Groundwater becomes polluted when contaminants on or near the surface dissolve in water and seep downward through soil until they reach the water table. This process can take days or decades depending on the soil type, the depth of the aquifer, and the chemical involved. The sources range from farm fields and leaking fuel tanks to landfills, road surfaces, and even the natural rock surrounding the aquifer itself.
How Contaminants Reach the Water Table
Rain and snowmelt soak into the ground through a process called infiltration. Once below the surface, water moves deeper through tiny spaces between soil particles and rock, a slower journey known as percolation. Eventually it reaches the saturated zone where every gap in the rock or sediment is filled with water. The top of that zone is the water table, and the water stored there is groundwater.
Anything dissolved or suspended in the water at the surface can travel along for the ride. Dense clay soils act like a filter and slow the process considerably, while sandy or gravelly soils let water (and whatever it carries) pass through quickly. Limestone and karst landscapes, where the rock is riddled with cracks and underground channels, are especially vulnerable because water can flow almost directly from the surface into the aquifer with very little natural filtering. The USGS describes karst aquifers as “highly vulnerable to land-surface contamination” because of their rapid recharge and conduit flow.
Farming and Fertilizer Runoff
Agriculture is one of the largest sources of groundwater contamination worldwide, primarily through nitrate. Nitrate comes from the nitrogen in synthetic fertilizers and animal manure. It dissolves easily in water and passes through soil to the water table with little resistance. Natural groundwater typically contains less than 2 milligrams per liter (mg/L) of nitrate. The EPA’s drinking water standard is 10 mg/L, a threshold that wells beneath heavily farmed land regularly approach or exceed.
How quickly nitrate reaches groundwater depends on two things: how well the soil drains and how much of the surrounding land is cropland versus forest or grassland. Sandy, well-drained soils under large swaths of row crops present the highest risk. Pesticides follow a similar path, though many break down faster than nitrate during the journey through soil. Atmospheric deposition of airborne nitrogen compounds adds a smaller but measurable contribution on top of what’s applied directly to fields.
Leaking Underground Storage Tanks
Millions of underground storage tanks (USTs) hold gasoline, diesel, and other petroleum products at gas stations, industrial facilities, and military bases. When these tanks corrode or their fittings fail, fuel seeps into the surrounding soil and eventually reaches groundwater. The main concern is petroleum hydrocarbons and fuel additives. MTBE (methyl tertiary butyl ether), once widely used as an octane enhancer, became one of the most common groundwater contaminants in the United States because it dissolves readily in water and moves quickly through soil. Older tanks that stored leaded gasoline also released lead scavenger compounds like ethylene dibromide and ethylene dichloride into the ground.
These plumes of contamination can spread outward from the leak point for years, affecting wells and water supplies far from the original tank. The EPA currently identifies roughly 1,200 hazardous substances that can be found at contaminated sites, many of them associated with petroleum storage and industrial chemical handling.
Landfills and Waste Disposal
When rain falls on a landfill, it percolates through layers of decomposing waste and picks up a cocktail of dissolved chemicals called leachate. This dark, concentrated liquid contains heavy metals including lead, cadmium, nickel, copper, iron, and manganese, along with organic compounds and nutrients. Iron and manganese tend to show up at the highest concentrations because they’re abundant in municipal solid waste.
Modern landfills use engineered liners to catch leachate before it reaches the ground, but no containment system lasts forever. Older landfills, many of which were built before liner requirements existed, sit directly on soil and have been leaking into aquifers for decades. Even at well-managed sites, monitoring wells around the perimeter track whether contaminants are escaping, because a single crack or seam failure in a liner can allow a slow, steady release.
PFAS: The “Forever Chemicals”
Per- and polyfluoroalkyl substances, known as PFAS, have been manufactured since the 1940s and used in firefighting foams, nonstick coatings, stain repellents, cosmetics, and paper products. They enter groundwater wherever they’re made, used, spilled, or disposed of. A classic contamination scenario is the repeated use of firefighting foam at military bases and airports, where the chemicals soak into the ground and migrate to the water table.
What makes PFAS different from most pollutants is their persistence. EPA researchers have found that these compounds don’t flush out of soil the way other contaminants do. Instead, they bind to soil particles in the unsaturated zone above the water table and slowly bleed into groundwater over long periods. This creates a source of contamination that continues releasing PFAS for years or decades after the original spill or application has stopped.
Septic Systems and Household Waste
About one in five U.S. households relies on a septic system to treat wastewater. When these systems age, crack, or are poorly maintained, they can discharge essentially raw sewage into the soil. A failing septic system releases pathogens like E. coli, along with nitrates and other chemicals, directly into groundwater. In rural areas where homes rely on both septic systems and private wells, a failing system on one property can contaminate a neighbor’s drinking water supply.
Even properly functioning septic systems release some nitrogen into the ground. In areas with high housing density on septic, the cumulative nitrogen load can push local groundwater nitrate levels above safe thresholds, particularly in sandy soils where there’s little natural filtration.
Roads, Parking Lots, and Urban Runoff
Urban surfaces collect a surprising mix of pollutants. Highway runoff contains petroleum hydrocarbons from fuel spills and asphalt leachate, heavy metals like chromium, copper, cadmium, lead, nickel, zinc, and manganese from brake pads and tire wear, and rubber particles ground off tires. In winter, deicing salts add sodium, calcium, and chloride in concentrations far above what’s found in natural water. Even the anti-caking compounds used to keep road salt granular contain cyanide.
When stormwater soaks into the ground, soil captures some of the suspended particles, but dissolved salts and many dissolved metals pass through. Areas with heavy vehicle traffic, gas stations, and industrial yards generate especially high concentrations of hydrocarbons and trace metals, making them “hotspot” sources of groundwater contamination in urban and suburban settings.
Naturally Occurring Contaminants
Not all groundwater pollution comes from human activity. Arsenic occurs naturally in certain rock formations and dissolves into groundwater without any help from surface contamination. The WHO recommends a maximum of 10 micrograms per liter in drinking water, but groundwater in parts of Bangladesh, India, Argentina, Chile, China, Cambodia, Mexico, Pakistan, Vietnam, and the United States naturally exceeds that level. Millions of people drink arsenic-contaminated well water simply because of the geology beneath their feet.
Other naturally occurring contaminants include fluoride, radon, and uranium, each tied to specific rock types. These are particularly tricky because drilling a new well just a few hundred meters away can tap into entirely different geology with much higher or lower concentrations. Testing is the only way to know what’s in a given well, since natural contamination is invisible and odorless.
Why Some Aquifers Recover and Others Don’t
The speed of cleanup depends on the contaminant and the aquifer. Nitrate from a farm field may flush out within years if farming practices change, because nitrate moves freely with water. PFAS, by contrast, cling to soil and release slowly, making contaminated sites extremely difficult to remediate. Petroleum plumes can persist for decades, though bacteria in the soil naturally break down some fuel components over time.
Aquifer depth matters too. Shallow aquifers are more vulnerable to contamination but also exchange water faster, giving them a better chance of recovery. Deep aquifers are better protected by thick layers of soil and rock, but once contaminated, their water may take centuries to turn over. In karst terrain, contamination can arrive fast and spread widely through underground channels, but the same rapid flow can also help flush pollutants if the source is eliminated.