Freshwater and Groundwater Contamination Around the World

The pollution of freshwater and groundwater is of critical environmental concern. The problem of pollution in drinking water is varied and diverse, with issues ranging from toxic metals and pesticides to bacteria growth and thermal pollution. Interconnected water systems allow for the free movement of pollution and for wide-ranging damage to drinking supplies and ecosystems.

Introduction

Earth’s water cycle connects the water of the world through a vast and widespread system. Because of this interconnection, pollution in a single area can have far-reaching consequences. Just as water systems are connected in nature, so too are issues of water pollution.

Agricultural Runoff

Agricultural runoff occurs when water, such as heavy rain, flows through farmland and washes fertilizers, soil, and pesticides into watersheds, those areas that drain into bodies of water, such as rivers, lakes, and streams. A major concern associated with agricultural runoff is the harmful effects of fertilizers.

Fertilizers, which are compounds used to replace nutrients in the soil for the growing of plants, are rich in the elements nitrogen, oxygen, and phosphorus; the most common combinations of these atoms in fertilizes are nitrates (NO₃^-) and phosphates (PO₄^-3). Nitrates and phosphates are both anions that are extremely effective at replenishing nutrients in the soil. The same properties that make these compounds effective fertilizers also make them potential environmental hazards when improperly managed.

Improper management of fertilizers includes over-usage of fertilizers and the application of fertilizers in areas close to bodies of water. Both phosphates and nitrates are negatively charged ions, so they are highly water-soluble; they easily dissolve in even small amounts of water runoff. The water runoff containing nitrates and phosphates can then be introduced into watersheds, where these anions undergo chemical reactions and impact ecosystems.

The introduction of excess phosphate into a watershed can significantly alter aquatic ecosystems and reduce the quality of surface waters. The presence of excess phosphate can lead to eutrophicationa build-up of minerals that leads to an increase in microorganism and plant growthand a significant increase in algae growth. A by-product of algae growth is an increase in the amount of organic matter and sediment in surface water. Excess sediment can cloud the water and reduce the amount of sunlight that reaches aquatic plants, causing the metabolic pathways of plants to slow, potentially leading to death. Sediment also can clog the gills of fish or smother fish larvae, further compromising ecosystems.

Additionally, the eruption of algae from eutrophication can cause further oxygen depletion in water systems where algae decrease the oxygen dissolved. Fish and other aquatic fauna are particularly sensitive to oxygen depletion in rivers and streams. Algae blooms can also impede recreational opportunities and create a foul taste and odor in drinking water. Small increases in the level of phosphates can be detrimental to water systems and can potentially lead to eutrophication, which alters the life cycle of aquatic organisms and harms water quality.

Unlike phosphates, nitrates do not have the same immediate effect upon water quality and aquatic ecosystems. However, nitrates have a much further reach of environmental effects because of their chemical reactivity. One of the direct effects of nitrates in the water supply is the potential for methemoglobinemia, or blue baby syndrome. Methemoglobinemia is a potentially fatal disease in infants caused by the ingestion of large quantities of nitrates. The disease causes oxygen deprivation to vital tissues, such as the brain.

Well water and drinking water obtained from sources close to farming areas are the most common water sources affected. Pregnant women, adults with reduced stomach acidity, and people deficient in the enzyme that returns methemoglobin to normal hemoglobin are all susceptible to nitrite-induced methemoglobinemia.

Nitrates are in a category of chemicals called oxidizers, and nitrates are considered to be strong oxidizers. As a function of their reactivity, nitrates can have a wide range of environmental effects, particularly when interacting with toxic metals.

Heavy Metals

Metals in ores and minerals provide material for a wide array of products and industrial processes. However, when mines are abandoned or left unchecked, issues with chemical leaching frequently arise, as metals can find their way into and contaminate water systems.

Some of these metals are necessary for human metabolic processes, but humans need only a small amount of these metals, including cobalt, copper, chromium, manganese, and nickel. These and many other heavy metals are toxic when present in drinking water or food. The metals attack specific bodily systems and organs. For example, the central nervous system is impaired by manganese, mercury, lead, and arsenic, while mercury, lead, cadmium, and copper affect the kidneys and liver. Nickel, cadmium, copper, and chromium affect skin, bones, and teeth.

The damage to metabolic processes is not limited to animals; plants and ecosystems can also be harmed by constant exposure to heavy metals in freshwater. In both animals and plants, metals can build up through bioaccumulation and can be passed up the food chain. Unlike organic toxins such as pesticides, which can degrade over time, heavy metal toxicity remains present as long as the metal is present. Therefore, heavy metal remediation—involving the removal of heavy metal pollution and contaminants from a given environment—poses a daunting challenge because of the complex chemistry of the metals and because of the size of the water system in which they are dissolved.

A growing concern is that of the synergistic effects of different types of pollution. Chemical leaching of ground-state heavy metals (metals with an oxidation state of 0) into water systems is typically a slow process. Nitrates from agricultural runoff can often exacerbate and accelerate chemical leaching by oxidizing toxic metals and making the metals more water soluble. As a result, the oxidized metals are more readily dissolved into water systems and drinking water.

Heavy metal pollution can also be introduced to water systems in industrial processes, such as the smelting of copper and the preparation of metals as nuclear fuels; heavy metals can also be introduced into water systems through coal-fired power plants. Additional chemical leaching issues associated with heavy metals from mines include radioactive heavy metals in groundwater and arsenic poisoning of livestock from mine runoff in freshwater.

Mercury poses a unique hazard among the heavy metals in its ability to form organomercury compounds, such as methylmercury and dimethylmercury, which are more toxic than ground-state mercury. These more toxic forms of mercury are often found in fish and shellfish because of mercury in water systems. The organomercury compounds can work their way up the food chain through bioaccumulation and, ultimately, can appear in larger fish and become part of the human food supply.

Acid Rain and Acid Runoff

Just as toxic metals can be chemically leached into watersheds from both abandoned and active mines, it is possible for ores and minerals containing sulfides to be leached into water systems. The sulfides that are leached into the water supply are then oxidized to form acidic compounds and acid runoff. The acid runoff produced from the sulfides causes a lowering of the pH in rivers, lakes, and streams, making water systems uninhabitable for native flora and fauna.

A prime example of how acid runoff can devastate a water system is that of the Ocoee River in the southern Appalachian Mountains of the United States. The Ocoee was polluted for one hundred years by acid runoff from a massive copper mine in Copperhill, Tennessee. Both copper metal and acidic runoff from the copper minerals would flow into the river from chemical leaching by rainwater and snowmelt.

A project administered by the U.S. Environmental Protection Agency initiated measures to treat the water runoff flowing into the Ocoee River by remediation of the copper and by neutralization of the acid runoff. This was achieved through the use of lime (a basic mineral containing calcium), wherein the water runoff was treated in holding areas. After being treated with lime, the metals settle out from the pH neutral water and then clean water is released into the river. After ten years of environmental cleanup, aquatic life began to return to the river near the mine.

A similar process gives rise to acid rain. Coal-burning power plants can produce oxidized sulfides from the coal they burn, resulting in these acidic compounds being driven into the atmosphere, where they become part of the water cycle. As part of the water cycle, these compounds form acid rain. Acid rain, like acid runoff, increases rates of erosion and has toxic effects upon ecosystems and water quality. Acid rain, however, has a much larger area of impact, and because of its low pH, it can facilitate chemical leaching of heavy metals. As precipitation, acid rain readily makes its way into rivers, lakes, and streams, where it has the same negative impact as acid runoff on drinking water quality and on native flora and fauna.

Pesticides

Pesticides are a class of chemical compounds that can repel, mitigate, or kill pests. They are often associated with agriculture or silviculture and are used to safeguard crops from invading species such as insects, weeds, and rodents.

In many developed nations, agencies regulate the sale and, in some cases, the use of pesticides. Specific pesticides, such as insecticides, fungicides, and rodenticides, require specific attention, as many of these compounds are toxic to humans. LD₅₀ values are used to describe the effectiveness of a particular pesticide so that a tangible measure can be used to assess toxicity.

There are two types of toxicity with regard to chemical compounds. The first type is acute toxicity, in which detrimental health occurs from a large dose of chemicals in a short time. The second type of toxicity is chronic toxicity, in which adverse health effects occur from continued exposure to smaller chemical doses over a prolonged period of time. The LD₅₀ values help to quantify toxicity in various organisms by considering the variables that affect toxicity.

With respect to pesticides in water and water systems, chronic toxicity is a much more common problem than is acute toxicity. Pesticides can be introduced to streams, lakes, and rivers through improper or excessive application. This can be exacerbated when heavy rains wash pesticides into watersheds. Pesticides and other chemicals that have been introduced to a stream are then deposited downstream and collect in reservoirs. Pollutants that are dissolved in reservoirs can be concentrated if evaporation rates are high, resulting in a higher level of exposure to toxic pesticides.

In developed nations, instances of poisoning from pesticides in drinking water are fairly rare; however, in developing nations, poisoning from pesticides in drinking water is more common because of overuse. Many developing countries do not regulate the sale or use of pesticides and have higher concentrations of toxic pesticides in their water supplies. These higher levels can cause problems with bioaccumulation, in which aquatic species, game animals, and livestock build up large amounts of pesticides in their tissue. This accumulation of pesticides not only has detrimental effects on the health of animals; it also has serious effects on animals that are higher up in the food chain.

Thermal Pollution

The temperature of freshwater is a physical property that directly influences the quality of water. Thermal pollution of water can be detrimental to agriculture and to the quality of drinking water. This type of water pollution occurs when water is introduced into another body of water at a temperature that is significantly above or below the body of water in question.

Of particular concern is when incoming water is significantly above the temperature of the accepting body of water, as is the case with some rivers, lakes, and streams. Serious issues then arise with land and water usage, leading to complications with naturally occurring flora and fauna near the affected body of water.

Coal-burning power plants and other industrial complexes utilize water as a cooling resource for various industrial processes. These facilities pull in large volumes of water from lakes, rivers, estuaries, and oceans to cool their machinery, and they then release water at an elevated temperature. Water at elevated temperatures can also arise from geothermal vents, wherein the energy from the earth’s core warms subterranean water supplies. This heated water can eventually rise to the surface and can mix with freshwater and groundwater.

Issues associated with heated thermal pollution most often concern water usage. For example, water that is above 25 degrees Celsius (77 degrees Fahrenheit) is not suitable for human consumption because it is considered a heated resource. Additionally, water with a temperature of more than 35 degrees Celsius (95 degrees Fahrenheit) can be used only for irrigation (with caution). Water that has an elevated temperature also is likely to have a higher concentration of metal cations, such as boron and lithium, and anions, such as phosphates.

Additionally, there exists an inverse correlation between water temperature and dissolved oxygen. As the temperature of water rises, a smaller amount of oxygen is dissolved in the water; as a result, less oxygen is available for aquatic flora and fauna. Elevated water temperatures typically increase biological activity, resulting in greater oxygen needs for animals and causing a strain upon the ecosystems. This type of scenario can result in one species replacing another. For example, a species that favors warm water, such as the walleye fish, might replace a species that favors cold water, such as trout.

The natural ecosystem is affected in a variety of ways by thermal pollution in that native flora and fauna can become environmentally strained from increased biological activity and decreased oxygen supply. In extreme cases, some native species are replaced by invading species that favor warmer temperatures. Furthermore, water intake from industrial complexes also traps and kills fish due to the intense rate at which water can be pulled from a body of water, thereby reducing native fish populations.

Bacteriological Contamination

Bacteriologic factors are frequently considered in determining if water is of satisfactory quality for drinking or recreation. A number of dangerous bacteria, including coliform, fecal streptococcus, and fecal coliform, can arise in freshwater streams. Each of these bacteria carries its own risks to water quality, especially making water unsafe for drinking.

If bacterial levels exceed certain concentrations, then drinking-water quality can be compromised and result in infection. Many of these bacteria are associated with the introduction of organic matter, including fecal matter and decomposing flesh, into rivers, lakes, and streams.

The principal sources of fecal coliform that can lead to high pollution levels include domestic livestock, game animals, and humans. The presence of these mammals in riparian areas serves as a vehicle for the introduction of bacteria; surface runoff from heavy rains, snowmelt, and irrigation outwash can deposit bacteria into groundwater. Studies have shown that riparian systems with grazing livestock nearby are five to ten times as likely to have increased fecal coliform than are riparian systems with no grazing livestock. These increases in fecal coliform generally exceed drinking water standards.

Bacteria counts of this type also can be tied to seasonal trends, as heavy rains and snowmelt can play a large role in the introduction of bacteria to a water system. A general trend is seen in which spring and summer periods have a higher level of bacteria incidence, while winter periods show a time of low bacteria activity.

A unique issue of water quality arises in India. Millions of liters of untreated human sewage are introduced to the Ganges River each day. This leads to large concentrations of fecal coliform bacteria and results in a fecal coliform count that is ten times greater than the acceptable amount for water used for bathing, much less drinking. This gives rise to an unusual, specific issue: The Ganges is a holy river in Hinduism. It plays a number of different roles in Hindu culture, including the ritualistic cleansing and bathing that occur daily for many Indians. The constant pollution of the Ganges exposes many people to dangerous levels of bacteria and does so as a function of their religious beliefs. This presents a unique situation in which drinking water, water pollution, epidemiology, and religion collide.

Saltwater Intrusion

Naturally occurring salt can intrude upon and contaminate both surface and groundwater supplies of freshwater. This can have dangerous consequences as different aquatic and semiaquatic organisms are adapted for life in freshwater, saltwater, or brackish water, and so are sensitive to changes in salt concentration, or salinity. Moreover, consumption of large amounts of saltwater can cause lethal dehydration in humans, among other terrestrial organisms.

Normally, freshwater bodies like rivers and streams flow into saltwater bodies such as seas and oceans, and the zone where these meet consists of brackish, or slightly salty, water. In certain coastal areas, fresh groundwater floats above saline groundwater as well. However, saltwater can contaminate freshwater when lowered levels of fresh groundwater, whether from overpumping or droughts, pull saltwater into the aquifer. Saltwater contamination can also happen when saline groundwater wells up and when seawater encroaches (for instance, because of seawater warming and expanding, causing sea-level rise). The geologic characteristics of a given area affect where and how far the saltwater intrusion may extend and how damaging it is to water quality.

Summary

Issues with heavy metals and pesticides can lead to the immediate contamination of drinking water, rendering a resource unusable because of the high toxicity of the pollutants. Other types of contamination, such as thermal pollution, acid rain, and acid runoff, have a chronic effect upon water quality, with repeated exposures harming ecosystems.

Agricultural runoff can have a synergistic effect upon other pollutants through chemical leaching and can degrade water quality through eutrophication. Bacteriologic contamination arises from the abuse of riparian areas and saltwater intrusion from human activities such as freshwater pumping, potentially leading to extreme health hazards from the consumption of contaminated water. All of these issues affect drinkable water throughout the globe and require constant monitoring and vigilance to protect Earth’s freshwater supply.

Most of the world's groundwater is freshwater, and pollution of both sources has been rising over the twentieth and twenty-first centuries. Researchers and scientists consider the pollution of groundwater one of the most crucial environmental concerns of modern times. However, solutions are possible with international collaboration and attention.

Principal Terms

acute toxicity: the effect of a toxic agent on an organism that manifests from a large dose in a short time

bioaccumulation: the accumulation of substances, such as pesticides and metals, in an organism

chemical leaching: the process of extracting a substance from a solid by dissolving it in a liquid

chronic toxicity: the effect of a toxic agent on an organism that manifests from long-term, repeated exposure to a substance

epidemiology: the evaluation and study of public health as it relates to the effects of environment, choice, and risk factors

eutrophication: process of nutrient enrichment leading to dense algae growth

ground state metal: metal in its elemental form with an oxidation state of zero

LD50: median lethal dose (LD) to 50 percent of subjects in a toxicity study

remediation: the removal of pollution or contaminants from environmental systems

riparian: transitional zones between terrestrial and aquatic systems that exhibit characteristics of both systems

silviculture: the practice of controlling the establishment, growth, health, and quality of forests

water cycle: the continuous movement of water above, below, and on the surface of the earth

watershed: the region draining into a river, lake, stream, or other body of water

Bibliography

Abraham, Wolf-Rainer. “Megacities as Sources for Pathogenic Bacteria in Rivers and Their Fate Downstream.” International Journal of Microbiology, 2011. doi.org/10.1155/2011/798292. Accessed 15 Apr. 2023.

Brooks, Kenneth N. Hydrology and the Management of Watersheds. Iowa State UP, 1997.

"Climate Adaptation and Saltwater Intrusion." US Environmental Protection Agency, 5 July 2022, www.epa.gov/arc-x/climate-adaptation-and-saltwater-intrusion. Accessed 15 Apr. 2023.

Krešic, Neven. Groundwater Resources: Sustainability, Management, and Restoration. McGraw-Hill, 2009.

Li, Peiyue, et al. “Sources and Consequences of Groundwater Contamination.” Archives of Environmental Contamination and Toxicology vol. 80, no. 1, 2021, pp. 1-10. doi:10.1007/s00244-020-00805-z. Accessed 1 Aug. 2024.

Machtinger, Erika T. Riparian Systems. Natural Resources Conservation Service, 2007.

Mather, John R. Groundwater Contaminants and Their Migrations. Geological Society, 1998.

Pauwels, H. et al. “The Combined Effect of Abandoned Mines and Agriculture on Groundwater Chemistry.” Journal of Contaminant Hydrology, vol. 115, 2010, pp. 64-78. doi.org/10.1016/j.jconhyd.2010.04.003. Accessed 15 Apr. 2023.

"Seawater Intrusion." California Water Science Center, US Geological Survey, US Dept. of the Interior, 13 Aug. 2018, ca.water.usgs.gov/sustainable-groundwater-management/seawater-intrusion-california.html. Accessed 15 Apr. 2023.

U.S. Environmental Protection Agency.Protecting Water Quality from Agricultural Runoff: Clean Water Is Everybody’s Business. Author, 2005. nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P10039OH.TXT. Accessed 15 Apr. 2023.