Groundwater is water held underground in the soil, or in the pores and crevices of rock, stored within geological formations called aquifers. This vast reserve constitutes roughly 99% of the planet’s liquid fresh water, making it a globally significant resource, often serving as the primary source of drinking water. Groundwater is typically of high quality due to the natural filtration that occurs as water slowly seeps through soil and rock layers. However, this subsurface water is not immune to contamination, which renders it unsafe for human consumption. Contamination occurs when harmful substances dissolve into the water or are physically carried into the aquifer, often remaining concentrated in a plume.
Natural Geological Sources
Contamination in groundwater is not exclusively the result of human activity, as certain geological settings can naturally introduce harmful substances through geogenic contamination. This occurs when water interacts with the surrounding rock and sediment, dissolving minerals that are naturally present in the earth’s crust. While these interactions control the water’s basic chemistry, high concentrations of specific elements can pose a health risk. These naturally occurring contaminants often lack taste or odor, meaning the water may appear perfectly clean despite being unsafe for drinking.
One of the most widespread geogenic contaminants is arsenic, which is typically released from mineral deposits into the groundwater under specific geochemical conditions. Long-term consumption of water with elevated arsenic levels is associated with severe health issues, including skin lesions, nervous system ailments, and cancers. Similarly, high concentrations of naturally occurring fluoride, derived from the dissolution of minerals like fluorite, can cause dental mottling and the debilitating condition of skeletal fluorosis.
Another natural hazard is radon, an odorless, colorless radioactive gas produced by the radioactive decay of uranium found in bedrock, such as granite. As the water flows through these rock formations, the gas dissolves into the groundwater. While it is primarily an inhalation risk when released indoors from water use, it also contributes to radiation exposure from ingestion.
Contamination from Surface Activities
A significant portion of groundwater contamination originates from human activities that take place directly on the land surface, where pollutants are spread over a large area before slowly infiltrating the ground. This non-point source contamination relies on the process of leaching, where water carries dissolved contaminants down through the soil and into the underlying aquifer. The widespread use of chemicals in modern agriculture is a major contributor to this type of degradation.
Agricultural runoff introduces excess nutrients, primarily nitrates and phosphates from fertilizers. The nitrate ion is highly soluble and is not naturally retained by soil particles, making it particularly prone to leaching into groundwater supplies. High nitrate levels in drinking water are a serious concern, especially for infants, as they can cause methemoglobinemia, often called “blue baby syndrome.” Pesticides and herbicides applied to crops are also carried downward by infiltrating rainwater, contaminating shallow groundwater systems, particularly in areas with highly permeable soils.
Residential wastewater disposal systems, such as septic tanks, represent another common surface-related source of contamination. If a septic system is improperly maintained or overused, it can fail to adequately treat the wastewater before it is discharged into the drain field. This introduces pathogens, including bacteria and viruses, as well as high concentrations of nitrates and phosphates, directly into the shallow subsurface. Finally, chemicals used for de-icing roads in winter, such as various salts, dissolve into snowmelt and rainwater, which then infiltrates the ground, increasing the salinity of the groundwater.
Hazards Related to Subsurface Infrastructure
Localized, highly concentrated sources of contamination often stem from failures or inadequate management of infrastructure built beneath the ground surface. One major contributor is the leakage from underground storage tanks (USTs), which store petroleum products like gasoline and heating oil. As these tanks age, they can develop leaks, allowing substances such as benzene, toluene, and methyl tert-butyl ether (MTBE) to seep into the soil. A small leak from a single UST can have a massive impact, as just one gallon of petroleum can contaminate millions of gallons of groundwater.
These contaminants form a plume of volatile organic compounds (VOCs) that can travel great distances within the aquifer, posing a long-term threat to drinking water wells. Landfills generate a toxic liquid called leachate when water percolates through the buried refuse. If the protective liners of these facilities are damaged or fail, the leachate can escape and infiltrate the underlying groundwater.
Improperly constructed or abandoned wells also act as direct conduits for surface contamination to bypass the natural filtration provided by soil layers. A well with a compromised casing or one that is simply left unsealed can allow contaminated surface water or shallow groundwater to flow straight into a deeper, previously protected drinking water aquifer. In addition, industrial activities sometimes use injection wells to dispose of wastewater or fluids, such as those generated by hydraulic fracturing, directly into deep subsurface layers. If these wells are not properly managed, the injected fluids can migrate upward, introducing industrial pollutants into shallower freshwater aquifers.