Pollution and Bioremediation
When a substance is released to the environment at a rate in excess of what can be safely assimilated, that substance becomes an environmental pollutant.
Properties of Pollutants
Three properties make environmental pollutants especially hazardous: resistance to decomposition (persistence); ability to bioaccumulate , which is related to insolubility in water (hydrophobicity) and solubility in oil (lipophilicity); and the concentration or quantity at which toxic effects occur (potency). Elements like inorganic mercury cannot be further broken down but can only be changed in form, so once a source is reduced or eliminated pollutants' concentrations decrease over time only as a result of dilution or burial in accumulating soil or sediments.
Synthetic organic substances (SOCs), like the pesticide DDT, can persist in the environment because they are only very slowly decomposed by natural physical, chemical, and microbiological processes. Sunlight can break down some SOCs, while others react with water molecules under acidic or basic conditions in a process called hydrolysis . The enzymes of some microbes that decompose wood can also decompose SOCs. The products of these reactions tend to be more water-soluble and less fat-soluble than the parent compounds, reducing their tendencies to bioaccumulate. In general, the breakdown products are also less toxic, although there are exceptions to this rule.
Some air pollutants, like sulfur trioxide and nitrogen dioxide, react with the water vapor in the air to produce acid rain. The high acidity (low pH) caused by acid rain in poorly buffered lakes has decimated fish populations. Acid rain also leaches metals from the soils in the watershed more efficiently than does normal rain, and some of these metals like copper are especially toxic to aquatic organisms.
The metal mercury presents an example of a pollutant that is persistent, bioaccumulates, and is extremely toxic. Mercury is released into the air in small quantities from burning coal and municipal, medical, and industrial waste, and into water as pollution from a variety of industrial chemical processes. Like other air pollutants, mercury can eventually deposit on watershed surfaces and be carried by storm runoff into the stream, wetland, or lake, or deposit on them directly. Once present in the lake, the inorganic mercury attaches to living (biotic) or nonliving (abiotic) particles and eventually settles into the sediments. There, oxygen-avoiding (anaerobic) bacteria synthesize methylmercury from inorganic mercury as a byproduct of their life processes. The methylmercury then moves out of the sediment and into the overlying water and is absorbed by microscopic plants and animals.
Methylmercury is rapidly taken up but only slowly eliminated from the bodies of aquatic animals, and elimination efficiency decreases with increasing size. Thus, at the top of the aquatic food chain, prized sport fish like the largemouth bass can bioaccumulate as much as ten million times the concentration of methylmercury in the water in which it lives. The birds that feed on top-predator fish, like the eagle and the osprey, can further biomagnify the methylmercury in their bodies. When eggs are laid, the methylmercury is deposited in the egg's albumin, or white.
Mercury is not only a threat to birds, but to all animals, including humans. Methylmercury crosses into the developing brain, disrupting brain (neural) development and thought processing. At low doses, methylmercury can slow the development of movement (motor) and learning (cognitive) skills. At high doses in humans, it can cause the severe retardation and twisted limbs now referred to as Minamata disease, after the small city (Minamata Bay) in Japan where the severest toxic effects of methylmercury poisoning were first observed.
Synthetic Organic Substances: Targets for Bioremediation
Typically, decomposition of SOCs occurs more rapidly by oxygen-loving (aerobic) bacteria and fungi than by the more primitive, oxygen-avoiding (anaerobic) bacteria. Unfortunately, many SOCs like the benzene in gasoline leach into the aquifers underlying leaking storage tanks or spills where there is little oxygen. This means that decomposition occurs only slowly, if at all. To speed up this process, engineers have developed ingenious systems to pump water supersaturated with oxygen (and sometimes with added bacteria) into the groundwater. Other systems rely on pumping the groundwater with substitute hydrogen peroxide for oxygen, which releases oxygen as it breaks down. Scientists are also working with the bacteria and fungi that decompose wood to develop a taste for SOCs. While weaning the organisms onto SOCs used to be a time-consuming process in the past, a new generation of genetically engineered organisms are being tested for their efficiency and safety.
Finally, some plants are capable of transporting volatile organic compounds (VOCs) like benzene through their roots and out their leaves, while others take up toxic metals through their roots and store them in their stems and leaves. The process of using plants to clean up (remediate) a contaminated site is called phytoremediation. These bioremediation alternatives are often preferable to such techniques as burying hazardous waste in clay-lined pits, stabilizing the contaminant in place using soil-cement mixtures, or turning the soil into glass one section at a time using a powerful electric current.
DDT: An Environmental Success Story
By the late 1930s the pesticide SOC named dichlorodiphenyltricholroethane (DDT) had been shown to be an effective pesticide against a wide variety of insects, including the mosquitoes, lice, and fleas that carried human diseases, and a wide variety of agricultural insect pests. By the mid-1950s, it was readily available to farmers, who hailed DDT as the beginning of a new era in agriculture, allowing them to plant more crops in greater densities with less pest damage, and the use of DDT expanded dramatically. It was also broadcast on lakes, ponds, and swamps for mosquito control. Soon thereafter, when environmental samples were analyzed from across the United States and then the world, scientists were shocked to learn that DDT was not only building up in the soils and soil organisms where it was being applied, but in the birds feeding on those soil organisms in the sediment and water in nearby lakes, in the fish in those lakes, and in the birds eating those fish.
This intrigued an unknown biologist named Rachel Carson, who in the late 1950s began to review the scientific literature and compile lay unpublished reports and anecdotal information on the toxic effects of DDT, ultimately resulting in her book Silent Spring (1962). In it she predicted that the indiscriminate use of DDT and related pesticides threatened nontarget species like worm- and fish-eating birds with local extinction.
Studies showed that DDT mimicked the hormone that controlled calcium deposition in egg shell formation. DDT was not directly toxic to adult birds or their offspring, but it caused eggshell thinning, so that the shells broke under the weight of the nesting mother. At the same time, field scientists were able to document the serious declines in populations of fish-eating birds, even along the shores of the seemingly pristine Great Lakes.
Goaded by these revelations, within five years of the publication of Silent Spring, the state of Wisconsin was the first to ban all uses of DDT. By 1972, the then two-year-old U.S. Environmental Protection Agency (EPA) banned DDT across the United States, although manufacture and export are still lawful. One by one, DDT's chemical cousins—aldrin, endrin, heptachlor, chlordane, lindane, and toxaphene—were also banned. Clearly, Silent Spring was one of the most influential books of the twentieth century, giving birth to both the environmental movement and the environmental science that would guide the implementation of laws to restore and protect clean air, water, and soil and the proper disposal of hazardous waste.
Endocrine Disrupters: A New Challenge
In the 1990s Theo Colborn has refocused attention on chemicals that mimic, suppress, or amplify the action of animal hormones, so-called endocrine disrupters. When an organism is exposed to these endocrine signal scramblers in the egg or uterus, normal sexual development can be disrupted, resulting in increased incidences of infertility, underdeveloped sex organs, possession of both sets of sex organs (hermaphrodism), masculinization of females, and feminization of males.
In 1996, Colborn and her co-authors released Our Stolen Future, which summarized the scientific literature on the effects of endocrine disrupters to animals in the laboratory and in the wild and linked the occurrence of similar effects in humans to exposure to endocrine disrupters. While the controversy surrounding Our Stolen Future can only be compared to that of Silent Spring, so too its almost immediate impact on national policy. Because of the potentially serious consequences of human exposure to endocrine disrupting chemicals, Congress included specific language on endocrine disruption in the Food Quality Protection Act and amended Safe Drinking Water Act in 1996. The former mandated that EPA develop an endocrine disrupter screening program, whereas the latter authorizes EPA to screen endocrine disrupters found in drinking water sources. As of 2000, scientists are still in the process of developing the standardized tests required to screen for endocrine disrupter effects in the more than seventy thousand chemicals produced commercially each year.
Carson, Rachel. Silent Spring. New York: Houghton Mifflin, 1962.
Colborn, Theo, Dianne Dumanowski, and John P. Myers. Our Stolen Future. New York: Dutton, Penguin Books, 1996.
Ehrlich, P. R., A. Ehrlich, and J. P. Holdren. Ecoscience. New York: W. H. Freeman and Company, 1977.
Kruger, Ellen L., Todd A. Anderson, and Joel R. Coates, eds. Phytoremediation of Soil and Water Contaminants. American Chemical Society, Symposium Series 664. Washington, DC: American Chemical Society, 1997.
Mercury Report to Congress. Washington, DC: U.S. Environmental Protection Agency, 1997.