Soil Contamination

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Soil Contamination

Introduction

Soil contamination refers to the presence of un-natural (human-made) chemicals or other substances in the soil. The pollutant can be a solid or liquid. Examples include gasoline released from the rupture of an underground tank at a gas station, runoff of oil from a parking lot onto adjacent ground, deliberate dumping of solid or liquid waste, and the uncontrolled use of pesticides or herbicides.

Oil-based compounds (hydrocarbons), heavy metals, and solvents that do not mix well with water also tend to adhere tightly to soil particles, making them harder to extract. This is also true if they enter the groundwater, as they can then be transported a considerable distance away from the original site of contamination.

With the growth of urban centers and their associated industries beginning in European countries and the United States in the early nineteenth century, the contamination of soil became extensive. Among the many examples in the United States is Love Canal, a neighborhood in Niagara Falls, New York, that was created on the site of land that had been used for the public and private disposal of toxic waste.

A legacy of Love Canal and other sites of soil contamination was the passage of legislation in 1980 that created what is popularly known as Superfund sites—long abandoned, heavily contaminated sites for which the U.S. Environmental Protection Agency (EPA) provides funds for clean up and restoration. As of 2008, nearly 1,300 sites had been identified for clean up under this program.

Cleaning contamination soil can involve the removal of the soil or on-site treatment. Since the promising results obtained with the use of bacteria to digest the oil-coated shoreline in the aftermath of the Exxon Valdez oil spill in Alaska in 1989, this strategy, which is known as bioremediation, has become an accepted means of treating contaminated soil. The approach can be beneficial if the added bacteria are able to use the soil contaminant as a food source.

Historical Background and Scientific Foundations

Although natural soil contamination has always occurred (the main example is the movement of salty water into soils in coastal regions), soil contamination is mostly associated with the expansion of human development. This dates back to the growth of industries and urban areas beginning in the late eighteenth century with the Industrial Revolution, and, later, with the use of pesticides and herbicides on agricultural land to increase crop production.

In many developed countries including the United States, the end of World War II (1939–1945) was a time of great economic prosperity and growth of industry and agriculture. The use of chemicals expanded. At the same time, environmental regulations were not nearly as stringent as in 2008. Although not widely recognized at that time, this combination led to the environmental release of chemicals that were both toxic and slow to degrade.

A turning point came in 1962, with the publication of Silent Spring. In the book, author Rachel Carson (1907–1964) warned of the consequences of the widespread and extensive use of the pesticide DDT on the natural environment and humans, given that relatively little was known of the interactions of DDT with living creatures. The outcry following the book’s publication spurred efforts to understand and deal with environmental contamination including that of soil.

Love Canal is one of the most infamous examples of soil contamination in U.S. history. The area took its name from William Love, who began to develop the land

WORDS TO KNOW

GROUNDWATER: Fresh water that is present in an underground location.

REMEDIATION: A remedy. In the case of the environment, remediation seeks to restore an area to its unpolluted condition, or at least to remove the contaminants from the soil and/or water.

in the early 1890s with the aim of building a canal, first as a means of generating electrical power and then to allow ships to bypass Niagara Falls, New York. The project was abandoned and in the 1920s the land was acquired by the City of Niagara Falls. The city elected to use the area to dispose of chemical waste, since the ground was mainly composed of clay, in which liquids do not penetrate well, and since the area was far from existing neighborhoods. Dumping expanded in the 1940s, when the Hooker Chemical and Plastics Corporation bought the site. Records show that from 1947 to 1952 alone, approximately 22,000 tons of toxic chemical waste were buried by the company. After the site reached capacity in 1952, the waste was overlaid with a thick layer of clay and the site was sold back to the city for one dollar.

In the 1950s, housing was permitted on the site; 100 homes and a school were built. By the 1960s, residents were complaining of health problems, and the presence of potential cancer-causing chemicals including benzene leaching from the soil had been detected. Initial tests did not conclusively link the chemicals to the health problems. By 1979, the EPA had established a link between the chemical waste and various health maladies including genetic damage.

The growing public outcry led U.S. President Jimmy Carter to declare a federal emergency at Love Canal in May 1980 and provide financial aid in relocating residents most at risk. Ultimately, more than 800 families were relocated.

Love Canal is an example that shows some soil contaminations are handled most quickly and economically by abandoning the site, as the scope of the clean up is too large for other options. However, the danger in this strategy is that contaminants can continue to migrate outward underground and spread over a wider area. In 2008, if a site is to be abandoned without remediation, the boundary of the contaminated site needs to contain a plastic liner that prevents escape of the contaminant.

More typically, the soil is treated. This treatment can be done on-site (also refereed to as in place or in situ remediation). This can involve drilling a well to the site of the contaminants and pumping them to the surface. However, since the contaminants may not be that easily extracted, often in situ treatment has to take the solution to the site of the problem. One example is bioremediation, where the bacteria and a food source (water can be sufficient) are pumped down into the subsurface. After a time to allow the bacteria to digest the pollutants, the subsurface water is pumped out through another well drilled nearby. The cycle is repeated until tests done on the extracted water show that the level of contaminants have dropped to an acceptable level.

In reality, the process is more difficult. But, bioremediation can be successful, particularly if carried out on contaminated soil that has been transported off-site. When treated away from the original site of contamination, the soil is placed on a liner that prevents leakage. Aside from bioremediation, soil can be exposed to steam or chemicals that disperse the pollutant from the soil.

The off-site treatment strategy has the advantage of better control over the decontamination, since the soil can be spread out to increase the surface area that can be treated, and reduce the depth of the soil. Afterward, the liner can be collected and destroyed.

Impacts and Issues

As shown by Love Canal, soil contamination is a serious health concern, causing illness (which can be long lasting) and genetic damage that can lead to cancer, developmental and nervous system damage in children (if lead or heavy metal contamination has occurred), and many other consequences. As two other examples, exposure to mercury-contaminated soil can cause kidney damage that may be permanent, and contamination with polychlorinated biphenyls (PCBs), which were widely used as coolants and insulating fluids in transformers until being banned in the 1970s, can damage the liver. Chemicals such as PCBs and DDT degrade in the environment very slowly. So, once present in soil and if not detected and removed, such chemicals can remain capable of causing illness for decades.

Contaminated soil harms other creatures as well as humans. Since an ecosystem is based on a food chain—where organisms serve as food for other organisms, which in turn can be a food source for another organism—disruption of soil populations can ripple upward through the food chain. Furthermore, some soil pollutants accumulate in animals that feed on contaminated prey (bioaccumulation). Also, as exemplified by DDT, which caused weakened egg shells in some exposed species of birds, a soil contaminant can cause population decrease due to reduced efficiency in producing offspring.

Vegetation can also be harmed by the absorption of toxins through their roots.

Until the 1970s, despite the Love Canal disaster, soil contamination was not regarded as being wide-

spread. However, as more sites were identified as a part of the Superfund initiative, the extent of soil contamination in the United States became apparent. As of 2008, the EPA has identified about 1,300 abandoned sites that are contaminated seriously enough to warrant clean-up funding. The actual number of contaminated sites is much higher—some estimates range up to 200,000.

As challenging as the U.S. soil clean-up efforts are, the situation is better than in developing nations, where disposal of toxic wastes can be less regulated, if at all, and where toxic chemicals banned in the United States are still in use (an example is DDT). In some African countries, where DDT has been sanctioned for use by the World Health Organization for the control of mosquito populations, and so as a malaria control, the application of the chemical is supposed to be strictly controlled to avoid contamination of soil and water.