Why soils in marshes matter

This exercise is a part of Educator Guide: Shores Still Reeling from 2010 Oil Spill / View Guide

Directions for teachers: Ask students to read the online Science News article “The Deepwater Horizon oil spill ruined long-term shore stability” and have them answer the following questions. A version of the article, “Shores still reeling from 2010 oil spill,” appears in the March 25, 2023 issue of Science News.

1. Why does Scott Zengel, an environmental scientist, consider the study highlighted in this article unique?

The scientists showed that the Deepwater Horizon spill changed the soil stability in marshes that were inundated with oil.

2. Outline the cascade of events that led to the destabilization of the Louisiana marshes and shoreline studied by the ecologists. Include at least four events.

The Deepwater Horizon oil rig exploded. Nearly 800 million liters of oil spilled into the Gulf of Mexico. The oil that spilled into the marshes killed the plants along the coast. The soil under the dead plants washed away. The oil that remained in the water then killed the next level of marsh plants and loosened more soil, thus speeding coastal erosion.

3. What was the rate of marsh loss in the area studied before and after the Deepwater Horizon oil spill? (Include the appropriate units.) What does this finding suggest? Are there any other factors that could contribute to the difference in rates?

Before the spill, the marshes under study had been retreating on average 0.8 meters per year. After the spill, the rate increased to approximately 1.7 meters per year on average, which suggests the oil spill has played a meaningful role in shore loss. However, scientists also have to consider the role played by coastal storms.

4. The article mentions two very different tools used by the ecologists to study the spill’s impact on the marsh. Name the tools and explain how they were used.

The scientists used a sheer vane and satellite images. The sheer vane was used to test soil strength, and the satellite images were used to see how the marshes had changed over time. The views from the air covered a 23-year period and included images from before, during and after the oil spill.

5. Before the 2010 oil spill, the soil concentration of oil aromatics (volatile compounds in oil) in the area studied by the scientists averaged 23.9 nanograms per gram of sediment. How high were the average aromatic concentrations in 2011? How many times greater is the post-spill average concentration than the pre-spill average concentration? What is important about this finding?

The concentration of aromatics averaged 17,152 nanograms per gram of sediment in 2011.  The 2011 average concentration is 717.7 times higher than the pre-spill average concentration. This measurement indicates that there were large amounts of oil introduced to the soil from the oil spill.

6. What scientific unit is used to measure soil strength? Describe the changes the scientists found in soil strength before and after the spill. What does the most recent soil-strength figure indicate?

Soil strength is measured in kilopascals. Before the oil spill, the average soil strength was 26.9 kilopascals. Soil strength declined to 11.5 kilopascals in 2011. By 2018, the average had risen to 16.4 kilopascals. What the 2018 figure indicates is that soil strength is improving from the 2011 level, but soil strength in the marshes has not yet returned to pre-spill levels.

7. How can coastal erosion be mitigated? Why could this action work?

Because marsh plants hold soil in place, replanting is a strategy that can slow coastal erosion.

8. The University of Louisiana scientists who did this research have studied the effects of the Deepwater Horizon oil spill for more than a decade. What have the scientists learned doing a long-term study that they might not have learned if they had done a short-term study right after the spill?

The scientists learned that soil strength was not permanently diminished, which might have been their conclusion if they had not looked at soil strength over time. Although soil strength is not at pre-spill levels, it is better than it was in the year after the spill. Taking a long view also made it possible to see how oil residues remaining in the water in the marshes continued to destabilize soil long after the oil spill.