Geoscience Reference
In-Depth Information
Case Study 9.4 Long-term persistence of oil residues in mangrove sediments, Panama
In April 1986, rupture of an oil store spilled between 75,000 and 100,000 barrels of medium-
weight crude oil, at Bahiá las Minas on the Caribbean coast of Panama (Case Fig. 9.4a).
Surveys conducted 2 months after the spill indicated that heavy oiling occurred along some
82 km of coastline between Isla Margarita and Islas Naranjos, which harbours around 16 km
2
of mangroves and 8 km
2
of coral reefs. The only areas along this section of coast to escape
heavy oiling were two restricted mangrove lagoons (Case Fig. 9.4a). The oil had a significant
negative impact upon the mangrove communities, as well as upon nearshore reefs and seagrass
infauna ( Jackson et al. 1989). Long-term (
>
5 yr) monitoring of mangrove sites along the coast
demonstrated:
1
the long-term persistence of residual oils that accumulated within organic-rich mangrove muds;
2
the potential for a range of aromatic hydrocarbon residues to be preserved in the sediments
and for intermittent release into the nearshore environment;
3
the impacts on the mangrove communities themselves.
Initial incorporation of oil into the mangrove sediments was rapid. Oil residues were found
at depths of 20 cm within 6 months of the spill, and are likely to have migrated into the sediments
via diffusive processes, through crab burrows and down dead or decaying mangrove root casts.
The oil residues, however, showed evidence, at most sites, of rapid compositional changes
resulting from a combination of weathering processes (including evaporation), dissolution,
microbial degradation and photochemical decomposition. Undegraded oil residues were found
only at a few sites and these are believed to have been preserved within anoxic sediment zones.
Despite the apparently rapid compositional changes that occurred, oil residue concentrations
remained high within surface sediments 5 years after the spill, and at many sites had increased
by an order of magnitude at depths of up to 20 cm (Burns et al. 1994).
(b)
Panama
Open coast
Islas
Naranjos
1
0
Bahia
Las
Minas
Channels
(a)
1
Punta
Muerto
Punta
Galeta
Isla
Margarita
R
0
Drainage streams
1
R Oil refinery
Coastline affected by heavy oiling
Areas free from heavy oiling
0 km 2
0
A N F M A N F M A N F M A N F M A N F M
86 87 88 89 90 91
Case Fig. 9.4
(a) Map showing the location of the Bahiá Las Minas oil refinery and the main areas of coast impacted by the
April 1986 oil spill. (Adapted from Guzmán et al. 1991.) (b) Plots showing the periodicity of secondary oiling episodes, as
indicated by the presence of oil on marker dowels, in different mangrove environments. Arrows denote periods when increased
impacts were reported on adjacent coral reef communities (data in Guzmán et al. 1994). These appear to immediately follow
each of the secondary oiling episodes. (Adapted from Burns et al. 1993.)
