Research > St. Louis Bay

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Sediment-Associated Pollutants and Water Quality in St. Louis Bay, Mississippi
Results from a recent study 


Protecting Our Coastal Resources:

Most people enjoy spending the day along the coast. But have you ever thought about the value of the coastal zone? Besides value to us in terms of aesthetic and recreational enjoyment, the coastal zone provides us value in terms of things like jobs, food, coastal protection, and natural water treatment. Consider the 2010 Deepwater Horizon well blowout. The oil spill caused the shutdown of many fisheries in the northern Gulf of Mexico, tourism was impacted, and coastal marshes were damaged. Clearly there was an economic value associated with these impacts and hence a value for the coastal zone itself. Economic valuation of ecosystems is difficult, but recent estimates suggest that the global coastal ecosystem provides us with services greater in value than the US gross domestic product (GDP). Just the Mississippi River delta itself is thought to provide services in the range of $12-47 billion annually. These values and estimated costs emphasize the importance of protecting our coastal resources from stresses such as environmental pollutants.

Understanding the fate and effects of pollution is an important aspect of protecting our coastal resources. The project discussed herein dealt with the transport of both organic and metal pollutants though St. Louis Bay, Mississippi. Specifically, the organic pollutants examined included dioxins, furans, and polychlorinated biphenyls (PCBs). These organic chemicals are known as persistent organic pollutants (POPs) because they tend to be highly toxic and don’t readily degrade in the environment. The major source of these compounds is human activities. Dioxins and furans can be produced when herbicides, paper and pulp are produced as well as through combustion. PCBs were used in certain electrical apparatus as well as in some machining, but their production has now been banned. These toxic materials can cause cancer and can interfere with hormones and immune system response.

Regarding metals, there are a wide variety of possible toxic effects, depending on the metal. Some elements, such as arsenic, lead, and mercury, have no known biological function and are toxic at very low levels. Other metals, such as zinc and chromium, are actually vital nutrients in low doses. However, at higher levels these, too, will be toxic. Metals don’t get broken down or destroyed, but they can end up in more or less toxic forms.

Our St. Louis Bay research examined mainly the sediments and water of the bay along with a few oyster samples. Burial in the sediments can effectively remove pollutants from active biological exposure. However, storm surge as well as dredging can sometimes bring those pollutants back into the realm where fish, shellfish, and ultimately humans can be exposed.


St. Louis Bay:

St. Louis Bay (SLB), Mississippi, is a small, shallow estuary with moderate local residential and industrial development. Over the last ~30 years, sediments and shellfish from St. Louis Bay have twice been analyzed for various forms of chemical contamination, once in 1978 and again in 2004.  Shortly after the 1978 baseline study, the DuPont titanium dioxide refinery at DeLisle, MS, began operation on the northern shore of the bay. According to the U.S. EPA, this refinery produces significant quantities of dioxins, furans, polychlorinated biphenyls (PCBs) and heavy metal byproducts, which are disposed of, in part, by point source air emissions and surface water discharge to St. Louis Bay. A follow-up study in 2004, noted increased concentrations of toxic dioxins in the sediments as well as increases in certain heavy metals in shellfish and sediments. Subsequently, during Hurricane Katrina in 2005, the region was impacted by a 25-foot storm surge which mobilized and re-deposited bay sediments.

Between 2010 and 2013, we re-sampled the bay sediments and waters to update the story on sediment-associated pollutants and water quality. This project produced both practical and fundamental results: practical in terms of useful information for the local community and fundamental in terms of scientific results of use to the broader scientific community.


Highlighted Results:

1: Cesium (Cs) in the water column and surface sediment.

Salts of cesium (Cs)….a naturally occurring heavy element with chemical properties similar to sodium and potassium….have a known use in the titanium dioxide refining process. We observed dissolved Cs enriched 100-fold or more above natural levels in some bay waters. We also observed Cs enriched several-fold above background in some surface sediments. The highest Cs enrichments occurred near the DuPont outfall in the northern part of the bay.

The high levels of Cs do not appear to be toxic, but are useful in forming a “dye” indicator that can help trace the dispersion of material from the outfall in the northern bay. We were able to track Cs in the water as it moved out of the bay into Mississippi Sound.

2: Impact of Hurricane Katrina on metal deposition in local salt marshes.

Salt marsh sediments generally undergo steady accumulation over time and thus are widely used to reconstruct the history of contaminants derived from atmospheric and river sources. However, major hurricanes can significantly affect the coastal landscape by eroding and re-distributing sediments. Four short sediment cores were collected from the fringing marshes of St. Louis Bay to investigate the impact of Hurricane Katrina in marsh sediment. Results do indicate a significant impact from the hurricane on trace metal deposition. A layer near the tops of the cores showed substantial mixing. Equally important, although we could see a Cs enrichment in the upper part of the cores, we saw little to no enrichment in the trace elements arsenic, chromium, nickel, and vanadium. This suggests minimal contamination of sediments of the fringing marsh with at least those four metals.

3: Dissolved metals in bay waters.

The waters of St. Louis Bay potentially receive dissolved metals from a variety of natural and human sources. We surveyed concentrations of various dissolved metals in waters of the bay eight times over the course of two years. Metals determined included copper, chromium, iron, molybdenum, nickel, and vanadium, among others. Because pre-human metal levels in this system are not known, it is impossible to say that there is no impact of human activities on the metal distributions. However, observed concentrations fell within the range observed in other less-perturbed systems. Additionally, we identified no “hot spots” of contamination. We did find that wind-induced sediment resuspension can increase metal concentrations in the water, thereby potentially helping to flush sediment-bound metals out of the bay.

4: Dioxins and furans in sediments and oysters.

Previous work found high concentrations of dioxins in sediments and oysters from St. Louis Bay and concluded that the titanium dioxide refinery was the prime source for it. This new study examined surface sediments and oyster samples from the bay for dioxins and poly-chlorinated biphenyls (PCBs)…..two types of highly toxic chemicals. While PCB concentrations were found to be very low (with undetectable levels in many samples), the dioxin concentrations found in sediments and oysters were similar to those from the 2004 study. 

5: Methane fluxes through St. Louis Bay.

Methane, which is the dominant component of natural gas, is an atmospheric greenhouse gas that has more than doubled in the atmosphere since the beginning of the industrial revolution. It is formed, in part, by micro-organisms in highly oxygen-depleted environments. We have begun examining methane concentrations in and fluxes through St. Louis Bay for several reasons. First, methane can be an indicator of groundwater discharge to the bay, which is another route by which some pollutants, such as excess nutrients, can enter the water. Also, wetlands naturally produce a significant amount of methane and this study can contribute to our understanding of how our coastal bays contribute to the global cycle of this important trace gas. Our results to date show a large flux of methane from bay sediments through the water and then to the atmosphere. The magnitude of this methane flux through the bay is not unlike what has been observed in other similar environments.

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For further information on this project, contact Dr. Alan M. Shiller, Professor of Marine Science, The University of Southern Mississippi; 228-688-1178;

This work was largely supported by the Mississippi Department of Marine Resources through the federal Coastal Impact Assistance Program. Additional funding was provided by the Northern Gulf Institute and the National Science Foundation.

August 2014.