Oil poses a significant concern along coastlines due to its toxic effects on life and its environmental persistence. Coal Oil Point has the greatest tar accumulation by several orders of magnitude than anywhere on the Pacific Coast (Lorenson et al., 2004). This tar affects beach goers, marine life, and littoral creatures, like the endangered snowy plover. Yet, little is known about this tar accumulation. Including basic questions such as how much tar accumulates on the beaches, where does the tar come from, and perhaps of greatest interest, what causes some days to be much worse than other days with regards to tar. This study seeks to characterize the spatial and temporal distribution of tar accumulation and to understand the underlying mechanisms of tar accumulation.
Sources of Tar and Beach Tar Variability
The source of tar to the area beaches is the nearby Coal Oil Point (COP) seep field, yet, clear geochemical evidence linking beach tar and seep field emission sources remains elusive. The COP seep field is among the largest seep fields in the world (Hornafius et al., 1999), and the best studied. Oil and gas escape to the ocean from the Miocene Monterey Formation reservoir underlying the COP seep field. Oil emissions have been estimated at a minimum of 100 barrels day-1, (Clester et al., 1996; Hornafius et al., 1999), with significant variability on a range of time scales.
Schematic illustrating some slick advection and chemical weathering processes. From Leifer et al., 2006.
Although some of the variability arises from emission variability, transport of oil slicks to the beach and chemical conversion (weathering) of oil slicks into tar are important processes to beach tar accumulation. Winds and currents are the most important forces underlying tar transport; however, many other oceanographic processes, such as langmuir circulation, wave breaking, and oil-specific processes such as Fahy gravitational spreading can be important. Further, additional complexity arises in coastal areas due to the effect of bathymetry and the coastline on currents, wind, and waves.
Although oil at the sea surface initially forms thin slicks, chemical weathering due to evaporation, photolysis, and other chemical processes, causes the oil to evolve into brown mousse, and then tar balls. Further, depending on the source oil, if sufficient volatiles are lost, the oil may lose its buoyancy and sink.
Finally, beach and surf zone processes can be important to beach tar accumulation. Ebbing tides tend to strand tar on the beach, with subsequent rising tides removing tar. The long shore current moves the tar along the beach, decreasing its residence time, and is related to the swell height. Sunlight, evaporation, and winds on the beach also cause weathering.
The fate of the tar after beaching is completely unknown; however, tar is never found buried in the sand - thus it must return to the ocean and then . . . . .
Map of the Coal Oil Point hydrocaron seep field, located near the University of California, Santa Barbara in the Santa Barbara Channel. Gas seepage regions are shown as gray areas as determined by sonar return. Explore the seep field with the interactive seep field map, which contains information on many named seeps, including images and movies.
Seepage in the COP seep field has been mapped with sonar and GPS. There are three seepage trends that emit oil, a deep trend including the Seep Tent Seeps and the La Goleta Seeps in about 70 m water depth, a middle trend (the Coal Oil Point Seeps) including Trilogy Seeps, and Horseshoe Seeps in about 45 m water, and an inner trend including Farrar seep JackPot Seep, IV Super Seep and Shane Seep, in about 20 m water. Shallower seeps do not produce oil.
In general, winds and currents are such that the deep seep trend is unlikely to contribute significantly to COP beach tar; although they may cause tar deposition further to the west or east.
Aerial image of Coal Oil Point with Transect lines (Red) overlayed. Blue lines delineate quadrats. From Del Sontro et al., 2007.
Tar accumulation was surveyed over an area of beach of 19,927 m2 centered on Coal Oil Point along twelve transects spaced 20-m apart and perpendicular to the bluffs.
Examples of tar pieces for each size class. Top right is size class 6. From Del Sontro et al., 2007.
Tar pieces in six size classes were counted in each 2-m quadrat along the transect between the bluffs and the swash zone. Tar mass for size classes 1-4 was measured for 100 representative tar pieces in the laboratory and used to convert number of tar pieces to tar mass. Size classes five and six were treated differently, the three dimensions of each tar piece was measured and tar mass calculated from the volume and assuming a density of 1 g cm-3. The mass of tar was then integrated between the transect lines to account for the irregular grid shape due to the geometry of the point, to calculate total tar for each transect and for the study area.
To understand the sources of variability, environmental oceanographic and meteorological data was collected and related to the time series of beach tar accumulation. These factors included:
East-west and north-south surface current components.
East-west and north-south wind components.
Significant swell height and direction.
Sea bottom temperature.
Sea surface temperature.
Time series of total beach tar during 2005. From Del Sontro et al., 2007
COP tar accumulation was surveyed on 57 days from February through December 2005 (Fig. 3a) with 17, 14, 9, and 17 days during winter, spring, summer, and autumn, respectively. Zero tar was observed on only two winter days (24 Feb. and 8 Mar.). Non-zero values of tar mass ranged from 0.10 kg (4 Nov.) to 39.11 kg (27 Feb.). For the entire study, mean tar (steady state tar accumulation) was 4.40 kg (~ 10 lbs). Based on observations, the beach tar residence time was between 1 and 2 tidal cycles. Observations showed flooding tides pushed tar higher on the beach and absent a prevailing onshore wind, ebbing tides removed tar. Further, consecutive day tar surveys showed that tar below the high tide level did not persist through a tidal cycle (Del Sontro et al., 2007).
|Thus, an upper flux limit, assuming both daily high tides reached the bluff and removed all tar, is ~ 9 kg day-1 (0.45 g m-2 day-1) for the study area. An approximate lower limit is 4.40 kg day-1 (0.23 g m-2 day-1) if tar persisted for only one high tide.|
Aside from Feb. 27, winter tar was an order of magnitude less than summer tar accumulation with spring tar intermediate. Further, the intra seasonal variability was much less than the inter season variability - indicating that their were distinct seasons with respect to tar accumulation. The seasonality is shown clearly in the accumulation with respect to transect and date, below.
Time series of beach tar on different transects during 2005. To the top is the east side of the point, the point is at transects 5-6. Hatched area represents no data for transects 11 and 12 which were added to the survey in spring. From Del Sontro et al., 2007.
There are several clear seasonal differences. During winter, not only is there less tar, but it is also sporadic, with many transects not having any tar. In contrast, during the summer tar is far more homogeneous, and all transects always have tar accumulation. Finally, in autumn, the tar distribution was intermediate between the summer and winter characteristics.
The seasonal trends must relate to a combination of environmental variables affecting transport, and seasonal variability in the source strength. Thus, swell is lower in the summer, leading to slower long-shore currents and hence longer beach residence time in the study area, as well as less turbulence and wave breaking in the swash zone. Northerly winds were generally more common in the spring and summer than winter and fall. North winds would enhance advection of slicks to COP beaches.
A more rigorous statistical (multiple regression model) analysis which allowed for time lags was conducted to study the relationship between tar accumulation and environmental factors. Several factors were found signicantly correlated with tar accumulation. Swell height was inversely related to tar with no time lag. Tar accumulation was also correlated with winds from the west 18 hours prior to a survey, followed by a northerly component 14 hours later. Since most surveys were conducted in the morning, this corresponds to winds from the west in the morning, shifting to a northerly component in the afternoon. Overall, this combination of variables was able to explain 34% of the variability in the tar accumulation (Del Sontro et al., 2007).
A portion of the remaining variability is likely explained by source variability. It is known that seepage gas emissions and oil emissions respond to tides. Long term trends in gas seepage have also been documented. This it is likely that there are also long term trends in oil emission, too.
Del Sontro, T., I.Leifer, B.P. Luyendyk, and B. Broitman, 2007. Beach tar accumulation and transport mechanisms at Coal Oil Point, CA Marine Pollution Bulletin, submitted.
Clester, S.M., Hornafius, J.S., Scepan, J., Estes, J.E., 1996. Quantification of the relationship between natural gas seepage rates and surface oil volume in the Santa Barbara Channel, (abstract). EOS (American Geophysical Union Transactions), 77 (46), F419.
Hornafius, S. B.P. Quigley, D.C., Luyendyk, B.P., 1999. The world’s most spectacular marine hydrocarbons seeps (Coal Oil Point, Santa Barbara Channel, California): Quantification of emissions. Journal Geophysical Research - Oceans 104 (C9), 20,703-20,711.
Leifer, I., B.P. Luyendyk, and K. Broederick, 2006. Tracking an oil slick from multiple natural sources, Coal Oil Point, California. J. Mar. Petrol. Geol. 23, 621-630.
Lorenson, T.D., Dougherty, J.A., Hostettler, F.D., Rosenbauer, R.J., 2004. Natural seep inventory and identification for the county of Santa Barbara, California. Final Report, prepared for County of Santa Barbara, CA. http://www.countyofsb.org/energy/information/NaturalSeepInventoryFinalReport.htm