INTERPRETING THE INFLUENCE OF CONTAMINANTS IN FLOOD WATERS FROM NEW ORLEANS ON LAKE PONTCHARTRAIN

During Hurricane Katrina concern was expressed about the effects of contamination from New Orleans on Lake Pontchartrain. Background data relevant to assessment of New Orleans' prized estuary was compiled in a comprehensive, quality-controlled sediment and contaminant inventory completed in 2002 by USGS with cooperators (http://pubs.usgs.gov/prof/p1634)(DB). These and post-hurricane data by the Louisiana Department of Environmental Quality (DEQ) and other agencies provide perspectives on the contaminant issue.

Contaminant metals

Except for samples in the vicinity of New Orleans canal mouths, mean concentrations of contaminant metals in analyzed sediment samples from Lake Pontchartrain (LP) were lower than those from Mississippi River (MR) suspended matter, and in the range of pristine sediments (e.g. Pb in MR 25.7, LP 17.5 ppm). This surprising result is explained by the fact that an estimated two thirds of the Lake had been dredged for (Rangia) clam shells prior to 1990. Core profiles (Flocks and Manheim, in DB) revealed that dredging mixed pristine (Pleistocene) sediments having very low metal concentrations with shallow surficial sediments that had increased contaminant levels. The mixing extended to depths of greater than 1 m.

Water sampling by the DEQ and partners has found detectable contaminant metals rare in city floodwaters. Hence, the likelihood of significantly increased metal contamination in the sediments outside areas adjacent to pumping station outfall appears small.

Toxic organic compounds

Principal toxic compounds include polyaromatic hydrocarbons (PAHs) found in petroleum and coal tar, polychlorinated biphenyls (PCBs), and pesticides and miscellaneous chemicals. PAH concentrations in Pontchartrain sediments (DB) were lower than expected. Besides mixing during dredging, this is ascribed to the breakdown of unchlorinated hydrocarbons by microbial attack in warm salty waters like those of Lake Pontchartrain. Even stations near earlier creosote spills on the north coast of Lake Pontchartrain showed only faint traces of what must have been heavy contamination from former Superfund sites. The DEQ report mentioned later found no oil sheen in offshore water sampling sites investigated. Evaporation and biochemical breakdown of petroleum may have already removed much of the oil dispersed in the floodwaters.

Early newspaper reports that some 78,000 barrels of oil had been spilled in New Orleans posed serious concern. This spill turns out to have occurred not in New Orleans, but at a storage tank site downstream in the Mississippi River near Cox Bay. The spill was large enough to pose serious short-term hazards to fish and other organisms in its path. However, in the longer term, it too can be expected to be dispersed by evaporation and microbial decomposition.

Chlorinated hydrocarbons are 100 times or more resistant to bacterial breakdown than PAHs in the natural environment. However, the earlier inventory showed that these compounds, too, were well below toxic levels in sediments, except for isolated occurrences in the immediate vicinity of New Orleans' canals. In the decades since the early 1970s, PCBs and long-lived pesticides like DDT, dieldrin, and aldrin have been systematically removed from use in the U.S., and none were detected in the initial DEQ samplings.

Superfund and other hazardous waste sites in New Orleans

Some newspapers during the hurricane crisis referred to as many as five Superfund sites within New Orleans flooded areas. However, the EPA Envirofacts database revealed only one, previously cleaned up Superfund site in New Orleans. Many other sites containing potentially hazardous materials are known in New Orleans, but the sampling information reported to date has not revealed evidence of significant leakage. It was reported that relevant sites, including those containing radionuclides for medical and other uses, would be checked on by DEQ and EPA.

Biological (sewage-related) and nutrient loads

Two contaminants prominently identified in New Orleans floodwaters were fecal coliform bacteria of sewage origin, and oil. Pathogens dispersed and diluted in Lake waters are expected to be broken down quickly. The load of sewage-derived organic matter is reflected in fecal coliform numbers greater than 2000 in a pumpage plume from the 17th St. Canal area, according to a post-Katrina water quality assessment by the Louisiana Department of Environmental Quality (DEQ) on September 20. However, all stations ½ mile from shore or more showed oxygen levels within the normal range for the area.

Once sewage-derived organic matter is broken down into its component nutrients – especially nitrogen - a secondary bloom of phytoplankton is expected. Such a plume may expand through dispersion and mixing. Information on such secondary productivity is not yet available. It can be tracked synoptically by multispectral satellite data as reported in by R. Stumpf in the USGS report cited above for the bloom following release of Mississippi River floodwaters through the Bonnet Carre Spillway in 1997. Any available late-breaking data will be shown in the oral presentation.

Summary

Through the efforts of relevant New Orleans and Louisiana agencies, stimulated by the urgings and studies of public organizations like Save Our Lake, coastal New Orleans beaches became swimmable and fishable in recent years. For the present and possibly next year nearshore recreational activities will be no doubt be impacted. Hurricane Katrina has also probably created large-scale sediment and bottom topographic changes that remain to be assessed. However, assuming appropriate restoration and upgrading of water treatment facilities and other appropriate measures in New Orleans, and barring factors not currently known, there is currently no information that would predict long-lasting contaminant influences on Lake Pontchartrain and its shores from Hurricane Katrina.