The 2005 storm season was the most devastating in Gulf Coast history. In October 2005, a team of scientists from the U.S. Geological Survey (USGS) and the Missouri University of Science and Technology (M S&T) deployed to southern Louisiana to collect perishable environmental samples of sediment deposited from the effects of hurricanes Katrina and Rita. The study area included the southern shoreline of Lake Pontchartrain and extended southward to the ‘Bird Foot’ Delta that extends into the Gulf of Mexico.
Samples collected in New Orleans, La., represented the entire flood-impacted area, in addition to areas that received hurricane damage but no flood waters. The origin of the sediment deposited in New Orleans was primarily from the channel systems designed to drain the city to Lake Pontchartrain. These sediments likely accumulated large concentrations of trace elements and organic compounds from years of municipal and industrial drainage. The mixing of these sediments with debris from the hurricanes may have been responsible for increases in trace element concentration.
The results discussed herein will serve as a planning tool for residents and debris removal teams as they make this region habitable once more.
Perishable samples collected for the investigation were subject to destruction or ruin by removal, mixing, or natural decay; therefore, collection was time-critical following deposition. The chemical constituents selected were thought to be those most likely to be bonded with sediments following the mixing of sediment and water with the diverse hurricane debris consisting of automobiles, boats, household wastes, building materials, industrial equipment and assorted chemicals. Analysis was limited to the determination and reporting of the distribution of collected data.
Gathering the samples within one month after hurricanes Katrina and Rita, the team was given the opportunity to catalog the effects that the storms had on the landscape before the debris and deposited sediments were removed from the affected area. For this analysis, 157 samples were used to characterize trace element, iron, total organic carbon, pesticide and polychlorinated biphenyl concentrations of deposited sediment and associated shallow soils.
Trace Element Results
In decreasing order, the largest variability in trace element concentration was for lead, vanadium, chromium, copper, arsenic, cadmium and mercury.
Lead. Lead was determined to be the trace element of most concern because of the large concentrations (4.50- to 551-milligrams per kilogram (mg/kg)) in the samples. The presence of lead in post- hurricane sediments likely is from redistribution of pre-hurricane contaminated soils and sediments from Lake Pontchartrain and the flood control canals of New Orleans.
The U.S. Environmental Protection Agency (EPA) has established a 400-parts per million (ppm) by weight lead standard for bare residential soils in child playground areas and a 1,200-ppm standard average concentration in non-playground areas. Results of this analysis identified only two samples with concentrations exceeding the lower standard; nevertheless, the EPA standard suggests a lead hazard may occur from the creation of dust during the sediment removal process in New Orleans. Sediment removal is not a concern in the Louisiana Delta region where the landscape primarily is rural and storm-related sediment deposition was minimal.
Arsenic. Although arsenic concentrations generally were small, all samples exceeded the EPA’s Human Health Medium-Specific Screening Level (HHMSSL) of .39-mg/kg and seven samples had concentrations above the non-cancer HHMSSL of 22-mg/kg. These screening levels are chemical concentrations that correspond to fixed levels of risk in soils, air and water. The cancer HHMSSL represents a one-in-one-million chance of contracting cancer at the established technical concentration; the non-cancer HHMSSL represents the concentration level associated with other health risks may occur that are not related to cancer. EPA notes that these are not considered regulatory ‘action’ or ‘cleanup’ levels, but can be used as a technical tool in evaluating potential risk to human health.
Mercury. Mercury concentrations ranged from .02-mg/kg to 1.30-mg/kg. A comparison of the mean mercury concentration present in post-hurricane samples with regional background data from the USGS National Geochemical Database indicated that mercury concentrations generally were larger in post-hurricane sediment. Mercury has known health risks when present in its methylated form. The elemental form is not widely present in the environment and rarely is considered toxic until it makes the transition to methyl mercury. EPA Region 6 has a HHMSSL for total mercury of 23-mg/kg dry; all samples analyzed had mercury levels below this screening level.
Sequential Extraction. To further investigate the nature of the trace elements and iron in environmental soil and sediment samples, scientists selected 51 of the 157 samples for a sequential extraction procedure to understand the form in which each element exists. Lead was the only element to have a substantial percentage of its total concentration in the highly mobile exchangeable and carbonate fractions. On the basis of these sequential extraction results, lead could be considered the most problematic trace element measured in the study, and could pose the greatest threat to the clean-up and repopulation of New Orleans. Sequential extraction analysis of lead indicates that 39.1 percent of the total lead concentration in post-hurricane sediment is associated with the iron-manganese oxide fraction. This fraction is considered extremely mobile under reducing environmental conditions, thereby making lead a potential health hazard. Arsenic and lead predominantly were contained in the iron-manganese oxides of samples. Mercury was not included in this procedure because of low levels detected in the single-extraction method and the potential for contamination in any one of the extraction steps.
The destruction caused by both hurricanes included the dispersion of organic chemicals throughout New Orleans and the Louisiana Delta. The field teams observed evidence of many oil slicks at high-watermark areas, fallen power lines and broken transformers. The organic chemical contents from local businesses and homes strewn about the landscape also were apparent. Analysis of the environmental samples was focused on organic compounds thought to be available to sediments in the affected region.
Pesticide concentrations in soil and sediment samples generally were small but there were exceptions. Prometon was the most frequently detected pesticide, with concentrations ranging from 2.4- to 193-micrograms per kilogram (µg/kg). Methoxychlor was the largest concentration measured and was present in 22 of the 157 samples measured for organic compounds with a concentration ranging from 3.5-µg/kg to 3,510-µg/kg. The 3,510- µg/kg value represents the largest concentration of any pesticide, but was well below the EPA’s High- Priority Screening Level for residential soils. The presence of methoxychlor in these samples does not pose a health threat at any level measured in this study. Smaller concentrations of atrazine, prometon, simazine and heptachlor also were observed in samples from New Orleans and the Louisiana Delta. The presence of these compounds in the reported concentrations is not a primary concern to repopulating residents.
Every scientific investigation following these devastating hurricanes provides the scientific community with a unique opportunity to help improve future planning, response and recovery efforts. The research and analytical effort employed by the USGS and M S&T team provides an essential contribution to that cause. Additionally, the research results herein provide vital information to resource managers, debris removal personnel and the repopulating residents of southern Louisiana.
No one could have predicted the extent of the destruction caused by hurricanes Katrina and Rita. The chemical status of post-hurricane sediments described in this report can help ensure a safer tomorrow for all of Louisiana’s residents. For a complete description of the study and analysis, please refer to U.S. Geological Survey Scientific Investigations Report 2008-5031.