Urban contamination

In urban areas where there has been extensive reshaping of the land surface, disturbance from building and road construction, land-filling, and additions of a variety of chemicals and minerals, there are many sources, causes, and types of urban contamination. Urban contamination is of concern from two perspectives: when new land is developed that was previously under agriculture or other rural land use , and when the central core industrial areas are redeveloped. Sources of contamination can be summarized in three very broad categories: agricultural, urban, and industrial.

Agricultural-sourced contamination affects urban areas in several ways. Use of some pesticides in agricultural or gardening situations has increased by significant amounts the levels of copper , arsenic , and mercury in soils. Long-term use of phosphorus fertilizers can lead to a buildup of cadmium—a contaminant in many fertilizers—in soils. Synthetic organic pesticides used in agriculture can also contribute to contamination. Organochlorine insecticides are persistent and accumulate in food chains. Examples of these types of compounds are DDT, aldrin, and dieldrin. Use of these insecticides has been discontinued since their negative impacts on the environment have been well documented. However, these compounds do not readily decompose and are very persistent in soils and sediment . Former agricultural areas that have become urbanized have residues of these chemicals and others that result in a risk for contamination of water supplies as well as soils. This is especially true for very localized areas where pesticides were stored, spilled, or that were regularly used to clean or load spray equipment.

Urban activities past and present also contribute to contamination. The most common and largest volume of contamination sources comes from construction or demolition debris. Much of the debris is made up of nonhazardous materials; however, some of the debris may contain asbestos , high levels of trace metals, or other problem-causing substances. Asbestos is of particular concern because it can cause cancer in humans when ingested through air or water.

Long-term use of lead-based paints and gasoline has led to considerable contamination. In 1976, the amount of lead used in gasoline in the United States was approximately 210 million tons (190.5 million metric tons). This has dropped to less than 50 million tons (45.4 metric tons) per year since the use of unleaded gasoline has replaced leaded fuel. Air emissions and deposition of lead from automobile exhaust near roads and highways have contaminated numerous urban areas.

Disposal of domestic wastes containing trace metals and other hazardous materials by land application or in landfills are also large sources of contamination. Smokestacks from incineration plants for wastes often contain high levels of trace metals. Polyaromatic hydrocarbons (PAHs) have been deposited in urban areas as the result of combustion of fossil fuels (coal , peat, etc.) for generating electrical power. Other urban activities that can lead to contamination include land application of soot, coal ash, sewage sludge , and corrosion from metals and buildings.

In addition to agricultural and general urban sources, industrial activities such as mining and smelting contribute significantly to localized soil metal contamination. Atmospheric transport of the emissions can contaminate surrounding areas. Many industries have contributed a number of different kinds of toxic materials to the urban environment and as those industries close or move away they have left the areas contaminated, making it difficult to use the sites for other purposes without expensive clean-up. Examples of toxic materials that are found in former industrial sites include asbestos, chromium, arsenic, and boron. Asbestos is found most often in areas that were formerly railroad lands, shipyards, or asbestos factories. Leather tanning industries or areas used for battery production leave residues of chromium and arsenic.

Locations near chemical manufacturing companies or storage and distribution centers are also places where there is considerable contamination. One of the extreme examples where such a site was redeveloped after which major problems arose is Love Canal in the state of New York. Up to 248 different industrial chemicals were deposited and buried at the site before 1953. People living there following redevelopment were evacuated after many serious health problems were reported and confirmed.

Many toxic solvents are used in chemical, paint, and dye industries, and frequently these solvents can be found in soils of former industrial sites. In the Silicon Valley of California, solvent leakages into soils from computer chip manufacturing have resulted in both soil and groundwater contamination. Trichloroethylene and chloroethylene are common solvents used in paper making, metal plating, electrical engineering, laundries, dry cleaners, and as degreasing agents. These solvents have caused not only soil contamination, but also groundwater contamination in numerous locations. These sites are very difficult and expensive to clean up after the industries leave.

Contamination by power stations arises from pulverized fuel or fly ash , which is alkaline (pH 10-12) and contains soluble salts, in particular boron, aluminum , copper, and arsenic. The ash results from burning coal and is extracted from the chimneys to prevent air pollution . Its extremely fine particle size makes it difficult to handle and dispose. Demolition debris from these sites often contains asbestos residues, trace metals, combustible materials, oil spillages, and polychlorinated biphenyls (PCBs).

This urban contamination is a major problem worldwide in industrialized countries. In the Netherlands alone there have been confirmed at least 10,000 contaminated sites, with 50% of them in residential areas, with total clean up costs estimated to be $25 billion. Western Germany has identified over 50,000 potentially contaminated sites. The Environmental Protection Agency (EPA) in the United States has identified 32,000 potentially hazardous waste sites, with an estimated clean-up cost of $750 billion. There can be large impacts on the growth of plants and on the health of humans and animals that come in contact with the contamination at the site through either the air, water, or soil. There are three main pathways by which the contaminants can enter the plants: direct uptake from soil, vapor uptake through the air, and sorption on plant surfaces. For organic types of contaminants the predominant pathways are through the roots or air vapors. In general, organic contaminants (including pesticides) can affect plant growth resulting in everything from depressed growth and reproduction rates to death.

The ability of the plant to take up metals and transmit them to animals or humans that eat the plants varies by plant and by metal. Since there are likely to be high concentrations of lead either from petroleum products or paint in contaminated sites, there is concern for backyard vegetable gardens or urban gardens where redevelopment has taken place. There are similar concerns for other metals such as zinc and cadmium . These metals are usually associated with electroplating industries or battery production and where municipal waste sludges have been land applied. High concentrations of trace metals can cause visible injury to plants and inhibit their growth.

The high concentrations of trace metals in many urban soils in inner-city and highly industrialized areas gives rise to concerns about potential human health effects. Lead is a trace metal of particular concern, and two predominant pathways for the uptake of lead have been identified in humans: soil and dust ingestion, and consumption of home grown foods. Depending on the extent of exposure, individuals may develop symptoms of acute or chronic lead poisoning . Symptoms or results of exposure include severe anemia , acute nervousness, kidney damage, irreversible brain damage, or death. Preschool children and women of child-bearing age are especially at risk. It has also been reported that people exposed to lead and cadmium in drinking water suffered from increased chronic kidney disease, skin cancer, heart disease, and anemia.

It is estimated that greater than 60% of human cancer is caused by contacting environmental chemicals containing PAHs. However, these compounds can be ingested through contaminated drinking water, but dietary intake of plant-based food is the largest single route of exposure.

Urban development and various industrial activities have resulted in extensive contamination in most industrial countries. When urban redevelopment or expansion is considered, the potential impacts of this contamination on plants, animals, and humans needs to be evaluated. Before the development occurs, contaminated sites need to be cleaned up, which can be very expensive. Urban populations that live in or near these sites already may be exposed to a wide range of contaminants that can have both acute and chronic health effects. As we push to revitalize our cities, this is an issue that will become increasingly important.

[James L. Anderson ]

RESOURCES

BOOKS

Air Pollution, People and Plants. St. Paul, MN: Sagar Krupa, The American Phytopathological Society, 1977.

Berrow, M. L. "An Overview of Soil Contamination Problems." In Chemicals in the Environment, edited by J. M. Lester. Lisbon: 1986.

Thornton, I. "Metal Contamination of Soils in Urban Areas." In Soils in the Urban Environment, edited by P. Bullock, and P.J. Gregory. Boston: Blackwell Scientific Publications, 1991.