Vol.1 part 4-5 - Department of Invertebrate Zoology

The test for independence is calculated as
xz =E (0U - E4) 2/E4
u
where OU is the observed number of items in row i, column
j . The test statistic follows an approximate chisquare
distribution with degrees of freedom equal to the
number of rows less one times the number of columns
less one .
For the factor of age, a x2 of 2 .98 with 4 degrees
of freedom was calculated . The tabled value at the
5-percent level is 9 .49 for 4 degrees of freedom, and the
results are clearly not significant . The conclusion is,
then, that no relationship can be assigned between platform
age and pollution level .
A relationship between the productivity of the
platform and the likelihood of trace metal contamination
of the surrounding area was examined in a similar
manner, using number of wells drilled as an indicator
of activity level . The numbers of wells were divided
into three classes : 1 to 7, 12 to 18, and 21 to 24 . The resulting
contingency table is given below (Table 16) .
The chi-squared value for this contingency
table was 8 .50 with 4 degrees of freedom, again falling
below the critical value of 9 .59 . The conclusion is that
no relationship may be assessed between the activity
level and the likelihood of pollution around the platform
.
The final investigation examined the
relationship between level of production of both oil and
gas and the likelihood of trace metal contamination .
Cumulative production figures were obtained for 17 of
the 20 study platforms . Oil production in millions of
barrels and gas production in MCF were used to construct
contingency tables (Table 17) .
The chi-squared values were 3 .50 with 4 degrees
of freedom and 10 .30 with 6 degrees of freedom
for oil and gas, respectively . The critical values from the
chi-squared distribution at the five-percent level are 9 .49
and 12.59 with 4 and 6 degrees of freedom, respectively .
Therefore, no relationship is assessed .
The overall conclusion is that if some portion
of the trace metal concentrations in sediments analyzed
in this study are due to releases from platforms, this
contamination cannot be related to the age of the
platform, the level of activity, or the level or type of
production .
2 . Task 2 : Correlation of Trace Metal Concentradons
is Downcore Sediments with Proximity to
and Age of Platforms
The investigation of trace metals in downcore
sediments with respect to age of the platform and distance
from it could not be successfully completed . The
dating of the cores revealed that either extensive mixing
had occurred naturally or during the sampling process
or the activity of supported Pb-210 was too large and no
accurate time frames could be established . In addition,
a particle size analysis of the top ten centimeters did not
correspond to the analysis of the surficial sediments
from the same sites . This raises the possibility that the
surficial layer was lost on sampling, which would also
adversely affect the results of this investigation . The
overall conclusion, then, is that no meaningful information
can be obtained from these data, and that no response
to this task is possible .
3. Task 2a : Correlation of Trace Metals in Downcore
Sediments with Historical Development of
the Study Area
The original plan to determine the petroleum
"predevelopment" strata in the downcore sediments (by
Pb-210 geochronological dating) was not accomplished
because of the apparent well-mixed character of the
upper (0 to 20+ cm) sediment layers . This well-mixed
upper layer was either (1) an artifact of the sampling effort
(see Volume I, Part 3, II .C .I .b .), (2) actually representative
of rapid sedimentation in this area of the
OCS, or (3) the result of the passage of a major storm
front (Tropical Storm Debra, 1978) .
Without the geochronological time frame of
reference the trace metal data from the downcore sediments
could not be analyzed to draw any further conclusion
than those stated in Section IV A .2 of this report .
4. Task 3: Determination of Impacts of Known
Spills, Discharges and Other Sources of Petroleum
Activity-Related Contaminants
a. Known Spills
Oil spills are known to have occurred near
S12 and S13 (Vol . I, Part 1, V.C .11) . Crude oils usually
have high concentrations of Ni and V, and concentrations
of these two metals would have been higher at
these two sites . However, no concentrations of trace
metals in surficial sediments were observed that could
be associated with those spills .
TABLE 16
No . of Wells
Probable
Pollution
Possible
Probable
Pollution No . Pollution
Total
1-7
0
0.7
0
0.8
2
0.5
12-18
21 - 24
4 .55 5 .2 5 .25
6 S 3
1 .75 2 1 .25
1 3 0
14
Total
20
28