Soil contamination

DEFINITION: Presence of hazardous substances in soil

Soils contaminated with high concentrations of hazardous substances pose potential risks to human health and Earth’s thin layer of productive soil.

To be productive, soil depends on bacteria, fungi, and other microbes to break down wastes and release and cycle nutrients that are essential to plants. Healthy soil is essential for humankind’s ability to grow enough food for the world’s increasing population. Soil also serves as both a filter and a buffer between human activities and natural water resources, which ultimately serve as the primary source of drinking water. Soil that is contaminated may cause water pollution through the leaching of contaminants into groundwater and through runoff into surface waters such as lakes, rivers, and streams.

The US government has tried to address the problem of soil contamination by passing two landmark legislative acts. The Resource Conservation and Recovery Act (RCRA) of 1976 regulates hazardous and toxic wastes from the point of generation to disposal. The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980, also known as Superfund, identifies past contaminated sites and implements remedial action.

Soils can become contaminated by many human activities, including fertilizer or pesticide application, direct discharge of pollutants at the soil surface, leaking of underground storage tanks or pipes, leaching from landfills, and atmospheric deposition. Additionally, soil contamination may be of natural origin. For example, soils with high concentrations of heavy metals can occur naturally because of their close proximity to metal ore deposits. Common contaminants include inorganic compounds such as nitrate and heavy metals (for example, lead, mercury, cadmium, arsenic, and chromium); volatile hydrocarbons found in fuels, such as benzene, toluene, ethylene, and xylene (BTEX) compounds; and chlorinated organic compounds such as polychlorinated biphenyls (PCB) and pentachlorophenol (PCP).

Contaminants may also include substances that occur naturally when they appear in concentrations that are elevated above normal levels. Examples are substances such as nitrogen- and phosphorus-containing compounds, which are often added to agricultural lands as fertilizers. Since nitrogen and phosphorus are typically the limiting nutrients for plant and microbial growth, their accumulation in the soil is usually not a concern, but the leaching and runoff of these nutrients into nearby water sources is a problem, as it may lead to oxygen depletion of lakes. Furthermore, nitrate is a concern in drinking water because it poses a direct risk to human infants (blue-baby syndrome).

Contaminants may reside in the solid, liquid, and gaseous phases of the soil. Most will occupy all three phases but will favor one phase over the others. The physical and chemical properties of the contaminant and the soil will determine which phase the contaminant favors. The substance may preferentially adsorb to the solid phase. This may include either the inorganic (mineral) or the organic (organic matter) fraction of the soil. The attraction to the solid phase may be weak or strong. The contaminant may also volatilize into the gaseous phase of the soil. If the contaminant is soluble in water, it will dwell mainly in the liquid-filled pores of the soil.

Contaminants may remain in soils for years or wind up in the atmosphere or nearby water sources. Additionally, contaminating compounds may be broken down or taken up by the biological component of the soil. This may include plants, bacteria, fungi, and other soil-dwelling microbes. The volatile compounds may slowly move from the gaseous phase of the soil into the atmosphere. The contaminants that are bound to the solid phase may remain intact or be carried off in runoff attached to soil particles and flow into surface waters. Compounds that favor the liquid phase, such as nitrate, will either wind up in surface waters or leach down into the groundwater.

Metals display a range of behaviors. Some bind strongly to the solid phase of the soil, while others easily dissolve and wind up in surface water or groundwater. PCBs and similar compounds bind strongly to the solid surface and remain in the soil for years. These compounds can still pose a threat to waterways because, over long periods of time, they slowly dissolve from the solid phase into the water at trace quantities. Fuel components favor the gaseous phase but will bind to the solid phase and dissolve at trace quantities into the water. However, even trace quantities of some compounds can pose serious ecological and health risks. When a contaminant causes a harmful effect, it is classified as a pollutant.

One of two general approaches is used in cleaning up a contaminated soil site: treatment of the soil in place (in situ) or removal of the contaminated soil to another location for treatment (ex situ). In situ methods, which have the advantage of minimizing exposure pathways, include biodegradation, volatilization, leaching, vitrification (glassification), and isolation or containment. Ex situ methods generate additional concerns about exposure during the process of transporting the contaminated soil. Ex situ options include thermal treatment (incineration), land treatment, chemical extraction, solidification or stabilization, excavation, and asphalt incorporation. The choice of method depends on the quantity and type of contaminants and the nature of the soil.

Bibliography

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Liu, Yu-Rong, et al. "Soil Contamination in Nearby Natural Areas Mirrors That in Urban Greenspaces Worldwide." Nature Communications, vol. 14, no. 1706, 2023, pp. 1-12, doi.org/10.1038/s41467-023-37428-6. Accessed 24 Sept. 2025.

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