Key Concepts

After completing this chapter, you will be able to:

1. Describe the ubiquitous distribution of elements in the environment and explain this phenomenon in terms of the difference between pollution and contamination.
2. Outline cases of natural pollution by toxic elements and explain how they provide insight into the effects of anthropogenic pollution.
3. Describe cases of anthropogenic pollution by metals and outline the resulting ecological damage.

Introduction

All of the naturally occurring metals and other elements are ubiquitous (found everywhere) in at least trace concentrations in soil, water, air, and organisms. As long as the detection limits of the available analytical chemistry are low enough, this universal contamination can always be demonstrated.

Organisms require some of the trace elements as essential micronutrients, including copper, iron, molybdenum, zinc, and in some cases aluminum, nickel, and selenium. Under certain conditions, however, these same elements can accumulate to high concentrations in organisms and cause ecological damage (see In Detail 22.1). Trace elements that are most often associated with environmental toxicity are the heavy metals cadmium, chromium, cobalt, copper, iron, lead, mercury, nickel, silver, tin, and zinc, as well as the lighter elements aluminum, arsenic, and selenium.

Some cases of elemental pollution are natural in origin. This usually involves metal-rich minerals being exposed at the surface and causing local ecological changes. However, human activities have caused additional examples of pollution by toxic elements, particularly in the vicinity of industrial sources such as smelters. In addition, emissions of mercury and lead from power plants and automobiles have caused widespread contamination of remote environments, although it is not yet certain that this is causing ecological damage.

There are cases of people having been poisoned by exposure to toxic elements in their environment. Some historians believe that the decline of the Roman Empire may have been hastened by neurotoxicity caused by chronic lead poisoning. The Romans had significant exposure to lead because they stored acidic beverages (such as wine) in pottery treated with pigments and glazes that contained lead. As well, their water piping was made of lead (the word “plumbing” is based on the Latin word for lead – plumbum). In nineteenth-century Britain, many people who made felt top-hats developed neurological damage because of their exposure to mercury compounds used to give a shiny finish to the hats – hence Lewis Carroll’s character in Alice in Wonderland, the “Mad Hatter,” and the expression “mad as a hatter.”

More recently, thousands of people suffered mercury poisoning during the 1960s after they ate grain that had been treated with mercuric fungicide. In one disastrous case in Iraq in 1971, more than 6,500 people were poisoned (about 500 died) when they ate food prepared from mercury-treated grain. The grain had been donated by a foreign aid program and was intended only for planting. Although the sacks of grain were labeled to indicate that the seeds were poisonous, many of the victims were illiterate or did not understand or ignored the implications of the message. About the same time, similar poisonings occurred when people ate mercury-treated grain in Guatemala, Iran, and Pakistan. To avoid these problems today, fungicide-treated seed-grain is usually dyed red, which warns people not to use it as food.

Mercury also caused thousands of cases of poisoning at Minamata, Japan. A factory there had discharged elemental mercury into Minamata Bay. In that form mercury is not very poisonous, but microbes in the sediment transformed the metal into methylmercury, which is extremely toxic and bio-accumulates in organisms in preference to the water of their aquatic environment. The methylmercury further biomagnified up the food web and caused extensive poisoning of fish-eating birds, domestic cats, and people (see In Detail 22.1 and Global Focus 22.1). In this chapter, we examine natural and anthropogenic pollution with toxic elements and the resulting ecological consequences.

Concentration and Availability

All of the naturally occurring elements are present in at least trace concentrations in all samples of water, soil and rocks, air, and organisms. The term background concentration refers to a presence that is not significantly influenced by either anthropogenic emissions or unusual natural exposures. The background concentration in soil and rocks is usually much higher than in water, and also generally higher than in the tissues of organisms (Table 22.1).

However, elements that are dissolved in water often occur in chemical forms (such as ions) that are relatively easily absorbed by organisms. For this reason, even a trace aqueous concentration may be toxic. In contrast, the much higher concentrations that commonly occur in soil and rocks are mostly insoluble, and therefore are not particularly bioavailable. Scientists determine the total concentration of metals in a component of the environment (such as soil, sediment, or rock) by digesting a sample in a hot mixture of strong acid. In contrast, the “available” concentration is determined from an aqueous (water) extract of a sample. In general, the available concentration of toxic elements in soil are much smaller than the total concentrations (generally less than 1% of the total value), and it is also much more relevant to potential toxicity.

Most elements are found in only trace concentrations in the environment (Table 22.1). In contrast, aluminum and iron are prominent constituents of rocks and soil, with concentrations typically about 8% and 3-4%, respectively. However, almost all of the aluminum and iron in soil and rocks occurs as insoluble minerals that are not readily available for uptake by organisms. For example, virtually all aluminum in soil occurs as insoluble silicate and clay minerals. Although aluminum in these forms comprises about 8% of the soil mass, it is not available for uptake by plants and is therefore non-toxic. However, much smaller concentrations of aluminum, typically only a few parts per million (ppm), are found as ions, either bound to organic matter and clay surfaces or freely dissolved in soil water. The ionic forms of aluminum are readily available for biological uptake and may cause toxicity to species that are sensitive to this metal.

Much higher concentrations of soluble available aluminum occur in strongly acidic environments, especially when the pH is less than about 5.5. (In fact, almost all metals are much more soluble under acidic conditions.) Aluminum solubility is also greater in strongly alkaline environments, with pH higher than about 8. Moreover, different ionic species of aluminum occur at different pH levels:

• Al³⁺ is dominant in strongly acidic environments with a pH less than about 5.0
• AlOH²⁺ and Al(OH)₂⁺ are important under less acidic conditions of pH 4.5–5.5
• Al(OH)₃ from pH 5.2–9.0
• AlOH₄^– in alkaline environments with pH greater than 8.5

Aluminum toxicity is a common problem for organisms that live in highly acidic or alkaline environments. This is because of the combined influences of greater solubility and the presence of relatively toxic ions under those conditions.

Table 22.1. Background Concentration of Elements in Selected Components of the Environment. Source: Data from Bowen (1979).

Toxicity

The toxicity of elements and other chemicals is related to two factors: (1) the exposure (dose) and (2) the vulnerability of an organism to the particular substance. The dose received is influenced by the available concentration in the environment and the period of exposure. Therefore, a long-term exposure to only a minute available concentration may cause toxicity, especially if the element is able to bioaccumulate and then biomagnify in the food web until it exceeds a threshold of biological tolerance.

Organisms vary greatly in their tolerance of exposures to toxic elements (and to all other poisons). Consequently, an intense exposure to a potentially toxic chemical may result in some species being poisoned, while tolerant ones may not be damaged and may even benefit from the demise of sensitive species in their community. In addition, there is usually genetically-based variation for tolerance within a species. This can lead to the evolution of populations (known as ecotypes) that are relatively tolerant of toxic exposures (we examine this topic in the next section).

The most common mechanism of poisoning by toxic elements is damage to an enzyme system. (Organisms have a huge diversity of enzymes, which are proteins that catalyze specific biochemical reactions and are critical to healthy metabolism.) The poisoning occurs because metal ions bind to specific enzymes, which changes their shape and results in a loss of their unique catalytic function. Toxic elements may also cause poisoning by binding to DNA or RNA, thereby disrupting transcription and translation, the processes by which genetic information is used to produce specific proteins (including enzymes; see In Detail 7.1). Toxic metals can also disrupt DNA replication and hence cell division.

Typical symptoms of acute poisoning caused by toxic elements in plants include abnormal patterns of growth, decreased productivity, impaired reproduction, the occurrence of disease, and ultimately death. Symptoms of chronic toxicity are harder to detect and may include a “hidden injury” such as a decrease in productivity that occurs without signs of acute damage. Animals can show a variety of symptoms associated with enzyme disruption, often including neurotoxicity and impaired functioning of the kidneys, liver, and other organs.

Natural Pollution

Localized natural pollution sometimes occurs when metal-rich minerals are present at the surface and are prominent in the chemistry of soil, surface water, and vegetation. These conditions can often be identified by a distinctive, stunted growth form of the vegetation, and sometimes by the presence of particular indicator plant species. In combination with chemical analyses, these biological indicators can be used to explore for metal-rich deposits, a technique known as biogeochemical prospecting.

In some cases, natural pollution by metals can be quite intense. For example, soil containing up to 3% lead plus zinc was found at a site on Baffin Island. In another case, peat filtering a spring of metal-rich groundwater in New Brunswick accumulated as much as 10% copper. High concentrations of metals in soil are also reflected in the chemistry of plants, especially in certain genetically adapted hyperaccumulator species that may occur in metal-rich habitats. For example, nickel concentrations as high as 10% have been measured in plants in the genus Alyssum growing in Russia, and up to 25% in the blue-colored latex of Sebertia acuminata from New Caledonia in the South Pacific. These hyperaccumulator plants grow on naturally metal-polluted sites.

Serpentine Soil and Vegetation

Some well-studied cases of natural pollution involve soil influenced by serpentine minerals, which are rich in nickel, chromium, and cobalt and are associated with asbestos deposits. Soil containing serpentine minerals is toxic to non-adapted plants because of the high concentrations of these metals, in combination with an imbalance of the nutrients calcium and magnesium. Serpentine soil typically contains several thousand parts per million of nickel, but can have as much as 25-thousand ppm (or 2.5%) of this metal.

The natural vegetation on serpentine sites is often distinctively stunted. Extensive serpentine “barrens” occur in eastern Quebec and western Newfoundland. Those habitats support tundra-like ecosystems in a landscape that is otherwise covered by boreal forest.

In some places, serpentine areas support plant species that occur only in that kind of habitat, a narrow distribution that ecologists refer to as endemic. In other cases, widespread species have evolved locally adapted populations that can cope with the toxic and nutritional stresses of serpentine soil – these are known as ecotypes. On non-serpentine sites, the specifically adapted endemics and ecotypes are quickly eliminated by competition with plants that are better competitors in less-stressful habitats.

Serpentine sites in northern California support relatively ancient vegetation, because the area was not recently glaciated (Image 22.1). These habitats contain at least 255 endemic species or subspecies of plants (Safford and Miller 2020). Some of the endemics occur only on particular serpentine sites in California and nowhere else in the world. In contrast, the serpentine barrens in eastern North America are relatively young, being released from glaciation only about 10-thousand or fewer years ago. Consequently, not enough time has passed to allow many serpentine endemics or ecotypes to evolve.

Image 22.1. Serpentine Site in California. Altogether, serpentine soils and rock comprise about less than 1% of Earth’s terrain. However, serpentine sites, like the one pictured above, are found widely throughout California, especially the Central Coast region. These sites are characterized by low levels of nitrogen, phosphorus, and potassium while being rich in several heavy metals toxic to most vegetation. The unusual chemical composition of these soils creates an environment in which only a limited number of plant species can prosper, some of which are considered to be rare and unique. Source: G. A. Meindl.

Seleniferous Soil and Vegetation

Semi-arid regions in various parts of the world often have areas with soil that contains high concentrations of selenium. These seleniferous habitats may support plants that hyperaccumulate selenium, such as species in the genus Astragalus (locoweeds). About 25 North American species of Astragalus are hyperaccumulators of selenium. They may contain up to 1.5% of selenium in their tissues, storing it in unique amino-acid-like biochemicals, such as selenomethionine. The Astragalus species also emit dimethyl selenide and dimethyl diselenide to the atmosphere, giving them a distinctive, unpleasant odor. Livestock that feed on these plants are poisoned by a toxic syndrome known as alkali disease or blind staggers.

Mercury in Aquatic Environments

Even in remote oceanic habitats, mercury often accumulates in high concentrations (as methylmercury, CH₃Hg) in fish, birds, and sea mammals. In marine waters off eastern and western North America, large fish may have mercury concentrations in their flesh that exceed the limit considered acceptable for human consumption (more than 0.5 ppm mercury on a fresh-weight basis; Figure 22.2). Analysis of old specimens of fish and seabirds in museums has revealed levels of mercury contamination similar to those in modern samples, which suggests that the phenomenon may be natural. The contamination of marine animals represents a substantial biomagnification from ambient seawater, which has a trace concentration of mercury of less than 0.1 ppb.

Figure 22.2. Mercury Contamination of Fish Captured Offshore of North America. The data show the average mercury concentration in the muscle of species of marine fishes. The data are in ppm, measured on a fresh-weight basis. Source: Data from Armstrong (1979).

Biomagnification occurs because of the progressive accumulation of mercury up the trophic web. Algae initially absorb mercury from the water (as methylmercury), and zooplankton accumulate even larger residues as they graze on the algae. Zooplankton-eating fish accumulate still larger quantities, but the largest residues occur in long-lived top predators, such as big fish and marine mammals (see In Detail 22.1).

Within any particular species of fish, larger (and older) individuals generally have higher mercury concentrations than smaller (and younger) ones. A study of swordfish caught off eastern Canada found that animals heavier than 45 kg had an average mercury concentration of 1.1 ppm, while those weighing 23-45 kg had 0.86 ppm, and those smaller than 23 kg had 0.55 ppm (Armstrong, 1979). It appears that mercury residues become more intense as the animals age and grow larger.

High concentrations of mercury also occur in fish-eating marine mammals and birds, which are top predators in their ecosystem. Studies of adult harp seals (Phoca groenlandica) in eastern Canada found an average mercury concentration of 0.34 ppm in muscle and 5.1 ppm in the liver (Armstrong, 1979). High mercury residues also occur in North Atlantic seabirds, with an average of 7 ppm found in feathers of northern skua (Catharacta skua), 5 ppm in puffin (Fratercula arctica), and 1-2 ppm in fulmar (Fulmarus glacialis), kittiwake (Rissa tridactyla), razorbill (Alca torda), and common murre (Uria aalge) (Thompson et al., 1991).

Mercury contamination of fish has also been observed in many remote lakes, which can be seen throughout the U.S. Fish consumption advisories have been issued to over 10 million acres of lakes and over 400,000 stream miles (Brigham, 2021). Mercury contamination is the main cause of these advisories, due to the harmful effects of mercury poisoning on fish and on human health. In general, freshwater fish that are top predators have the highest residues of mercury, and larger or older individuals are the most contaminated.

In the U.S., the accumulation of mercury in fish varies statewide depending on the bodies of water and species of fish present. Federal, state, and local governments are responsible for issuing any necessary advisories when a body of water is found to be contaminated with not only mercury, but also chlordane, dioxin, DDT, and PCB’s (EPA, 2020b). The FDA and EPA have created guidelines pertaining to mercury accumulation in order to inform people on which fish are safe to eat and which ones to avoid. These guidelines are especially important for the elderly, pregnant women, nursing mothers, and children. The typical safety threshold for mercury consumption in the U.S. is 0.1 microgram per kilogram of body weight. In general, this means that if a fish contains more than 0.46 micrograms/gram it should be avoided altogether, anything between 0.46 micrograms/gram and 0.23 micrograms/gram should be eaten in moderation, and the safest choices are below 0.15 micrograms/gram (EPA, 2021).

The causes of mercury contamination of lakes are not known for certain. It seems likely that the phenomenon may be natural in regions that are remote from sources of emission. However, anthropogenic mercury is contributing to the problem closer to large emissions sources, such as coal-fired generating stations, municipal incinerators, and smelters. In the U.S., 70% of mercury deposition is derived from anthropogenic sources, 55% of which coming from coal combustion sources (Wentz et al., 2014). In addition, discharges from chlor-alkali and acetaldehyde factories and some older pulp mills have caused local mercury pollution, resulting in high residues of methylmercury in fish and other animals. The case of Minamata Bay, Japan, involved an acetaldehyde plant (Global Focus 22.1). A less severe case in Canada, which affected parts of the English and Wabigoon Rivers in northwestern Ontario, involved a pulp mill.

Significant bioaccumulation of mercury also occurs when hydroelectric reservoirs are developed (see Chapter 24). Flooding leaches naturally occurring soil mercury into the reservoir, where bacteria in oxygen-poor sediment transform it into methylmercury that is biomagnified by fish. This process occurs more rapidly in acidic lakes because that condition favors the production of methylmercury in the sediment, compared with less available dimethylmercury in non-acidic waterbodies.

Although there is some controversy about the relative importance of natural and anthropogenic sources of mercury to remote lakes, it is reassuring to know that the overall emissions have been greatly reduced in recent decades. This occurred because of improved emissions controls at industrial facilities, including the closing of several metal smelters and coal-fired power plants. However, there is still a tremendous amount of mercury pollution globally. Annual global mercury emissions from anthropogenic sources are approximately 2,220 metric tons per year (Figure 22.3). Globally, artisanal and small-scale gold mining is the largest source of anthropogenic mercury emissions (37.7%), followed by stationary combustion of coal (21%). Other large sources of emissions are non-ferrous metals production (15%) and cement production (11%) (EPA, 2020e).

Figure 22.3. Global Mercury Emissions. Total mercury emissions from all sources in 2015 by geographic area. Source: EPA.

Anthropogenic Sources

Industrial processes used to mine, process, and use metals can result in the pollution of air, water, and land.

Mining Residues

Areas near mine sites may be badly damaged by the dumping of metal-rich excavation waste (rocks whose metal concentration is not high enough to be considered commercial ore). Because these materials may be toxic, vegetation development can be restricted to early successional communities, such as sparse grassland. In some cases, soil toxicity is severe enough that few plants manage to establish even after hundreds of years. This can be seen on mine wastes from 2000-year-old Roman lead workings in England and Wales.

Ecologists studying British sites polluted by mine wastes have found that these habitats often support plant ecotypes that are genetically tolerant of the metals that are present. The locally adapted ecotypes can grow in metal-polluted soils, where non-tolerant plants are eliminated by the toxic stress. Conversely, the tolerant ecotypes are poor competitors in non-polluted environments, and so are rare in habitats unaffected by metal toxicity.

Research into metal-tolerant ecotypes has provided insights into the process of evolution (Chapter 6). Metal-tolerant individuals do occur in populations growing on non-polluted sites, but they are rare. However, the frequency of tolerant genotypes increases quickly after metal pollution occurs. In places with sharp boundaries between polluted and non-polluted soils, a tolerant population can maintain itself over a distance of only a few meters. This is possible because the intense toxicity of polluted soil strongly favors the survival and reproduction of tolerant individuals. Such a population-level change in genetically based characters, occurring in response to an agent of natural selection (in this case, metal pollution), is a demonstration of evolution (more specifically, microevolution).

Metal-tolerant ecotypes have been studied near Sudbury, Canada, where pollution by nickel and copper has been caused by emissions from smelters and roast beds (see Chapter 19). Plant communities of polluted sites are dominated by metal-tolerant ecotypes of several grasses, particularly Agrostis gigantea and Deschampsia caespitosa. Meadows of these grasses developed soon after the extremely tall “superstack” was commissioned in 1972. Because it dispersed emissions widely, the superstack greatly reduced ground-level SO₂ pollution. However, soil in the area remained acidic and polluted with metals. The local ecotypes of these grasses can tolerate toxic stress from acidity and metals, but are intolerant of SO₂, which is why the grasslands did not develop until after the superstack began to operate.

The metal tolerance of the grass Deschampsia caespitosa has been studied. Plants were grown in solutions containing the metals of interest, and were compared with controls (Figure 22.4). The data show that the Sudbury population is tolerant of nickel and copper, which occur in their native soil at concentrations of about 400 ppm, compared with 20 ppm at non-polluted reference sites. The Sudbury population is also more tolerant of aluminum. This is a response to the greater solubility and toxicity of aluminum in acidic soil near the smelters (which had a pH of 3.5-3.9, compared with pH 6.8-7.2 at reference sites).

Figure 22.4. Tolerance of a Grass to Metals. Populations of the hairgrass (Deschampsia caespitosa) were collected from metal-polluted places near Sudbury and from reference sites where metals are not a problem. The index of metal tolerance is based on the root growth that occurs when plants are grown in solutions containing metals, compared with a no-metal control. The larger the index number, the greater the tolerance. In all comparisons presented here, the two populations had statistically significant differences in tolerance to the metal tested, with a probability level of <0.001, except for aluminum (p < 0.05; note: a probability value of <0.001 means that there is less than a 0.1% (or 1/1000) likelihood that the difference between the two populations is due to chance alone; p <0.05 means there is less than a 5% chance). Source: Data from Cox and Hutchinson (1979).

Metal-Containing Tailings

Once ore is mined, it is ground to a fine powder in a process called milling. The powder is then separated into a valuable metal-rich fraction, which is roasted and smelted, plus large quantities of waste tailings. In most cases, the tailings are dumped into a low-lying contained area, which when full is covered with vegetation as a stabilization measure. Although the tailings are a waste product, they still contain high concentrations of metals, and that can make it difficult to establish vegetation after a dump is filled. In addition, if sulfide minerals are present, acidity is generated when they become oxidized by bacteria, and that makes the toxicity worse. Toxic tailings can be easily spread and contaminate the environment through runoff into waterways and leaching through the soils (Table 22.2). Acidic tailings are especially toxic, because metals are much more soluble and bioavailable under acidic conditions.

Table 22.2. Water Quality Standards in the Evaluation of the Atlas Mill Site located in Ouray Silver Mines, Ouray, CO. The data below is from a 2020 Draft Engineering Evaluation and Cost Analysis report. The data summarizes sampling of various metals in water sources both upstream and downstream from the Atlas Mill Site adopted from an Alpine Environmental Consultants LLC., 2018 assessment report. The 2020 report stated that the Atlas Mill may cause a slight increase in dissolved cadmium, dissolved silver and dissolved zinc concentration in Sneffels Creek. Source: Geosyntec Consultants via U.S. Forest Service.

The U.S. has various methods for the cleanup and remediation of mining waste areas. Each state may have its own rules and regulations on top of federal requirements. One recent technique specific to metal tailing sites is referred to as bioremediation. In this case, microbiota (bacteria and other microorganisms) as well as biological nutrients are added to the waste site to aid in the breakdown or immobilization of hazardous compounds (EPA, 2000). In addition, other methods of containment include building dams to sequester polluted water; however, if their associated dams and berms are not structurally sound and become breached by high water flows during severely rainy weather, tailings-disposal areas can be a source of massive water pollution.

On August 5th, 2015, the EPA was working to assess the Gold King Mine waste site near Silverton, Colorado. The goal was to treat the contained mine water and aid in the remediation process. In the process of excavation, pressurized water began leaking from the mine tunnel ultimately bursting and releasing about 3 million gallons of water containing heavy metals such as cadmium and lead directly into Cement Creak which leads into the Animas River (Image 22.2). This highly acidic and polluted water had a devastating effect on the surrounding environment, wildlife, and even residential areas. Those living near the river were advised to have their water tested before consuming it. The EPA has assumed full responsibility for the water spill, and is still working on a total cleanup. Future potential outcomes are still unknown (EPA, 2020a).

Image 22.2. Animas River after Gold King Mine Waste Water Spill. The image above shows the Animas River within 24 hours of the spill. The bright orange color of the river shows the extreme levels of pollution that resulted from such a catastrophe. Source: “Animas River spill” by Riverhugger is licensed under CC BY-SA 4.0.

If a filled-up tailings-disposal area is to have a stable cover of vegetation established on top, its contents must be treated to reduce the toxicity. If the tailings are acidic, a liming treatment is needed to raise the pH to a neutral level and so reduce the availability of metals. Fertilizer may also be used to alleviate nutrient deficiency and organic matter added to improve soil structure and water-holding capacity, and then plants are sown. Sometimes, novel techniques are used, such as the use of acid- or metal-tolerant ecotypes in the planting mixture. If the tailings are extremely toxic or acid-generating, they may have to be covered with a locally available overburden, such as glacial till, which is then vegetated (Image 22.3). Canadian Focus 22.1 describes the reclamation of tailings-disposal areas in the vicinity of Sudbury, Canada.

Image 22.3. Reclaimed Area of Tailings. Tailings are the fine waste that remains after ore is ground and processed to remove metal-rich minerals. Tailings contain high concentrations of metals and can generate acidity when exposed to the atmosphere. These conditions make it difficult to establish vegetation after disposal sites are filled. This is a view of a reclaimed area of tailings near Sudbury, Canada. Most of the vegetation was sown, but native shrubs and trees are also becoming established. Source: B. Freedman.

Smelters

A smelter is a large industrial facility where ore is roasted. This is done to oxidize sulfide minerals, a process that results in large amounts of waste SO₂ and metallic particulates. In most cases today, pollution-control technologies are used to recover much of the SO₂ and particulates before the flue-gases are vented to the atmosphere. In the past, however, those wastes were emitted into the environment, causing intense pollution and ecological damage. As recently as several decades ago this was a common practice, and it still is for some older smelters. Newer smelters operate much more cleanly.

A smelter is a point source of toxic stress to surrounding ecosystems. Emissions may result in well-defined spatial gradients of both pollution and its resulting ecological damage, which diminish with increasing distance. Studies of damage near smelters indicate the following generalizations:

• Close to the point source, pollution by atmospheric SO₂ and metals in soil is most severe
• The intensity of pollution decreases rapidly (more or less exponentially) with increasing distance from the smelter.
• Damage to vegetation varies with the intensity of toxic stress and includes decreases in biomass, productivity, and species diversity, with only a few low-growing species occurring in the most polluted habitats
• Ecological processes such as nutrient cycling and decomposition are disrupted by toxic metals, gases, and acidity

The pattern of metal pollution around a point source can be illustrated by the Copper Cliff smelter near Sudbury, Canada. Figure 22.5 shows that metal concentrations in the environment decline rapidly with increasing distance from that smelter. These data specifically refer to the forest floor, but similar observations are seen in soil, vegetation, lake water, and other components of the ecosystem.

Figure 22.5. Metal Pollution near Sudbury, Canada. Decades of emissions of metals from the Copper Cliff smelter caused an accumulation of nickel and copper in the environment. The most intense pollution occurs close to the point source. These data are for metals in the forest floor, which is the organic-rich layer that overlies the mineral soil. The forest floor binds metals in organic complexes and accumulates higher residues than the underlying soil. The samples were collected along a transect running south from the smelter. The spatial patterns of copper and nickel are highly correlated, with a coefficient of 0.98. Source: Data from Freedman and Hutchinson (1980).

It is difficult to determine the specific role of toxic metals in causing ecological damage. One way to investigate the influence of metals is to grow plants in polluted soil in a greenhouse, where SO₂ is not present. These bioassay experiments have demonstrated that soil collected near the smelters is toxic, mainly because of its high concentrations of metals. To a substantial degree, the toxicity persists even after the soil acidity is neutralized by adding lime.

Not all smelters emit both SO₂ and metals. The ecological damage that results from those that emit only metallic particulates has consequently been caused by metal pollution. One well-studied smelter, at Gusum, Sweden, has operated since 1661 (Tyler, 1984). Zinc is an important pollutant there, reaching concentrations as high as 2% (20-thousand ppm) in surface organic matter close to the point source, compared with less than 200 ppm farther than 6 km away. Copper pollution is similar, reaching 1.7% within 0.3 km, compared with 20 ppm beyond 6 km. The zinc and copper pollution has caused local ecological damage. Pine and birch trees have died or declined close to the source, and understory plants, mosses, lichens, and soil-dwelling invertebrates have been damaged. Rates of decomposition and nutrient cycling are also impaired in the most polluted sites. Some plants, however, are tolerant of the metal pollution at Gusum. They include the grass Deschampsia flexuosa and the moss Pohlia nutans, which do relatively well in sites that are toxic to other plants.

Use of Inorganic Pesticides

Until the 1970s, inorganic chemicals were widely used as pesticides in agriculture (see also Chapter 26). This was especially true in fruit orchards, where pesticides based on lead arsenate, calcium arsenate, copper sulfate, and related compounds were used to control fungal diseases and arthropod pests. These compounds have now been largely displaced by synthetic organic pesticides.

However, until the mid-1970s, annual spray rates of lead in North American orchards were as high as 8.7 kg/ha, while arsenic treatments reached 2.7 kg/ha, zinc 7.5 kg/ha, and copper 3.0 kg/ha (Frank et al., 1976). The spray rates depended on the crop being grown, the pest being managed, and the pesticide used, but in some cases all of these toxic elements were applied in the same orchards.

Residues of these chemicals accumulated in the soil of treated orchards. Studies of apple orchards found residues as high as 890 ppm of lead and 126 ppm of arsenic in surface soil, compared with background levels of <25 ppm lead and <10 ppm arsenic (Figure 22.6). The accumulations were caused by up to 70 years of spraying lead arsenate as an insecticide, mostly against the codling moth (Laspeyresia pomonella), a pest that causes “wormy” apples. This in particular is an issue in Central Washington State where lead arsenate was used as an insecticide to combat infestations of apple orchards with codling moth until 1950 (Scheffer, 2020).

Figure 22.6. Accumulation of Arsenic and Lead in Orchards. Lead arsenate was used as an insecticide to combat infestations of apple orchards with codling moth. These data show the progressive accumulation of arsenic and lead in soils of orchards in southern Ontario. The largest residues are in the oldest orchards, which had been sprayed for many years. The background concentration for lead is 20 ppm, and for arsenic it is 10 ppm. Source: Data from Frank et al. (1976).

Agricultural soil can also be contaminated by the use of mercury-containing fungicides, especially those that protect newly germinated seedlings from a fungal infection known as damping-off. This pathogen attacks seedlings at the soil-air interface and causes the weakened plant to fall over and die. Mercury-containing pesticides are also used to control turfgrass diseases on golf-course putting greens. In March of 2015, the Minnesota Department of Agriculture issued an advisory stating that the greens of golf courses as well as tee boxes contained high levels of mercury and warned nearby residents to test their water and soils (Minnesota Department of Agriculture, 2015). The sowing of seed coated with mercuric fungicide has caused poisoning of wild animals that consumed the planted grain or ate herbivores that did so. Alkyl-mercury compounds such as methylmercury are especially hazardous in this respect because this form is extremely toxic and readily assimilated by animals from their food. Use of these fungicides was common until the early 1970s.

Beginning in the late 1960s, most developed countries prohibited the use of alkyl-mercury fungicides as seed dressings. This ban resulted from the recognition of ecological problems associated with use of these chemicals, especially the poisoning of wild animals. Sweden, for example, prohibited the use of these pesticides in 1966, while approving the use of alkoxyl-alkyl-mercury compounds, which are much less toxic, as replacements. This action rapidly led to decreased mercury contamination of wildlife, such as predatory birds (Figure 22.7). The U.S. took similar action, although several years later.

Figure 22.7. Mercury Contamination of Swedish Hawks. (a) Mercury in feathers of goshawks (Accipiter gentilis), during various time periods; (b) Mercury in feathers of marsh harriers (Circus aeruginosus). Note the large increase in contamination caused by the use of alkyl-mercury fungicides and the rapid decrease that followed the banning of these chemicals in 1966. Source: Data from Johnels et al. (1979).

As was noted in the introduction to this chapter, humans have also been poisoned by inadvertently eating mercury-treated seed grain.

Birds and Lead

Millions of birds have suffered lead poisoning in North America each year because they ate spent shotgun pellets. Most of the spent shot was associated with hunting. More than 100 species of birds have been weakened or killed due to the ingesting of lead shots (Kimmel & Tranel, 2007). Although more localized, skeet shooting was also a problem because of the large amount of shot deposited, up to tonnes of lead each year.

After being ingested by a seed-eating bird, lead shot may be retained in the gizzard, a muscular forepouch of the stomach. Hard grit is normally retained in the gizzard and used to grind tough-coated seeds, aiding in their digestion. Unfortunately, shotgun pellets are similar in size and weight to the grit that many birds select for this purpose. The shot becomes abraded in the gizzard, and the bits are swallowed and dissolved by acidic stomach fluid. The lead is then absorbed into the bloodstream, allowing it to poison the nervous system of the bird, leading to death.

Waterfowl have been especially widely affected, with 2-3 million individuals, or 2-3% of the North American population, dying each year from lead-shot poisoning in the early 1990s. The retention of just one or two pellets in its gizzard can poison a duck, causing a wasting away of 30-50% of its body weight, neurological toxicity, and ultimately death. Typically, about 10% of the waterfowl surveyed in North America had one or more shotgun pellets in their gizzard. Larger aquatic birds, such as swans, are known to retain lead fishing weights in their gizzard. Lead sinkers or shot were cited as the cause of 20-50% of the mortality of trumpeter swans (Cygnus buccinator) in western North America. Lead sinkers are also known to poison tundra swans (C. columbianus) wintering in the eastern U.S., mute swans (C. olor) in Europe, and common loons (Gavia immer) in Canada and the U.S. A related syndrome, caused by ingesting lead shot and bullets, afflicts birds that scavenge dead carcasses. Although the numbers are not well documented, this poisoning is known to kill vultures, eagles, and other scavenging birds. The critically endangered California condor (Gymnogyps californianus) has been relatively well studied – about 60% of its known deaths in the wild between 1980 and 1986 were caused by toxicity from ingested bullets in carrion. Because of the widespread poisoning of birds by lead shot, regulators have now restricted its use. Lead shot is banned over most of the U.S. The use of lead shot for hunting is being replaced mostly by steel shot, and to a lesser degree by bismuth shot. The restricted use of lead shot has caused some controversy because many hunters believe that the alternative shot types might cause more crippling deaths. However, field tests have shown this effect to be marginal, as long as the inferior ballistic qualities of the alternatives are compensated for by shooting at closer distances or by using a larger size of shot. Furthermore, in the U.S., anglers report to losing 0.18 sinkers per hour and 0.23 hooks and lures per hour, both often containing lead. This has led to an estimated 450 million toxic fishing sinkers entering the environment in per year (Keats, 2011).

Automobile Emissions of Lead

Lead emitted by automobiles has contributed to a general contamination of urban environments. From 1923, but particularly after 1945, tetraethyl lead was added to gasoline as a so-called “anti-knock” compound. The lead increases mechanical efficiency and gasoline economy, while decreasing engine wear. In 1975, about 95% of the gasoline used in North America was leaded at concentrations as high as 770 ppm. In 1987, only 35% of the gasoline was leaded, and the maximum permitted then was 290 mg/L. The decreased use of lead between 1975 and 1987 was mostly due to the increased use of catalytic converters to reduce emissions of other automobile pollutants, especially carbon monoxide and hydrocarbons. Automobiles equipped with a catalytic converter can only use unleaded gasoline, because the catalysts, usually platinum, are rendered inactive by lead.

After 1990, the use of leaded gasoline was banned in the U.S. (the only exceptions were low-lead fuels [up to 30 ppm] for use in some farm vehicles, marine engines, and large trucks). Consequently, emissions of lead from automobiles in the U.S. decreased by 98% between 1980 and 2019 (EPA, 2020c; 2020d).However, many other countries, particularly in the less-developed world, continue to allow the use of leaded fuels.

Almost all of the lead in gasoline is emitted as particulates through the vehicle tailpipe. The larger particulates settle out close to the roadway. This results in the buildup of a well-defined gradient of lead pollution, the intensity of which is related to traffic volume. This pattern of roadside pollution is illustrated in Figure 22.8 (this study was made prior to the banning of leaded fuels). Finer lead particulates are more widely dispersed in the atmosphere and contribute to the general contamination that occurs in cities. Not surprisingly, studies have shown some effects of lead on urban wildlife. For example, pigeons (Columba livia) living in cities can have significant residues of lead and may exhibit symptoms of acute poisoning.

Figure 22.8. Lead Pollution and Vehicular Traffic. In a study in northern England, lead concentrations in soils were highest along the immediate border of roadsides, and decreased with increasing distance from the road border. Source: Akbar et al. 2006.

Overall, there have been large reductions in the emissions of lead in the U.S., and also in other developed countries (Figure 22.9). This improvement in environmental conditions has occurred because of the banning of leaded gasoline as well as improved emissions controls at smelters and other industrial facilities.

Figure 22.9. Anthropogenic Lead Emissions in the U.S. The decrease in nationwide lead emissions was largely due to the elimination of leaded gasoline. Restrictions on lead emissions were strengthened in the 2008 National Ambient Air Quality Standards, which specified the limits for lead concentration in outdoor air. Source: EPA.

Conclusions

All of the naturally occurring elements are present in at least a trace level of contamination in all components of the environment – in the air, water, soil, and organisms. Sometimes their concentration is naturally elevated, as occurs when an ore body is present at the surface of the ground. Increasingly, however, anthropogenic activities are responsible for large emissions of toxic elements to the environment, and in some cases this has resulted in serious damage to ecosystems and in toxicity to people. The worst cases of pollution involve industrial practices that are no longer allowed in the U.S. or other wealthy countries, such as uncontrolled emissions of metals from smelters, the dumping of mercury into aquatic environments, the use of leaded gasoline, and the use of lead shot for hunting. Nevertheless, pollution by toxic elements is still an important problem. Damage is still being caused to ecosystems and organisms by releases of lead, mercury, and other toxic elements. This is true of all parts of the world, although pollution by toxic elements in poorer countries is much less controlled than in wealthier ones.

Questions for Review

1. How can we identify normal (or reference) levels, contamination, and pollution by metals and other elements given that these substances are ubiquitous in the environment?
2. What are the important sources of metal emissions to the environment?
3. What is the difference between the total and available concentrations of metals?
4. Describe the spatial pattern of metal pollution around a large point source of emissions, such as a smelter.

Questions for Discussion

1. Important environmental benefits have been gained by banning the use of leaded gasoline in the United States. Why were there long delays in taking similarly vigorous actions against the use of lead shot in hunting and skeet shooting and lead weights in fishing?
2. Pick an element that was examined in this chapter and research its benefits, toxicity, effects on the environment, control, and mitigation.
3. Explain the principles of bioaccumulation and biomagnification using the case of methylmercury in aquatic ecosystems. Why do you think these phenomena were unanticipated “surprises” to environmental scientists?

Exploring Issues

1. Assume that the U.S. and Canada are negotiating a treaty to govern their emissions of mercury to the environment. You are a science advisor to the American team. Some members of the team want to press for a “zero emissions” policy, believing that no emissions of mercury to the environment are acceptable. They ask for your advice on this issue. What kinds of information about the toxicity of mercury, to humans and to wild ecosystems, do you need in order to give the team objective advice about the proposed zero-emissions policy? Also, is it physically possible to have zero emissions?

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