By Alisha A. Renfro, Ph.D., National Wildlife Federation
The Mississippi River Delta is experiencing the highest relative sea-level rise in the U.S. due to the combination of global sea-level rise and local subsidence (sinking land). High subsidence is typical of river deltas where rapid sediment accumulation also traps a great deal of water. Over time, as layers of new sediment are deposited, water is squeezed out of the underlying deposits, causing compaction and the land surface to sink.
Subsidence rates in the Mississippi River Delta are even further increased due to the large amount of fluid withdrawal in the northern Gulf of Mexico. Oil and gas extraction leads to increased soil compaction over a shorter amount of time. The coastal land loss that can be attributed to this exacerbated subsidence rate is difficult to quantify, but it is suspected to be significant.
A recent study lead by Alexander S. Kolker, “An evaluation of subsidence rates and sea-level variability in the northern Gulf of Mexico” (Geophysical Research Letters, Vol. 38), calculated various subsidence rates from tidal gauge records and examined the relationship between subsidence, oil and gas production in south Louisiana, and coastal land loss.
The study compared long-term tidal gauge records (50+ years) in the northern Gulf of Mexico at Grand Isle, La. and Pensacola, Fla. The Pensacola gauge is in an area that is not subsiding and was used as a baseline to isolate the subsidence rates at the Grand Isle gauge from other drivers that effect relative sea-level rise in the northern Gulf of Mexico. The long-term subsidence rate at Grand Isle averaged 0.3 inches per year, but when the long-term trend was broken into six-year increments, distinct variations with time became evident. The subsidence rate at Grand Isle from 1947 to 1952 was 0.14±0.11 inches per year, peaked from 1965 to 1970 at 0.62±0.12 inches per year, and declined to -0.06±0.24 inches year from 2001-2006.
The onshore production of oil in south Louisiana followed a similar pattern as the subsidence rate, with maximum production peaking in 1968 and very low onshore production occurring in the present day. The link between higher subsidence rates and human activity has been indicated in other research, but the results of Kolker’s study suggest that fluid withdrawal from oil production can rapidly influence subsidence rates.
To explore the link between subsidence and land loss in coastal Louisiana, researchers from the United States Geological Society (USGS) compared subsidence rates at Grand Isle to land loss rates calculated for Barataria Bay, La. in their report, "Land Area Change in Coastal Louisiana from 1932 to 2010." Variations in subsidence over time showed a similar pattern of land loss, with the highest rates of loss occurring in Barataria Bay from 1973 to 1975, when the subsidence rate was at its peak and onshore oil production was at its maximum. While subsidence is not the only process driving land loss in the Mississippi River Delta, high subsidence rates increase the vulnerability of coastal wetlands to other destructive processes, such as sea-level rise and hurricanes.
Subsidence rates are a key variable considered when calculating the sustainability of Louisiana’s coastal system. Kolker’s research suggests that human activities can intensify subsidence, but the subsidence rates calculated for the present time period — -0.06±0.24 inches per year — are lower than they have been in the past. This suggests that bold, strategic coastal restoration may fare better — even with the predicted increase in global sea-level rise — than other research has suggested.