Today’s guest post comes from Dr. Reda Amer in the Department of Earth and Environmental Sciences at Tulane University. The followers of this blog may have noticed that, in recent months, a number of our posts have addressed issues specific to the Gulf of Mexico and the coastal south of the United States. Followers may also recall much earlier posts that talked about the use of Landsat satellite images and the collaboration between the National Museum of African Art, the National Air and Space Museum, and our non-Smithsonian partners at NASA. In an interesting twist, Dr. Amer brings these factors together to reveal how satellite images are now being used to detect plumes and concentrations of run-off into the Gulf of Mexico. Truly, the technology is remarkable and hopefully paves the way for cleaning up and improving the health of our environment.
Thermal Remote Sensing Analysis for Submarine Groundwater Discharge Plumes along the Gulf Coast
Dr. Reda Amer, Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118
Fresh groundwater flowing through the inland aquifers and discharge into the Gulf of Mexico. Groundwater discharge into Gulf water occurs as a slow diffusive flow but can be concentrated at fractures zones. Shallow groundwater aquifers contain high concentration of dissolved inorganic nitrogen, soluble phosphate, and fecal bacteria from the agricultural and human activities. Exiting of this groundwater into Gulf water can result in changes in water salinity and decreased oxygen content; which known as eutrophication. This process impacts fragile coastal ecosystems (e.g. estuaries, coral reefs, fish kills, and shifts in the dominant flora).
Because of the different thermal signatures between groundwater and ocean water, remote sensing thermal infrared images are useful to trace groundwater outflow and can identify the location of concentrated discharge along the Gulf coast. The images employed in this study are from the Earth Explorer database provided by the USGS taken during Landsat missions between September and November of 1999. Landsat is a multispectral satellite mission, and the thermal band provides 60 m resolution. The attached figure exemplifies using thermal infrared imagery to identify groundwater discharge plumes into the Gulf water southeast Louisiana.
The Landsat thermal infrared image was processed using ENVI software where the pixels digital numbers were converted into temperature in Kelvin. The image shows that the lowest temperature is about 300 K and indicated by blue colors, and the highest is 329 K indicated by red colors. At lower latitudes groundwater is generally has cooler signature than the ocean water. The anomalously cooler plumes are indicated as yellow colors within pale-red and red color of Gulf water. The salinity data from Shiller and Mao (1999) was used to validate the results of thermal image analysis. Comparison of salinity (green points) and thermal signatures revealed that there are several zones where lower salinity values correspond to lower temperature values, specifically at the salinity values 20.2 psu and 16.2 psu (indicated with blue circles). The results show that thermal infrared remote sensing imagery can be used as a time and cost-effective tool for identifying submarine groundwater discharge plumes.
Image: Landsat Thermal Infrared images of the Gulf coast along southeastern Louisiana. Blue colors indicate low temperature, red colors indicate high temperature. Salinity data from Shiller and Mao (1999) represented by green points. The blue circles indicate a correlation between lower temperatures and lower salinity.