Brian Barton
While human activity is responsible for much of the groundwater pollution, it can also occur naturally due to the presence of a minor and unwanted constituent, contaminant, or impurity in the groundwater. Some substances found naturally in rocks and soils, such as arsenic, iron, chlorides, sulfates, fluoride, or radionuclides, can become dissolved in groundwater. Other naturally occurring substances, such as decaying organic matter, can move in groundwater as particles.
| Characteristics | Values |
|---|---|
| Natural Sources of Groundwater Pollution | Arsenic, iron, chlorides, sulfates, fluoride, radionuclides, decaying organic matter, radon, boron, radionuclides, calcium, magnesium |
| Arsenic Sources | Aquifer sediments contain organic matter that generates anaerobic conditions in the aquifer, leading to the release of arsenic |
| Natural Processes Leading to Pollution | Weathering, mining and quarrying |
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What You'll Learn
Arsenic in soil and water
Arsenic is a metalloid that can occur naturally in groundwater, particularly in Asia, including China, India, and Bangladesh. It is also found in the Puget Sound soil of western Washington, where the Asarco copper smelting plant was a source of arsenic contamination from 1890 to 1986. The area surrounding the plant is now an EPA superfund site due to serious arsenic and lead contamination.
The natural arsenic pollution in groundwater occurs because aquifer sediments contain organic matter that generates anaerobic conditions. These conditions result in the microbial dissolution of iron oxides in the sediment and the release of arsenic, which is usually strongly bound to these iron oxides, into the water. Inorganic arsenic, the most common type of arsenic in soil and water, is extremely toxic and can be produced as a byproduct of industrial production, such as copper smelting.
Arsenic does not decompose, biodegrade, or move downward through soils. Instead, it remains permanently in the top layers of soil unless removed. The Washington State Department of Ecology has determined that the natural background level of arsenic in Puget Sound soil is 7 ppm (parts per million). For reference, it is recommended to avoid gardening in soils with arsenic in excess of 20 ppm.
If you suspect your soil is contaminated with arsenic, it is important to take steps to reduce exposure, especially for children who may play in the dirt and inadvertently ingest it. Some recommended exposure reduction measures include dampening soils with water before gardening to limit dust inhalation and frequently washing toys, pacifiers, and other items that go into children's mouths.
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Radionuclides in rocks
While human activity is to blame for much of the groundwater pollution today, some natural processes can also contaminate groundwater. Radionuclides, which are radioactive elements, are one such contaminant that occurs naturally in many rocks and minerals.
Uranium, for instance, is a radionuclide that has been present in rocks since the Earth was formed. Uranium can be concentrated in rocks through igneous and sedimentary processes, and certain rocks, such as granites, have higher concentrations of uranium. Uranium ores have been mined and milled to produce nuclear fuels, particularly in the western United States. Uranium-238 is a common trace element in many rock types, and its decay can produce other radionuclides like radon and radium.
Radon is another radionuclide that is frequently found in rocks and soils, usually in small amounts. It is radioactive and decays into polonium, which is also radioactive and can be harmful to human health. Radium, specifically radium-226 and radium-228, are the two most common isotopes of radium and are both carcinogenic. They are produced by the decay of uranium. While uranium and radium concentrations in groundwater depend on the geochemical conditions in an aquifer, their mobility factors differ, resulting in rare coincidences of high concentrations of both in principal aquifers.
The presence of radionuclides in groundwater can pose risks to human health, as some are toxic or carcinogenic. They can accumulate in bones and other tissues, increasing the risk of cancer. Radionuclides are also useful for determining the age of groundwater in an aquifer or the age of sediment at the bottom of a water body. This information can indicate how long the groundwater has been in the aquifer and provide insights into the water's quality.
In addition to radionuclides, other naturally occurring substances can also contaminate groundwater. For example, arsenic, a common contaminant found in soil and water, can be released into groundwater through the microbial dissolution of iron oxides in aquifer sediments. Fluoride, chlorides, sulfates, iron, and decaying organic matter are other natural contaminants that can be found in groundwater.
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Mining and quarrying
The heavy use of water in processing ore and the water pollution from discharged mine effluent and seepage from tailings and waste rock impoundments can also affect groundwater quality. Mine waste rock and tailings, which often contain acid-generating sulphides, heavy metals, and other contaminants, may need to be managed for decades or even centuries after a mine's closure. Acid Mine Drainage (AMD), a natural process where sulphuric acid is produced when sulphides in rocks are exposed to air and water, can severely degrade water quality, kill aquatic life, and make water virtually unusable. AMD can also accelerate the leaching of heavy metals from waste rock.
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Natural gas drilling
While groundwater is generally safer and more reliable than surface water, it can still be contaminated by pollutants from various sources. Natural gas drilling has been identified as a potential source of groundwater pollution, particularly through hydraulic fracturing or "fracking".
Fracking is a process used to extract natural gas from underground reserves by injecting large amounts of water, sand, and chemicals into the ground to crack open the rock formations and release the gas. This process has been associated with several environmental concerns, including groundwater pollution. The high water consumption associated with fracking can impact aquatic habitats and water availability for other uses. Additionally, the chemicals used in fracking fluids can be toxic and contaminate groundwater sources if not properly treated and disposed of.
The U.S. Environmental Protection Agency (EPA) and other organizations have recognized the potential risks associated with natural gas drilling and fracking. In some cases, homeowners near drilling sites have reported methane seeping into their water supplies, with at least one incident resulting in an explosion that tragically killed three people. The EPA has established guidelines to reduce methane and other harmful air pollutants released during natural gas drilling.
To mitigate the environmental impacts of natural gas drilling, several measures can be implemented. These include proper wastewater treatment and disposal, using steel tanks instead of open pits for wastewater, and monitoring for potential leaks or spills. Additionally, horizontal and directional drilling techniques can help reduce the land area disturbed during the drilling process.
While natural gas is often promoted as a cleaner alternative to other fossil fuels, the potential risks to groundwater and drinking water sources cannot be overlooked. It is crucial to balance the development of natural gas resources with the protection of valuable water resources through stringent regulations, monitoring, and cleanup efforts.
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Decay of organic matter
While human activity is responsible for much of the groundwater pollution today, it can also occur naturally. Some substances found naturally in rocks and soils, such as arsenic, iron, chlorides, sulfates, fluoride, or radionuclides, can become dissolved in groundwater.
The decay of organic matter is a natural process that can also contribute to groundwater pollution. This occurs when organic matter, such as plant and animal remains, decomposes and breaks down into simpler compounds. This decomposition is carried out by microorganisms, such as bacteria and fungi, which use the organic matter as a source of energy and nutrients. The rate of decomposition is influenced by factors such as temperature, pH, oxygen availability, and the chemical composition of the organic matter.
In the case of groundwater pollution, organic matter can decay anaerobically, without the presence of oxygen. This process can release pollutants, such as nitrogen and phosphorus compounds, into the surrounding environment, including groundwater. Additionally, the decay of organic matter can also influence the behaviour and mobility of other pollutants in the soil and water. For example, organic matter can adsorb and bind to certain pollutants, affecting their solubility and movement in the groundwater system.
The decomposition of organic matter can also be influenced by the presence of other substances in the environment. For instance, in a study of a shallow groundwater system in northern Poland, the decay of organic matter was found to be strongly correlated with the concentration of Fe2+, suggesting that the "iron index" may be a useful indicator of organic matter decay intensity in such systems.
Furthermore, the decay of organic matter can have complex interactions with pollutants. In some cases, organic matter can enhance the dissipation of certain pollutants, such as triazines, in groundwater. However, if the long-term persistence of pollutants in the organic matter decay process is not adequately addressed, it can lead to an underestimation of the pollution levels and subsequent environmental impact.
While the decay of organic matter is a natural process, human activities can accelerate or alter this process, leading to potential groundwater pollution. For example, the application of organic fertilisers in agriculture can introduce organic matter that affects the behaviour and mobility of pollutants in the groundwater system.
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Frequently asked questions
Groundwater pollution, also known as groundwater contamination, occurs when pollutants are released into the ground and make their way into groundwater.
Naturally-occurring sources of groundwater pollution include arsenic, iron, chlorides, sulfates, fluoride, radionuclides, radon, and boron.
Arsenic occurs naturally in some soils and rocks. Arsenic is released into the water when aquifer sediments contain organic matter that generates anaerobic conditions, resulting in the microbial dissolution of iron oxides in the sediment and the release of arsenic, which is normally strongly bound to iron oxides.
Fluoride can become dissolved in groundwater and is found in rock-dominant semi-arid regions.
Mining and quarrying can release pollutants that were previously trapped in rocks into surrounding underground water sources.
