Aquifer Pollution: Understanding The Contamination Risk

Aquifers are underground reservoirs that store almost all of the planet's liquid freshwater. They are a crucial source of drinking water for billions of people worldwide. However, aquifers are vulnerable to pollution from various sources, which can render them unsafe for human consumption and harm wildlife and plants. Some of the main ways in which aquifers can become polluted include surface contamination, where pollutants such as agricultural pesticides and fertilizers, oils, and salts from roadways are carried by surface water into the groundwater; subsurface contamination, where corroded underground storage tanks leak their contents into an aquifer; and landfill and waste disposal issues, where runoff and leaching carry pollutants from dumpsites and landfills into nearby water sources. Atmospheric contamination, where pollutants released into the atmosphere eventually return to Earth through precipitation, and saltwater contamination, where overdrawing of groundwater causes saltwater to flow into and contaminate freshwater sources, are also significant contributors to aquifer pollution.

Landfills and waste disposal

The liners used in modern landfills are designed to prevent leachate from escaping. However, these liners are not always effective, and even small defects can result in leaks. Over time, liners can also degrade, tear, or crack, allowing contaminants to migrate into the surrounding soil and groundwater.

The impact of landfills on groundwater can be mitigated by setting an isolation distance between the landfill and nearby drinking water wells. This distance varies depending on the type of aquifer and the hydrogeological parameters such as groundwater gradient and hydraulic conductivity. Isolation distances can range from 106 metres to over 50 kilometres.

The construction of landfills near residential areas can have adverse effects, including the accumulation of gases, groundwater contamination, and property destruction. In rural areas, landfills may have less impact on the quality of life but can still affect property values.

To minimise the risk of groundwater contamination, proper waste disposal and burial methods are essential. Open dumping should be avoided, and controlled landfills should be used instead. Regular monitoring of landfills and the surrounding environment is also necessary to detect and address any leaks or contamination issues promptly.

Atmospheric contamination

Atmospheric contaminants are another source of aquifer pollution. As groundwater is part of the hydrologic cycle, contaminants in other parts of the cycle, such as the atmosphere, can eventually be transferred into groundwater supplies.

The atmosphere can contain a wide range of physical, inorganic chemical, organic chemical, bacteriological, and radioactive contaminants. These contaminants can enter the atmosphere through human activities such as industrial processes, agricultural practices, and the burning of fossil fuels. Once in the atmosphere, these contaminants can be transported over long distances and deposited onto the land surface through precipitation or dry deposition.

As precipitation infiltrates the soil, it can carry these atmospheric contaminants into the groundwater. Atmospheric deposition can also impact surface water bodies, such as lakes and rivers, which are fed by groundwater. This means that atmospheric pollution can indirectly affect groundwater resources by contaminating the surface water that interacts with aquifers.

Additionally, atmospheric contaminants can also be released directly into the groundwater through leakage or spills from underground storage tanks, pipelines, or waste disposal sites. Human activities, such as industrial processes and waste management, can result in the release of toxic chemicals and pollutants that can eventually find their way into aquifers.

Furthermore, natural processes, such as volcanic eruptions and forest fires, can also release contaminants into the atmosphere, which can then affect groundwater quality. While natural contamination occurs less frequently than human-induced pollution, it can still have significant impacts on aquifer pollution.

To prevent atmospheric contamination of aquifers, it is essential to implement measures such as reducing emissions from industrial facilities, improving waste management practices, and protecting natural areas that contribute to groundwater recharge. Regular monitoring of groundwater quality is also crucial to detect any potential atmospheric contamination and take appropriate remedial actions.

Saltwater contamination

There are several factors that contribute to saltwater contamination:

• Recharge by irrigation water and contaminated surface water: While this typically leads to modest increases in salinity, it can still impact groundwater quality.
• Contamination by oil-field brine and salt-mine waste: These activities can result in highly concentrated salt contamination, although it tends to be localized.
• Natural sources: Saltwater contamination can occur naturally due to salt- and saltwater-bearing bedrock formations. In some cases, freshwater aquifers are separated from saltwater-containing aquifers by confining beds, which are clay or bedrock layers that slow down or prevent vertical water movement. However, when these confining beds are absent or compromised, saltwater may leak upward and contaminate the freshwater aquifer.
• Groundwater extraction: Removing groundwater from coastal freshwater wells is a primary cause of saltwater intrusion. As water is extracted, it lowers the level of the freshwater table, reducing the pressure exerted by the freshwater column. This allows the denser saltwater to move inland laterally. This has been observed in locations like Cape May, New Jersey, where groundwater extraction has lowered water tables and resulted in the contamination of water supply wells.
• Canals and drainage networks: The construction of canals and drainage networks can facilitate saltwater intrusion by providing conduits for saltwater to move inland. This has been observed in the Sabine Lake Estuary in the Gulf of Mexico, where large-scale waterways have enabled saltwater intrusion. Additionally, drainage networks constructed to drain flat coastal areas can lower the freshwater table, reducing water pressure and allowing saltwater to move inland.
• Sea level rise and climate change: The rising sea levels caused by climate change contribute to saltwater intrusion in coastal areas.
• Extreme events: Natural disasters like hurricane storm surges can exacerbate the problem of saltwater intrusion.

Surface contamination

Agricultural practices, such as the use of pesticides and fertilizers, contribute to surface contamination. These chemicals can accumulate in the soil and eventually reach the groundwater, impacting both human and wildlife health. Similarly, oils and salts from roadways can also seep into the groundwater through surface runoff.

Above-ground storage tanks and pipelines for petroleum products and other chemicals pose another risk for surface contamination. Leaks or breaks in these structures can release their contents, allowing them to seep into aquifers. Additionally, mine tailings and industrial waste stored in surface pits can leach toxic substances into the groundwater.

The impact of surface contamination on aquifers is particularly concerning due to the slow rate of groundwater renewal. Unlike surface water, which renews every 20 days on average, groundwater renewal takes approximately 1,400 years. As a result, pollutants in aquifers continue to accumulate, turning them into sinks for contamination.

Furthermore, surface contamination can also occur through atmospheric deposition. Pollutants released into the atmosphere eventually return to the earth through precipitation, finding their way into surface water sources and subsequently contaminating groundwater supplies.

Subsurface contamination

In addition, landfills and waste disposal sites can also cause subsurface contamination. While modern landfills that meet federal standards are designed to prevent runoff from leaching into the groundwater, older dumps and landfills did not have these protections. Improperly discarded hazardous wastes, such as those buried or placed in open pits, or just dumped on the land, can filter through the soil and find their way into the groundwater.

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