Characterization of Gulf Region Tar Balls Following the Deepwater Horizon Oil Spill

Transcription

1 Characterization of Gulf Region Tar Balls Following the Deepwater Horizon Oil Spill Jaap de Zeeuw, BARRY BURGER, Jack Cochran, Chris English, Jim Whitford Restek Corporation, Bellefonte, PA, USA

2 Abstract After nearly two years cleanup efforts continue in the gulf following the second catastrophic oil platform disaster in that region. IXTOC I & Deepwater Horizon both are estimated to have spilled nearly 5 million barrels of oil. A year after the Deepwater Horizon spill significant amounts of heavy oil and tar balls are washing up on the beaches of the gulf coast. Tar near where the IXTOC I oil rig sank can still be found by digging a few centimeters into the ocean floor 31 years later. In 1990 the Oil Pollution Act (OPA) was promulgated under the direction of the National Oceanic and Atmospheric Administration (NOAA) following the Exxon Valdez oil spill in March of These regulations require a Natural Resource Damage Assessment (NRDA) following a release of oil into the nation s waterways. Currently NOAA is conducting a NRDA to determine the impact of the Deepwater Horizon oil spill. There are several NRDA technical working groups (TWGs) assembled to determine; baseline conditions before the oil spill, impacts to plant and animals following the spill, and the current conditions of the marine ecosystem. The trustees are also evaluating impacts from the response, to include use of dispersants.

3 Types of Analyses There are a variety of methods for the determination of weathered crude oils. This paper will discuss the analysis and characterization of different crudes for volatiles, semivolatiles and biomarkers. Tar balls that have been collected on the beaches of Florida over the course of a year will be evaluated. Attempts will be made to characterize these samples using a variety of techniques to include: Simulated Distillation chromatography, gas chromatography mass spectrometry, and multidimensional gas chromatography.

4 C10 Injector: COC (oven track) Typical Detector: Crude FID 330 Analysis C GC Oven Conditions: 50C (no hold) 10C/min to 300 (hold 20 min). Carrier gas: 40cm/sec Sample: Light Crude Oil W / C10-40 overlay Concentration: 500 PPM Sample size: 1ul C Time (min) Figure 1: Analysis of MC252 Crude oil with an overlay of alkanes from C10 to C40. The majority of this crude is in the range of C3 to C36. Purge-able range covers up to 1-methylnaphthalene (shaded area). Biomarkers are found a retention time of 23 minutes to 25 minutes.

5 C5 Injector: COC (oven track) Detector: 430 C Oven: 53 C (no hold) 10 C/min to 430 C (5 min hold) Carrier: 140 cm/sec (18ml/min) constant flow Sample: 1ul CS2 of C5-C100 1% reference standard C40 C Time (min) Figure 2: Conditions used for the initial evaluation of Florida tarballs. Chromatograms are found on slides 2 through 5 as black FID traces which correspond to each of the samples listed.

6 Target Compounds ASTM D e1 Distribution of Petroleum Simulated distillation analysis requires the sample to be diluted in carbon disulfide and analyzed by cool-on-column injection with a final oven temperature of 430 C. Crude oils and tarballs were analyzed under the conditions shown in figure 2.

7 Tarballs Explained Weathered oil is characterized in three ways; Supratidal Buried Oil (SBO), Small Surface Residual Balls (SSRB) and Surf Zone Submerged Oil Mats (SOM). SBOs can remain in the environment for years and maintain high PAH levels. For instance following the Valdez Oil Spill this type of weathered oil was responsible for contaminating bivalves affecting the food chain, most notably see ottters, over a decade later. Small surface residual balls are not the typical tarballs found on the beach. One example of a SSRB is shown in slide #4. Generally they are smaller than 1 in diameter and are on average ¼ (5mm OD) with a composition of 90% sand to 10% oil. Dispersants are believed to minimize SSRBs reaching the shore. Following mechanical or manual beach cleanup SSRBs are left behind since removing this oil is believed to be more detrimental than allowing it to degrade on its own. Surf Zone Submerged Oil Mats (SOM) are less weathered than oil on the beaches and contain the highest amount of individual PAHs. SOMs that break up in the tide into discrete chunks wash up on the shore as Tarballs, but once on the shore can quickly weather and mix with sand to become SSRBs. The mechanism by which the oil is weathered is as important as the time the oil has been exposed to the environment.

8 Weathered Oils Found in Florida (Tarballs) While many Surf Zone Submerged Oil Mats contain high percentages of sand, several of our SOM samples had a weathered outer crust with a soft core, like a jellybean and consisted of oil only. One example, Tar ball #11 (slide 5) contained UV active material and tested positive for Polycyclic Aromatic Hydrocarbons (PAHs), while a similar tar ball was highly degraded and lacked cyclics or light alkanes (tar ball#13 not shown). Some of the samples were so highly degraded they appeared as a hump in the chromatogram as shown in slides 2 & 3 (Tar ball #1 & 3). Interestingly the appearance of the tar ball was not an accurate predictor of the composition. The SimDist analysis show in Figure 2 was our initial screen of the sample to determine overall range of petroleum products.

9 Phenanthrenes/Anthracenes Observations Slide #1 Riser Pipe Oil MC252 Napthalenes Alkanes

10 On going clean-up efforts FL 2011 Tar ball # 1: Oily Mix Sampled: Location: Ed Walline Park, FL Slide # Time (min) Time (min)

11 Tar ball # 4: SOM, tarball Sampled: Location: TopSail SP, FL Slide # Time (min) Slide #4

12 Tar ball # 5: SSRB Sampled: Location: Dune Allen Beach, FL Slide # Time (min)

13 Tar ball # 11: SOM Sampled: Location: Ed Walline SP, FL Slide #5 Biomarkers Time (min)

14 Slide #6 Tar ball # 11 Biomarkers Tar ball # 1 Biomarkers

15 5α-Cholestane m/z α(H),21β(H)-Hopane m/z 191 Molecular Fossils Biomarkers are traces of plant matter that have remained in the oil nearly unchanged. These molecules are resistant to heat, weathering and other types of degradation. The specific ratio of biomarkers is useful in determining the source of the oil. Biomarker concentrations in oil can range from single digit parts per million to over 1000 ppm. Cyclic terpenoids include steranes (cholestane) and terpanes (hopanes) as well as aromatic variations. In theory there are thousands of cyclic terpinoids, however, only cyclopentyl and cyclohexyl are found in petroleum.

16 GC-MS SIM Analysis Single ion monitoring was used to compare sterane m/z 217 and hopane m/z 191 patterns to reference oils. The MC252 Crude oil is the lightest crude tested (shown on far right). It was the only crude oil that could be drawn up into a syringe needle with a density of Maya crude is considered a medium sour crude, that is composed of higher molecular weight fractions and contains sulfur (density 0.93) The three crudes on the left represent the light, medium and heavy crudes. Crude C is rich in asphaltenes; not too dissimilar from Prudhoe bay crude oil and is considered a heavy sour crude oil with a density of 0.97.

17 Source Oil Comparisons Reference oils and samples were compared for biomarker patterns. Figures 3 and 4 compare MC252 to tarball #11 with striking similarities between the reference and weathered sample. Additional information is provided on slide #5. If tarball #11 is from the gulf oil spill the biomarkers have remained intact for over 15 months in the ocean.

18 Figure 3: GC-MS SIM m/z 191 DWH MC Tarball #

19 Figure 4:GC-MS SIM m/z 217 DWH MC Tarball #

20 Figure 5: GC-MS SIM m/z 217 DWH MC Crude A (light crude)

21 Conclusions Weathered oil undergoes significant changes over the course of days to years, especially in an aquatic environment. High temperature analysis can determine the range of hydrocarbons remaining in a weathered sample. Further analysis using GC-MS can differentiate between PAHs and straight chain hydrocarbons. Determining the source of a tarball or spilled oil requires a total understanding of the sample. One tool is the examination of the distribution of biomarkers. Differences in the concentration of the cyclic terpinoids was observed in the source oils tested using GC-MS and GCxGC-FID. One example is provided above, but other examples have proven to require a higher degree of precision since the patterns of these biomarkers between different oils is similar.

22 Acknowledgments: Benefits of GC x GC Susan Forsyth: Tar balls & Photographs Steve Kolian: Seawater Samples, Biota EPA Region 6: Mississippi Canyon Block 252 Crude