Natural Seep Project Summary
May 24, 2004
The Natural Oil Seep Inventory was conducted
from April, 2002, to March, 2004, by:
Santa Barbara County, Planning and Development
Department, Energy Division, and
U.S. Geological Survey, Western Coastal
and Marine Geology Team, Menlo Park,
CA
The study was funded in part by: State
of California Resources Agency, Coastal
Resources Grant Program, Project No.
42-A-01, Contract No. OCA13008
The following is excerpted from the
County’s summary final report to
the California Resources Agency, May
17, 2004.
Abstract
This project was conducted in conjunction
with a U.S. Geological Service (USGS)
project funded by the Minerals Management
Service (MMS), utilizing the expertise
of USGS research scientists and resources
of their Menlo Park laboratory. We
conducted monthly sampling at 10 Santa
Barbara County beaches in order to
establish a baseline estimate of the
amount of tar present. Field crews
composed of geology students from the
University of California at Santa Barbara
(UCSB) weighed tar and counted tar
balls during a 12-month period. Sampling
was extended an additional 4 months
by USGS personnel. 346 tar samples
were collected in 637 beach transects.
More than 182 of the beach tar samples
were analyzed for persistent hydrocarbons
(“biomarkers”) and isotopic
composition. The samples analyzed include
these beach tar samples, plus additional
samples collected by the USGS from
beaches, natural offshore oil seeps,
and offshore production platforms.
Biomarker ratios were computed from
the analytical data. Clustering and
principal component methods were applied
to the biomarker data, resulting in
a classification of the tar samples
into 9 groups. The transect sampling
data were used to estimate the amount
of tar present per kilometer of shoreline
of each beach during each month of
the project. The locations of samples
from the various tar groups were mapped
using color coded dots, providing a
graphical picture of the location of
seep sources in relation to beaches
where corresponding tar was found.
Estimated beach tar amounts were presented
in bar graphs.
The study has provided significant new
information about the deposition of tar
originating in offshore natural oil seeps
onto Santa Barbara’s beaches. The
tar “fingerprint” database
and data on amounts of tar deposited
on the beaches developed during the study
contributes to the knowledge of baseline
beach tar conditions. However, the study
is not conclusive. Additional analysis
of samples from natural seeps and production
platforms will be needed to determine
sources of some of the beach tar samples,
and to refine the classification model
and test its limitations in discriminating
natural seep oil from platform-produced
oil. The estimates of beach tar amounts
indicate that the amount of tar present
is highly variable. It may be possible
to reanalyze the data set in the future,
controlling for some of the factors responsible
for the variability, such as tides and
currents. An error analysis of the tar
deposition data should be done.
Objectives & Accomplishments
The overall objectives of this project
are to advance the state of knowledge
of the patterns of transport and deposition
of oil that enters the ocean through
natural seeps offshore of Santa Barbara
County, and to provide the beginnings
of a library of chemical “fingerprints” of
natural seep tar and oil produced at
offshore platforms. Such knowledge,
beyond its inherent scientific value,
has practical importance for Santa
Barbara County and the State of California.
A record of baseline beach tar deposits
and their variability will be valuable
information for resource damage assessment
in the aftermath of possible future
oil spills. An extensive and reliable
library of chemical fingerprints of
oil and tar could be instrumental for
identifying the responsible party and
distinguishing spilled oil from natural
seep oil.
As noted in the project work statement,
more than a single year of observations
would be needed to fully understand and
characterize tar deposition on the County’s
beaches. In the following, it can be
seen that the specific objectives listed
in the project work statement were accomplished.
Although the results of the study are
not conclusive, the study represents
significant progress in characterizing
seep tar deposition and composition in
Santa Barbara County.
Specific Objectives Accomplished:
1. Extend the systematic tar sampling
at all previously sampled beaches
for one additional year, using the existing
U.S.G.S. sampling protocols.
This study built on the foundation of
work by the USGS under MMS funding. The
study extended the original USGS study
for 12 months (from June, 2002, to May,
2003), employing crews of UCSB students
for field sampling. The USGS research
team further extended sampling through
August, 2003. The student teams operated
under a sampling routine and protocol
adapted from the methods developed by
the USGS team.
2. Add 4 or 5 more beaches along the
south coast of the County into the sampling
regime.
In addition to the 4 beaches sampled
by USGS prior to May, 2002, the student
teams sampled 6 beaches along the
County’s
southern coast.
3. Intensify
sampling to a monthly frequency, in order
to better characterize the variability
of tar deposition.
Sampling was conducted at each beach
at monthly intervals.
4. Analyze an additional 150-200 tar
samples.
In excess of 182 samples were analyzed
by gas chromatography/mass spectrometry
and isotope analysis in the USGS laboratory.
5. Analyze
and archive the resulting data.
The data resulting from the chemical
analysis were analyzed to determine
ratios of persistent biomarkers. The
ratios were used in combination to “fingerprint” the
oil samples. The data have been organized
into a database, and additional samples
have been archived for future use.
Objectives
Partially Achieved:
1. Correlate sources of oil with depositional
locations.
The analyses have successfully shown
that tar deposited varies along the
coast, so that the assortment of tar
types varies from one beach to the
next. The tar classification model
developed by USGS during this project
to classify tar into biomarker-based
groups distinguishes 9 different source-related
tar groups that are found on County
beaches. The tar deposition patterns
are complex, so that at some beaches
several tar types are found. Relatively
few oil samples have been collected
at seep sources for analysis, whereas
there are hundreds of active seeps
in the region. And, the tar is moved
about by complex and variable current
patterns in the Channel. Thus, the
identification of tar deposits with
their seep sources is a “work in
progress.” We anticipate that
the tar classification model will be
further refined during the remaining
months of the MMS-funded part of the
USGS project, so that the depositional
patterns will be better understood.
2. Determine how well biomarker analysis
can distinguish natural seep tar from
oil spills.
The method effectively discriminates
beach tar from oil produced at some
local platforms. However, so far the
method has not been successful in discriminating
tar deposited on beaches from oil produced
at Platform Holly and Platform “A.” It
is unclear whether further refinements
in the method will make that discrimination
possible.
3. Characterize the baseline rates
of tar deposition, and how the
rate varies from beach to beach
and from month to month.
The USGS lab developed estimates of
the amount of tar present at each beach
at the time of each sampling trip. At
the outset, we hoped that the data would
enable us to characterize rates of tar
deposition at different beaches. Although
some general patterns can be inferred
from the data, understanding and confidence
in baseline rates is limited by high
variability of the data. USGS plans to
further analyze the estimation error.
Findings & Conclusions
Patterns of Beach Tar Deposition
The tar and oil samples analyzed in
this study could be classified into 9
groups, based on differences in chemical
composition, as reflected in isotope
analysis and ratios of various persistent
hydrocarbons (“biomarkers”). Each beach
tar sample falls into one of the groups.
In some cases, oil sampled at a natural
seep and tar collected at nearby depositional
beaches fall into the same group. In
other cases, nearby seeps that match
the beach tar are not identified. It
appears that some samples may originate
in seeps near the Channel islands. The
tar groups represented in the samples
vary from beach to beach. At some beaches
(e.g., Sacate Beach), almost all samples
are associated with no more than two
groups, while the tar collected at some
other beaches is more diverse. The correspondence
between tar sources and deposition is
incomplete. Collection and analysis of
additional oil from offshore seeps will
be needed to fill in the gaps. The depositional
patterns are discussed and mapped in
the accompanying USGS report.
Discrimination of Seep Tar from Oil
Produced at Platforms
Biomarker analysis is able to distinguish
oil from distinct sources with confidence.
Thus, beach tar believed to originate
from natural seeps can be distinguished
from oil produced at some platforms (e.g.,
Point Arguello platforms; Irene). However,
samples of oil produced at Platform Holly
(offshore of Coal Oil Point) and Platform “A” (in
the eastern Santa Barbara Channel) are
very similar to much of the oil found
on Santa Barbara’s southern beaches.
The biomarker method so far has been
unable distinguish these oils. It is
uncertain whether the inability to distinguish
them represents a limitation of the method
(which potentially could be overcome
with refinements to the method), or whether
the sources of the produced oil and beach
tar are identical or nearly so.
Tar Deposition Rates and Variability
The amount of tar collected at each
beach was converted into estimates of
tar mass and number of tar balls per
kilometer of beach. Comparisons of tar
deposition at the 10 beaches are presented
in graphs in the attached report. There
are obvious differences in tar mass
and numbers of tar balls among the
beaches, with generally greater amounts
occurring on northern beaches. Among
the South Coast beaches sampled,
Coal Oil Point receives the largest amount
of tar, whereas Jalama and Surf beaches
are most heavily tarred of the beaches
north of Point Conception. Such differences
are expected, due to the location
of offshore seeps relative to the depositional
beaches, the prevailing current patterns,
the orientation of the shoreline,
etc. On average, tar balls found on the
four northern beaches were substantially
larger than those found on the South
Coast beaches. Many large tar blobs
and ropes were found on the north
beaches, whereas small tar balls and
tiny flecks were more typical in the
south. This finding is consistent with
the fact that oil in the more northerly
fields is generally heavier and more
viscous than that found off the South
Coast. Tar deposition appears to vary
seasonally, with the greatest accumulations
observed during summer and fall. The
data shows large variability in tar deposition
from month to month, and differences
in this variability among beaches.
Factors responsible for the variability
are discussed below.
Complicating Factors
Part of the variability observed tar
deposits may be attributable to under-sampling,
both spatially and temporally. Part
may be due to the fact that the accumulated
tar deposit can change greatly over
a single tidal cycle. A likely major
cause of variability is that differences
in wind, current, and surf energy
affect how much tar can reach and strand
on a beach; these effects vary among
beaches and seasonally. Wind speed and
surface chop affect natural dispersion
of floating oil in the water column.
Current direction affects the trajectory
of floating oil and weathered tar balls.
Surf and tidal action can wash the beach
clean, or bury tar under the sand, or
release previously sequestered tar. Variations
in seep activity of different seeps
may also be an important factor.
It may be possible, in a future analysis
of the tar sample data, to explain some
of the variability by taking into consideration
wind, currents, and tides at the time
of sampling, in relation to the relative
locations of seeps and the beaches. However,
at this point, the variability cannot
be adequately explained.
Establishing a Baseline of Beach Tar
Deposition
The tar “fingerprint” database
contributes to the understanding of baseline
conditions, and may aid in determining
the cause and responsible party in the
case of future unusual beach oiling events.
The database constitutes a record of
what types of oil, as classified by biomarker
composition, are commonly found at points
along the coastline (during the year
2002-2003).
The study has established the normal
range of tar mass deposited during the
year, and a qualitative description of
its appearance. This information could
aid in judging whether a future tarring
event is unnatural and in determining
whether or beach tar is likely to have
originated from ongoing, natural seepage.
However, the description of baseline
conditions developed in the study is
not definitive. In some cases it may
not be possible to distinguish spilled
oil from natural seep tar, based on the
current database and tar “fingerprinting” methods.
Further analysis of oil samples from
platforms and natural seeps, and further
refinement and validation of the tar
classification model, should lead to
improved differentiation of tar groups
in the future.
For additional information on this project
and results, please follow the link to
Natural
Seep Inventory Final Report.
Please contact Dr.
John Day for more information.
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