Contaminated Soil

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 infiltration of rain and snowmelt, and decreases the amount of potential leachate in the surrounding soil. Scrubbers and filters on energy plant smokestacks prevent contamination of rainwater by dissolved chemicals and particulate matter. Vertical slurry 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 and residual treatment strategies are after-the-fact remediation methods that reduce the toxicity and volume of soil contaminants. Treatment procedures are generally categorized as either extractive or in situ measures. Extractive options involve physical removal of the contaminated soil, off-site treatment of the contaminants by incineration or chemical neutralization, and disposal in a landfill. In situ processes treat the soil in place.

In situ options include thermal, biological, and separation/extraction technologies. Thermal technologies involve heating soils in place. The thermal desorption process breaks the chemical bonds between contaminants and soil particles. Vitrification, or glassification, involves melting the mineral component of the soil and encapsulating the contaminants in the resolidified glassy matrix. Biological treatment includes biodegradation by soil fungi and bacteria, ultimately rendering contaminants into 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 pore fluids, and force them to the surface. Soil vapor extraction can successfully remove volatile organic compounds by enhancing volatilization with externally-forced sub-surface air flow. Stabilization, or chemical fixation, uses additives that bind dissolved organic pollutants and heavy metals to eliminate contaminated leachate. Soil washing and flushing processes use dispersants and solvents to dissolve contaminants such as PCBs and enhance their removal. Groundwater pump-and-treat schemes purge the contaminated soil with clean water in a flushing action. A number of these in situ approaches are still experimental, notably soil vitrification and enhanced bioremediation using engineered microorganisms.

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.