The presence of toxic and radioactive chemicals in soil at concentrations above trace levels poses potential risks to human health and ecological integrity. Soil can be contaminated by many human actions, including discharge of solid or liquid materials to the soil surface, pesticide and fertilizer application, subsurface release from buried tanks, pipes, or landfills, and deposition of atmospheric contaminants such as dust and particles containing lead.
Contaminants can be introduced into the soil horizon at discrete locations called point sources, or across wide areas called non-point sources. Point source contamination typical of leaking tanks, pipes, and landfills is often concentrated and causes rapid, dramatic effects in a localized region near the original spill or leak. Soil contaminated by leakage from a point source is, however, often easier to identify and remediate than the diffuse pollution caused by non-point sources like agriculture runoff or airfall from coal-burning energy plants. Governmental programs established to clean up contaminated soil in the United States (such as Superfund) have made progress in cleaning up the nation’s most polluted sites, but the technical difficulty and expense of remediation has made prevention the clear solution to soil contamination issues.
Frequently observed soil contaminants include hydrocarbons (compounds that contain both hydrogen and carbon), such as benzene, toluene, ethylene, and xylene, and alkanes found in fuels. Noxious compounds used in chemical processing such as paraffin; chlorinated organic compounds such as polychlorinated biphenyls (PCBs) that were used as coolants and lubricants in electrical equipment; pesticides and wood preservatives such as pentachlorophenol; and inorganic compounds of heavy metals like lead, cadmium, arsenic, and mercury are all additional contaminants found in soil. Soil can also become contaminated with radioactive waste. Often, soil is tainted with a mixture of contaminants. The nature of the soil, the chemical and physical characteristics of the contaminant, environmental factors such as climate and hydrology, and proximity to human agricultural and municipal water sources interact to determine the accumulation, mobility, toxicity, and overall significance of the contamination in any specific instance.
Contaminants in soil may be present as solids, liquids, or gases. When liquids are released, they move downward through the soil and may fill pore spaces, absorb onto mineral or organic surfaces, dissolve into soil water, or volatilize into the soil atmosphere. Most hydrocarbons exist in more than one location in soil. For example, insoluble soil contaminants, which do not mix with other soil constituents, travel downward to reach the saturated zone, or water table, where spaces between soil particles are filled with fluid. Their behavior in the water table depends on their density. Light compounds float on the water table, while denser compounds may sink deeper into the water. Although many hydrocarbon compounds are not very soluble in water, even low levels of dissolved contaminants may produce unsafe or unacceptable groundwater quality. Other contaminants such as inorganic salts, nitrate fertilizers for example, are highly soluble and move rapidly through the soil environment. Metals like lead, mercury, and arsenic demonstrate a range of behaviors; some chemically bind to soil particles and are held within a small region, while others can dissolve in water and move far from their point of entry.
Soil contaminants that enter the water table can be transported by the water flow. The flow can move horizontally and vertically away from the contaminant source. Movement up or down in the groundwater is possible, forming a three-dimensional zone of contaminant that decreases in concentration with distance from the source and time since introduction of the contaminant. A portion of the contaminant is left behind as groundwater flow passes, resulting in a longer-term contamination of the soil after the contaminant plume has receded. If the groundwater velocity is fast, the zone of contamination may spread quickly, potentially affecting wells, surface water, and plants that extract the contaminated water in a wide area.
Over years or decades, especially in sandy and other porous soils, groundwater contaminants and leachate may be transported over distances of miles, resulting in a situation that is extremely difficult and expensive to remedy. In such cases, immediate action is needed to contain and clean up the contamination. However, if the soil is largely comprised of finegrained silts and clays, contaminants will spread slowly. Comprehensive understanding of site-specific factors is important in evaluating the extent of soil contamination, and in selection of an effective cleanup strategy.
Prior to the 1970s, inappropriate waste disposal practices like dumping untreated liquids in lagoons and landfills were common and widespread. Some of these practices were undertaken with willful disregard for their environmental consequences (and occasionally for existing regulations), but many types of soil contamination occurred as a result of scientific ignorance. The toxicity of many contaminants, including PCBs and methyl mercury, was discovered long after the chemicals were spilled, dumped, or sprayed into soils. The ability of groundwater flow to transport contaminants was likewise unknown until decades after many contaminants had flowed away from unlined landfills, sewage outlets, and chemical dumps.
The presence and potential impacts of soil contamination were finally brought to public attention after well-publicized disasters at Love Canal, New York, the Valley of the Drums, Kentucky, and Times Beach, Missouri. Congress responded by passing the Comprehensive Environmental Response, Compensation, and Liability Act in 1980 (commonly known as CERCLA, or Superfund) to provide funds with which to remediate contamination at the worst sites of point-source pollution. After five years of much litigation but little action, Congress updated CERCLA in 1986 with the Superfund Amendments and Reauthorization Act.
In 2003, a total of 1, 300 sites across the United States were designated as National Priorities List (NPL) sites that were eligible under CERCLA for federal cleanup assistance. By 2005, the number of qualifying sites had risen to over 1, 600. These Superfund sites are considered the nation’s largest and most contaminated sites in terms of the possibility for adverse human and environmental impacts. They are also the most expensive sites to clean. While not as well recognized, numerous other sites have serious soil contamination problems.
The U.S. Office of Technology Assessment and the Environmental Protection Agency (EPA) have estimated that about 20, 000 abandoned waste sites and 600, 000 other sites of land contamination exist in the United States. These estimates exclude soils contaminated with lead paint in older urban areas, the accumulation of fertilizers and pesticides in agricultural land, salination of irrigated soils in arid regions, and other classes of potentially significant soil contamination. Federal and state programs address only some of these sites.
Currently, CERCLA is the EPA’s largest program, with expenditures exceeding $3 billion during the 1990s. However, this is only a fraction of the cost to government and industry that will be needed to clean all hazardous and toxic waste sites. Mitigation of the worst 9, 000 waste sites is estimated to cost at least $500 billion and to take at least 50 years to complete. The problem of contaminated soil is significant not only in the United States, but in all industrialized countries.
U.S. laws such as CERCLA and the Resource Conservation and Recovery Act (RCRA) also attempt to prohibit practices that have led to extensive soil contamination in the past. These laws restrict disposal practices and ensure that the money for cleanup, personal injury, and property damage costs is recovered. Furthermore, the laws discourage polluters from deliberately contaminating soil by making it possible that offenders could go to jail.
The cleanup or remediation of contaminated soil takes two major approaches: (1) source control and containment and (2) soil and residual treatment and management. Typical containment strategies involve isolating potential contamination sources from surface and groundwater flow. Installation of a cover over the waste limits the entry of rain and snowmelt and decreases the amount of contaminant that escapes to the surrounding soil. Scrubbers and filters on energy plant smokestacks prevent contamination of rainwater by dissolved chemicals and particulate matter. Vertical walls may control horizontal transport of pollutants in near-surface soil and groundwater. Clay, cement, or synthetic liners encapsulate soil contaminants. Groundwater pump-and-treat systems, sometimes coupled with injection of clean water, hydraulically isolate and manage contaminated water and leachate. Such containment systems reduce the mobility of the contaminants, but the barriers used to isolate the waste must be maintained indefinitely.
Soil decontamination can involve removal of the soil and its transport to a site where the decontamination is done. In some cases, the soil can be treated at the original site. Such so-called in situ options include the use of heat, microorganisms, and chemical or physical techniques. Heating the soil can break the chemical bonds between contaminants and soil particles. Biological treatment includes biodegradation by soil fungi and bacteria to produce carbon dioxide, other simple minerals, and water. This process is also called mineralization. Biodegradation may, however, produce long-lived toxic intermediate products and even contaminated organisms. Separation technologies attempt to isolate contaminants from fluids and force them to the surface.
The number of potential remediation options is large and expanding due to an active research program driven by the need for more effective, less expensive solutions. The selection of an appropriate cleanup strategy for contaminated soil requires a thorough characterization of the site and an analysis of the cost-effectiveness of suitable containment and treatment options. A site-specific analysis is essential, because the geology, hydrology, waste properties, and source type determine the extent of the contamination and the most effective remediation strategies. Often, a demonstration of the effectiveness of an innovative or experimental approach may be required by governmental authorities prior to its full-scale implementation. In general, large sites use a combination of remediation options. Pump-and-treat and vapor extraction are the most popular technologies.
The cleanup of contaminated soil can be expensive and can pose a risk of further contamination. In general, containment of a spill is cheaper and has fewer environmental consequences than actually trying to treat the soil to remove the contaminant. Excavation and incineration of contaminated soil can cost $1, 500 per ton, leading to total costs of many millions of dollars at large sites. (Superfund clean-ups have averaged about $26 million.) In contrast, small fuel spills at gasoline stations may be mitigated using vapor extraction at costs under $50, 000. However, in situ options may not achieve cleanup goals.
Unlike air and water, which have specific federal laws and regulations detailing maximum allowable levels of contaminants, no levels have been set for contaminants in soil. Instead, the federal Environmental Protection Agency and state environmental agencies use subjective, case-specific criteria to set acceptable contaminant levels. For Superfund sites, cleanup standards must exceed applicable or relevant and appropriate requirements (ARARs) under federal environmental and public health laws. Cleanup standards are often determined by measuring background levels of the offending contaminant in similar, nearby, unpolluted soil.
In some cases, soil contaminant levels may be acceptable if the soil does not produce leachate with concentration levels above drinking water standards. Such determinations are often based on a test called the Toxics Characteristic Leaching Procedure, which mildly acidifies and agitates the soil, followed by chemical analysis of the leachate. Contaminant levels in the leachate below the maximum contaminant levels (MCLs) in the federal Safe Drinking Water Act are considered acceptable. Finally, soil contaminant levels may be set in a determination of health risks based on typical or worst case exposures. Exposures can include the inhalation of soil as dust, ingestion (generally by small children), and direct skin contact. The flexible definition of acceptable toxicity levels reflects the complexity of contaminant mobility and toxicity in soil, and the difficulty of pinning down acceptable and safe levels.
Calabrese, Edward J., Paul T. Kostecki, and James Gragun. Contaminated Soils, Sediments and Water: Science in the Real World. New York: Springer, 2004.
Sherrow, Victoria. Love Canal: Toxic Waste Tragedy. Berkeley Heights, NJ: Enslow Publishers, 2001.
Agency for Toxic Substances and Disease Registry. January 6, 2003 <http://www.atsdr.cdc.gov/> (accessed October 23, 2006).
United States Environmental Protection Agency. “Superfund.” January 7, 2003 <http://www.epa.gov/superfund//> (accessed October 23, 2006).