Naming a single “most polluted” lake in the United States is impossible because water quality assessment involves multiple, distinct criteria. The label depends entirely on the type of contamination, the volume of the affected water body, and its regulatory history. Determining which lake is the most polluted requires examining the various metrics scientists and regulators use to define pollution severity.
The Challenge of Defining “Most Polluted”
Pollution severity is classified using a combination of scientific and regulatory metrics that reflect different environmental problems. One primary measurement is Toxicity, which focuses on the concentration of dangerous chemical substances within the water and sediment. This involves measuring contaminants such as mercury, polychlorinated biphenyls (PCBs), and industrial solvents, which pose direct risks to human health and aquatic life.
Another measure is the total Volume or mass of pollutants present, which determines the scale of a necessary cleanup effort. A lake with a lower concentration of toxins but a massive volume of contaminated sediment may require a far greater restoration effort than a smaller lake with higher concentrations. A third, widespread measure is Trophic Status, which assesses the level of biological productivity driven by nutrient loading. This is classified using the Trophic State Index (TSI), which uses factors like total phosphorus, chlorophyll a, and water clarity to determine if a lake is moving toward an over-enriched, or eutrophic, state.
Case Studies of Severely Contaminated U.S. Lakes
Onondaga Lake in New York is a prime example of a water body severely impacted by historical industrial waste. Beginning in the 1880s and continuing for over a century, industries discharged significant amounts of waste, including mercury, ammonia, organic compounds, and PCBs, into the lake and its tributaries.
The lake was designated a federal Superfund site in 1994 due to the complexity and severity of the toxic waste contamination. An estimated 165,000 pounds of elemental mercury alone were discharged into the lake between 1946 and 1970, contaminating the lake bottom sediments. The contamination was so severe that swimming was banned by 1940 and fishing was prohibited in 1970, although massive cleanup efforts have since led to significant improvements.
In contrast, Lake Erie represents a large-scale contamination problem primarily driven by nutrient loading and eutrophication. The shallowest and warmest of the Great Lakes, it is highly susceptible to massive, recurring harmful algal blooms (HABs). These blooms, which are excessive growths of cyanobacteria, are fueled by an overabundance of nutrients, primarily phosphorus, washing into the lake.
The nutrient source is overwhelmingly agricultural runoff, where rainfall washes fertilizer and manure from farm fields into the lake’s tributaries, particularly the Maumee River watershed. These toxic blooms threaten the drinking water supply for over 11 million people, deplete oxygen, and harm aquatic life.
Primary Sources of Lake Contamination
The sources of lake pollution are generally categorized as point sources and non-point sources. Point Sources are identifiable, single locations, such as a pipe or a ditch, where pollutants are discharged directly into a water body. Historically, this included industrial discharge from factories and effluent from municipal sewage treatment plants.
This type of pollution is easier to monitor and regulate because the source is discrete and measurable. Through the implementation of environmental laws, significant progress has been made in controlling the pollutants released from these specific industrial and municipal locations. Facilities that discharge pollutants from a point source must obtain a permit that controls the type and quantity of contaminants released.
Non-Point Sources are diffuse and scattered, making them much harder to trace, regulate, and control. This pollution results from rainfall or snowmelt moving over the ground and picking up contaminants before depositing them into waterways. The largest contributor is agricultural runoff, which carries excess nutrients like phosphorus and nitrogen, along with pesticides, from fields into lakes.
Other major non-point sources include urban runoff, which delivers oil, grease, heavy metals, and litter from streets and sidewalks, and atmospheric deposition, which introduces pollutants like mercury into water bodies through the air.
Remediation and Restoration Efforts
Addressing severe lake contamination requires using the federal Superfund program, a primary tool for cleaning up the nation’s most contaminated sites. The program provides funding and a mechanism for the cleanup of complex toxic waste sites, often involving multiple sub-sites and responsible parties.
For contamination caused by nutrient loading, the Clean Water Act mandates the use of Total Maximum Daily Loads (TMDLs). A TMDL determines the maximum amount of a specific pollutant a water body can receive while still meeting water quality standards. This process sets pollutant reduction targets and allocates the necessary load reductions among point and non-point sources.
Common remediation techniques deployed in severely polluted lakes include dredging, which physically removes contaminated bottom sediments for disposal in secure containment facilities. Another technique is capping, where a layer of clean material is placed over the contaminated sediment to isolate it from the water column. For nutrient-rich lakes, management practices focus on reducing inflow, such as implementing best management practices (BMPs) in agricultural areas and treating tributaries to inhibit the formation of harmful toxins.