Brian Barton
The Mississippi River is one of the most polluted rivers in the world. With a relatively low flow for its size, the Mississippi has been affected by a variety of pollutants, including agricultural and urban runoff, oil spills, and plastic waste. While certain toxic chemicals have been banned and pollution from cities and towns has decreased, water quality issues remain. The river is listed as impaired by the EPA for both mercury and PCBs, and agricultural runoff, the leading cause of water pollution in the US, has resulted in eutrophication and the creation of a 'dead zone' in the Gulf of Mexico.
| Characteristics | Values |
|---|---|
| Sedimentation | Filling in the Mississippi River and its backwaters with silt and sand particles |
| Toxic Chemicals | Mercury, PCBs, chloride, nitrate, microplastics |
| Human Activities | Urban pollutants, agricultural runoff, fertilizer use, soil conservation techniques |
| Water Quality Improvements | Reduction in total suspended solids (TSS), improvements in wastewater treatment, ban on phosphorus fertilizer |
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What You'll Learn
Mercury and PCBs
The Mississippi River is the main source of drinking water for about 18 million Americans in 50 cities across the United States. The river is also a migratory route for 40% of North American waterfowl and shorebirds and is home to 260 species of fish, 50 species of mammals, and 145 species of amphibians and reptiles.
Polychlorinated biphenyls (PCBs) are man-made chemicals that are colorless and odorless. They were widely used in industrial applications until the late 1970s due to their chemical and thermal stability. PCBs were used in fluorescent light fixtures, carbonless paper, adhesives, plastics, paints, varnishes, sealants, and electrical equipment. The United States produced 1.5 billion pounds of PCBs between 1930 and 1975.
PCBs were later discovered to be hazardous compounds that do not readily degrade into less toxic chemicals. As a result, PCBs are highly persistent in the environment. PCBs have been detected throughout the country, including areas with no known PCB manufacturers. It is estimated that over seven billion pounds of PCBs have been released into the environment through air, land, and water discharges. When PCBs are released into rivers, they enter the food chain and become increasingly concentrated in higher trophic levels through a process called biomagnification.
Mercury is another contaminant of concern in the Mississippi River. Mercury pollution in the river primarily originates from coal combustion and mining activities. Mercury levels in the river peaked in the 1960s but have significantly decreased over the past few decades due to increased control measures. Mercury accumulates in the muscle tissue of fish, leading to fish consumption advisories for lakes and streams in the region.
Both mercury and PCBs have been monitored in the Mississippi River for over 20 years. Sedimentation, the filling in of the river and its backwaters with silt and sand, is a significant problem as it affects habitats and transports pollutants like PCBs and mercury downstream. While the Upper Mississippi River is cleaner today than it was 30 years ago due to increased awareness and legislation, human activities continue to impact water quality and nutrient balance.
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Agricultural runoff
Agricultural practices lead to nutrient loading, which causes eutrophication. This occurs when a body of water becomes enriched with dissolved nutrients, stimulating the growth of aquatic plant life and resulting in the depletion of dissolved oxygen. Nitrogen and phosphorus are the most common agricultural pollutants in the Mississippi River Basin. These chemicals are applied to farmland as fertilisers to increase crop yields. However, when excess fertiliser is applied, it can be carried into bodies of water by runoff or leaching. Runoff occurs when water from rainfall or melting snow moves over and through the ground, picking up excess nutrients and other sediments.
The runoff of nitrogen and phosphorus contributes to the development of hypoxic zones or "dead zones", where there are low levels of dissolved oxygen. These dead zones are caused by the rapid growth and death of algae in nutrient-rich areas. As algae grow and die, they use up the available oxygen in the water, leading to the asphyxiation and death of other marine life. The Mississippi River is a significant contributor to the Dead Zone in the Gulf of Mexico, an area devoid of life that forms each summer due to hypoxia.
In addition to fertilisers, animal waste from livestock operations can also be a source of agricultural runoff. Concentrated Animal Feeding Operations (CAFOs) are large industrial operations that raise high numbers of animals for food. Improper storage or application of animal waste as fertiliser can result in runoff, washing pollutants into waterways.
While some states have passed laws to address agricultural pollution, regulating non-point source pollution, such as agricultural runoff, remains challenging. The Clean Water Act (CWA) regulates both point source and non-point source pollution, but the latter is more difficult to control due to its diffuse nature. To improve water quality in the Mississippi River and reduce the impact of agricultural runoff, a combination of regulations, conservation practices, and nutrient reduction strategies are needed.
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Sedimentation
One of the primary consequences of sedimentation is the accumulation of silt in the backwaters, which fills in important habitats for fish and wildlife. This silt covers up spawning grounds and transports pollutants like PCBs and heavy metals in the water. The depth of accumulated sediment can reach significant levels, burying roads, buildings, and bridges. While soil conservation techniques have helped minimize hillside erosion, the mobilization of sediment remains a concern, especially during flood events when unprotected stream banks are more vulnerable.
However, the overall impact of sedimentation on the Mississippi River is complex. While it can help bury and contain pollutants, it also disrupts habitats and contributes to the transport of pollutants downstream. The accumulation of silt and sediment can alter the river's flow and natural flood patterns, leading to ecological degradation and the creation of hypoxic zones, or "dead zones," where water contains dangerously low levels of dissolved oxygen, resulting in the death of marine life.
Agricultural runoff, including fertilizer use and animal waste, is a significant contributor to the sedimentation and pollution of the Mississippi River. Excess nutrients and sediments from farmland are carried into the river during rainfall or snowmelt, leading to eutrophication and algal blooms. As the algae die, they sink to the bottom, where bacterial decomposition consumes oxygen, contributing to the creation of dead zones.
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Urban pollution
The Mississippi River has been polluted by urban and residential areas, as well as agricultural and industrial activities. Urban pollution of the Mississippi River is largely due to the introduction of pollutants from industrial activities, waste disposal, and chemical runoff from urban areas.
Engineering projects aimed at enhancing navigability and preventing flooding have also led to severe ecological degradation. The construction of levees, locks, and dams has altered natural flood patterns, creating areas of hypoxia, or "'dead zones', particularly at the river's mouth. These areas of hypoxia are caused by the concentration of pollutants within the river's main channel, as the reduction of wetlands environments along the banks has left fewer natural filters for the water.
Cities and towns have often dumped toxic waste into the Mississippi River, including chemicals such as furan, trichlorobenzene, DDT, TCA, and PCBs. These toxins have polluted the waterway from its northernmost reaches down to the Gulf of Mexico, with consumption advisories in place for fish and the water sometimes being unsafe for swimming.
Nonpoint source pollution, especially runoff from agricultural regions, has also contributed to the degradation of the Mississippi River. Fertilizer, oil, grease, sediment, and other materials from urban areas are washed into the river by rainfall or snowmelt, depositing a variety of pollutants that harm the river's ecosystems.
Efforts to reduce pollution and restore wetlands have been undertaken by the government and private groups, with some success in reducing pollution from specific sources such as refuse dumped by businesses or cities. However, nonpoint source pollution remains a significant challenge, and the Mississippi River continues to be one of the most polluted rivers in the United States.
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Microplastics and unregulated chemicals
The Mississippi River is the largest river in the United States and the third largest in the world. It supplies drinking water to 70 cities in the US and is used for agricultural purposes. The river has been a subject of concern due to the presence of microplastics and unregulated chemicals, which pose a risk to the environment and human health.
Microplastics
Microplastics have been found in the Mississippi River and are considered a significant source of plastic pollution. A large-scale study quantified microplastics in the water column from St. Louis, Missouri, to New Orleans, Louisiana, and found that microplastic concentrations increased downriver. The river's aquatic and terrestrial expanse suggests that it may contain large amounts of microplastics, impacting the watershed's biodiversity. Microplastics have been detected in various fish species, including Gizzard Shad, Largemouth Bass, Bluegill, and Flathead Catfish, indicating their presence in the food chain.
Unregulated Chemicals
The Mississippi River Basin, which drains approximately 3,200,000 km2, has seen extensive chemical use in agriculture. More than 100,000 metric tons of pesticides and 6,500,000 tons of commercial nitrogen fertilizers are applied to cropland within the basin annually. As a result, pesticides and nitrates have made their way into the region's streams, aquifers, and atmosphere, ultimately reaching the Gulf of Mexico.
Additionally, certain unregulated chemicals, known as "forever chemicals," have been detected in the Lower Mississippi River. These chemicals, classified as PFAS (per- and polyfluoroalkyl substances), have been linked to various health risks. PFAS chemicals are ubiquitous in the United States and are used in consumer products like non-stick cookware and plastic containers. While the EPA has not yet published maximum contaminant levels for certain PFAS chemicals, states have enacted their own regulations to address the potential hazards associated with these chemicals.
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Frequently asked questions
The Mississippi River has been polluted by agricultural runoff, urban runoff, and toxic chemicals. The river is on the Wisconsin DNR state impaired water list for both mercury and PCBs.
The Mississippi River has been identified as a significant contributor to the hypoxic zone or "dead zone" in the Gulf of Mexico, where the water contains less than two parts per million of dissolved oxygen. The river also impacts communities downstream, with pollutants accumulating and causing severe problems in areas such as Lake Pepin and the Gulf of Mexico.
Efforts have been made to reduce certain types of pollutants in the Mississippi River, such as dangerous bacteria like E. coli, and total suspended solids (TSS). Policy changes and initiatives, such as the Clean Water Act, have also led to improvements in water quality. However, challenges remain, and a broad coalition of public, private, and nonprofit partners are working to further reduce pollution and enhance the river's ecology.
