Groundwater is the water found beneath the Earth’s surface in saturated zones of rock and soil pore spaces. This vast reservoir is held within geological formations known as aquifers, which are permeable enough to yield usable quantities of water. Groundwater is a major global source of fresh water, often supplying close to half of the public drinking water in countries like the United States, and is relied upon heavily for agricultural irrigation. Environmental threats impact both the volume and purity of this resource.
Excessive Withdrawal and Depletion
The primary threat to groundwater volume is over-extraction, which occurs when water is pumped out faster than nature can replenish it through rainfall and snowmelt. This imbalance causes the water table to fall progressively lower. When a well actively pumps water, it lowers the water level in a conical shape around the wellbore, creating a “cone of depression.”
If heavy pumping continues, this cone expands and deepens, potentially causing nearby, shallower wells to run dry. The lowering of the water table also impacts surface ecosystems, as many streams, rivers, and wetlands rely on groundwater discharge to maintain flow, especially during dry seasons. Excessive withdrawal can capture water that would have otherwise sustained these features, leading to the drying up of streams and the loss of associated habitats.
Climate change compounds this issue by altering global precipitation patterns, which directly influences aquifer recharge rates. Reduced rainfall and increased evaporation due to warmer temperatures slow the rate at which water seeps back into the ground. This decreased natural replenishment forces users to draw more heavily from existing reserves, accelerating depletion.
Contamination from Human Activities
Groundwater quality is threatened by pollutants introduced through human activities. These contaminants often leach slowly through soil and rock layers, making the pollution plume difficult and expensive to clean once it reaches the aquifer. Contamination sources are broadly categorized by the type of activity that generates the waste.
Agricultural Sources
Agricultural practices represent a significant source of non-point source pollution, spreading contaminants across a wide area. Fertilizers contain nitrogen compounds that transform into highly soluble nitrates, which easily infiltrate the soil and enter the groundwater. Nitrates pose health risks, particularly to infants, and are a common indicator of agricultural impact.
Pesticides and herbicides also percolate down into aquifers, sometimes persisting for years due to slow groundwater movement. Trace amounts of compounds, such as the weed killer atrazine, can be detected in supplies. Animal waste from concentrated feeding operations contributes pathogens, bacteria, and additional nitrates to the subsurface environment.
Industrial and Urban Sources
Industrial and urban activities are major contributors to point source pollution, involving specific sites or localized discharges. Leaky underground storage tanks (USTs), particularly those holding petroleum products like gasoline and diesel, are a common source. Hydrocarbons from these leaks can form a plume that travels with the groundwater flow, rendering the water unusable over a large area.
Other industrial wastes, including solvents, heavy metals (like lead and arsenic), and various organic chemicals, can be spilled or improperly discharged. These substances can mobilize and travel long distances in the aquifer, especially if they are dense non-aqueous phase liquids (DNAPLs) that sink below the water table. Road salts used for de-icing also seep into the ground, increasing the salinity and chloride content of local supplies.
Waste Disposal
Waste disposal sites and septic systems threaten groundwater quality with pathogens and nutrients. Unlined or poorly managed landfills produce a toxic liquid known as leachate, rich in heavy metals, household chemicals, and organic compounds. Effluent from millions of septic systems used in areas without municipal sewer lines can introduce viruses, bacteria, and nitrates into shallow aquifers if poorly maintained. The increasing presence of pharmaceuticals and other trace organic contaminants, often originating from wastewater discharge, represents an emerging challenge.
Naturally Occurring Water Quality Issues
Not all water quality issues result from human-introduced pollution; some threats stem from natural geological or hydrological processes. These contaminants often become problematic when human actions, such as excessive pumping, upset the natural balance of the aquifer system.
Saltwater Intrusion
Saltwater intrusion is a significant threat to coastal aquifers, which naturally contain a boundary between lighter freshwater and denser saltwater. Excessive groundwater pumping lowers the freshwater pressure near the coast, allowing the denser seawater to move inland and upward into the freshwater zone. The resulting increase in salinity makes the water unfit for drinking and irrigation, sometimes forcing the abandonment of wells.
Geogenic Contaminants
Geogenic contaminants are naturally occurring minerals and elements present in the rock and soil that dissolve into the groundwater. Depending on the local geology, elements such as arsenic, radon, and fluoride can be found in concentrations that exceed safe drinking water standards. Arsenic is mobilized from certain sulfide minerals under specific geochemical conditions and is a widespread problem globally.
Other heavy metals and radioisotopes, including uranium and radium, also dissolve into the water through water-rock interactions. While these contaminants are natural, human activities like mining can increase their mobilization by exposing new rock surfaces or altering the local pH, which increases the solubility of these elements.