Module #42

Heavy Metals and Other Chemicals

Module Introduction:

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• We have seen that some compounds such as nitrogen and phosphorus cause environmental problems by over-fertilizing the water.
• Other inorganic compounds, including heavy metals (lead, arsenic, and mercury), acids, and synthetic compounds (pesticides, pharmaceuticals, and hormones), can directly harm humans and other organisms.
• In this module, we will examine how each of these chemicals enters water bodies and the effects that they can have on ecosystems and humans.

Module 42: Heavy Metals and Other Chemicals

Essential Knowledge

8.1 Sources of Pollution (Modules 41, 42, 43, 57)

• A point source refers to a single, identifiable source of a pollutant, such as a smokestack or waste discharge pipe.
• Nonpoint sources of pollution are diffused and can therefore be difficult to identify, such as pesticide spraying or urban runoff.

8.3 Endocrine Disruptors (Module 42)

• Endocrine disruptors are chemicals that can interfere with the endocrine system of animals.
• Endocrine disruptors can lead to birth defects, developmental disorders, and gender imbalances in fish and other species.

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Essential Knowledge

8.2 Human Impacts on Ecosystems (Modules 25, 41-44, 51-54, 57, 60)

• Organisms have a range of tolerance for various pollutants. Organisms have an optimum range for each factor where they can maintain homeostasis. Outside of this range, organisms may experience physiological stress, limited growth, reduced reproduction, and in extreme cases, death.
• Coral reefs have been suffering damage due to a variety of factors, including increasing ocean temperature, sediment runoff, and destructive fishing practices.
• Oil spills in marine waters cause organisms to die from the hydrocarbons in oil. Oil that floats on the surface of water can coat the feathers of birds and fur of marine mammals. Some components of oil sink to the ocean floor, killing some bottom-dwelling organisms.
• Oil that washes up on the beach can have economic consequences on the fishing and tourism industries.

Essential Knowledge

8.2 Human Impacts on Ecosystems (Continued)

• Oceanic dead zones are areas of low oxygen in the world’s oceans caused by increased nutrient pollution.
• An oxygen sag curve is a plot of dissolved oxygen levels versus the distance from a source of pollution, usually excess nutrients and biological refuse.
• Heavy metals used for industry, especially mining and burning of fossil fuels, can reach the groundwater, impacting the drinking water supply.
• Litter that reaches aquatic ecosystems, besides being unsightly, can create intestinal blockage and choking hazards for wildlife and introduce toxic substances to the food chain.
• Increased sediment in waterways can reduce light infiltration, which can affect primary producers and visual predators. Sediment can also settle, disrupting habitats.
• When elemental sources of mercury enter aquatic environments, bacteria in the water convert it to highly toxic methylmercury.

Heavy Metals are Highly Toxic

Lead

• Lead contaminates water bodies through lead lined pipes,
• Lead is a soft, malleable metal that has been used for a variety of purposes including lead pipes and as an additive in paint and gasoline.
• Fetuses and infants are most sensitive to lead (teratogen) and exposure can damage the brain, nervous system and kidneys (neurotoxin).
• The combustion of coal produces a number of air pollutants including SO_x, NO_x, particulates, CO₂ and multiple heavy metals including mercury and lead.

Children are generally more vulnerable to toxins and pollutants. Their immune systems and livers are not fully developed, their bodies are still maturing and their smaller body size effectively increases the concentration relative to an adult.

Arsenic

• Arsenic is a carcinogen that can cause cancer of the skin, lung, kidney and bladder.
• Arsenic is typically found dissolved in groundwater and fracking can exacerbate this issue.
• The highest concentrations of arsenic are generally found in the upper Midwest and the West.

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Carcinogen: a chemical that causes cancer

Mercury

• Mercury is a neurotoxin that damages the central nervous system and increases in toxicity as it moves up the food chain (aka. biomagnification).
• Elemental, inorganic mercury is released into the environment, where microorganisms convert it into methylmercury, an organic form that can bioaccumulate in living organisms like fish.
• Figure Right: Mercury emissions from human activities vary greatly among regions of the world.

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Neurotoxin: a chemical that disrupts the nervous system of animals.

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Biomagnification: The increase in chemical concentration in animal tissues as the chemical moves up the food chain.

Chemical Pollution: Acid Deposition�

• Acid deposition and acid mine drainage affect both terrestrial and aquatic ecosystems.
• Acid deposition occurs when burning coal and volcanic activity releases sulfur dioxide and nitrogen dioxide into the air.
• In the atmosphere, these chemicals are converted to sulfuric acid and nitric acid, which fall back to Earth as acid deposition. 
• Sulfur dioxide can also block incoming solar radiation, providing a climate cooling effect such as the Mount Tambora eruption in 1815 (“Year Without a Summer”).
• Acid deposition reduces the pH of water bodies to levels that are lethal to many organisms. 
• Many coal-burning facilities have installed coal scrubbers to combat this problem.

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Acid deposition: Acids deposited on Earth as rain and snow or as gases and particles that attach to the surfaces of plants, soil, and water.

Impacts of Acid Deposition in Aquatic Environments

• As it flows through the soil, acidic rainwater can leach aluminum from soil clay particles and then flow into streams and lakes. The more acid that is introduced to the ecosystem, the more aluminum is released.
• Some types of plants and animals are able to tolerate acidic waters and moderate amounts of aluminum. Others, however, are sensitive and will be lost as the pH declines and they are exposed to aluminum.
• Generally, the young of most species are more sensitive to environmental conditions than adults. At pH 5, most fish eggs cannot hatch.

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Impacts of Acid Deposition in Terrestrial Environments

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• Dead or dying trees are a common sight in areas affected by acid rain. Acid rain leaches aluminum from the soil. That aluminum may be harmful to plants as well as animals. Acid rain also removes minerals and nutrients (e.g. NPK cations) from the soil that trees need to grow.
• At high elevations, acidic fog and clouds might strip nutrients from trees’ foliage, leaving them with brown or dead leaves and needles. The trees are then less able to absorb sunlight, which makes them weak and less able to withstand freezing temperatures.

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Chemical Pollution:

Synthetic Compounds

• Synthetic or human-made compounds can enter the water supply from industrial point sources or from nonpoint sources when they are applied over large areas. 
• These compounds include: pesticides, pharmaceuticals, military compounds, and industrial compounds.
• Synthetic organic compounds can be toxic, cause genetic defects, and interfere with growth and sexual development.

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Chemical Pollution: Pesticides

• Pesticides serve an important role in helping to control pest organisms that pose a threat to crop production and human health.
• However, pesticides can have unintended impacts on non-target species including other pests as well as non-pest species.
• For example, DDT—designed to kill mosquitoes—can move up an aquatic food chain to birds that consume fish. Bald eagles that consumed DDT-contaminated fish produced eggs with thinner shells that broke too easily and the population crashed. After the United States ban DDT in 1972, the bald eagle and other birds of prey increased in numbers. NJ removed bald eagles from it's endangered species list in 2025.

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Chemical Pollution:

Pharmaceutical Drugs and Hormones

• Many pharmaceuticals act as endocrine disruptors that interfere with hormones in the body (e.g. atrazine).
• Atrazine is a pesticide (herbicide) that also acts as an endocrine disruptor, causing demasculinization and feminization in male African Clawed frogs resulting in reduced breeding ability and the potential for population crashes.
• Figure Right: Streams contain a wide variety of chemicals including pharmaceutical drugs and hormones. These come from a combination of wastewater inputs, agriculture, forestry, and industry.

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Chemical Pollution:

Perchlorates

• Perchlorates sometimes contaminate the soil in regions of the world where military rockets are manufactured, tested, or dismantled.
• It used to be common for manufacturers in the United States to dump industrial compounds directly into bodies of water.
• Human exposure to perchlorates comes primarily through contaminated food and water.
• In the human body, perchlorates can affect the thyroid gland and reduce the production of hormones necessary for the proper functioning of the human body making it an endocrine disruptor.

Perchlorates: A group of harmful chemicals used for rocket fuel.

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Chemical Pollution:

Polychlorinated Biphenyls

• PCBs are used to produce plastics, including the water bottles that people drink from.
• PCBs are very stable, tasteless and odors and an example of a biomagnitive persistent organic pollutant (POP) meaning they are very stable as well as tasteless and odorless.
• “PCBs can accumulate in the leaves and aboveground parts of plants and food crops. They are also taken up into the bodies of small organisms and fish. As a result, people who ingest fish may be exposed to PCBs that have bioaccumulated in the fish they are ingesting.” (EPA, 2017)

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Polychlorinated biphenyls (PCBs): A group of industrial compounds used to manufacture plastics and insulate electrical transformers.

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PCBs

Sources

• Poorly maintained hazardous waste sites that contain PCBs.
• Illegal or improper dumping of PCB wastes.
• Leaks or releases from electrical transformers containing PCBs.
• Disposal of PCB-containing consumer products into municipal or other landfills not designed to handle hazardous waste.
• Burning some wastes in municipal and industrial incinerators.

Human Health Impacts

• Teratogen: Birth defects
• Neurotoxic: Nervous system damage
• Brain damage and learning disabilities
• Endocrine disruptor: feminization, low sperm counts, hermaphroditism
• Carcinogenic: Cancer
• Other impacts
• Paralysis
• Attention deficit disorder
• Damage to the reproductive system

Persistent Organic Pollutants (POPs)

• Persistent organic pollutants (POPs), (ie. "forever chemicals"), are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes.
• Because of their persistence, POPs tend to bioaccumulate and subsequently biomagnify with adverse impacts on human health and the environment.
• The inherent danger of POPs is that they are not selective, but remain in the environment for so long: nontarget species can come into contact with the POP and suffer the impacts.
• Major examples include: DDT and PCBs as well as a variety of other pesticides, industrial compounds and byproducts.

Bioaccumulation and Biomagnification

• Another challenge of chemical pollutants is their tendency to persist, bioaccumulate and biomagnify.
• Persistent organic pollutants (POPs) like PCBs and DDT frequently exhibit all three of these traits.
• Persistence: The length of time a chemical remains in the environment.
• Bioaccumulation: An increased concentration of a chemical within an organism over time.
• Biomagnification: The increase in chemical concentration in animal tissues as the chemical moves up the food chain.

The Effects of Biomagnification

• The initial exposure is primarily in a low trophic group such as the plankton in a lake.
• Consumption causes the upward movement of the chemical where it is accumulated in the bodies at each trophic level.
• The combination of bioaccumulation at each trophic level and upward movement by consumption allows the concentration to magnify to the point where it can be substantially more concentrated in the top predator than it was in the water thereby amplifying the impact of the pollutant.

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Module Review:

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• In this module, we learned that a wide variety of chemicals in the water can have harmful effects on humans and other organisms.
• These chemicals include heavy metals, acids, and synthetic organic compounds.
• Through legislation and cleanup efforts, some of these compounds are now found less frequently in the environment.
• However, some chemicals continue to make their way into water bodies and others persist over time.