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Both TCE and PCE are organic molecules composed mainly of carbon and hydrogen atoms; they are unsaturated aliphatic chlorinated hydrocarbon molecules because of the C=C double bonds, and both have the absence of the aromatic ring. Examining the properties of these molecules in comparison with the characteristics of water, shows that both solvents are colorless liquids at room temperature, are less viscous than water, will sink in water, and will dissolve slightly in water. Once discharged onto the ground surface, these chemicals will percolate very quickly into the unsaturated zone (the soil zone between the surface and the soil/water interface, also called the vadose zone). Rainwater and moisture in the soil facilitates rapid transportation of the chemicals to the groundwater. Once the chemicals reach the groundwater, they sink through the water, deeper underground. The liquids will continue to sink until they reach a less permeable layer of earthen material (called a confining layer). There, they will spread out under the influence of gravity or sink through holes in the confining layer.

Remediation of groundwater impacted by dense phase chlorinated solvents is more difficult than spills of chemicals such as gasoline or diesel fuel. Gasoline and diesel fuel are less dense than water and tend to float near the surface of the watertable. Recovery of contaminants dissolved in the water or floating on the water is undertaken through the installation of one or more recovery wells. Such wells are installed to a depth that will facilitate the creation of a cone of depression in the watertable that will capture the contaminant plume. By pumping the groundwater out to a reservoir, and then passing them through a combination activated carbon filter and vapor extraction unit, the solvents can be recovered as waste, and clean water returned to ground. However, this is not as easy as it sounds, because the nature of these solvents can make recovery very difficult at times. Since these solvents will sink though the water, sometimes hundreds of feet, the volume of contaminated water can be much greater. Just one typical industrial drum (55 gallons in size) can impact a very large area and volume

About 15 gallons of TCE can impact an area 1,000 meters in length, 100 meters in width, and 20 meters in depth with an average concentration of 100 ppb, or roughly 528 million gallons of groundwater. These factors make TCE, TCA, and PCE very difficult, and expensive, to clean up.

A further complication is the biotransformations of the hydrocarbons in soil and groundwater. In the early 1980s, chloroethene, cis- and trans-l,2-dichloroethene were detected in groundwater beneath industrial sites that traditionally only used TCE and PCE as degreasers or solvents. Because microorganisms present within the sources of recharge for the area aquifers were found to be active in the decomposition of organic matter, researchers speculated that the organisms could also act on the hydrocarbons, resulting in exotic organic byproducts that are poisonous themselves. It was concluded that some organisms can promote successive transformation of PCE to TCE, and of PCE and TCE to other compounds. Some of the chemical transformations are illustrated in Figure 17. As a result, there are several concerns when dealing with groundwater remediation. First, because of daughter compounds or by-products of environmental transformations, the identification of the source of contamination becomes much more difficult. As a result, the abatement of the source of discharge takes more time, during which additional contaminants may be introduced into the environment. Second, treatment methods developed to remove one compound may prove ineffective in removing the breakdown compounds.

Figure 17. Biotransformations of PCE and TCE.

Third, the chlorinated hydrocarbons may be transformed into contaminants that have lower MCLs or higher MCLs, making remedial activities much more difficult to accomplish. This can mean then that activated carbon, and other standard technologies such as thermal treatment and extraction may have to be used in combination to achieve proper levels of cleanup. Unfortunately many groundwater contamination sites are never fully characterized to the extent of defining satisfactory levels of cleanup. This in fact becomes an argument that once groundwater contamination has occurred, there is essentially permanent impact (or at least many, many years) to the environment and to property values. Table 10 provides you with a summary and comparison of common treatment technologies that can separate organic chemicals from liquid wastestreams.

Table 10. Compares Treatment Technologies that Separate Organics from Groundwater.

Treatment Technology

Feed Stream Properties

Output Stream Characteristics

Carbon adsorption

Aqueous solutions; typical concentration < 1 %; SS < 50ppm

Adsorbate on carbon; usually regenerated thermally or chemically

Resin adsorption

Aqueous solutions; typical concentration < 8 %; SS < 50 ppm; no oxidants

Adsorbate on resin; always chemically regenerated

Ultrafiltration

Solution or colloidal suspension of high molecular weight organics

One stream concentrated in high molecular weight organics; one containing dissolved ions

Air stripping

Solution containing ammonia; high pH

Ammonia vapor in air

Steam stripping

Aqueous solutions of volatile organics

Concentrated aqueous streams with volatile organics and dilute stream with residuals

Solvent extraction

Aqueous or non-aqueous solutions; concentrations < 10 %

Concentrated solution of organics in extraction solvent

Distillation

Aqueous or non-aqueous solutions; high organic concentrations

Recovered solvent; still bottom liquids, sludge, and tars

Steam distillation

Volatile organics, nonreactive with water or steam

Recovered volatiles plus condensed steam with traces of volatiles

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