Deepwater Horizon: Where Did the Oil
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THE SEA GRANT and GOMRI DEEPWATER HORIZON: WHERE DID THE OIL GO? PARTNERSHIP Monica Wilson, Larissa Graham, Christine Hale, Emily Maung-Douglass, Stephen The mission of Sea Grant is Sempier, Tara Skelton, and LaDon Swann to enhance the practical use and conservation of coastal, marine, and Great Lakes During the Deepwater Horizon (DWH) oil spill, approximately 200 million resources in order to create a sustainable economy and gallons of oil flowed from the Macondo well. Of the 200 million gallons, environment. There are 33 responders recovered 17 percent directly from the wellhead, releasing university–based Sea Grant programs throughout the 172 million gallons of oil into the Gulf of Mexico. Due to the size and scope coastal U.S. These programs of the spill, people wanted to know where the oil would travel. Some oil are primarily supported by the National Oceanic and accumulated at the shoreline, on the ocean’s surface, and in an underwater Atmospheric Administration plume. Unexpectedly, a portion of the oil also found its way to the seafloor. and the states in which the programs are located. In the immediate aftermath of the Deepwater Horizon spill, BP committed $500 million over a 10–year period to create the Gulf of Mexico Research Initiative, or GoMRI. It is an independent research program that studies the effect of hydrocarbon releases on the environment and public health, as well as develops improved spill mitigation, oil detection, characterization, and remediation technologies. GoMRI is led by an independent and academic 20–member research board. The Sea Grant oil spill science outreach team identifies the best available science from projects funded by GoMRI and others, and only shares peer- reviewed research results. Oil from Deepwater Horizon washes ashore. (NOAA, Dari Knight) Texas • Louisiana • Florida Gulf residents wanted to know the oil’s path the tool established seven categories to Mississippi-Alabama and its potential impacts on the marine describe the fate of the spilled oil (Figure gulfseagrant.org environment. In 2010, a group of scientists 1).1 For example, responders burned or developed an oil budget calculator to skimmed some surface oil immediately, estimate what happened to the oil and to but other oil lingered in the environment. provide a status update to the Incident The Sea Grant publication Top 5 Frequently gulfresearchinitiative.org Command. Scientists who developed Asked Questions about the Deepwater FIGURE 1. Scientists have estimated BIODEGRADATION approximately 11 to 25 percent of Oil seeps occur naturally in the marine the 200 million gallons of oil from environment, so microorganisms in the 1 Deepwater Horizon is unaccounted for. ocean have evolved to be able to consume, Recent studies show that three to five percent of the total oil made its way or eat, oil. In the Gulf of Mexico hundreds onto the seafloor.20 (Florida Sea Grant/ of species of microbes such as bacteria, Anna Hinkeldey) archaea, and fungi degrade oil. This is known as biodegradation. Microbes play a key role in the biodegradation of oil in the ocean and can reduce the overall environmental impact.6,7 Horizon Oil Spill details the following categories: disperse, dissolve, or be broken down by microbes • recovered at the wellhead, (called biodegradation) or the sun.4,5 The study also did • skimmed, not calculate the amount of oil recovered by the Vessels- • burned, of-Opportunity program and the amount that sank to • chemically dispersed, the bottom.3 • naturally dispersed, • evaporated or dissolved, or OIL AT THE WATER’S SURFACE • unaccounted for.2 Because oil is less dense than water, most spilled oil floats A few points not covered in the original study include on the water’s surface. Currents, tides, river discharge, the amount of weathering of surface oil.3 Over time, waves, and wind can move this surface oil, affecting where oil weathers as it interacts with the surrounding it winds up.8 Scientists use computer models to track the environment. It can evaporate, emulsify, naturally movement of oil at the water’s surface and predict its path. 2 TABLE 1. Number of miles of shoreline oiled by state.14 OIL ON THE SHORELINE State Miles of Percent of total During the DWH oil spill, a portion of the surface oil oiled shoreline oiled shoreline moved under the influence of the wind and ocean Texas 35 3% currents to make its way to the shore. Texas, Mississippi, Louisiana 847 64% Alabama, and Florida coasts all experienced shoreline Mississippi 158 12% oiling to different degrees (Figure 3). The oil impacted Alabama 95 7% approximately 1,313 miles (37 percent) of the 3,540 miles Florida 178 14% of Gulf of Mexico coastline. Total 1,313 100% Table 1 lists the amount of shoreline oiled by state. Louisiana received 64 percent of the oil that made it to synthetic Scientists also used satellite imagery called shore. The two shoreline habitats primarily affected were aperture radar (SAR) to calculate the area affected beaches (46 percent) and coastal wetlands (52 percent). during the 87 days oil flowed freely into the Gulf. Results The remaining two percent included all other shoreline indicated surface oil reached a cumulative area of 57,000 types.14 square miles (Figure 2).9 Scientists found that winds and large river outflow from the Mississippi River kept OIL BELOW THE SURFACE WATER a portion of surface oil offshore.10 More oil could have Approximately one month after the spill began, scientists traveled offshore in the Loop Current but was trapped aboard the R/V Pelican first found evidence of an oil in a large eddy on the northern part of the Loop Current. plume just under the water’s surface, called a subsurface The eddy remained stationary for several months, plume. Certain chemical compounds in oil emit a light that keeping the oil from reaching South Florida.11,12 allow them to be detected by fluorescence techniques. Evaporation Using colored dissolved organic matter (CDOM) Oil weathers over time as it interacts with the fluorometers, scientists discovered the subsurface oil surrounding environment. The amount of weathering, plume. They found elevated levels of CDOM at water the type of oil spilled, and the conditions during and depths greater than 3,280 feet and as far as eight miles after a spill influence how oil will move. Evaporation is from the wellhead. They took water samples from an important part of the weathering process for most areas with elevated levels of CDOM and found elevated oil spills.4,5,13 Typical crude oils can lose up to 45 percent levels of polycyclic aromatic hydrocarbons (PAHs). of their initial volume due to evaporation in the first few The PAH concentrations confirmed that the elevated days following a spill.13 A significant fraction of the leaked levels of organic matter (CDOM) were related to oil DWH surface oil evaporated.4,5 contamination.15 FIGURE 2. During the Deepwater Horizon oil spill, the extent and amount of oiling on the sea surface varied across the Gulf of Mexico. Colors indicate the average volume of oil in each cell area, with the highest con- centrations (red) occurring near the wellhead.9 (Reprinted from MacDonald et al., 2010) 3 FIGURE 3. Varying amounts of oil reached the shorelines and impacted beaches (top) and coastal wetlands (bottom). (Reprinted from Nixon et al., 2016) Using similar techniques other scientists analyzed over Other samples containing elevated fluorescence and low 5,000 water samples to track the deep water plume. In dissolved oxygen signals indicated the presence of oil as far addition to looking for oil droplets, they searched for other as 256 miles from the wellhead (Figure 4).16 evidence of contamination at the plume depth, including Other scientists confirmed the subsurface plume using the elevated PAHs, dissolved gas, fluorescence, and lowered autonomous underwater vehicle (AUV) Sentry (Figure 5). dissolved oxygen levels. Water samples containing oil They collected and tested water samples and found the droplets were found as far as 96 miles from the wellhead. subsurface oil plume between depths of 3,380 and 4,265 feet in late May and early June of 2010. Although more OILED WASTE MATERIAL than 22 miles in length and existing for months after the Approximately 100 million pounds of oiled waste oil spill, the subsurface plume was not a river of oil floating materials were removed from sand beaches during under water.17 The plume was simply an area that had response activities.28 (adapted from Michel, 2015) elevated amounts of PAHs compared to other areas.18 The subsurface plume trapped approximately 35 percent of State Prior to June 2011 - June 2011 February 2014 the spilled oil, but scientists are uncertain of the plume’s (pounds) (pounds) ultimate fate.18,19 Texas 17,454 0 OIL ON THE SEAFLOOR Louisiana 76,062,800 15,176,296 Estimating the total amount of oil that made its way to the Mississippi 3,886,415 113,061 seafloor is difficult. Deep sea currents move the deposited Alabama 2,569,200 930,656 oil, causing it to accumulate in dips and depressions on Florida Not available 66,276 the seafloor. Marine animals that live on the bottom bury 4 a. FIGURE 4. Scientists took offshore water samples to track the deep plume, mapping surface oil spreading out from the wellhead in red and the deep plume going towards the southwest in yellow/green (a). In a three- dimensional underwater model (b), a column of oiled samples rises from the wellhead to the surface while the deep plume extended 256 miles southwest from the wellhead. Colors relate to the phases of each sample’s oil—samples with b. Deepwater Horizon oil droplets (red); dissolved oil without oil droplets (blue); or containing poorly defined oil fingerprints but with other evidence of oil such as dissolved oxygen or fluorescence (yellow/green).16 (Adapted from Payne and Driskell, 2015) it, move it, and change its distribution.