Groundwater is water held underground in the soil or in the pores and crevices of rock formations. The geologic material that yields a usable quantity of water is called an aquifer. Groundwater is a primary source for drinking water globally, making its purity a matter of public health. Once pollutants enter the aquifer, the slow movement of water and the large volume of rock material make remediation exceedingly difficult and costly.
Contamination from Agricultural Practices
Farming activities represent a widespread, non-point source of groundwater contamination, primarily due to the use of chemicals and the management of animal waste. The most common chemical pollutant is nitrate, originating from the over-application of synthetic nitrogen fertilizers and animal manure. The nitrate ion is highly mobile and negatively charged, meaning it is not retained by soil particles. Instead, it is easily carried downward by infiltrating water, a process known as leaching, especially in porous soils.
The use of pesticides and herbicides also introduces synthetic organic compounds into the subsurface. A chemical’s potential to contaminate groundwater is determined by its persistence (half-life) and how tightly it binds to soil organic matter (soil sorption coefficient). Compounds with high persistence and low binding affinity, such as the degradation products of herbicides like atrazine and metolachlor, are the most likely to migrate into the water table.
Concentrated animal feeding operations (CAFOs) contribute a complex mix of pathogens, antibiotics, and antibiotic resistance genes (ARGs). The massive volume of manure generated is often stored in earthen lagoons or spread on fields, allowing contaminants to infiltrate the soil. Pathogenic bacteria, such as Salmonella or E. coli, and antibiotic residues can reach shallow groundwater through runoff and infiltration, posing a health risk and creating a reservoir for resistance.
Pollution from Waste Disposal Infrastructure
Centralized and decentralized waste management systems can become localized sources of groundwater pollution if their containment structures fail. Municipal landfills generate leachate, a concentrated liquid formed when water percolates through the buried solid waste. Leachate contains a variety of hazardous substances, including heavy metals (like lead and nickel), volatile organic compounds, and high concentrations of ammonia-nitrogen.
If the landfill’s protective liners are compromised or if the site is an older, unlined facility, this toxic leachate can migrate downward and form a persistent plume of contamination in the aquifer. Decentralized septic systems, common in rural and suburban areas, rely on the soil below the drain field to filter and treat household effluent. Failure, often due to hydraulic overloading or poor maintenance, causes partially treated sewage to bypass the soil’s natural biological treatment zone.
The effluent from failing septic systems introduces pathogens, phosphates, and high levels of nitrogen, which quickly convert to leachable nitrate in the shallow groundwater. A third source is deep-well injection, used to dispose of industrial and hazardous wastewater, such as fluids from oil and gas operations, far beneath usable aquifers. Contamination risk arises if the injection well’s casing or seals fail, allowing waste to migrate upward into shallower drinking water sources. Furthermore, the high-pressure injection of large fluid volumes has been linked to altering subsurface pressures, which can induce seismicity and create new fracture pathways for fluid migration.
Industrial and Commercial Spills and Leaks
Accidental releases of synthetic chemicals from industrial and commercial activities create highly localized, toxic point-source contamination. Leaks from aging or corroded Underground Storage Tanks (USTs) at gas stations or industrial facilities are a major source of petroleum hydrocarbon contamination. The most mobile and toxic components of gasoline and diesel are the BTEX compounds (Benzene, Toluene, Ethylbenzene, and Xylenes). Benzene is highly soluble and mobile, often serving as a fast-moving indicator of a leak in the groundwater.
Another significant group of industrial contaminants are chlorinated solvents, such as Trichloroethene (TCE) and Tetrachloroethene (PCE), historically used as metal degreasers and dry-cleaning agents. These chemicals are classified as Dense Non-Aqueous Phase Liquids (DNAPLs) because they are denser than water and exhibit limited solubility. When released, DNAPLs sink through the water-saturated zone until they pool on a low-permeability layer, like clay or bedrock, or become trapped as residual saturation in the soil matrix.
These DNAPL pools act as long-term, continuous sources, slowly dissolving into the passing groundwater over decades, making their remediation difficult. Mining operations also contribute pollution through a process called Acid Mine Drainage (AMD). AMD begins when sulfide minerals, most commonly pyrite, are exposed to atmospheric oxygen and water during excavation. The oxidation of pyrite generates sulfuric acid, which drastically lowers the water’s pH. This highly acidic water then mobilizes toxic heavy metals, including copper, zinc, and lead, from the surrounding rock, allowing them to dissolve and enter the groundwater system.
Naturally Occurring Contaminants
Not all groundwater contamination is the result of human activity; some originates from the natural interaction between water and the geological materials it passes through. Certain elements present in high concentrations within specific rock formations can dissolve into groundwater over time, a process known as geogenic contamination. Arsenic and fluoride are two significant examples of this natural contamination.
Arsenic is mobilized into groundwater through the reductive dissolution of iron oxyhydroxides in anoxic (low-oxygen) sedimentary aquifers or the oxidative dissolution of sulfide minerals like arsenopyrite. Fluoride is released through the weathering and dissolution of fluoride-bearing minerals in crystalline rocks, such as fluorite and fluorapatite, with higher concentrations often found in alkaline water conditions.
Radon is an odorless, colorless, radioactive gas formed by the natural decay of uranium and thorium in soil and rock. It easily dissolves into groundwater as the water flows through uranium-rich formations, particularly granite. The contaminated groundwater then acts as a transport mechanism, carrying the dissolved radon until it is released into indoor air when the water is used for showering or washing.
Saltwater intrusion is a major concern in coastal aquifers, where freshwater bodies are in direct contact with the ocean. Freshwater is less dense than saltwater, creating a natural boundary between the two. When excessive pumping reduces the freshwater pressure (hydraulic head) in the aquifer, the denser, deeper saltwater moves inland and upward, contaminating the usable freshwater supply.