Poor water quality results from a mix of human activities and natural processes, ranging from agricultural chemicals washing into rivers to arsenic leaching from underground rock. Globally, 1 in 4 people, roughly 2.1 billion, still lack access to safely managed drinking water. Understanding what degrades water helps explain why contamination is so widespread and what makes some water sources more vulnerable than others.
Agricultural Runoff
Farming is one of the largest contributors to water pollution in the United States and worldwide. Each year, approximately 12 million tons of nitrogen fertilizer, 4 million tons of phosphorus fertilizer, and half a million tons of pesticides are applied to crops across the continental U.S. Rain and irrigation wash a portion of these chemicals off fields and into nearby streams, rivers, and lakes. Unlike a factory pipe that discharges from a single point, agricultural runoff seeps in from broad stretches of land, making it difficult to track and control.
The nitrogen and phosphorus that reach waterways act as a fertilizer for algae, triggering explosive growth known as algal blooms. As those algae die and decompose, bacteria consume the available oxygen in the water, creating hypoxic (low-oxygen) zones where fish, shellfish, and other aquatic life suffocate. This process, called eutrophication, has created massive dead zones in places like the Gulf of Mexico and Lake Erie. Animal manure from feedlots and pastures adds to the problem, carrying both excess nutrients and disease-causing bacteria into surface water.
Industrial and Chemical Discharge
Manufacturing, mining, and chemical processing release a wide range of pollutants into water systems. In 2023, industries in the U.S. reported releasing roughly 186 million pounds of chemicals into water, with nitrate compounds making up about 90 percent of the total. The remaining 10 percent included metals like manganese, zinc, and barium, along with chemicals such as methanol, ammonia, and sodium nitrite. Heavy metals like lead and mercury, along with persistent chemicals such as dioxins, are particularly concerning because they accumulate in the food chain and resist natural breakdown.
Mining operations pose a specific risk. When rock containing sulfide minerals is exposed to air and water during excavation, it produces acidic runoff that dissolves metals from surrounding soil and rock. This acid mine drainage can contaminate rivers and groundwater for decades after a mine closes, turning streams orange and making them toxic to aquatic life.
PFAS and Synthetic Chemicals
Per- and polyfluoroalkyl substances, commonly called PFAS or “forever chemicals,” represent a newer category of water contaminant. These synthetic compounds resist heat, water, and oil, which made them useful in nonstick cookware, food packaging, stain-resistant fabrics, and firefighting foams. That same durability means they persist in the environment almost indefinitely.
PFAS contamination in water systems has been linked to firefighting foam used at airports and military bases, industrial facilities like chrome plating operations, landfills where consumer products break down, and agricultural land treated with contaminated biosolids (processed sewage sludge). Because PFAS dissolve readily in water, they migrate easily through soil into groundwater, where they can travel long distances from their original source. Communities near airports, refineries, and older industrial sites face the highest exposure levels.
Harmful Algal Blooms and Cyanotoxins
When nutrient pollution combines with warm temperatures, calm water, and ample sunlight, cyanobacteria (blue-green algae) can multiply rapidly into harmful algal blooms. These blooms produce toxic byproducts called cyanotoxins, which are released into the surrounding water mostly when the cells die and rupture, though some species excrete toxins continuously. The most commonly measured cyanotoxins in the U.S. include microcystins, which can damage the liver, and anatoxins, which are neurotoxins that interfere with the nervous system. Saxitoxins, the same family of toxins responsible for paralytic shellfish poisoning, are also produced by certain freshwater cyanobacteria.
Algal blooms can render a lake or reservoir temporarily unusable for drinking water, recreation, or fishing. They’re increasingly common as climate change raises water temperatures and intensifies the storms that flush nutrients into waterways.
Naturally Occurring Contaminants
Not all water pollution comes from human activity. Groundwater, which many communities depend on because it’s naturally filtered by soil and rock, can contain dangerous concentrations of arsenic and fluoride that leach from geological formations. These are the two most significant natural groundwater contaminants worldwide in terms of both health effects and geographic reach.
What makes natural contamination tricky is its unpredictability. Safe and unsafe wells can exist in close proximity, or at different depths, because of complex underground flow paths and varying geochemical conditions. A neighbor’s well might test clean while yours exceeds safety limits. Geology, climate, topography, and soil composition all influence where these contaminants concentrate, and researchers now use machine learning models with large global datasets to predict high-risk areas.
Aging Water Infrastructure
Even when the water source is clean, the pipes that deliver it to your tap can introduce contaminants. Corrosion of metallic pipes and fittings creates breaches that allow outside pollutants to enter the system. More commonly, the pipes themselves are the source: lead and galvanized steel service lines corrode over time, leaching lead directly into the water. The EPA has set the maximum contaminant level goal for lead at zero, recognizing there is no safe level of exposure. If more than 10 percent of tap water samples in a water system exceed 15 parts per billion of lead, additional corrective actions are required.
Iron pipes present a different problem. As they corrode internally, the rough, pitted surfaces provide ideal habitat for biofilms, colonies of bacteria that cling to pipe walls. These biofilms are partially shielded from the disinfectants added to keep water safe. Older pipes can also leach organic contaminants or even asbestos fibers, depending on the materials used when they were installed. Many water systems in the U.S. rely on infrastructure that is 50 to 100 years old, meaning millions of people drink water that has passed through pipes never designed to last this long.
Bacteria, Viruses, and Parasites
Biological contamination remains one of the most immediate threats to water quality, especially in systems with inadequate treatment or distribution failures. The CDC identifies a long list of germs that can contaminate tap water, including bacteria like E. coli, Salmonella, Shigella, Campylobacter, and Legionella, viruses like norovirus, rotavirus, hepatitis A, and enterovirus, and parasites like Cryptosporidium and Giardia.
These pathogens typically enter water supplies through sewage overflows, septic system failures, or animal waste washing into source water. Cryptosporidium is especially problematic for water treatment plants because it forms a hard outer shell that resists chlorine disinfection. Outbreaks often follow heavy rainfall, which can overwhelm sewer systems and flush contaminated runoff into reservoirs and rivers. Boil-water advisories, while inconvenient, exist precisely because these organisms can survive standard treatment under certain conditions.
Stormwater and Urban Runoff
In cities and suburbs, rain hits rooftops, parking lots, and roads rather than soaking into soil. This stormwater picks up motor oil, brake dust, heavy metals, trash, pet waste, and lawn chemicals as it flows across impervious surfaces and into storm drains. Most storm drains discharge directly into rivers, streams, or the ocean without any treatment.
The concentration of pollutants is highest during the “first flush,” the initial surge of runoff at the start of a rainstorm, when accumulated contaminants are swept off surfaces all at once. As urban areas expand, so does the volume of untreated stormwater reaching natural water bodies. Combined sewer systems, found in many older cities, make this worse by mixing stormwater with raw sewage and discharging the overflow during heavy rains.