Marine Pollution Bulletin 89 (2014) 201–208
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Polycyclic aromatic hydrocarbon concentrations across the Florida Panhandle continental shelf and slope after the BP MC 252 well failure Richard A. Snyder ⇑, Melissa Ederington-Hagy, Fredrick Hileman, Joseph A. Moss, Lauren Amick, Rebecca Carruth, Marie Head, Joel Marks, Sarah Tominack, Wade H. Jeffrey Center for Environmental Diagnostics and Bioremediation, University of West Florida, Pensacola, FL 32514, United States
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Article history: Available online 16 October 2014 Keywords: PAH Sediment BP oil spill Gulf of Mexico
a b s t r a c t The Florida Panhandle continental shelf environment was exposed to oil from the BP oil well failure in the Gulf of Mexico during 2010. Floating mats of oil were documented by satellite, but the distribution of dissolved components of the oil in this region was unknown. ShipekÒ grab samples of sediments were taken during repeated cruises between June 2010 and June 2012 to test for selected polycyclic aromatic hydrocarbons (PAHs) as indicators of this contamination. Sediments were collected as composite samples, extracted using standard techniques, and PAHs were quantified by GC/MS-SIM. PAHs in samples from the continental slope in May 2011 were highest near to the failed well site and were reduced in samples taken one year later. PAHs from continental shelf sediments during the spill (June 2010) ranged from 10 to 165 ng g1. Subsequent cruises yielded variable and reduced amounts of PAHs across the shelf. The data suggest that PAHs were distributed widely across the shelf, and their subsequent loss to background levels suggests these compounds were of oil spill origin. PAH half-life estimates by regression were 70–122 days for slope and 201 days for shelf stations. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Petroleum hydrocarbons are common to the northern Gulf of Mexico, from natural seeps (MacDonald et al., 2010; Wade et al., 1989), and anthropogenic extraction and industrial activities (Overton and Aston, 2004). Petroleum presence increased significantly with the Deepwater Horizon drilling accident at the British Petroleum Macondo Field 252 well site (BP MC 252). Dispersed oil created midwater plumes extending east and west of the DWH (Camilli et al., 2010; Hazen et al., 2010; Edwards et al., 2011; Montagna et al., 2013), and floating oil was distributed over a broad area, including a substantial portion of the Florida Panhandle Shelf (http://gomex.erma.noaa.gov/erma.html#/x=-89.37870&y= 29.14486&z=6&layers=5723+23036). The fate and impacts of the oil have been the subject of study, experimentation, and speculation. Many were surprised by the rapid microbial response and disappearance of the oil (Hazen et al., 2010; Edwards et al., 2011). Modeling has suggested that water mass mixing accelerated biodegradation rates of deep-water plumes (Valentine et al., 2012), but the full extent of subsurface oil ⇑ Corresponding author at: CEDB-UWF, 11000 University Parkway, Pensacola, FL 32514, United States. Tel.: +1 850 474 2806. E-mail address:
[email protected] (R.A. Snyder). http://dx.doi.org/10.1016/j.marpolbul.2014.09.057 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved.
distribution remains unknown, and concern that some of the floating mats of oil sank to the benthos remains. It has been suggested that chemicals from the spill were widely distributed, even as far as the West Florida Shelf (Paul et al., 2013; Weisburg et al., 2014), but information on contamination of the Florida Panhandle Shelf sediments has been missing. Potential mechanisms for oil compounds to reach the sediments include: (1) direct sediment contact with dissolved or dispersed oil in mid-water plumes (Montagna et al., 2013), (2) direct sedimentation of floating oil mats as the material weathered and transitioned from positive to negative buoyancy (Stevens, 2014 MS Thesis, Louisiana State University), and (3) biological processes in the water column concentrating and precipitating dissolved and dispersed chemicals with water column organic materials. These biological processes could include the formation of marine snow (Silver et al., 1978; Alldredge and Silver, 1988; Passow, 2012) similar to the microbial interactions that enhance flocculation in wastewater treatment systems (Curds, 1982), and the production of negatively buoyant fecal material from plankton and nekton. These biological processes could scavenge and concentrate chemicals from low dissolved background concentrations and facilitate their deposition on the sediments. Chemicals incorporated into the biological components of the ecosystem may also prolong residence time in the system over chemicals free in the physical environment and
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subject to direct microbial degradation. Prolonged persistence of PAHs in mesozooplankton in the northern Gulf of Mexico has been documented by Mitra et al. (2012). PAHs in fish liver and gall bladder samples from the area also suggest that these compounds had an extended residence time in the biotic phase through the summer of 2011 (Murawski et al., 2014). Thus, sediment petroleum loads reflect both direct deposition of oil chemicals and water column processes that may have continued to load material to the benthos after direct sources of the compounds had been eliminated. Very little pre-spill PAH data from the offshore region of the Northeastern Gulf of Mexico is available. Prior to the tightening of bilge and ballast water regulations in the 1980s, petroleum in the form of ‘‘tar balls’’ was anecdotally common along Florida seashores. PAH concentrations in dated sediment cores from the northwestern Gulf were reported by Overton and Aston (2004), showing increases above an approximately 200 ng g1 background, coincident with offshore extraction activity expanding after the 1950’s. Gearing et al. (1976) reported total hydrocarbons in sediments off the Florida coast averaged 3.06 ± 41% lg g1. A station deep in DeSoto Canyon near the Florida Panhandle Shelf yielded sediment containing 80.1 and 147 ng g1 total PAHs in repeated samples (Wade et al., 2008). Thus, the background levels for PAH concentrations on the continental shelf of the Florida Panhandle prior to the spill are largely unknown. Some sampling results are available for Florida Panhandle beaches. Using SPME resins to absorb PAHs from the water, Allan et al. (2012) recorded a brief peak of PAHs occurring inside Pensacola Pass in August and September of 2010, lagging behind sustained elevated PAHs recovered from sites in Louisiana, Mississippi, and Alabama. The USGS published pre- and post-spill analysis of sediment PAHs for Gulf of Mexico shorelines and found fewer samples exceeding a probable effects threshold after the spill (1) than before (10) for the Panhandle of Florida (Nowell et al., 2013). A recent report from our lab documented PAH concentrations in surf sand and intertidal Coquina clams (Donax spp.) on the beaches along the Florida Panhandle (Snyder et al., 2014). PAHs in surf sand disappeared within a year following beach oiling, but higher concentrations of PAHs were found in clam tissues relative to ambient sands, and PAHs in these organisms returned to background levels 2 years after oil impacted the beaches. Offshore from the beaches, the Northeastern Gulf of Mexico continental shelf is dominated by a geological feature called DeSoto Canyon. This rift in the continental shelf with a steep shelf break to the west and a gradual slope to the east affects the physical oceanography of the system. Deep water upwelling through the canyon and over the shelf break, loop current eddies, wind driven surface currents, and river plumes from the Mississippi River and coastal bays all mix in the region (Huh et al., 1981; Yang et al., 1999; Hamilton et al., 2000; Sturges and Lugo-Fernandez, 2005; Morey et al., 2003; Schiller et al., 2011; Kourafalou and Androulidakis, 2013), creating a very complex distribution of nutrients, pollutants, and biological responses. Sediments of the continental shelf of the Florida Panhandle are dominated by quartz sands transitioning to marl and clays at the shelf break (Balsam and Beeson, 2003). The complexity of the system combined with the toxicity of the polycyclic aromatic hydrocarbon (PAH) fraction of the DWH petroleum, and concerns about the distribution and fate of oil spill hydrocarbons, prompted us to establish a sampling plan to obtain time series data over a spatial scale covering the continental shelf of the Florida Panhandle between Cape San Blas and Perdido Key, centered around the head of DeSoto Canyon. Additional samples were obtained at the DWH site and at increasing distance east towards DeSoto Canyon on the continental slope. Three PAH classes were targeted that would be indicative of BP MC 252 crude oil.
2. Materials and methods 2.1. Sampling Three transects (Fig. 1) were established with stations at 5–10 mile intervals to cover the shelf and the head of DeSoto Canyon from the shoreline out to 500 m depth: due south of Pensacola Bay, FL (P transect), due south of Choctawhatchee Bay, FL (C transect), and running SSW from St Andrews Bay, FL (A transect). Location coordinates, depths, and sediment types are listed in Table 1. Ship time was provided by the Florida Institute of Oceanography (FIO) on the R/V Bellows and the R/V Weatherbird II. The P transect was sampled on 11 cruises from June 2010 through May 2012. C and A transects were sampled on 10 cruises from January 2011 through May 2012. Some stations/samples were missing from the dataset due to weather and equipment difficulties. Sediment samples were obtained from the P transect on the R/V Bellows in June 2010 during the oil spill event while floating mats of oil were over the Florida Panhandle Shelf. Additional sediment samples were taken at the DWH well site and along the continental slope to the east in May 2011 and May 2012 during cruises aboard the R/V Weatherbird II. Triplicate sediment samples were obtained at each shallow station. Some deep stations (>250 m) were only sampled by a single grab, depending on the weather, time and equipment. Samples were obtained with a stainless steel ShipekÒ grab with buckets covering 0.04 m2 each. The sampler adequately preserved the sediment–water interface to allow removal of the surface sediment (1 cm depth) with a pre-cleaned stainless steel spoon as a composite sample in 250 ml certified pre-cleaned wide mouth sample jars. Samples were stored at 4 °C on board ship and back in the laboratory until processed, typically within one week of sampling. All surfaces that contacted sediment samples were cleaned between stations with dish detergent followed by a seawater rinse, an isopropyl or ethyl alcohol rinse, a final deionized water rinse, and then allowed to air dry. Exposed clean surfaces were covered with new aluminum foil until use. 2.2. Extraction and analysis Sand samples were dried and homogenized prior to removal of approximately 100 g. Deeper sediments comprised mostly of silt/ clay particles required the addition of sodium sulfate as a drying agent. Sediments were extracted three times with methylene chloride. A water bath at 38 °C and TurbovapÓ was used to evaporate the sample to 1 ml of added n-octane. The analysis was performed by gas chromatography/mass spectroscopy- selective ion monitoring (GC/MS-SIM). Naphthalene, phenanthrene, chrysene, and respective alkylated compounds (C0-C3) were targeted as major PAH components from BP MC252 raw and weathered crude (Snyder et al., 2014). An internal standard included in all samples was composed of d8 naphthalene, d10 phenanthrene, and d12 chrysene for PAHs. Every ten samples included a sediment blank, a replicate sample and two spiked samples. Spiked samples received a low concentration of evaporated (‘‘weathered’’) source oil and the internal standards listed above. The reporting limit for PAHs in sediments using this method was 4 ng g1. Standard deviations for replicate composite samples ranged from 1.55 to 4.52 ng g1. 2.3. PAH half-life estimates Decay coefficients (k) and half-lives of the PAHs found in the deep slope samples were estimated from the two time point samples assuming first order exponential loss by the formula:
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Fig. 1. Study area map with station locations. DWH and DS stations were sampled May 2011 and May 2012. Continental shelf stations on the P, C, and A transects (only oddnumbered stations labeled) were sampled on repeated cruises from June 2010 through May 2012. Station DS3.4 was located within 200 m of DS4 indicated on this chart.
k¼
ln ½PAH0 ln ½PAHt t
where k is the extinction coefficient, [PAH]0 is the first PAH concentration measured, [PAH]t is the second PAH concentration measured, and t is the time in days between sampling. The half-life was calculated as 0.693/k. Extinction coefficients (k) and half-lives for the PAHs recovered in repeated continental shelf sampling of the P transect were estimated as the slopes of regression lines fit to natural log transformed PAH concentrations as a function of time in days (JMP software, SAS, Inc.). Half-lives were obtained by dividing 0.693 by the slope values (k) estimated by the regression analysis. 3. Results Naphthalenes (Naph), phenanthrenes (Phen), and chrysenes (Chry) were recovered from surface sediments on the continental slope and on the continental shelf. The data have been archived and are available at: https://data.gulfresearchinitiative.org/data/ Y1.x047.000:0001/. No samples were found to have concentrations that exceeded a NOAA 4000 ng g1 sediment level expected to produce a probabilistic biological effect 10% of the time (Long and Morgan, 1990). On the shelf, sediment from station P4 during the spill (June 2010) had naphthalene and phenanthrene concentrations that exceeded the state of Florida Threshold Effects Level for sediments (34.6 ng g1 and 86.7 ng g1 respectively, 108 ng g1 for chrysene; TEL; MacDonald, 1994) and naphthalene again exceeded the TEL for station P9 in June 2011. The deep water slope stations DS3.0 and DS3.1 exceeded the TELs for all three PAHs in the May 2011 samples, and station DS3.4 additionally exceeded the TEL for phenanthrene. In the repeat sampling conducted May 2012, none of the samples exceeded TELs. The State of Florida Probable Effects Levels (PEL) for sediments (naphthalene 391 ng g1, phenanthrene 544 ng g1, chrysene 846 ng g1; MacDonald, 1994) were never exceeded for any of the stations sampled in this study. Continental slope samples taken near the Deep Water Horizon well and rig failure site (DWH), and at variable distance from that site, were muds. In May 2011, Naph + Phen + Chry levels at DS3.0 were the highest at 634 ng g1, and the sum of these PAHs
Table 1 Station locations for repeated sampling and analysis of PAH content of sediments on the Florida Panhandle continental shelf and slope. Station ID
Latitude (N)
Pensacola Bay (P transect) P1 30.250 P2 30.167 P3 30.083 P4 30.000 P5 29.917 P6 29.833 P7 29.750 P8 29.583 P9 29.417
Longitude (W)
Depth (m)
Sediment type
18 22 33 32 48 81 152 289 478
Sand Sand Sand Sand Sand + shell Sand Mud Mud Mud
25 25 30 51 95 112 127 185 272
Sand Sand Sand Sand + shell Fine sandy mud Mud Mud Mud Mud
Panama City-St Andrews Bay (A transect) A1 30.133 85.775 A2 30.067 85.817 A3 29.992 85.867 A4 29.921 85.921 A5 29.967 85.967 A6 29.783 86.058
18 21 26 35 37 40
A7 A8 A9
86.058 86.108 86.158
48 54 89
Sand + shell Sand Sand + shell Sand + shell Fine Sand Fine Sand + shell Shell Fine Sand Fine sandy mud
88.390
1503
Mud
88.268 88.251 87.748 86.735 87.749
1248 1250 990 653 903
Mud Mud Mud Mud Mud
87.250 87.250 87.250 87.250 87.250 87.250 87.250 87.250 87.250
Destin-Choctawhatchee Bay (C transect) C1 30.20 86.40 C2 30.15 86.40 C3 30.10 86.40 C4 30.05 86.40 C5 30.00 86.40 C6 29.55 86.40 C7 29.50 86.40 C8 29.40 86.40 C9 29.30 86.40
29.713 29.646 29.579
Base of the continental slope DWH well 28.741 site DS3.0 28.826 DS3.1 28.838 DS3.4 29.184 DS3.6 29.234 DS4.0 29.182
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decreased with distance from the DWH (Fig. 2). One year later in May 2012, the DWH site yielded a sample containing 32 ng g1, the DS3.0 station sample contained 23 ng g1, and the concentrations again generally decreased in samples with increasing distance from DWH (Fig. 2). On the continental shelf, the only visible oil on the sediments was found in January 2011 at the near coastal station (P1) off of Santa Rosa Island, as fine
