download 21.6mb - Oil-Spill-Info.com

ably reflect the chemistry of sediments under relatively calm conditions.
Mixing which occurs as a result of tidal or storm driven wave
action might alter the depth to which oxygen can penetrate sediments.
This should vary with the nature of the sediment so that muds should be
less affected than sandy sediments on high-energy beaches. Over seasonal
time intervals, it is likely that the aerobic/anoxic boundaries
predicted by Eh profiles are preserved in the muddy sediments sampled.
However, it is likely at the oiled beach AMC-4 that erosion and deposition
created considerable instability in the depth of oxygen penetration
and could even have caused vertical redistribution of sediments
(Gundlach and Hayes, 1978). It is also possible that oxygen could be
introduced to depths below the lower boundary of its diffusion by sediment
infauna which can burrow into anaerobic sediments.
The vertical distribution of the dominant anaerobic process, sulfate
reduction, indicated maximum activity in the surface 0-3 cm interval
at all stations. The obligately anaerobic sulfate-reducing and
methane-producing bacteria were also present in maximum number in the
0-3 cm depth interval (Winfrey and Ward, submitted). These observations
suggest that at least portions of the 0-3 cm interval at all
sites were sufficiently anoxic to allow survival and activity of obli-
gately anaerobic microorganisms.
A survey of the various sediments sampled confirmed the presence
of AMOCO CADIZ pollutants in oiled sites. Although control sites were
not polluted by the AMOCO CADIZ spill, each contained some hydrocarbons
of anthropogenic origin. The extent of oiling was greatest at the sediment
surface where anaerobic processes were greatest. Oiling decreased
with depth, but there was clear evidence of AMOCO CADIZ hydrocarbons in
sediments likely to be free from exposure to oxygen. Sediments below
the aerobic/anoxic boundary and above the deepest level of penetration
of AMOCO CADIZ pollutants provided an environment suitable for the enrichment
of anaeroDic hydrocarbon-degrading microorganisms, and appropriate
for comparison to aerobic surface sediments to study the differences
in weathering in situ due to different exposures to oxygen.
Because of the rapid biodegradation of aliphatic components and
relative enrichment of aromatic components of the spilled oil (Atlas,
et al, 1981; Ward, et al, 1980), ratios of naphthalenes, phenanthrenes
and dibenzothiophenes to the more persistent C -DBT were used as an
index of weathering. This index should be independent of absolute
amounts of oil within sediment samples which could vary due to patchy
distribution of oil. Changes in sediment aromatic hydrocarbons occurred
in all sediments and at all sediment depths where comparisons
were made for one year or were noted in
longer.
surface sediments of
The
the
greatest and most rapid changes
beach station AMC-4 where most
compounds had decreased by one year after the spill and extensive
losses had occurred by about 20 months after the spill. This seems
consistent with the expected mixing and oxygenation of this high energy
beach sediment. In muddy sediments slower , changes in relative amounts
of aromatic compounds were noted. Extensive losses were observed mainly
among the naphthalenes and DBT's. This may have been related to the
low energy nature of these sediments and/or the corresponding lack of
oxygenation indicated by reducing conditions. Decreases in these com-
183