Date Published: March 18, 2019
Publisher: Public Library of Science
Author(s): Karin L. Lemkau, Christopher M. Reddy, Catherine A. Carmichael, Christoph Aeppli, Robert F. Swarthout, Helen K. White, Qi Fu.
Prior to Hurricane Isaac making landfall along the Gulf of Mexico coast in August 2012, local and state officials were concerned that the hurricane would mobilize submerged oiled-materials from the Deepwater Horizon (DWH) spill. In this study, we investigated materials washed ashore following the hurricane to determine if it affected the chemical composition or density of oil-containing sand patties regularly found on Gulf Coast beaches. While small changes in sand patty density were observed in samples collected before and after the hurricane, these variations appear to have been driven by differences in sampling location and not linked to the passing of Hurricane Isaac. Visual and chemical analysis of sand patties confirmed that the contents was consistent with oil from the Macondo well. Petroleum hydrocarbon signatures of samples collected before and after the hurricane showed no notable changes. In the days following Hurricane Isaac, dark-colored mats were also found on the beach in Fort Morgan, AL, and community reports speculated that these mats contained oil from the DWH spill. Chemical analysis of these mat samples identified n-alkanes but no other petroleum hydrocarbons. Bulk and δ13C organic carbon analyses indicated mat samples were comprised of marshland peat and not related to the DWH spill. This research indicates that Hurricane Isaac did not result in a notable change the composition of oil delivered to beaches at the investigated field sites. This study underscores the need for improved communications with interested stakeholders regarding how to differentiate oiled from non-oiled materials. This is especially important given the high cost of removing oiled debris and the increasing likelihood of false positives as oiled-materials washing ashore from a spill become less abundant over time.
The 2010 Deepwater Horizon (DWH) disaster resulted in the release of 4.1 million barrels of oil from the Macondo well (MW) , contaminating over 1700 km of the Gulf of Mexico coast [2,3] including beaches [4,5] and salt marshes [6–10]. The spill had substantial economic impacts on Gulf Coast communities and fisheries [11–13]. While extensive response activities were conducted in the months and years following the spill, not all residual oil from the event has been removed from the environment . Residual oiled material exists in the surf zone in the form of submerged oil mats and is regularly deposited on beaches [4,15–18,2]. In the years since the spill, discrete areas of shoreline have experienced periodic remobilization of weathered oil and sand mixtures or “sand patties”, also known as surface residual balls [4,19,20]. These are believed to originate from submerged oil mats located offshore . Monitoring the composition of these sand patties has shown changes in their composition since the spill, including formation of environmentally recalcitrant oxygenated hydrocarbons [19,21]. Ongoing field investigations of oil from the DWH spill in the coastal environment can provide useful information to inform future cleanup efforts.
We assessed the impact of the hurricane on the resuspension of oil residues through examination of sand patty density and chemical analysis. While the DWH spill was expected to remain the major source of oil, it was hypothesized that changes in sand patty density would be observed if the hurricane remobilized sand patties with a different degree of weathering or oil from different subsurface reservoirs. We also examined the dark-colored mat samples to determine their hydrocarbon content and source.
We sought to address two questions: 1) Did Hurricane Isaac affect the weathering state, chemical composition or density of sand patties regularly washed ashore along the Gulf Coast? and 2) What was the composition of the unknown mat samples washed ashore following the storm and were they related to the DWH spill? The effect of Hurricane Isaac on the remobilization of oiled material was examined in several ways. First, sand patty densities were compared for all available samples collected since April 2011 (n = 565; S1 Table). These samples were collected at numerous Gulf Coast sites before and after Hurricane Isaac to explore the possibility that the high energy of the hurricane delivered denser sand patties to the beach (i.e. sand patties with a lower weight percent oil) than observed under normal wave-conditions. Second, the oil content of the mat samples collected from the beach at Fort Morgan, AL was compared to sand patty samples collected at the same site several months earlier (May 8th, 2012), one week prior (August 19th and 20th, 2012), and immediately following the landfall of Hurricane Isaac on September 1st, 2012. Finally, we sought to determine the source of the dark-colored mat samples by characterizing their grain size distribution, TOC, and bulk δ13C composition.
This study expands our understanding of the need for thorough examination of the oil content of materials washing ashore along the Gulf Coast and has implications for accurate and cost-effective shoreline cleanup, assessment and treatment endpoint determination following an oiling event.