Water becomes impure when anything other than water molecules is present in it, whether that’s a living organism, a dissolved chemical, a suspended particle, or a radioactive element. The EPA groups water impurities into four broad categories: biological, chemical, physical, and radiological. Each enters your water supply through different routes and poses different risks.
Biological Impurities
Bacteria, viruses, parasites, and other microorganisms are among the most common and immediately dangerous water impurities. These typically enter water when human or animal waste reaches a water source through sewage overflows, broken sewage systems, polluted stormwater runoff, or agricultural runoff from farms.
Some of the most well-known biological contaminants include E. coli (often traced to cattle farms), Campylobacter, Giardia, and Cryptosporidium. All of these cause gastrointestinal illness. Legionella bacteria are a slightly different case: they occur naturally in freshwater but become a health risk when they multiply in building water systems like hot water heaters, cooling towers, and hot tubs.
Public water systems test for total coliforms, a group of bacteria that aren’t necessarily harmful on their own but signal that more dangerous pathogens may be present. This is one of the most basic indicators of whether water is pure or contaminated.
Chemical Impurities
Chemical contaminants include both naturally occurring elements and synthetic compounds. Some dissolve into water as it passes through rock and soil. Others come from industrial discharge, household plumbing, or agricultural activity.
Heavy metals are a major concern. Mercury, lead, chromium, cadmium, and arsenic are the five most commonly associated with human poisoning. Lead often enters tap water through corroded household plumbing rather than from the water source itself, and it’s especially dangerous for infants and children because it can delay physical and mental development. Arsenic erodes from natural rock deposits and can also come from petroleum refinery discharge; long-term exposure is linked to skin damage, circulatory problems, and increased cancer risk.
Nitrogen and phosphorus from fertilizers are another widespread source of chemical impurity. About 12 million tons of nitrogen fertilizer and roughly half a million tons of pesticides are applied to crops in the continental United States each year. These chemicals don’t stay where they’re applied. Rain, snowmelt, and irrigation carry them into streams, rivers, and groundwater. Nitrate, a nitrogen compound, is particularly dangerous for infants under six months old, who can become seriously ill from water that exceeds safe limits.
Pesticides, including herbicides, insecticides, and fungicides, follow the same path from farmland into water systems. Agricultural activity is the single largest source of nutrient contamination in major U.S. river basins.
PFAS: The “Forever Chemicals”
A newer category of chemical impurity involves PFAS, synthetic compounds used in nonstick coatings, food packaging, and firefighting foam. These are called “forever chemicals” because they break down extremely slowly in the environment. In April 2024, the EPA established the first legally enforceable limits for two specific PFAS compounds in drinking water, giving public water systems until 2029 to comply. Drinking water systems don’t produce these chemicals themselves; polluters contaminate the surface water and underground aquifers that supply communities.
Disinfection Byproducts
Ironically, the process of making water safe can introduce its own impurities. Chlorine and chloramines are added to kill harmful microorganisms, but they react with naturally occurring organic matter in the water to form byproducts like trihalomethanes and haloacetic acids. Both are linked to increased cancer risk with long-term exposure and are regulated under federal drinking water standards.
Physical Impurities
Physical contaminants change the appearance or texture of water without necessarily dissolving in it. Sediment, silt, clay, and organic matter suspended in water are the most common examples. These particles make water cloudy, a property measured as turbidity. High turbidity typically signals erosion, either natural or caused by construction, farming, or other land disturbance.
Microplastics are an increasingly studied physical impurity. Tiny fragments of plastic from packaging, synthetic clothing, and industrial processes show up in both tap water and bottled water. Research on Amsterdam’s water supply found that treatment plants removed 97 to 98% of microplastics, bringing tap water concentrations down to an average of about 0.21 micrograms per liter. Bottled water generally contains higher concentrations of microplastics than tap water, likely because the plastic packaging itself sheds particles into the liquid.
Radiological Impurities
Radioactive elements like radium, uranium, and radon occur naturally in rock and soil and can dissolve into groundwater as it flows through underground formations. Radon, a radioactive gas, forms as radium decays and can be present in well water and other groundwater sources. These contaminants emit ionizing radiation, and the EPA sets maximum contaminant levels for radionuclides in public drinking water. Cesium and plutonium are also classified as radiological contaminants, though these are more associated with industrial or nuclear contamination than natural sources.
Impurities That Affect Taste and Appearance
Not all impurities pose a direct health threat. The EPA maintains a separate set of guidelines for 15 “nuisance” contaminants that affect how water looks, tastes, or smells. Iron above 0.3 milligrams per liter gives water a rusty color, metallic taste, and can leave reddish-orange stains on sinks and laundry. Manganese above 0.05 milligrams per liter turns water dark brown or black and adds a bitter metallic flavor. Sulfate above 250 milligrams per liter makes water taste salty.
Corrosive water can also dissolve copper from pipes, creating a blue-green tint and metallic taste. Excess chloride, foaming agents, and high levels of total dissolved solids all contribute to off-putting taste or odor. These standards are not legally enforceable the way primary health standards are, but they represent the threshold where most people would notice something is wrong with their water.
How These Impurities Reach Your Tap
Water impurities enter the supply chain at nearly every point. At the source, rivers and lakes pick up sediment from erosion, agricultural chemicals from farmland, and microorganisms from animal waste. Groundwater absorbs minerals, metals, and radioactive elements as it moves through rock. During treatment, disinfection chemicals can create byproducts. In the distribution system, aging pipes can leach lead and copper. Even at the tap, building plumbing and water heaters can harbor bacteria like Legionella or introduce metals from corroded fittings.
The distinction between pure and impure water is ultimately about what’s dissolved or suspended in it. Perfectly pure water, containing nothing but H₂O molecules, doesn’t exist in nature. Every water source contains some level of dissolved minerals or organic matter. The practical question is whether those impurities are present at levels that affect your health, the taste and appearance of your water, or both.