Groundwater is the water found beneath the Earth’s surface in the cracks and spaces within soil, sand, and rock formations known as aquifers. This subterranean reservoir represents the largest source of usable freshwater storage on the planet. Groundwater provides drinking water for a significant portion of the global population, including over 50% of people in the United States and nearly all of its rural residents.
Contamination poses a major public health and environmental concern. Aquifers are vulnerable because the pathways that allow for natural replenishment, such as rainfall seeping through the soil, can also serve as routes for pollutants to migrate from the surface. Vulnerability depends on factors like the thickness of protective soil layers and the speed at which water moves through the ground.
Widespread Chemical Threats from Surface Sources
Groundwater contamination often stems from common sources near the land surface, primarily linked to agricultural and residential practices. The most widespread chemical contaminant detected in U.S. groundwater is nitrate. This compound is a byproduct of nitrogen-based fertilizers, animal waste from feeding operations, and effluent from septic systems.
Because nitrate is highly soluble, it easily leaches through the soil into shallow aquifers following rainfall or irrigation. The health risk is acute for infants under six months of age. When ingested, nitrate converts to nitrite, which interferes with the blood’s ability to carry oxygen, causing methemoglobinemia, or “blue baby syndrome.”
The U.S. Environmental Protection Agency (EPA) set a safety standard of 10 milligrams per liter for nitrate in drinking water to protect against this threat. Pesticides, also widely applied in agriculture, represent another surface-source threat. These chemicals similarly percolate through the soil, posing risks depending on their specific toxicity and persistence.
Naturally Occurring and Industrial Inorganic Pollutants
Inorganic contaminants, often persistent and toxic metals or metalloids, form a separate category of groundwater pollutants. Arsenic is a prime example, frequently occurring naturally in certain geological formations. Although arsenic is naturally present, human activity can inadvertently mobilize it from harmlessly bound sediments into the water.
Intensive groundwater pumping, such as for irrigation, can alter the aquifer’s flow and redox conditions. This change may trigger the microbial dissolution of arsenic-bearing iron oxides, releasing the toxic metalloid into the water. Arsenic exposure is linked to long-term chronic issues, including skin lesions, cardiovascular disease, and increased cancer risk.
Other heavy metal threats are primarily industrial, including lead and hexavalent chromium (Cr(VI)). Lead contamination is often linked to the leaching of legacy industrial waste, such as from former smelters or battery recycling facilities. Improper disposal of waste from industries like leather tanning and chrome plating introduces hexavalent chromium, a known carcinogen, into the subsurface.
Emerging and Persistent Organic Contaminants
A complex challenge comes from emerging contaminants, substances whose health impacts are still being studied but are increasingly detected in water sources. Per- and polyfluoroalkyl substances (PFAS) are the most prominent, characterized by an extremely strong carbon-fluorine bond that makes them highly resistant to environmental breakdown. This persistence earned them the nickname “forever chemicals.”
PFAS are found in thousands of consumer and industrial products, including non-stick coatings, fire-fighting foams, and stain-resistant fabrics. Once they enter the environment through industrial discharge or landfill leachate, their high mobility allows them to travel easily through soil and contaminate underlying aquifers for decades.
Long-term exposure to certain PFAS compounds has been associated with adverse health effects, including:
- Immune system suppression.
- Endocrine disruption.
- Chronic kidney disease.
- An elevated risk of specific cancers.
Another group of emerging threats includes Pharmaceuticals and Personal Care Products (PPCPs), encompassing chemicals like antibiotics, antidepressants, and insect repellents. These substances enter groundwater mainly through wastewater discharge and septic systems, as conventional treatment plants are often not designed to completely remove them.
PPCPs like the insect repellent DEET and the seizure medication carbamazepine are frequently detected in groundwater near riverbanks, indicating infiltration from surface water. While the full health implications of low-level, long-term exposure to PPCPs are not yet fully understood, their presence in drinking water sources is a growing concern.
Protecting Groundwater Quality
Preserving groundwater quality relies heavily on preventing contamination at the source. This requires implementing robust source water protection measures, such as controlling the application of agricultural fertilizers and pesticides and properly managing industrial and municipal waste disposal. Effective regulation and monitoring of surface activities reduce the inflow of new pollutants into aquifers.
For private well owners, regular testing is the primary defense against unseen contaminants like arsenic or nitrate, as private wells are not subject to public monitoring. When contamination occurs, various remediation technologies are available, though their effectiveness varies based on the pollutant type. Pump-and-treat systems, which extract contaminated water for surface treatment, are a common approach.
Advanced treatment methods include specialized filtration, such as using granular activated carbon to adsorb organic contaminants or media selective for inorganic compounds like arsenic. Bioremediation offers an in-situ alternative, using naturally occurring or introduced microorganisms to break down or transform harmful substances into less toxic forms. These solutions ensure the sustainability of this water resource.