Groundwater is the water held underground in the cracks, spaces, and pores of soil and rock formations beneath the Earth’s surface. These saturated zones, known as aquifers, act as vast underground reservoirs that supply a significant portion of the world’s drinking water, particularly in rural areas. Aquifers are naturally recharged by rain and melting snow that slowly seeps downward. Once a pollutant enters this system, it is often difficult and extremely expensive to remove because the water moves slowly, allowing contaminants to remain concentrated for long periods. Preventing contamination is vastly more effective and cheaper than attempting complex remediation efforts.
Agricultural Runoff and Chemical Contamination
Farming activities represent a widespread non-point source of pollution. The heavy use of synthetic fertilizers and animal manure introduces substantial quantities of nitrogen and phosphorus. Excess nitrogen converts to highly soluble nitrate, which is easily carried downward through the soil. This process, called leaching, is accelerated by irrigation and heavy rainfall, allowing the nitrate to bypass the soil’s natural filtering mechanisms and infiltrate the aquifer.
Nitrate is the most common chemical contaminant found in groundwater, posing a serious public health risk. Elevated nitrate levels can lead to methemoglobinemia, often called “blue baby syndrome,” where an infant’s blood loses its capacity to carry oxygen. Pesticides and herbicides, such as atrazine, are also applied to fields and can leach into the groundwater, even decades after being banned due to persistence in the environment. These toxic chemicals can affect human health at very low concentrations.
Onsite Wastewater Treatment Systems
Decentralized sewage disposal systems, such as septic tanks, are a significant source of localized groundwater pollution, particularly outside of urban areas. A typical septic system collects household wastewater in a tank, where solids settle, and the liquid effluent flows into a drain field. This effluent is intended to be treated by soil filtration as it percolates through the unsaturated zone before reaching the water table.
System failure, poor maintenance, or high-density placement can overwhelm the soil’s capacity to purify the water, leading to contamination. Key contaminants include pathogens, such as bacteria and viruses, which cause waterborne diseases. The effluent also carries high concentrations of nutrients like nitrogen and phosphorus, along with household chemicals and pharmaceuticals. When these substances enter the groundwater, they compromise water potability and can cause environmental harm.
Leaking Underground Storage Tanks
Underground Storage Tanks (USTs) are large containers used to store commercial products, most often petroleum products like gasoline, diesel, and heating oil at service stations. Corrosion of older steel tanks, faulty installation, or poor piping connections can lead to leaks, creating a highly concentrated, point-source contamination event. Because these leaks occur directly below ground, contaminants are introduced close to the water table, bypassing the upper soil layer.
The most concerning pollutants are volatile organic compounds (VOCs), especially the BTEX group: benzene, toluene, ethylbenzene, and xylene. Benzene is a known human carcinogen, and these compounds are highly toxic even in small amounts. Due to their low solubility and persistence, these petroleum components form dense, long-lasting contamination plumes that are difficult and costly to track and remediate.
Hazardous Waste and Landfill Leachate
Landfills become a source of groundwater contamination when rainwater filters through buried refuse, creating a toxic liquid known as leachate. This complex liquid contains substances including heavy metals like lead and arsenic, industrial solvents, and various toxic chemicals. Older, unlined municipal landfills pose a significant threat because leachate can seep directly into the ground and migrate toward the nearest aquifer.
Modern, regulated landfills are constructed with thick, impermeable liners and sophisticated collection systems to capture and treat leachate before it enters the subsurface. Even with these safeguards, liners can eventually fail or be compromised, allowing the hazardous liquid to escape. The complexity of the chemicals in landfill leachate makes it a persistent and challenging source of long-term groundwater pollution.
Reducing the Risk of Groundwater Pollution
Mitigating the risk of groundwater pollution requires a combination of regulatory measures and responsible individual actions.
Agricultural Practices
In agricultural settings, implementing best management practices involves precision farming techniques to optimize fertilizer application. This ensures chemicals are used only in necessary amounts, minimizing excess nitrate available to leach into the water table.
Household and Septic Management
Individuals can make a difference by properly disposing of household hazardous waste, such as old paints, motor oils, and cleaning agents, rather than dumping them. For homes using septic systems, regular maintenance, including periodic pumping and inspecting the drain field, is necessary to ensure the system functions correctly.
Regulatory Oversight
Strict regulatory oversight and consistent monitoring of underground storage tanks are required to promptly detect and address any leaks before a major plume of contamination can form.