Sifting Slush: Speciation of Heavy Metals in Katrina Sediments from New Orleans, Louisiana
Hurricane Katrina, a Category 4 storm on the Saffir-Simpson scale, struck the Gulf Coast of Louisiana on August 29th, 2005. The storm surge resulted in breaches of the levee system in New Orleans at least at two locations, flooding up to 80% of the city. The floodwater also brought sediment which was left behind once the water receded after a few days.
The flood water was sampled in the Mid-City and Lakeview areas of New Orleans within five days by Dr. John Pardue and his co-workers from the Civil and Environmental Engineering Department of LSU. The concentrations of metals such as iron, zinc and copper in the flood water were often elevated compared to EPA drinking water standards, but rarely exceeded the limits. The metal concentrations conformed to those typically expected in urban storm water.
Dr. Pardue and his group went back to New Orleans after the flood water receded and collected sediment samples. Dr. Amitava Roy and his colleagues at CAMD collaborated with Dr. Pardue to investigate the speciation of metals in Katrina sediments.
Figure 1
Sediment samples were obtained from the Lakeview and Mid City areas of New Orleans (Figure 1). The sediments contained highly diverse materials, including broken Mardi Gras beads.
Iron oxide coated particle in the sediment from Lakeview neighborhood (left) and a clay mineral-rich particle.
Sediment samples from Mid City. There are particles of asphalt, quartz, and rounded glass. The reddish areas are coating of iron oxide.
Figure 6
Chromium K edge spectra from specimens from two sites show a distinct pre-edge peak at around 5993 eV (Figure 6) due to hexavalent chromium. The peak is more prominent for the sample from Mid City New Orleans. Least squares fitting for these spectra showed the Mid City sample contained ca. 10% by weight of this toxic metal or a total of 3 ppm.
There is quite a bit of variation in speciation of the metals at the two sites and even within a single neighborhood. One sample from the Mid City region showed both copper and zinc in reduced (metallic) state but iron appeared in II+ oxidation state.
The variation in speciation to some extent represents the variation in the sources themselves. There is a wide variation in mineralogy and chemistry of the sediments in a neighborhood and between them. This suggests that there sources could be different and/or there is mixing of sediments coming from different sources. The floodwater not only entrained the local particulate matter but also brought in sediments from the surrounding water bodies, such as Lake Pontchartrain and the canals joining the Mississippi River with the Gulf of Mexico.
