Chemical Concentration Change (CCC) method
Highlights
A new method is developed to compute oil exposure dates and regions.
Dates and regions were computed from concentrations of shoreline sediments.
Differences were observed against exposure dates predicted by oil spill trajectories.
Nearshore circulation features appear to account for some differences.
Cyclones also appear to influence exposure dates.
Abstract:
Oil spills can result in changes in chemical contaminant concentrations along coastlines. When concentrations are measured along the Gulf of Mexico over time, this information can be used to evaluate oil spill shoreline exposure dates. The objective of this research was to identify more accurate oil exposure dates based on oil spill chemical concentrations changes (CCC) within sediments in coastal zones after oil spills. The results could be used to help improve oil transport models and to improve estimates of oil landings within the nearshore. The CCC method was based on separating the target coastal zone into segments and then documenting the timing of large increases in concentration for specific oil spill chemicals (OSCs) within each segment. The dataset from the Deepwater Horizon (DWH) oil spill was used to illustrate the application of the method. Some differences in exposure dates were observed between the CCC method and between oil spill trajectories. Differences may have been caused by mixing at the freshwater and sea water interface, nearshore circulation features, and the possible influence of submerged oil that is unaccounted for by oil spill trajectories. Overall, this research highlights the benefit of using an integrated approach to confirm the timing of shoreline exposure.
Method Description
Chemical concentration change method is a method based on large amount of sediment sampling data. By comparing the concentrations of one particular chemical between two consecutive sampling dates, the increase in concentrations can be computed. The change in concentrations were therefore used to indicate whether the coastal region was exposed to oil. A large increase indicated that the chemical impacted the region between the two sampling dates.
Results
Figure 1. Split the research region into 20 segments.
The exposure region based on remote sensing has a latitude range of 1.5° (166.5 km) and a longitude range of about 17.1° (1700.76 km). The longitude range was one order of magnitude larger compared with the latitude range. Because of the large horizontal expanse of the sampling area, we chose to convert the 2-dimensional problem into a 1-dimensional problem by neglecting the latitude change. Thus, the longitude was used to assign a sampling point to a particular segment. Considering the density of the sampling points, the area where the largest density of data points was taken, from longitude -97 to -83, was split into 20 segments, where each segment length was 49.6 km (0.5°).
Figure 2. Trajectories from GNOME model and remote sensing image.
Based on the segment classification, the first date that the nearshore trajectories extended to one segment was regarded as the exposure date of that segment.
Thus, we have the exposure date based on GNOME model and remote sensing trajectories.
The trajectories were from ERMA website.
Click here to see all the trajectories on ERMA.
The function of this method:
r: The increase ratio of chemical concentration.
C: Concentration for different dates.
Figure 3: Number of days after the oil spill (20 April 2010) that the segment was exposed to oil based on the CCC method for the time period from 20 April 2010 to 31 July 2010 (panel a) and from 20 April 2010 to 31 August 2010 (panel b), remote sensing images (panel c) and oil spill transport model (panel d).
The yellow bars in each panel indicate that the first oil exposure date for each segment and the purple rhombus surrounded by a circle shows the location of the wellhead.
For panel a) and b), the black solid line at the top of the panel means the end date of that group of data. The dashed lines with different colors indicate the date that tropical cyclones impacted the Gulf of Mexico. The paths of the cyclones are shown in green (Tropical Storm Bonnie) and blue (Tropical Depression Five) in panels a) and b).
The error bars for each segment show the confidence range of the exposure dates for each segment. The number above the bar corresponds to the number of chemicals that were used to estimate the exposure date. For reference red lines across the coastline correspond to beach areas.
The yellow bars in panel d) indicate the result based on beached oil data and the blue dash lines indicates the result based on exposure zone trajectories from the oil transport model.
The red shading in panel a) and b) showed the influence of the two cyclones happened around August.
The blue frames in panel a) and c) showed the almost not impacted segments.
The purple frames in panel a), b) and c) showed the segments that were late exposed.
Figure 4. Results based on CCC method compared with SCAT* report.
The results based on July 31th dataset are very similar to the SCAT result. The red shading means the regions have a high possibility being exposed to oil, the yellow shading means medium possibility and the green shading means low possibility.
*SCAT: Shoreline Cleanup and Assessment Technique