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Soil Remediation

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Soil Remediation By Ben McLaughlin On August 6, 1983, when most CWRU students were only toddlers, South Africa saw one of the worst oil spills in its history when the hull of the Castillo de Bellver ruptured, pouring 1,392,700 barrels of oil into the water off of the coast. The Castillo de Bellver was one of 35 oil tankers to dump oil into the waters of the world during 1983. Since then, countless other oil tanker accidents have happened. Environmental and civil engineers have been working for years to improve the clean-up methods employed after these disasters occur. But while oil tanker accidents attract great media attention, they aren't the most common contamination problem. Environmental problems caused by oil from cars and trucks, leaky containers, industrial accidents, and poorly disposed of wastes are much more common cause for concern. While they don't cover vast areas of ocean with a shadowy dread, spills that contaminate dirt can eventually taint our drinking water. In 1995 the US Air-Force alone had 2,000 sites in need of soil remediation.
The most common soil contaminants are petroleum-based. Hydrocarbons from diesel fuel and gasoline are widespread problems, as are polyaromatic hydrocarbons (PAH). Many PAHs are known carcinogens, and others are suspected problem chemicals that need to be kept from contaminating drinking water. Chemicals tend to spread through soil by diffusion and convection. Diffusion is a molecular transport that is motivated by differences in concentration. Chemicals are a lot like people, in that if you put a bunch of them in one area but there aren't many in the surrounding region they'll slowly sprawl out into the unpopulated region. Convection is a means of molecular motion where the driving force is provided by a fluid such as rain or wind. Water will pick up pollutant particles as it soaks into the earth and carry them further from the initial spill. Fortunately, the mechanisms that spread pollutants can also be used to remove them.
There are two distinct classes of soil remediation: in-situ, or on-site, and ex-situ, or off- site. On-site cleanups are often preferred because they are cheaper. On the other hand, excavating a contaminated area and transporting it to a remote site before cleaning it can often be more complete. Ex-situ remediation also has the added bonus of taking the bulk of contaminants off-site before they can spread further. In addition, in-situ situations are limited because only the topside of the soil is accessible. Some drilling can be done, but ultimately the contaminants must fight gravity and surface. These environmental limitations force in-situ remediation to fall into three categories: washing, venting, and bioremediation. Off-site facilities have the luxury of more complete control over the cleaning chemical processes. Soil can be isolated in a controlled environment, without weather problems. More caustic chemicals can be used and acid leaching can be conducted ex-situ without harming the environment. Soil is often put into a sealed dome, and cleansing fluids are pumped in while wastes are pumped out and treated.

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