 June 2016 
© The Author(s) 2016. This article is published with open access at Springerlink.com 
Abstract Oil dispersed in the water column remains sheltered 
from wind forcing, so that an altered drift path is a key con 
sequence of using chemical dispersants. In this study, ensem 
ble simulations were conducted based on 7 years of simulated 
atmospheric and marine conditions, evaluating 2,190 hypo 
thetical spills from each of 636 cells of a regular grid covering 
the inner German Bight (SE North Sea). Each simulation com 
pares two idealized setups assuming either undispersed or 
frilly dispersed oil. Differences are summarized in a spatial 
map of probabilities that chemical dispersant applications 
would help prevent oil pollution from entering intertidal coast 
al areas of the Wadden Sea. High probabilities of success 
overlap strongly with coastal regions between 10 m and 20 
m water depth, where the use of chemical dispersants for oil 
spill response is a particularly contentious topic. The present 
study prepares the ground for a more detailed net environmen 
tal benefit analysis (NEBA) accounting also for toxic effects. 
Introduction 
The intertidal sand and mud flats of the Wadden Sea cover an 
area of about 4,700 km 2 along the Dutch, German and Danish 
coasts (Reise et al. 2010). Their rich and biologically diverse 
Responsible guest editor: C. Winter 
G3 Fabian Schwichtenberg 
fabian.schwichtenberg@hzg.de 
1 Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, 
Max-Planck-Str. 1, 21502 Geesthacht, Germany 
2 Bundesamt fur Seeschifffahrt und Hydrographic, 
Bemhard-Nocht-Str. 78, 20359 Hamburg, Germany 
ecosystem is a migration stopover and wintering site for many 
birds (e.g. van Beusekom et al. 2012). Due to their proximity 
to busy shipping lanes, some of the Wadden Sea habitats are 
potentially endangered by nearshore oil spill incidents. Such 
an accident already happened in October 1998, when the car 
go vessel PALLAS ran aground close to the island of Ammm 
(Reineking 1999). The subsequent oil spill affected about 
12,000 sea birds residing in the North Frisian Wadden Sea 
(Fig. 1). 
So far, contingency planning for the German North Sea 
coast considers only the use of mechanical devices because 
of concerns regarding toxic effects of chemical dispersants. 
Application of dispersants is regarded as a last resort response 
option (EMSA 2014). If mechanical cleaning is not promis 
ing, then dispersants could be applied without restriction in 
areas deeper than 20 m, whereas it is strictly prohibited in 
areas shallower than 10 m. At intermediate water depths 
(10-20 m), dispersants might potentially be used depending 
on the outcome of a case-specific assessment of environmen 
tal damages and benefits. With today’s third-generation chem 
ical dispersants being much less toxic (EMSA 2009), the op 
tion of using them particularly in inshore zones needs to be 
reassessed. 
The advantages of using chemical dispersants are twofold; 
in the first place, dispersants reduce the pollutant volume on 
the water surface. In the second place, they facilitate biodeg 
radation processes by increasing the reactive surface of the oil. 
However, their effectiveness much depends on the kind of oil 
spilled, its state of weathering (viscosity and degree of water 
in oil emulsions) and the hydraulic energy in the polluted area. 
Other factors of influence are salinity, turbidity and tempera 
ture (EMSA 2009). 
Initially, concentrations of dispersed oil may be quite 
high before they get decreased by dilution and biological 
degradation. Even when toxicity of the dispersant itself is