Productive soil depends on bacteria, fungi, and other soil microbes to break down wastes and release and cycle nutrients that are essential to plants. Healthy soil is essential for growing 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 serve as a source of water pollution through leaching of contaminants into groundwater and through runoff into surface waters such as lakes, rivers, and streams.
The U.S. government has tried to address the problemof 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.
Sources of Contamination
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.
Contaminants may also include substances that occur naturally but whose concentrations are elevated above normal levels. For example, nitrogen and phosphorus-containing compounds are often added to agricultural lands as fertilizers. Since nitrogen and phosphorus are typically the limiting nutrients for plant and microbial growth, accumulation in the soil is usually not a concern.
The real concern is the leaching and runoff of the nutrients into nearby water sources, which may lead to oxygen depletion of lakes as a result of the eutrophication encouraged by those nutrients. Furthermore, nitrate is a concern in drinking water because it poses a direct risk to human infants (it is associated with 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, either the inorganic minerals or the organic matter. The attraction to the solid phase may be weak or strong. The contaminant may also volatize 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 the soil for years or wind up in the atmosphere or nearby water sources. Additionally, the 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.
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 water-ways 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 a serious ecological or health risk. When a contaminant causes a harmful effect, it is classified as a pollutant.
There are two general approaches to cleaning up a contaminated soil site: treatment of the soil in place (in situ) or removal of the contaminated soil followed by treatment (non-in situ).
In situ methods, which have the advantage of minimizing exposure pathways, include biodegradation, volatilization, leaching, vitrification (glassification), and isolation or containment.
Non-in situ methods generate additional concerns about exposure during the process of transporting contaminated soil. Non-in situ options include thermal treatment (incineration), land treatment, chemical extraction, solidification or stabilization, excavation, and asphalt incorporation.
The choice of methodology will depend on the quantity and type of contaminants, and the nature of the soil. Some of these treatment technologies are still in the experimental phase.